Meeting In Memory Of Piero Elter the relationships between northern apennine and western alps: state of the art fifty years after the “ruga del bracco” Abstract Volume volume Editors: Michele MARRONI, Giancarlo MOLLI, Luca PANDOLFI and Piero PERTUSATI - Dipartimento di Scienze della Terra, Università di Pisa Pisa, June 26-27, 2014 Franco Marco ELTER - Dipartimento di Scienze della Terra dell’Ambiente e della Vita, Università di Genova MEETING IN MEMORY OF PIERO ELTER the relationships between northern apennine and western alps: state of the art fifty years after the “ruga del bracco” Pisa, June 26-27, 2014 ABSTRACT VOLUME volume Editors: Michele MARRONI, Giancarlo MOLLI, Luca PANDOLFI and Piero PERTUSATI - Dipartimento di Scienze della Terra, Università di Pisa Franco Marco ELTER - Dipartimento di Scienze della Terra dell’Ambiente e della Vita, Università di Genova Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Collision, Collapse and Delamination: The making of the Alps-Apennine connection Andrea Argnani ISMAR-CNR, Bologna, Italy The relationship between the Alps and Apennines represents a critical issue of the Italian geology and has major relevance on the geological evolution of the Central Mediterranean region (e.g., Elter and Pertusati, 1973; Argnani, 2009). The long term evolution of the Alps-Apennine connection can be related to the Mesozoic palaeogeography of the region (Fig. 1A). The collision of the Adria promontory played a key role in imposing the SW-ward extent of the Alpine belt and in locating of collapse of the Alpine belt during the progressive NE-ward shift of the promontory along the plate boundary. The NE-ward shift of continental collision that results from kinematic reconstruction represents a viable alternative to the occurrence of a microcontinent located between Africa and Eurasia (e.g., Molli and Malavieille, 2011), for which there is little room in geological reconstructions (Argnani, 2012; van Hinsbergen et al., 2014). The collapse of the Alpine belt was promoted by NEward motion of Adria relative to Eurasia, that placed the orogen above an Appennine oceanic subduction (Fig. 1B). A major effect of this change in polarity of subduction was to disactivate the collisional regime since late Eocene, as illustrated in the geological evolution of Alpine Corsica. A similar setting is currently present offshore north Taiwan, as the colliding Luzon volcanic arc shifted south-ward along the plate boundary. The plate kinematic evolution of the last 10 Myr or so of the system that encompasses the collision of the Luzon arc with Eurasia in Taiwan, passing to the north to the retreating subduction of the Philippine Sea plate, represents the best current analogue of the Oligocene-Miocene Alps-Apennine relationships (Argnani, 2009, 2012). The post-collisional extension of the Alpine belt of Corsica occurred before the rollback of the Adriatic slab, and was driven by gravity during the change from continent collision to oceanic subduction with opposite polarity. As a result of this kinematic evolution, the region comprised between the Alpine and Apennine belts, and now mostly located in the northern Tyrrhenian Sea, is floored by orogenic units (Argnani, 2009). Within the reduced convergence that followed the continental collision of Adria, the oceanic lithosphere of the Alpine Tethys was subject to sinking and rollback, creating the backarc opening of the Balearic basin, and starting the related calc-alkaline volcanism. The consuption of the African (Adriatic) oceanic lithosphere was completed during the rotation of Corsica-Sardinia, leading to a soft-collsiion with the Adriatic continental margin. The subsequent evolution of the Apennine orogen was controlled by continental delamination of the Adriatic lithosphere (Argnani, 2002). The most notable effect of this last stage evolution has been the onset of the Tuscan magmatism (Serri et al., 1993). The east-ward extent of the Tuscan magmatism can be taken as conservatively indicating the hinge of the subducted Adriatic lithosphere. An interesting implication concerns the structure of the Adriatic lithospheric mantle. It is known that the SKS mantle anisotropy in Tuscany describes a pattern of NW-trending fast axes, which contrasts with the supposed retreat of the Adriatic slab (Salimbeni et al., 2013). To account for this “anomalous” pattern it has been suggested that it is related to toroidal mantle flow, that should bring in material from the mantle below the Adriatic slab, entering counterclock-wise around the northern tip of the Adriatic slab. The width of the area with northwest-trending fast axes, however, seems exceedingly large, and the lack of fast directions with retreating (northeast-ward) signal is a bit intriguing, as slab retreat is suggested as driving the mantle flow. An alternative interpretation is here proposed, which considers the mantle fabric related to the Mesozoic extension. The axes of fast directions, in fact, result sub-parallel to the trend of the main fracture zone (Adria Fracture Zone; Fig. 1A) that bounds to the west the Adria promontory, and that is inferred to link the two subduction zones with opposite polarity of the Alps and Apennines (Argnani et al., 2006; Argnani, 2012). 3 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Figure 1. Frames of tentative palaeogeographic reconstruction. A) Santonian (83 Ma). The Adria continental promontory has a large oceanic embayment on its western side. Continental collision s.s. was limited to the Alps and Corsica, and did not extend to the SW, where an Apennine vergent oceanic subduction was present. The polarity of subduction changes across the Adria Fracture Zone . B) Chattian (25.5 Ma). The Apennine subduction has progressively taken over in Alpine Corsica, as Adria moves N-ward, leaving an inactive subduction. The progressive substitution from Alpine to Apennine subduction caused a sector of the Alpine belt to collapse gravitationally (gray field). REFERENCES - Argnani, A. (2002). The Northern Apennines and the Kinematics of Africa–Europe convergence. Boll. Soc. Geol. It., 1, 47–60 (Vol. Spec). - Argnani, A. (2009). Plate tectonics and the boundary between Alps and Apennines. It. J. Earth Sci., 128, 317–330. - Argnani, A. (2012). Plate motion and the evolution of Alpine Corsica and Northern Apennines. Tectonoph., 579, 207–219. - Argnani, A., Fontana, D., Stefani, C., Zuffa, G.G. (2006). Palaeogeography of the Upper Cretaceous–Eocene carbonate turbidites of the Northern Apennines from provenance studies. In: Moratti, G., Chalouan, A. (Eds.), Tectonics of the Western Mediterranean and North Africa: Geological Society, London, Special Publications, 262, pp. 259–275. - Elter, P. & Pertusati, P. (1973). Considerazioni sul limite Alpi-Appennino e sulle sue relazioni con l’arco delle Alpi Occidentali. Memorie della Società Geoligica Italiana 12, 359–375. - Molli, G. & Malavieille, J. (2011). Orogenic processes and the Corsica/Apennines geodynamic evolution: insights from Taiwan. Int. J. Earth Sci., 100, 1207–1224. - Salimbeni, S., Pondrelli, S. & Margheriti, L. (2013). Hints on the deformation penetration induced by subductions and collision processes: Seismic anisotropy beneath the Adria region (Central Mediterranean). J. Geoph. Res., 118, 1–13, doi:10.1002/2013JB010253. - Serri, G., Innocenti, F. & Manetti, P. (1993). Geochemical and petrological evidence of the subduction of delaminated Adriatic continental lithosphere in the genesis of the Neogene-Quaternary magmatism of central Italy: Tectonoph., 223, p. 117–147. - van Hinsbergen, D.J.J, Vissers, R.L.M. & Spakman, W. (2014). Origin and consequences of western Mediterranean subduction, rollback, and slab segmentation. Tectonics, doi: 10.1002/2013TC003349 4 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 A new stratigraphic and tectonic model of the South eastern Monte Antola Unit and its relationship with the Monte Gottero Unit (eastern Liguria) Pietro Balbi, Franco Marco Elter, Egidio Armadillo and Michele Piazza DISTAV - Department of Earth Science, Environment and Life, University of Genova, Italy Within the complex and frequently discussed junction between the Alps and the Apennines, two cretaceous turbiditic Tectonic Units outcrop: M. Antola (UA) and M. Gottero (UG). The study area is located in eastren Liguria (Northern Italy) between the cities of Genova and Chiavari, where the Monte Antola Unit (Upper Campanian – Paleocene, Corsi et al., 2001), the upper tectonic unit of the Ligurian stack, overlays the Internal Ligurian Units, particularly the Monte Gottero Unit (Valanginian – Paleocene, Marroni, 1991). The aim of this work is to present new stratigraphic data on the UA and to discuss its stratigraphic and tectonic relationships with the UG in the area of the eastern Fontanabuona Valley (GE). These two units are made up of upper Cretaceous calcareous and silicoclastic Flysch, and they show an intense pre-Oligocene brittle-ductile deformation followed by a mainly fragile deformation. In the study area the UA is usually considered to be very thin (up to about 200 m) and thrusted over the basal complex of the UG (Lavagna shales and Manganesiferi shales). Recent literature (e.g. Marroni et al., 2004) describes an about N100° thrust fault in the southern side of Fontanabuona Valley, that tectonically displaces the Antola Formation directly onto the UG with a top-to-NE movement, but an accurate and detailed field survey doesn’t confirm this interpretation. Evidences of stratigraphic contact between the Calcari del M. Antola Formation and the pelagic rocks underneath suggest a new stratigraphy of the UA and a slightly northward shift of the thrust surface. It is therefore possible to draw a new and more complete lithostratigraphy of the Unit in this area that can help to reconstruct its Cretaceous position. The above-mentioned pelagic rocks lie at the base of the UA and are here informally named respectively from the bottom to the top “argilloscisti di Maxena formation” and “Sanguineto member”. These rocks show interesting lithological and chronological analogies with some basal formations of the UG, respectively with the “Scisti Ardesiaci di M. Verzi Formation” and the “Scisti Manganesiferi Formation”. It is interesting to underline the peculiar shape as drawn in the map of this part of the UA, NW-SE oriented while the rest of the Unit is NNE-SSW oriented. This fits nice- ly with the very well known differences in orientation of the main fold axis of the south eastern UA relatively to the main body of the Unit (Marini, 1981), being NW-SE oriented while the others trend NNE-SSW. Within this new scenery, we propose that the eastern part of the UA is certainly recognizable as a Cretaceous Flysch, but it shows important stratigraphic differences from the proper UA. In the study area it is possible to state that the Gottero and Antola Units have an approximately coeval and partially common sedimentary history, followed by a similar succession of deformational events (mainly compressional, known as the “Ligurian Deformational Phases”) during which the UA overthrusted the UG with a top to N-NE movement. During these events, because of the thrust surface, the UA got antiformally folded while the UG got synformally folded and experienced an epizonale metamorphism (Bonazzi et Al. 1987). REFERENCES - Bonazzi A., Cortesogno L., Galbiati B., Reinhardt M., Salvioli Mariani E., Vernia L (1987). Nuovi dati sul metamorfismo di basso grado nelle unità liguridi interne e loro possibile significato nell’evoluzione strutturale dell’Appennino settentrionale - Acta. Nat. Ateneo Parmense, 23, 17-47. - Corsi B., Elter F.M., Giammarino S. (2001). Structural fabric of the Antola Unit (Riviera di Levante, Italy) and implications for its alpine versus apennine origin. Ofioliti, 26 (1), 1-8. - Marini M. (1981). Analisi geologico- strutturale ed interpretazione paleogeografica e tettogenica dei calcari del M. Antola (Appennino Ligure). – Ofioliti, 6 (1), 119-150. - Marroni M., 1991. Deformation history of the Mt. Gottero Unit (Internal Ligurid Units, Northern Appennines). Boll. Soc. Geol. It., 110, 727-736. - Marroni M., Meneghini F., Pandolfi L (2004). From accretion to exhumation in a fossil accretionary wedge: a case History from Gottero Unit (Northern Appennines, Italy). Geodinamica Acta 17, 41-53. 5 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Broken and dismembered formations in the Monviso Meta-ophiolite Complex (Western Alps) Gianni Balestro(1), Andrea Festa(1), Paola Tartarotti(2) (1) Dipartimento di Scienze della Terra, Università di Torino, Italy (2) Dipartimento di Scienze della Terra, Università di Milano, Italy The Monviso Meta-ophiolite Complex (axial sector of the Western Alps) represents a eclogitized remnant of the Ligurian-Piedmont oceanic lithosphere, tectonically superposed on the Dora Maira Unit (i.e. the European continental margin), and tectonically overlain by the Queyras Schistes Lustrés (i.e. the fossil accretionary wedge) (CASTELLI et alii, 2014, and references therein). The Monviso Meta-ophiolite Complex consists of different W-dipping tectonic units that are currently alternatively interpreted as i) a pile of imbricated tectonostratigraphic units (LOMBARDO et alii, 1978), ii) a fossilized serpentinite subduction channel wherein ophiolite “blocks” were tectonically and chaotically juxtaposed (GUILLOT et alii, 2004), and iii) an original almost continuous portion of oceanic litosphere subducted to 80 km depth and deformed by eclogite-facies shear zones (ANGIBOUST et alii, 2011). These tectonic units consist of serpentinite, Mg-Al and Fe-Ti metagabbros, basalt-derived metabasite and different type of metasediments, and were affected by three main deformation phases corresponding to i) a first subduction-related deformation phase (D1), represented by an early eclogitic foliation (S1) overprinting primary surfaces, ii) a subsequent collision-related deformation (D2), resulting in non-cylindrical W-verging folds that developed the blueschist- to greenschist-facies regional foliation (S2), and iii) a late-metamorphic deformation (D3) developing SW- and NE-dipping conjugate transtensional shear zones (BALESTRO ET ALII, in press). The lower part of the northern Monviso Meta-Ophiolite Complex (i.e. the Pellice Valley; BALESTRO et alii, 2011) particularly consists of serpentinites and metagabbros covered by a quite heterogeneous metasedimentary succession, made up of calcschist, micaschist and quartzite, metabreccia and metasandstone of gabbroic composition, and carbonate-bearing mafic schist. This metasedimentary succession is peculiarly characterized by the occurrence of block in matrix structures (sensu FESTA et alii, 2010), corresponding to broken and dismembered formations. The latters differ each other in the degree of stratal disruption (from dismembered interbeds to isolated blocks) of the massive metabreccia and metasandstone horizons that are enveloped in a matrix of calcschist, micaschist and mafic schist. Both broken and dismembered formations are the product of polyphasic deformation related to the superposition of D1, D2 and D3 structures. During the D2 deformation, the S1-parallel metasedimentary succession was affected by progressive layer-parallel extension, boudinage and non-cylindrical folding, that induced significant stratal disruption, shearing and transposition of rootless fold hinges. Subsequent D3 deformation favored mixing processes by dissecting the earlier disrupted succession with extensional shearing concentrated within thin (centimeters-to decimeters thick) and penetrative shear zones. The recognition of the mechanisms forming these blockin-matrix structures provided us important information on both original stratigraphy and tectonic evolution of the Monviso Meta-ophiolite Complex. Documenting mode and time of the processes forming block-in-matrix units, is thus a relevant approach in studying the Alpine meta-ophiolites and modeling exhumed convergent plate margins. REFERENCES - Angiboust S., Agard P., Raimbourg H., Yamato P. & Huet B. (2011), Subduction interface processes recorded by eclogite-facies shear zones, Lithos, 127, 222-238. - Balestro G., Fioraso G. & Lombardo B. (2011), Geological map of the upper Pellice Valley (Italian Western Alps), Journal of Maps, 2011, 634-654. - Balestro G., Lombardo B., Vaggelli G., Borghi A., Festa A. & Gattiglio M. (in press), Tectonostratigraphy of the northern Monviso Meta-ophiolite Complex (Western Alps). Italian Journal of Geosciences. - Castelli D., Compagnoni R., Lombardo B., Angiboust S., Balestro G., Ferrando S., Groppo C., and Rolfo F., 2014. The Monviso meta-ophiolite Complex: HP metamorphism of oceanic crust & interactions with ultramafics. Geological Field Trips, ISSN: 2038-4947.8.35. 6 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 - Festa A., Pini G.A., Dilek Y. & Codegone G. (2010), Mélanges and mélange forming processes: Historical overview and new concepts. International Geology Review, 52, 1040–1105. - Guillot S., Schwartz S., Hattori K., Auzende A. & Lardeaux J. (2004), The Monviso ophiolitic Massif (Western Alps), a section through a serpentinite subduction channel. In Beltrando M., Lister G., Ganne J., and Boullier A. (eds.), Evolution of the western Alps: insights from metamorphism, structural geology, tectonics and geochronology. Journal of the Virtual Explorer, 16, 3. 7 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Geological setting of the southern termination of the western Alps arc (Maritime and Western Ligurian Alps, Nw Italy) Barale L.(1), Battaglia S.(2), Bertok C.(1), d’Atri A.(1), Ellero A.(2), Leoni L.(3), Martire L.(1), Piana F.(4) (1) Dipartimento di Scienze della Terra, Università di Torino, Italy (2) CNR - Istituto di Geoscienze e Georisorse, Pisa, Italy (3) Dipartimento di Scienze della Terra, Università di Pisa, Italy (4) CNR - Istituto di Geoscienze e Georisorse, Torino, Italy The southern termination of the Western Alps arc (Maritime and Western Ligurian Alps) tails off the orogenic belt along a complex transpressive system (Michard et al., 2004) comprised between the External Briançonnais Front, the boundary faults of the Argentera Massif and the Limone-Viozene Zone (Piana et al., 2009). This region, in which the transition from internal high pressure metamorphic rocks to external very low grade and non-metamorphic rocks occurs, consists of an assemblage of juxtaposed tectonic units elongated on average ESE-WNW direction (Malaroda, 1970; Gidon, 1972) and separated by double-vergent (to SW and NE) thrusts and strike-slip faults (among which the Stura Fault Auct. is the more largely known, although strongly debated). The stratigraphic and structural setting of these units has been recently revised by mapping their macroscale boundary faults and evaluating the affinity degree of their stratigraphic successions with those of the main paleogeographic domains (Briançonnais, Subbriançonnais, Dauphinois, Provençal domains and the Eocene-Oligocene Alpine foreland basin) (Piana et al., 2009; Piana et al., 2014). A deep revision of the relations between the Alpine tectonic units and the Meso-Cenozoic paleogeographic domains has been done. Great efforts have been also dedicated in discovering the pre-Alpine physiographic features such as paleo-escarpments and stratigraphic discontinuities that controlled the facies lateral variations within each unit (Bertok et al., 2011; 2012). This resulted in a partial re-interpretation of the overall tectonic setting and evolution. Researches also focused on Mesozoic faults and fractures activity by studying the petrogenetic processes induced by hosted fluid circulation, consisting in the dolomitization of huge rock masses (Barale et al., 2013; Martire et al., 2014) and generation of hydrothermal marbles (Rossetti et al., 2014). Furthermore, the crucial role, for kinematic reconstruc- tions, of the Early Oligocene chaotic complexes of the Alpine foreland basin succession was investigated (Perotti et al., 2012). Finally, reconstruction of the metamorphic evolution has been performed by analyses of the illite and chlorite Crystallinity Index (Piana et al., 2014) in some tectonic units placed above and below the External Briançonnais Front (recording both anchimetamorphic conditions), the tectonic slices of Helminthoides Flysch involved in the main ESE-WSW transpressive shear zones (also affected by anchimetamorphic conditions) and the S.Remo-M.Saccarello Ligurian Unit placed at the geometric top of the Western Ligurian Alps, that conversely show only diagenetic conditions. In conclusion, our researches indicate that the southern termination of the Western Alps was characterized by: - marked tectonic activity since Mesozoic times (important hydrothermal activity and tectonic control on sedimentary evolution in the Cretaceous-Paleogene time span); - deposition of Helminthoides Flysch-type successions on the Provençal European paleomargin and involvement of Helminthoides Flysch slices in the regional transpressive shear zones since the very beginning of the Alpine tectonic stage; - persistence of convergent-wrench tectonics since Early Cretaceous up to at least Oligocene times; - absence of metamorphic gap across the External Ligurian Briançonnais Front and moderate dip-slip displacement across it, if compared with the width of the associated shear zone system. These data and interpretation should be considered for the reconstruction of the Alpine-Apennine system evolution, where major E-W transfer zones have been often invoked as crucial kinematic features connecting, since the Late Eocene, the Ligurian Alps and Northern Apennines structural domains. 8 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 REFERENCES - Barale, L., Bertok, C., d’Atri, A., Domini, G., Martire, L. & Piana, F. (2013), Hydrothermal dolomitization of the carbonate Jurassic succession in Provençal and Subbriançonnais Domains. Comptes-Rendus Geoscience, 345, 47–53. - Bertok, C., Martire, L., Perotti, E., d’Atri, A. & Piana, F. (2011), Middle–Late Jurassic syndepositional tectonics recorded in Ligurian Briançonnais. Swiss J. Geosci., 104, 237–255. - Bertok, C., Martire, L., Perotti, E., d’Atri, A. & Piana, F. (2012), Kilometre-scale palaeoescarpments as evidence for Cretaceous synsedimentary tectonics in the External Briançonnais. Sedim. Geol., 251, 58–75. - Gidon, M. (1972), Les chaînons briançonnais et subbriançonnais de la rive gauche de la Stura entre Bersezio et le Val de l’Arma (Italie). Géol. Alpine, 48, 87–120. - Malaroda, R. (Ed) (1970), Carta Geologica del Massiccio dell’Argentera, 1:50.000. Mem. Soc. Geol. Ital., 9. - Martire L., Bertok C., d’Atri A., Perotti E. & Piana F. (2014), Selective dolomitization by syntaxial overgrowth around detrital dolomite nuclei: a case from the Jurassic of the Ligurian Brianconnais. Journ. Sed. Res. 84, 40-50. - Michard, A., Avigad, D., Goffé, B. & Chopin, C. (2004), The high-pressure metamorphic front of the south Western Alps. Schweiz. Min. Petr. Mitt., 84, 215–235. - Perotti, E., Bertok, C., d’Atri, A., Martire, L., Piana, F. & Catanzariti, R. (2012), A tectonically-induced Eocene sedimentary mélange in the West Ligurian Alps, Italy. Tectonophysics, 568–569, 200–214. - Piana, F., Musso, A., Bertok ,C., d’Atri, A., Martire, L., Perotti, E., Varrone, D. & Martinotti, G. (2009). New data on post-Eocene tectonic evolution of the External Ligurian Briançonnais. It. J. Geosci., 128, 353–366. - Piana, F., Battaglia, S., Bertok, C., d’Atri, A., Ellero, A., Leoni, L., Martire, L. & Perotti, E. (2014), Illite and chlorite cristallinity as a constraint for the kinematic evolution of the External Briançonnais front in the Western Ligurian Alps. Accept. paper, It. J. Geosci. - Rossetti, P., Barale, L., Bertok, C., d’Atri, A., Martire, L. & Piana, F. (2014), The Valdieri marbles: the result of a localised recrystallization and metasomatism related to a focused flow of REE-rich fluids. Congr. SGI-SIMP, Milano, Sept. 10–12, 2014. 9 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Geological and geophysical evidences for diapirism in south-eastern Sicily (Italy) and implications on paleo-tectonics of the western Ionian domain Giovanni Barreca, Carmelo Monaco Department of Biological, Geological and Environmental Science, University of Catania, Italy A recent investigation on the northern margin of the Hyblean Plateau in south-eastern Sicily (S. Demetrio horst) highlights the occurrence of a clayey diapiric intrusion, previously unidentified into the foreland carbonate series. A ~270 long and ~30 m high quarry wall-section provided the opportunity to study excellent exposure of the rocks and their reciprocal geometrical relations. Results from field analysis revealed that rocks consist of volcanic products and sedimentary beds resting on greenish chaotic clays, that are unusual deposits for the Hyblean Plateau. Green clays are internally affected by contractional deformation whereas the adjacent rocks are deformed only by a set of low-angle and domino-arranged normal faults. The occurrence of two distinct deformation patterns (contraction and extension), confined within the same structural level, suggests that their nucleation could be intimately related. Geometry of normal faults (LANF) indicates that they nucleated in a gravity gliding deformation context triggered by the strong uplift due to the diapiric emplacement. Available seismic reflection sections confirms that mud diapirism occurs also in the adjacent marine setting, about 18 km southeast from that observed on-land. Previous petrological studies (Manuella et al., 2012) elucidated the composition of the green clays that consist of hydrocarbons-rich serpenitinite-bearing muds interpreted as the product of flocculation from a mixture of hot Si-rich fluids and cold seawater in a Early to Middle Triassic (~246 My, see Sapienza et al., 2007) serpentinite-hosted hydrothermal system (Scribano, 2006). Taking into account that clays formed as a result of alteration of serpentinite rocks and that serpentinization commonly takes place in oceanic ridges (Karson et al., 1987; Cannat et al., 1995) or along highly fractured transform zone (Cann et al., 2001; Ildefonse et al., 2007) an ancient oceanic transform (Permian-Triassic according to Catalano et al., 2001; Early to Middle Permian according to Vai, 2003), probably linking distinct segments of the Ionian oceanic ridge (Catalano et al., 2001),can be imaged as a source environment for serpentinites and their associated products of alteration (e.g. the green clays). The mud diapirs originated from the uprising of pre-existing serpentinite bodies and others products of alteration probably developed along the inferred ancient ridge-transform intersection (here named the “Hyblean Transform”) where a hydrothermally altered mantle wedge occurred. This interpretation is supported by seismic, magnetic and gravimetric anomalies beneath the analyzed area and has implications on its geodynamic evolution. REFERENCES - Cann, J.R., Prichard, H.M., Malpas, J., Xenophontos, C., 2001. Oceanic inside corner detachments of the Limassol Forest area, Troodos ophiolite, Cyprus. J. Geol. Soc. Lond. 158, 757–767. - Cannat, M., Mevel, C., Maia, M., Deplus, C., Durand, C., Gente, P., Agrinier, P., Belarouchi, A., Dubuisson, G., Humler, E., Reynolds, J., 1995. Thin crust, ultramafic exposures, and rugged faulting patterns at the Mid-Atlantic Ridge (22–24°N). Geology 23, 49–52. - Catalano, R., Doglioni, C., Merlini, S., 2001. On the Mesozoic Ionian Basin. Geophys.J. Int. 144 (2001), 49–64. - Ildefonse, B., Blackman, D.K., John, B.E., Ohara, Y., Miller, D.J., MacLeod, C.J., 2007. Oceanic core complexes and crustal accretion at slow-spreading ridges. Geology 35, 623–626. - Karson, J. A., Thompson, G., Humphris, S. E., et al. 1987. Along axis variations in seafloor spreading in the MARK area. Nature, 328:681-685. - Manuella, F.C., Carbone, S., Barreca, G., 2012. Origin of saponite-rich clays in a fossil serpentinite-hosted hydrothermal system in the crustal basement of the Hyblean Plateau (Sicily, Italy). Clays and Clay Minerals 60, 18–31. - Sapienza, G., Griffin, W.L., O’Reilly, S.Y., Morten, L. 2007. Crustal zircons and mantle sulfides: Archean to Triassic events in the lithosphere beneath south-eastern Sicily. Lithos, 96, 503–523. 10 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 - Scribano, V., Sapienza, G.T., Braga, R., and Morten, L. 2006. Gabbroic xenoliths in tuff-breccia pipes from the Hyblean Plateau: insights into the nature and composition of the lower crust underneath South-eastern Sicily, Italy. Mineralogy and Petrology, 86, 63–88. - Vai, G.B., 2003. Development of the palaeogeography of Pangaea from Late Carboniferous to Early Permian. Palaeogeography, Palaeoclimatology, Palaeoecology196, 125–155. 11 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The concept of ophiolites from Alexandre Brogniart to Piero Elter Daniel Bernoulli Department of Earth Sciences, Swiss Federal Institute of Technology (ETH), Zürich, and Institute of Geology, University of Basel, Switzerland The term ophiolite, introduced by Brogniart in 1813, was originally synonymous with serpentinite; however, as early as 1821, Brogniart recognized the close association of serpentinites, gabbros, “variolite” and jasper (chert) in the Apennines of Liguria and Tuscany and included them into his “ophiolitic or serpentine formation”. In the 19th century, this association was recognized in many, particularly Alpine-type mountain belts and later became known as the Steinmann Trinity. The rocks of the Steinmann Trinity were thought to be a consanguineous association of intrusive (e.g., Steinmann) or extrusive (e.g., Lotti) igneous rocks or a combination of both, related in one way or another to the evolution of the mountain belts. Because of the perceived antiquity and permanence of oceans and continents, mountain chains were generally thought to originate from narrow, elongated, unstable belts, the geosynclines, circling the continents or following “zones of crustal weakness” from which geanticlines and finally mountain belts inevitably would develop. However, the anactualistic concept of the geosyncline had already met with major critiques, and as early as 1905, Steinmann considered the association of peridotite, “diabase” (basalt/dolerite) and radiolarites in the Alps and Apennines as characteristic of the deepocean floor; eventually Argand and E.B. Bailey placed Steinmann’s interpretation of the ophiolites into the context of continental drift and ocean evolution. Between 1930 and 1960, new problems were arising. The results of experimental petrology showed that peridotic magmas “were not possible” under reasonable conditions in the crust, and that, on the other hand, serpentine minerals were unstable above 550°C. Large stratiform mafic-ultramafic complexes could form at best by fractional crystallization of a basaltic magma and gravitational segregation. However, this interpretation was not applicable to the large ophiolite complexes of the eastern Mediterranean area because of their high ultramafic/mafic ratios. The problem could only be solved when it was recognized that Alpine-type peridotites were displaced fragments of the Earth mantle tectonically incorporated into mountain belts, representing either exhumed sub-continental mantle or oceanic mantle lithosphere associated with igneous rocks derived from partial melting of the mantle. In contrast to the east-Mediterranean area, where the standard model of ophiolite “stratigraphy”, canonized 1972 by the Penrose Field Conference on Ophiolites, was developed in line with the model of the oceanic lithosphere, the succession in west-Mediterranean did not meet this model. These ophiolite associations are characterized by relatively small amounts of partial melting, the gabbros forming smaller intrusive bodies within the mantle rather than a continuous oceanic layer 3; the absence of a sheeted-dyke complex; and the frequent occurrence of oceanic sediments resting with an unconformable depositional contact on mantle-derived serpentinized peridotites or exhumed plutonic rocks. Such a scenario is best explained by extension of both the continental and the oceanic lithosphere, exhumation and serpentinization of the mantle rocks at the sea floor, intrusion and exhumation to the sea floor of gabbroic rocks and submarine volcanicity. Indeed, such a scenario, now found along ocean–continent-transitions of passive margins and Atlantic-type slow-spreading ridges, where extension is accommodated by tectonic processes rather than by magmatic activity, was for the first time described in an orogen in a coherent way by Piero Elter and his co-workers, anticipating much of the current concepts. During the last forty years, Elter’s views have been modified and refined by comparisons of Alpine ophiolites with extant Atlantic-type passive margins and slow-spreading ridges, but they will remain a brilliant example of a combination of meticulous field observation and inspired interpretation. 12 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Reconciling the Geology of the Emilia Apennines and Tuscany across the Livorno-Sillaro Lineament, northern Apennines, Italy Giuseppe Bettelli (1), Filippo Panini(1), Francesca Remitti(1), Paola Vannucchi(2) (1) Department of Chemical and Geological Sciences, University of Modena and Reggio Emilia, , Italy (2) Department of Earth Sciences, University of Florence and Department of Earth Sciences, Royal Holloway University of London, UK Surface expression of lithospheric faults may vary greatly as they can develop a wide range of geomorphic/topographic features and various kinds of superficial geological/structural mismatchings. The “Livorno-Sillaro Lineament” (Nirta et alii, 2007; Pascucci et alii, 2007; Bettelli et alii, 2012) is one of the most important transverse lineaments of the Northern Apennine orogen. The lithospheric-scale role of this structure has been recognized long time ago by various authors on the base of different geophysical, geological and geomorphic data, although its origin is still not well defined. Also the exact surface characters of this structure are still not well-defined, we think because they are mainly based on old and out-of-date geological data. We present a review of the more recent stratigraphical and structural data related to the geology across the “Sillaro Lineament”, SL, the northeasternmost segment of the “Livorno-Sillaro Lineament”. Based on a re-examination and reinterpretation of the existing information about the regional geology of the Northern Apennines we conclude that the supposed mismatching of the Ligurian/Subligurian Units on the two sides of this lineament is mainly due to a lack of knowledge and to an inadequate correlation between corresponding units. Nevertheless, we recognize that this structure (along with the Secchia transverse lineament) greatly influenced the growth and the evolution of the oceanic accretionary prism/Ligurian/Subligurian thrust-nappe from the late Eocene to the late Serravallian, and also later on. In particular, we point out that at least the easternmost segment of this structure not only played an important role on the differential growth of the Ligurian/Subligurian accretionary prism-thrust nappe, but that it was responsible for the different amount of translation of the Ligurian Units on both side of the lineament. Our conclusions and interpretations include: 1) the Sillano/Mt Morello succession, typically cropping out SE of the SL in eastern Tuscany, represents the source rocks of the Ligurian blocks forming the Sestola-Vidiciatico tectonic unit and similar units (e.g., Coscogno-Montepastore tectonic unit: Remitti et alii, 2013) cropping out NW of the SL and along the SL itself; 2) the External Ligurian unit variously named as Samoggia/Val Sillaro/Val Marecchia Varicoloured Shales, AVS, and the overlying lower to middle Eocene turbidites (e.g., Savigno Fm) cropping out in the Emilia Apennines - i.e., NW of the SL – represents a lateral and more internal equivalent of the Sillano/Mt Morello succession. The AVS were extensively present also SE of the SL, as testified by the large klippen in the Romagna Apennines (Savio and Marecchia valleys) and many small klippens in the Umbria area (Umbertide-Gubbio area); 3) along and SE of the SL the AVS form the stratigraphic base of the Mt Morello Fm. Therefore, also this unit is present on both sides of the SL; 4) the pre-middle Eocene Subligurian Units cropping out NW of the SL (Argille e Calcari di Canetolo Fm and Calcari del Groppo del Vescovo Fm) do not correspond to the so called Subligurian Units cropping out SE of the SL (i.e., in Tuscany). The latter are the result of the sedimentation in a particular paleogeographic domain, transitional to the Tuscan domain, absent or not preserved NW of the SL. This seems to represent the only real difference in the geology of the Ligurian/Subligurian thrust nappes NW and SE of the SL. All the available data show that until the late Serravallian the thrust front of the Ligurian nappe was located in the same position across the SL. However, starting from the early-late Tortonian a differential translation of the Ligurian nappe NW of the SL took place, progressively reaching the present day position. With the exception of the Marecchia area, in the Romagna and Umbria Apennines (SE of the SL), instead, the thrust front of the Ligurian nappe remained more or less in the same position it reached in the late Serravallian. This implies that in the Northern Apennines the transverse SL played also an important role in the different amount of translation of the Ligurian thrust-nappe. 13 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 REFERENCES - Bettelli, G., Panini, F., Fioroni, C., Nirta, G., Remitti, F., Vannucchi, P. & Carlini, M. (2012), Revisiting the Geology of the “Sillaro Line”, Northern Aprnnines, Italy. Rendiconti Online Società Geologica Italiana, 22, 14-17. - Nirta, G., Principi, G. & Vannucchi, P. (2007), The Ligurian Units of Western Tuscany (Northern Apennines): insight on the influence of pre-existing weakness zones during ocean closure. Geodinamica Acta, 20/1-2, 71-97, doi:10.3166/ga.20.71-97 - Pascucci, V., Martini, I.P., Sagri, M. & Sandrelli, F. (2007), Effects of transversal structural lineaments on the Neogene-Quaternary basins of Tuscany (inner Northern Apennines, Italy). In: G. Nichols, E. Williams & C. Paola (Eds.), Sedimentary Processes, Environments and Basins: a Tribute to Peter Friend (pp.155182). Special Pubblication no. 38 of the International Association of Sedimentologists. - Remitti, F., Balestrieri, M.L., Vannucchi, P. & Bettelli, G. (2013), Early exhumation of underthrust units near the toe of an ancient erosive subduction zone: A case study from the Northern Apennines of Italy. Geological Society of America Bullettin, 125, 1820-1832, ISSN: 0016-7606. 14 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Piero Elter: from the Apennines to the Atlantic Enrico Bonatti Lamont Doherty Earth Observatory, Columbia University, USA Istituto Scienze Marine, CNR Bologna, Italy During the mid- and late-sixties of the last century, Piero Elter was often taking off from Pisa to roam around the mountains and hills of the western Apennines, doing field work also in the Ligurian ophiolites. We were then in the very early stages of the Plate Tectonic revolution: the nature and origin of ophiolites was a matter of debate and speculation. Elter and coworkers (in particular F. A. Decandia) in a couple of slim papers put forward some original ideas on the nature of the Ligurian ophiolites, suggesting that ophiolitic serpentinite-gabbro-basalt associations originated in young mid ocean ridge systems during the early stages of opening of an ocean. The initial stages in the exploration of the mid Atlantic Ridge, taking place at about that time, and subsequent studies of the Red Sea rift, gave strong support to Decandia and Elter’s intuitions. On a more personal note, I would like to recount how a “tesina” I did in Pisa under the supervision of Piero Elter contributed soon after to my focusing my research on the geology of the oceans. 15 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The geological sheet n° 261 – Lucca ( CARG): main results and some suggestion Andrea Cerrina Feroni (1) and Alberto Puccinelli (2) (1) Centro di Geotecnologie, Università di Siena, Italy (2) Dipartimento di Scienze della Terra, Università di Pisa, Italy The “official” geological cartography of the national territory has been improved by the scientific community (University and CNR). The CARG Project (1:50,000), now in stand by for lack of economic resources, represents a significant opportunity in the development of regional geology. This branch of the Earth Sciences, whose laboratory is the mountain, needs for its increase in new and methodologically updated geological mapping. The Geological Sheet N°261-Lucca, of CARG Project, is in progress by CGT UNISI. The Sheet N° 261 – Lucca (approximately 590 kmq) includes an area of low mountains and hills of the Serchio River Middle Valley. An important role is played by the Lucca Plain. In fact, it represents a continental sedimentary basin structured between Rusciniano (? lower Villafranchian) and Holocene, together with the clastic and terrigenous units outcropping in Montecarlo-Cerbaie hills. In this poster paper the Authors illustrate the most significant results and expose, in particular, their points of view on the structure of the Tuscan Nappe in the Pescaglia region. In this territory, located to the Southeast of the Apuan Alps tectonics window, the Tuscan Nappe is exposed with excellent outcrops and represents the main topic of recent structural studies (Carosi et al. 2005 ) In comparison with the past knowledge, we propose a new geometric and kinematic resolution of the major folds overturned, N120 -N160 strike, and facing to SW. The schematic geological section of fig 1 shows the polyphasic character of the Tuscan Nappe structure near Pescaglia. Axial planes of subisoclinal folds D1 (Campore T. Anticline, Mt Piglione Syncline and Pescaglia Anticline) are deformed till to overturning in the Mt. Prana synforme (D2). The original facing to NE of D1 folds appears so inverted in the overturned limb of second phase (D2) Mt. Prana synforme. The Mt. Prana synforme is consequently the only major fold in Pescaglia area with real facing to SW. In D1 phase there is the development of low-angle shear zones major structures scale (duplex structures) and mesoscale (s-c tectonite ). Duplex structures were reconstructed at different levels between the Cavernoso Limestone and Macigno Sandstone. In particular, the redoubling of lithofacies coupled Scaglia Fm. between Maiolica Limestone(Floor thrust) and Macigno Sandstone (Roof thrust) (Freddana valley) are consider as duplex structures Shear zones are developed in D1 phase even in the most rigid units of the sequence as in the “Scaglia calcarea” case (Coniacian ?) of Freddana valley (surroundings of Montefegatesi). In these outcrops, the involvement of D1 fabrics in the D2 phase overturned folds is evident. In Massarosa Mts.(near Chiatri area), the polyphasic stacking is well documented by the doubling of the Maiolica-Scaglia coupled (D1), under the Macigno sandstone, involved in the overturn in folds N160 strike facing to SW (D2) Fig 2 shows s-c tectonite (top to NE) borne in Marne a Posidonomya Fm (Dogger) into the shear zone of Foce di Bucine, along the road to the Matanna refuge at the base of the of Maiolica Limestone (Neocomiano). In D1 phase, the development of penetrative slaty cleavage (S1) slightly tilted on the layering (So) in less rigid or ductil behaviour units of Tuscan stratigraphic succession (Marne a Posidonomya, Scaglia) is also widespread. The Foce di Sella Fault subtend the overturned limb of the Mt. Prana synforme (fig. 1). This fault and in general all high-angle faults analysed shows evidences of Figure 1 16 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Figure 2 polyphasic kinematic. It is often an association of transpressional to compressional kinematic with extensional kinematic. The analytical data collected suggest that high-angle faults are generated in transpressive stress field and that only in the final stage their activities acted as normal faults. In this conceptual model, the folds of late stage D2 with the high angle faults NW-SE strike appear compatible. The overturned Mt. Prana synforme takes the meaning of a synclinal footwall in relation to the Foce di Sella Fault in its early transpressive step. A low-angle Extensional Tectonics, clearly post phase D2 ,probably related to structuration of Tyrrenian Basin, is very well expressed at the major scale and mesoscale in the Oltre Serchio and Massarosa Hills. REFERENCES - Carmignani L., Giglia G. & Kligfield R. (1978). Structural evolution of the Apuane Alps: an example of continental margin deformation in the Northern Apennines, Italy. Jour. Geol., 86: 487-504. - Carmignani L. & Kligfield R. (1990). Crustal extension in the Northern Appennines: thetransition from the compression to extension in the Alpi Apuane core complex.Tectonics, 9: 1275-130. - Carosi R., Montomoli C. & Pertusati P. C. (2004). Late tectonic evolution of the Northern Apennines, the role of contractional tectonics in the exhumation of the Tuscan unit. Geod. Acta, 17: 253-273. - Carosi R., Frassi C., Montomoli C. & Pertusati P. C. (2005). Structural evolution of the Tuscan Nappe in the southern sector of the Apuan Alps metamorphic dome (Northern Apennines, Italy). Geol. Jour., 40: 103-119. - Cerrina Feroni A. & Patacca E. (1975). Considerazioni preliminari sulla paleogeografia del Dominio Toscano Interno tra il Trias superiore e il Miocene medio. Atti soc. Tosc. Sci. Nat., Serie A, 82: 43-54. - Cerrina Feroni A., Plesi G., Fanelli G., Leoni L. & Martinelli P. (1983). Contributo alla conoscenza dei processi metamorfici di grado molto basso (anchimetamorfismo) a carico della Falda Toscana nell’area di ricoprimento apuano. Boll. Soc. Geol. It., 102: 269-280. - Fazzuoli M. (1981). Considerazioni preliminari sul Calcare selcifero della Val di Lima (Giurassico superiore) Toscana nord-occidentale. Mem. Soc. Geol. It., 21: 193-201, 3 figg., Roma. - Giannini E. & Nardi R. (1965). Geologia della zona nord - occidentale del Monte Pisano e dei Monti d’Oltre Serchio (prov. Di Pisa e Lucca). Boll., Soc. Geol. Ital., 84: 198-270. - Pertusati P. C., Plesi G. & Cerrina Feroni A. (1979). Alcuni esempi di tettonica polifasata nella Falda Toscana. Boll. Soc. Geol. It., 96: 587-603. - Trevisan L., Brandi G. P., Dallan L., Nardi R., Raggi G., Rau A., Squarci P., Taffi L. & Tongiorgi M. (1971). Note illustrative alla carta geologica d’Italia alla scala 1:100.000. Foglio 105 Lucca. Ministero dell’Industria, del Commercio e dell’Artigianato, Direzione Generale delle Miniere, Servizio Geologico d’ Italia, 1-51. 17 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The Portofino Conglomerate (Eastern Liguria): stratigraphic notes Barbara Corsi(1), Franco Marco Elter(2), Michele Piazza(2) (1) Corso Garibaldi 52/10 - Chiavari (Genova), Italy (2) DISTAV - Departement of Earth Science, Environment and Life, University of Genova, Italy The Portofino Conglomerate (CdP) crops out in the Eastern Liguria (Riviera di Levante), unconformably overlies the tectonostratigraphic Antola Unit and is composed of an about 460 m thick succession of poorly beded conglomerates with interbedded few sandy and/ or silty beds and lenses. On the whole, the conglomerates are moderately to poorly sorted, clasts-supported, with sandy to sandy-silty matrix and carbonate cement. Compositional evidences allow to distinguish three different lithofacies (Corsi, 2003), from bottom to top with gradational passages: Paraggi, Monte Pallone and Monte Bocche Lithofacies. The Paraggi Lithofacies (about 250 m in thickness) is characterized by the dominance of carbonate (marls, marly-limestones, calcarenites, biocalcirudites) clasts (60-88%), the low occurence of volcanic (pillow lavas/ porphiric diabases, 1-2%) and metamorphic (calcschists and marbles, 1-2.5%) clasts and by carbonate matrix. The Monte Pallone Lithofacies (about 60 m in thickness) is characterized by the decrease of carbonate (marls, marly-limestones, limestones with cherts) clasts (2-29%) and the increase of quartz clasts (5-32%), HP-metabasites, (4-30%), metamorphic (marbles, calcschists, micaschists 4-70%), and by quartz-rich sandy matrix and carbonate cement balanced proportion. The Monte Bocche Lithofacies (about 150 m in thickness) is characterized by the strong decrease of the carbonate clasts (0-15%) and the dominance of marbles/dolostones (4-50%), HT-metamorphic (gneiss, migmatites, orthogneiss, 4-20%) and noncarbonate sedimentary (anagenites, green/whitish quartzites, (10-37%) clasts, and the subordinate (less than 10 %) presence of intrusive igneus (granitoids and gabbros) clasts, and by the occurrence of quartz-domi- nated matrix and minor carbonate cement. In terms of provenance, the carbonate clasts appear to be supplied by the Antola Unit flysch or by a similar sedimentary complex; the metabasite, marble, calcschist, and volcanic clasts may be related to the oceanic Piedmontese Units, while the acid metamorphic, granitoid, HT-metamorphic, and quartz-rich sedimentary ones might be supplied by the Briançonnais Units. According to the geologic and sedimentologic evidence, the CdP can be regarded the result of a delta deposition in coastal to shallow marine environment; paleocurrents seem to suggest a feeding area located at the west of the present-day CdP position (Corsi, 2003). The CdP age is generally reported as “Oligocene”, but its fossil content is very poor. Until now, only one specimen of the plaktonic foraminifer Globigerinatheka (middle - upper Eocene) in the matrix of the Paraggi Lithofacies conglomerates, and coal lenses, coalified remains and rare bioclasts, unevenly distributed throughout the succession, were found. The few biocalcirudite clasts occurring in the Paraggi Lithofacies exhibit a fossil content (discocyclinids and red calcareous algae) that indicates an Eocene age. Finally, the relationship with the, probably, coeval conglomeratic units cropping out in the surrounding areas (i.e.: San Paolo Fm., Molare Fm., Pianfolco Fm., Savignone Conglomerate) remains unclear. REFERENCES - Corsi B. (2003). Eventi tettonico-sedimentari del settore tra Chiavari e Genova Nervi nel quadro dell’evoluzione geodinamica del sistema Ligure Balearico e Tirrenico. PhD Thesis - Università di Genova, 257 pp. 18 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Struttura ed evoluzione della catena alpina: dallo Structural Model of Italy al Progetto Crop ed ai loro sviluppi Giorgio V. Dal Piaz Accademia delle Scienze di Torino, Italy La geologia alpina, fortemente influenzata dai modelli di Emile Argand e Rudolf Staub, subisce alla fine degli anni 60’ l’impatto della plate tectonics, senza partecipare alla sua formulazione. I primi tentativi di applicare i canoni della nuova tettonica globale all’evoluzione convergente delle Alpi sono rappresentati da alcuni modelli a piccola scala di H.P. Laubscher (1969, 1970) che, incerto nello stabilire il ruolo delle placche in gioco, concepisce la contemporanea subduzione di entrambe verso una comune zona radicale, con struttura analoga al “tectogene” di Hess (1939). La subduzione litosferica ed il suo modello termico forniscono adeguata spiegazione alla genesi del metamorfismo eclogitico (Dal Piaz 1971; Ernst 1971; Fry & Fyfe 1971), ben noto dai tempi di Franchi, Zambonini ed altri grandi dell’epoca. Nelle Alpi occidentali, il metamorfismo eclogitico nel sistema tettonico Sesia-Dent Blanche (Dal Piaz & Nervo 1971; Dal Piaz al. 1972; Compagnoni & Maffeo 1973) e quello nelle unità europee distali del Monte Rosa-Gran Paradiso (Compagnoni & Lombardo 1974; Dal Piaz 1974; Frey et al. 1974) sono la prova che non solo la litosfera oceanica, ma anche coerenti sezioni di crosta continentale leggera erano discese nella zona di subduzione sino a profondità sottocrostali, vincendo la propensione al galleggiamento postulata dai fisici. La scoperta di coesite nel Dora-Maira (Chopin 1984) aumenta la profondità massima raggiunta da unità del margine continentale passivo europeo sino a valori prossimi ai 100 km, complicando ulteriormente il problema dell’esumazione. Il Progetto Finalizzato Geodinamica è un momento di rinascita e di grande fervore delle Geoscienze italiane, promuove una fattiva cooperazione tra geologia e geofisica, sdogana definitivamente in Italia i canoni della tettonica delle placche e fornisce le prime solide basi per la valutazione dei rischi naturali e la loro mitigazione. Il sottoprogetto “Modello Strutturale d’Italia”, coordinato da Paolo Scandone, realizza negli anni 80’ la “Synthetic structural-kinematic map of Italy” (1989) e lo “Structural Model of Italy”, in cinque fogli alla scala 1:500.000: il modello copre l’intera catena delle Alpi, dalla pianura padana all’avanpaese europeo Figura 1 – Carta tettonica delle Alpi, evoluzione schematica dello Structural Model. Austroalpino occidentale (WA) ed orientale (EA), Alpi Meridionali (SA); Unità oceaniche ed europee: Zona Pennidica (P) e ofioliti (blu), con i Klippe delle Prealpi Romamde e del Chiablese (Pk) e le finestre tettoniche dell’Ossola-Ticino (oyw), Engadina (ew), Tauri (tw) e Rechnitz (rw); Elvetico (H). Lineamento Periadriatico (pl), Avanfossa della Molassa (M), Avampaese europeo (EF), Avampaese padano-adriatico (PA), Bacino Pannonico (PB), Dinaridi (DI), Appennini (AP) (Dal Piaz, Bistacchi & Massironi, Episodes 2003). e da Vienna alla Provenza (Fig. 1). La legenda, impostata su base strutturale, distingue la catena collisionale a vergenza europea dalle Alpi Meridionali a vergenza padano-adriatica, indica le varie unità in successione da tetto a letto e le completa con distinzioni di carattere litostratigrafico e metamorfico. Ad esempio, rappresenta le unità policicliche di basamento e quelle di copertura, aderenti o scollate, le unità continentali ed oceaniche con metamorfismo alpino di subduzione, le unità ofiolitiche riferibili al bacino ligure-piemontese, estese dalle Alpi occidentali alle finestre dei Tauri e di Rechnitz, e quelle attribuibili ai bacini vallesano e nord-pennidico, confermando precedenti correlazioni tra le unità continentali interne e medie della Zona pennidica con quelle della finestra dei Tauri. Rappresenta plutoni, apofisi e la precisa ubicazione (asterisco) di centinaia di filoni del magmatismo calc-alcalino ed ultrapotassico di età oligocenica (32-30 Ma) nel settore interno del prisma collisionale e, almeno in parte, di età pre-Adamello nelle unità sudalpine a vergenza padano-adriatica. 19 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Figura 2 – Crocodile vs Ductile extrusion model (Transalp Working Group 2002). Come osservato da Castellarin et al. (2006), è preferibile il secondo modello; il primo attribuisce al cuneo pennidico, caldo (metamorfismo in facies anfibolitica) e soffice, l’improbabile ruolo di indenter in grado di penetrare profondamente nella crosta continentale sudalpina, più fredda e rigida. E’ stato un lavoro immane, senza supporto digitale, con carte disegnate a mano ed allestimento per la stampa alla vecchia maniera, ed alla fine è mancata la forza di scrivere le note illustrative: un’occasione perduta. Il secondo progetto di grande respiro per la ricerca italiana è stato il CROP, una serie di esperimenti di sismica a riflessione crostale, finanziati dal CNR con vari partners e realiz- zati tra il 1986 e il 1991 (CROP Atlas, 2003). Il progetto è iniziato con il profilo CROP-ECORS, tracciato attraverso le Alpi nord-occidentali, coordinato da R. Nicolich e R. Polino e realizzato in collaborazione con i francesi (Nicolas et al. 1990; Polino et al. 1990). L’esperimento ha fornito la prima chiara immagine dell’esistenza di due Moho indipendenti e ben separate, quella europea, in subduzione sotto il prisma collisionale, e quella della placca superiore adriatica, al posto della singola e continua Moho dei modelli classici, depressa a guisa di sinforme sotto la catena collisionale ispessita: oggi può sembrare banale, ma è stato un risultato rivoluzionario. Analoghe rotture litosferiche sono state messe in luce dal profilo CROP Alpi Centrali (Montrasio et al., 2003) e dal Progetto Transalp (Castellarin et al. 2006; Luschen et al. 2006; Lammerer et al. 2008), un profilo sismico crostale esteso da Treviso a Monaco, attraverso il settore occidentale della finestra dei Tauri. I rapporti tra il prisma collisionale e la fronte della crosta sudalpina sono stati interpretati in modo diverso (Crocodile vs Ductile extrusion model; Fig. 2) sulla base di differenti modelli reologici, mentre vi è stato accordo nell’attribuire le discontinuità profonde dell’immagine sismica alla subduzione della placca europea sotto il prisma collisionale e l’indenter sudalpino. La complessa struttura interna del prisma collisionale è confermata dallo studio geologico-strutturale del settore occidentale della finestra dei Tauri, effettuato per il progetto della nuova galleria di base del Brennero (Bistacchi et al. 2007, 2010; Dal Piaz et al. submitted). 20 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The tectono-stratigraphic evolution of distal magma-poor rifted margins: examples from the Alpine Tethys, the E-Indian and Newfoundland-Iberia rifted margins Alessandro Decarlis, Gianreto Manatschal, Isabelle Haupert, Emmanuel Masini IPGS/EOST, Université de Strasbourg, France The tectonostratigraphic evolution of distal, magma-poor rifted margins still remains a poorly-known topic despite of the increasing interest driven by new research opportunities in hydrocarbon exploration. Seismic imaging of modern distal margins suffer the technical tethers of geophysical exploration and moreover the general lacking of drill holes limits the observations only to the structure of sedimentary successions. On the contrary, fossil distal margins preserved inside mountain belts allow the direct observation of rock features, but exhibit major overprint due to the orogenic deformation and metamorphism that usually concentrates in distal domains. To avoid the incompleteness of the above cited examples, in this study we couple the observations on the different passive distal margin datasets. We use two well-imaged seismic profiles, Iberia-Newfoundland SCREECH1-ISE1, East India ION GXT-1000 and an onshore database represented by selected stratigraphic sections from the Jurassic Alpine Tethys. For the latter, we focus on the sedimentary successions of different domains of the former distal rifted margins: the ones from the Ligurian Prepiedmont and Piedmont domains, outcropping in the Western Alps (European margin) and those belonging to the Lower Austroalpine exposed in the Central Alps in SE Switzerland (Err, Bernina units; Adria margin). We choose these domains because of their stratigraphic completeness and the correlatability of the sedimentary successions. We stress that, with a certain degree of approximation, these areas can be considered as part of a former “conjugate” rift system during Jurassic. The aim of this study is to examine the different datasets into an iterative process where seismic sections act as a framework to determine first-order tectonic features for non-volcanic rifted margins and field observations that integrate the seismic datasets as virtual boreholes, into an attempt to point out the overall tectonostratigraphic evolution. As a result, once the upper and lower plate geometry sensu rifting has been defined in the three examples (i.e. respectively the footwall and the hangingwall of the main exhumation fault), the primary features of sedimentation and their relations with tectonics have been compared. This lead (1) to a better comprehension of the sedimentary environment evolution in the distal margin during the Alpine rifting. More in general, it allows to (2) decipher the interactions between tectonics, sediment supply and accumulation in distal passive margins, unravelling the different basin-style and the sediment distribution/composition. 21 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Ophiolitic units of the Northern Apennines as a tool to unravel Schistes Lustres stratigraphy of the Western Alps and their geodynamic context: the Urtier Valley example, Cogne, Aosta Valley Alessandro Ellero(1), Andrea Loprieno(2) (1) CNR-IGG, Pisa, Italy (2) Viale Cavagnet 45, Cogne, Aosta, Italy One of the most significant scientific contribution that Piero Elter has left us, is undoubtedly represented by the definition of the stratigraphic features and geodynamic significance of the ophiolitic units of the Northern Apennines (NA) (Elter & Raggi, 1965a, b; Decandia & Elter, 1972; Elter, 1975a, b; Elter & Marroni, 1991) and their relationships with the Western Alps (WA) (Elter et al., 1966; Elter & Pertusati, 1973; Elter, 1993). Two topics emerge from the extensive work of Elter: i) the stratigraphic and geodynamic distinction between an Internal Ligurian Domain, representative of the Ligure-Piemontese oceanic basin, and an External Ligurian Domain, derived from the Adria distal continental margin, and ii) different significance of depositional setting for the ophiolitic detritus occurring, at different stratigraphic levels, within the Ligurian units of the NA. After his extensive field experience in the Ligurian Units (LU), which lasted many years, once back in the Schistes Lustres of the Western Alps, Elter was always surprised by clear similarities in their stratigraphy and shared his idea that the Ligurian Units (LU) may represent a valuable tool in the study of the ophiolitic units in the Western Alps. In this paper we summarize the main results of such stratigraphic and structural analyses, originally performed exactly twenty years ago. The investigated area extends in the Urtier Valley which is located at the northern flank of the Gran Paradiso Massif, E of Cogne. Our attempt has been to study schistes lustres ophiolitic units, characterising a stratigraphy comparable to that of the LU. In this way it has been possible to unravel the geometry and the geodynamic context for the ophiolitic units of the Urtier Valley, similar to that suggested for the LU of the NA. Five different tectonic units were mapped in the area, from the geometrically lowermost to the uppermost they are: 1) the Gran Paradiso basement unit; 2) the Péne Blanche unit (part of the so-called “Fasceaux de Cogne”); 3) the Broillot unit (BU); 4) the Bardonney mélange unit (BM); 5) the Tour Ponton and Acque Rosse units. The two recognized ophiolitic units BU and BM, despite a pervasive metamorphism and an intense deformation, preserve a very characteristic stratigraphic succession, allowing an immediate correlation with similar sequences of the LU. The BU is characterized by ophiolitic bodies and related sedimentary cover, represented by the corresponding metamorphic terms of chert (metaquarzite), Calpionella limestone (impure marble) and Palombini shale (calcschist). The Palombini calcschists are overlain by metapelites and micaschists, alternated with arenaceous calcschists and detritic prasinites (metabasic sandstones). This succession has been interpreted as a turbiditic deposit comparable with the Val Lavagna Shale Formation of NA. Similar correlations were suggested for other alpine detritic successions (e.g. Col Agnel formation of Cottian Alps by Lagabrielle, 1987). The BU shows all the features typical of the Internal Ligurian Domain, so can be interpreted as derived from the Ligure-Piemontese oceanic domain. The BM is constituted by ophiolites (mainly metabasites) only represented by clasts and/or slices from centimeter to pluridecameter in size within an abundant carbonate/quarzitic and/or metabasic matrix. This unit has been correlated with the youngest formation of the Internal Liguride Units, named Bocco Shale or Colli-Tavarone Formation which consists of pebbly mudstones, pebbly sandstones and ophiolites slide-blocks in a varicoloured shaly matrix. It is interpreted as an early Paleocene lower slope deposits sedimented on the top of the Internal Liguride Units as they approach the accretionary prism (Elter & Marroni, 1991). Overall, integrating the stratigraphic sequences of the two ophiolitic units of the Urtier Valley, is possible to reconstruct a stratigraphic succession perfectly correlable with those belonging to the Internal Ligurian Domain of the NA, interpreted as recording trench-ward motion of the oceanic lithosphere (Marroni & Pandolfi, 1996). A similar analogy between LU successions and ophiol- 22 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 itc sequences in the Western Alps was proposed for the Lago Nero Unit, in the Chenaillet Massif (Burroni et al., 2003). The structural evolution of the Urtier Valley is characterized by three major deformation stages and related structures (Loprieno et al., 2009, 2014). D1 structures are represented by relict microfabric associated with eclogite-facies assemblages and small-scale isoclinal folds. It is also responsible for early stage of nappe stack formation. D2 is the dominant deformation stage, refolding the tectonic contacts between the units, showing D2 isoclinal folds on all scales, associated with S2 greenschist facies pervasive mylonitic fabric that represents the main foliation at regional scale. D2 folds axes and L2 lineation show a common orientation with E-W trend. D3 is instead characterized by open to close folds with NE or SW gently dipping axial plane that refold the composite main foliation S1/S2. At the scale of the study area, the reconstruction of the general architecture of the stacked units is facilitated by stratigraphic markers within the ophiolitic units, showing how the nappe pile as a whole is deformed by a D2D3 interference structure, with a large scale D2 synform refolded by a large scale D3 Urtier antiform. Despite intense metamorphic overprint and deformation history, this contribution testify how a combined structural and stratigraphic approach based on the knowledge about LU stratigraphy, represents a valuable tool for the study of the Schistes Lustres ophiolitic units of the Western Alps. REFERENCES - Burroni A., Levi N., Marroni M. & Pandolfi L. (2003). Lithostratigraphy and structure of the Lago Nero Unit (Chenaillet Massif, Western Alps): comparison with Internal Liguride Units of Northern Apennines. Ofioliti, 28, 1-11. - Decandia F.A. & Elter P. (1972). La zona ofiolitifera del Bracco nel settore compreso tra Levanto e la Val Graveglia (Appennino Ligure). Mem. Soc. Geol. It., 11, 503-530. - Elter P. & Raggi G. (1965) (a). Contributo alla conoscenza dell’Appennino ligure: 1)- Osservazioni preliminari sulla posizione delle ofioliti della zona di Zignago (La Spezia). 2) Considerazioni sul problema degli olistostromi. Boll. Soc. Geol. Ital., 84, 303-322. - Elter P. & Raggi G. (1965) (b). Contributo alla conoscenza dell’Appennino ligure: 3)- Tentativo di interpretazione delle brecce ofiolitiche cretacee in relazione con movimenti orogenetici nell’Appennino ligure. Boll. Soc. Geol. Ital., 84 (5), 1-12. - Elter G., Elter P., Sturani C. & Wiedmann M. (1966). Sur la prolongation du domaine de l’Appennin dans le Monferrat et les Alpes et sur l’origine de la Nappe de la Simme s.l. des Préalps romandes et chaiblaisiennes. Arch. Soc. Phys. Nat. Genève, 19, 1002-1012. - Elter P. & Pertusati P.C. (1973). Considerazioni sul limite Alpi-Appennino e sulle relazioni con l’arco delle Alpi occidentali. Mem. Soc. Geol. It., 12, 359375. - Elter P. (1975) (a). Introduction à la géologie de l’Apennin Septentrional. Bull. Soc. Geol. France, 17, 956-962. - Elter P. (1975) (b). L’ensemble ligure. Bull. Soc. Geol. France, 17, 984-997. - Elter P. & Marroni M. (1991). Le Unità Liguri dell’Appennino Settentrionale: sintesi dei dati e nuove interpretazioni. Mem. Descr. Serv. Geol. Italiano, XLVI, 121-138. - Elter P. (1993). Detritismo ofiolitico e subduzione: riflessioni sui rapporti Alpi e Appennino. Mem. Soc. Geol. It., 49, 205-215. - Lagabrielle Y. (1987). Les ophiolites: marquer de l’histoire tectonique des domains oceaniques. Le cas des Alpes franco-italiennes (Queyras, Piémont). Comparaison avec les ophiolites d’Antalya (Turquie) et du Coast Range de Californie. These de Doctorat d’Etat, Univ. de Bretagne, 350pp. - Loprieno A., Ellero A., Elter P. & Molli G. (2009). Structural geometries and tectonic evolution of ophiolite and continental units in the Urtier Valley, Cogne (Western Alps). 9th Alpine Workshop “AlpShop 2009”, Cogne, Italy. - Loprieno A. & Ellero A. (2014). Structural analysis of the nappe stack in the Urtier Valley, Cogne (Western Alps). Meeting in memory of Piero Elter. The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” Pisa, June 26-27, 2014. - Marroni M. & Pandolfi L. (1996). The deformation history of an accreted ophiolite sequence: the Internal Liguride units (Northern Apennines, Italy). Geodin. Acta, 9 (1), 13-29. 23 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The Late-Variscan tectonic barriers: the beginning of Alpine Wilson Cicle? Franco Marco Elter(1), Laura Gaggero(1), Enrico Pandeli(2) (1) DISTAV - Departement of Earth Science, Environment and Life, University of Genova, Italy (2) Departement of Earth Science, University of Firenze, Italy Recent studies suggest that the Variscan orogeny developed through a process of accretion of peri-Gondwanian terranes (e.g. Avalonia, Armorica) and wider continental masses (e.g. Baltica, Laurentia, and Laurussia, Padovano et al., 2012, 2014). The origin of the peri-Gondwanian terranes is linked to a continuous fragmentation of the northern Gondwana margin which started, at least, during the Cambrian with the opening of the Rheic Ocean and the separation of Avalonia from. During Devonian time, the opening of Palaeotethys led to a further fragmentation of the northern Gondwana margin, with the formation of a narrow terrane known as the Hun superterrane or the Galatian ribbon continent. The closure of the Palaeotethys and Rheic Oceans started at ~360 Ma, and led to the final collision and amalgamation of Gondwana, Gondwana-derived continents, and Laurussia, with the formation of Pangea at ~300 Ma (Figure 1). The continental collision involved plates characterized by irregular boundaries, and thus resulted in a complex pattern of coeval transpressional and/or transtensional strike–slip shear zones (Figure 1, Elter et al. 2010, Elter & Padovano 2010, 2013, Padovano et al., 2012, 2014). The East Variscan Shear Zone (EVSZ, Padovano et al., 2012, 2014) affected numerous Variscan massifs scat- Figure 1 tered in the western Mediterranean area in the time interval 330–300 Ma . The Gondwana continent is characterized by the presence, on the east, of the Paleotethys and by the Elbe Shear Zone system (running from Boemia to Moesia) while the western boundary is characterized of the peri-Gondwanian terranes affected by intracontinental strike-slip shear zones. The EVSZ, running from Slovenia to the Atlas system, is one of the major strike-slip shear zone at present known (Padovano et al., 2012, 2014). We suggest that the EVSZ and the ESZ could represent the tectonic lineaments (preexisting tectonic barriers) along those, the opening of Piedmontese-Ligure Basin on the west and the new Tethys on the east develop in the Permian-middle Jurassic time. The opening of these two oceans can give the beginning of the Alpine Wilson Cicle, involved in the Alpine Belt (on the East - Dinaridi-Hellenides system, on the west the Alps system). REFERENCES - Elter F.M. & Padovano M. (2013). Late Carboniferous Transpressional shear-zones in NE Sardinia (Italy): geodynamic implications. In: “Crustal melting within the European Variscan belt”- BRGM-SGF-Orleans- France, 7. - Elter F.M., Padovano M. & Kraus R.K. (2010). The Variscan HT metamorphic rocks emplacement linked to the interactiuon between Gondwana and Laurussia plates: structural constraints in NE Sardinia (Italy). Terra Nova, 22, 369-377. - Padovano M., Elter F.M., Pandeli E. & Franceschelli M. (2012). The East Variscan Shear Zone: new insights into its role in the Late Carboniferous collision in southern Europe. International Geology Review, 54, 8, 957970. - Padovano M., Dorr W., Elter F.M. & Gerdes A. (2014). The East Variscan Shear Zone: geochronological constraints from the Capo Ferro area (NE Sardinia, Italy). Lithos 196-197, 27-41. 24 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Olistostromes and mélanges in the External Ligurian Units in Monferrato (NW Italy) Andrea Festa(1), Gian Andrea Pini(2), Yildirim Dilek(3) (1) Dipartimento di Scienze della Terra, Università di Torino, Italy (2) Dipartimento di Matematica e Geoscienze, Università di Trieste, Italy (3) Department of Geology and Environmental Earth Science, Miami University, Oxford, OH, USA During 60’s and 70’s Piero Elter detailed worked and described the olistostromes of the Northern Apennines, introducing new terms (e.g. “endolistostrome” and “allolistostrome” of Elter and Raggi, 1965; “precursory olistostrome” of Elter and Trevisan, 1973) and concepts on their significance in the framework of the tectono-stratigraphic evolution of the Northern Apennines. His research significantly influenced the subsequent literature on mélanges and olistostromes throughout the world. About 50 years later and strongly inspired from those papers, we report the occurrence of olistostromes and mélanges in the External Ligurian Units in Monferrato. Following Elter et al. (1966), we describe in detail the internal structure of the External Ligurian Units which can be correlated with the Cassio Unit. Particularly, we demonstrate that they consist of different polygenetic chaotic bodies formed by the superposition of tectonic, diapiric and sedimentary processes which occurred during the Late Cretaceous – late Oligocene tectonic evolution of the Ligurian Accretionary Wedge (Festa et al., 2013). A Broken Formation is the oldest unit in the Ligurian Accretionary Wedge, showing bedding-parallel boudinage structures, which developed as a result of layer-parallel extension at the toe of the internal part of the Alpine wedge front during the late Cretaceous–middle Eocene. This Broken Formation experienced an overprint of tectonic, diapiric, and sedimentary processes as a result of continental collision in the late Oligocene. Contractional deformation produced a structurally ordered block-inmatrix fabric through mixing of both native and exotic blocks, forming a Tectonic Mélange. Concentration of overpressurized fluids along thrust faults triggered the upward rise of shaly material, producing a Diapiric Mélange, which provided the source material for the downslope emplacement of the youngest, late Oligocene Olistostromes (i.e., sedimentary mélange). The Olistostromes unconformably cover the thrust faults, constraining the timing of the youngest episode of contractional deformation in the Ligurian Accretionary Wedge. The interplay and superposition of tectonic, diapiric and sedimentary processes plays a significant role in the dynamic equilibrium of accretionary wedges. Our findings provide useful criteria to differentiate between different types of polygenetic mélanges in ancient and modern accretionary wedges. REFERENCES - Elter, P., & Raggi, G. (1965), Contributo alla conoscenza dell’Apennino ligure: 1. Osservazioni preliminari sulla posizione delle ofioliti nella zona di Zignago (La Spezia); 2. Considerazioni sul problema degli olistostromi. Bollettino della Società Geologica Italiana, 84, 303–322. - Elter, P., & Trevisan, L. (1973), Olistostromes in the tectonic evolution of the Northern Apennines. In K.A. De Jong, & R. Scholten (Eds.), Gravity and tectonics (pp. 175–188). New York, John Willey and Sons. - Elter, G., Elter, P., Sturani, C., & Weidmann, M. (1966), Sur la prolongation du domaine ligure de l’Apennin dans le Monferrat et les Alpes et sur l’origine de la Nappe de la Simme s.l. des Préalpes romandes et chablaisannes. Archives des Sciences de Genève, 19, 279–377. - Festa A., Dilek., Y., Codegone G., Cavagna, S., & Pini, G.A. (2013), Structural Anatomy of the Ligurian Accretionary Wedge (Monferrato, NW-Italy), and Evolution of Superposed Mélanges. Geological Society of America Bulletin, 125 (9/10), 1580-1598. 25 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Ductile shear zone in the thermo-metamorphic aureole of Monte Capanne: data from Cavoli-Colle Palombaia area (South-western Elba Island, Italy) Riccardo Giusti(1), Enrico Pandeli(1,2), Franco Marco Elter(3) (1) Department of Earth Sciences, University of Florence, Italy (2) CNR-Institute of Geosciences and Georesources, Section of Florence, Italy (3) DISTAV, University of Genoa, Italy The Elba Island is located in the Northern Tyrrhenian Sea at midway between Tuscany (Northern Apennines Chain) and Corsica (Alpine Corsica structural pile). The complex Elba Island stack of nappes, which is considered the innermost outcrop of the tertiary Northern Apennines Chain, is also well known for its Fe-ore bodies and the relationships between the emplacement of the Mio-Pliocene acidic magmatic bodies and tectonics (Bortolotti et alii 2001). The emplacement of the acidic plutons caused the thermal metamorphism in the host units (oceanic Ligurian and Piedmontese and continental Tuscan units) and remobilized the original tectonic stacking of the nappes. In particular the plutons caused deformations in both brittle and ductile regime in the surrounding units (e.g. detachements and foldings, BORTOLOTTI et alii 2001 and references therein). The plutonic complex of Monte Capanne (6.9 Ma Dini et alii, 2002), which dominates the western part of the island (Fig.1), was emplaced in the Upper Jurassic-Lower Cretaceous ophiolitic successions (Bortolotti et alii 2001), after the intrusion of acidic dikes and laccoliths (“Christmas-tree” laccolithic complex, Dini et alii, 2002: 8-8.5 Ma Capo Bianco Aplite, 8 Ma Portoferraio Porphyry and 7.4-7.2 Ma San Martino Porphyry). The rocks of M. Capanne aureole show different thermal metamorphic imprint ranging from high to low grade in the different parts of the “ring”, as a function of distance from the pluton and of the circulation of fluids. Locally, an anomalous increase of metamorphic grade is evident and can be likely related to variation of permeability in the host rocks for the ascent of the metasomatic fluids. Furthermore in the Fetovaia Promontory area, the un-metamorphic Eocene Fetovaia Flysch unit tectonically lies above the metamorphosed ophiolitic successions. This tectonic attitude is related to brittle low-angle detachements occurred in the cover units of the pluton during its uplift, as it is well known in central Elba for the eastward sliding of the Marina di Campo Fm. (CEF, Maineri et alii, 2003). Subsequently, these rocks have been displaced by high-angle normal faults (e.g. EBF, Maineri et alii, 2003). In any case, the primary relationships between intrusive bodies and their covers are often evident in many places, as well as the Fig. 1 - Geological sketch map of Western Elba Island 26 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 evidence of polyphasic both ductile and brittle tectonic activity due to the different evolutive stages of emplacement of the pluton. In the thermo-metamorphosed ligurian units, the ductile deformation occurred especially within the carbonate lithologies, while the pelitic lithologies show basically a brittle behavior, coherently with static recrystallization phenomena. The paper presents for the first time data on the ductile shear zone found in the ophiolitic units of Cavoli-Colle Palombaia outcrops, with the development of mylonites, decimeter to plurimetric in thickness, consisting mainly of crystalline carbonate foliated bodies (derived from the Calpionelle Limestones) in which enclaves of meta-cherts and meta-basalts are present. The analysis, performed on the kinematic indicators (e.g. σ and δ type porphyroclasts, asymmetric folds and domino structures) and fold structures, show that the main direction of the mylonitic foliation and the shear sense are connected with a transport towards the outside of the intrusive massif of M. Capanne. In particular the direction of movement is towards ESE. So, the location of the outcrops with respect to the main body of the pluton of M. Capanne is consistent with a radial pattern of the stress field. This radial trend is a typical feature of the discharge phenomena related to the up-rising of a sub-spherical body. In several outcrops of the mylonite it is evident the presence of different deformation regimes, both simple shear and pure shear. Locally the occurrence of the combination of the two end-member (sub-simple shear) testify the extreme variability and complexity of the processes at the time of the development of shearing. Furthermore, the microscopic analysis point that the foliation and associated microstructures are related to metamorphic events of medium to high grade (HT-LP) linked to the dynamo-thermal effects of the M. Capanne pluton emplacement. REFERENCES - Bortolotti V., Fazzuoli M., Pandeli E., Principi G., Babbini A. & Corti S. (2001), Geology of the central and eastern Elba Island, Italy. Ofioliti, 26 (2a), 97-150. - Dini A, Innocenti F., Rocchi S., Tonarini S. & Westerman D.S. (2002), The magmatic evolution of the late Miocene laccolith-pluton-dyke granitic complex of Elba Island, Italy. Geological Magazine, 139, 257-279. - Maineri C., Benvenuti M., Costagliola P., Dini A., Lattanzi P, Ruggieri G. & Villa I.M. (2003), Sericitic alteration at the La Crocetta deposits (Elba Island, Italy): interplay between magmatism, tectonic and idrotermal activity. Mineralium Deposita 38, 67-86. 27 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The role of the Ottone-Levanto line in the geodynamic evolution of the Northern Apennine: evidences from the high Sturla Valley, Liguria, Italy Noah Hobbs, Edoardo Luvisi, Michele Marroni, Francesca Meneghini, Luca Pandolfi Dipartimento di Scienze della Terra, Università di Pisa, Italy In the Northern Apennine, the structural setting is represented by a pile of tectonic units assembled from different pelogeographic domain during the closure of the Ligure-Piemontese oceanic basin and the following continental collision. This pile is cut by several high-angle shear zones of regional extent, showing different kinematics, age and geodynamic role. Among these lines, the Ottone-Levanto line (Elter and Pertusati, 1973) is regarded in literature as one of the most important structural element that played a fundamental role in the geodynamic evolution of the Northern Apennine, at its junction with the Western Alps. Despite its importance, no meso- and microstructural data are available in order to constrain the kinematics of the Ottone-Levanto line. A shear zone representative of the deformation along the Ottone-Levanto line has been identified in the high Sturla valley (Liguria). In the field, the shear zone is represented by 5 to 15 m thick foliated cataclasites, associated with meter thick bodies of unfoliated cataclastic serpentinites. The foliated cataclasites originated from the brittle dismemberment of an assemblage of peridotites, gabbros, granites. They display a crude mesoscopic layering defined by the alternation of mm- to cm-thick bands of different colours, from green to dark grey. Bands display locally an anastomosing network, and are often characterized by a sharp thickness reduction along their length. The dark grey bands show a fine-grained, homogenous granular texture, whereas the green ones consist of matrix-dominated rocks where mm to cm size angular fragments of serpentinites, limestones and gabbros are floating in a fine-grained matrix with well-developed penetrative foliation. Foliation surfaces dip toward the SW, and show a strike ranging from N30°/N40° to N80°/ N90°, as a result of a deformation by NW-SE trending folds with gently dipping axial plane. More over, the foliated cataclasites show an internal deformation by open to close folds with E-W trending axes that does not affect the surrounding lithotypes. In the NW-SE striking and SW dipping foliated cataclasites, the facing of these folds indicate a top-to-the SE sense of shear. The cataclastic unfoliated serpentinites occur as meter-thick dark green bodies characterized by a complex network of quartz veins. These bodies, even if massive, reveal a rough banding of thick levels, featuring relics of pyroxene, and a poorly modified peridotite primary structure, alternating with levels showing a strong grain size reduction. The structural analysis at meso-scale indicates a deformation involving components of contraction with topto-the-NE thrusting, and contemporaneous top-to-theSW sinistral shearing and dip-slip shearing. At the micro-scale, the foliated cataclasites are characterized by elongated, angular to sub-rounded clasts of serpentinite, and subordinately calcite, both deriving from the rock types characterizing the hanging wall and footwall units. Serpentinite clasts are generally asymmetric with δ-type shapes, and the asymmetry is evidenced by pressure shadows at clasts edges defined by re-crystallization of micro-crystalline quartz. In oriented samples, the clasts asymmetry is indicative of a sense of shear toward the eastern sectors. Calcite in the clasts is characterized by thin and thick, regular straight twins, ascribable to the Type I and II twin morphology classes of Burkhard (1993) and Ferrill et al. (2004). The foliation of the cataclasites is a planar anisotropy defined by the millimeter-scale alternation of phyllosilicate layers and granoblastic quartz-rich layers. Phyllosilicates layers are made by chlorite crystals preferentially oriented parallel to banding to define the foliation. Granoblastic layers are characterized by re-crystallized fine- to very fine-grained quartz crystals that are frequently associated with microcrystalline serpentine and calcite. Chlorite also occurs as an alteration product of the serpentinite clasts, whereas calcite is also found in veins possibly related to alteration and fracturing of the serpentinite. Quartz is dominantly affected by intracristalline deformation (mainly undulose exctintion), but commonly shows as well evidence of dynamic recrystallization. These data coupled with regional evidences suggest that the Ottone-Levanto line can be regarded as a sinistral 28 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 traspressional shear zone developed in the Late Eocene-Early Oligocene time. The line can be thus correlated with others lines of the Alpine-Apennine system, as the Insubric Fault, the Sestri-Voltaggio line and the Central Corsica shear zone. All these lines devel- oped during the collisional tectonics resulting in both east- and westward thrusting of the internal zone of the appennine-alpine belt onto the continental margin domains, coeval with the northward displacement of the Adria plate. 29 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Distribution of differing stress regimes in the Western Central Alps controlled by inner arc bending? Kraus R.K.(1), Elter F.M.(1), Elena E.(2), Solarino S.(2) (1) DISTAV, University of Genova, Italy (2) INGV, Genova, Italy The slightly N-S bent Western Alps, with the adjacent Po Plain and Ligurian Sea are characterized by many seismic events. The pattern of earthquakes shows various patterns: arcuate, cluster and random distribution. We observe on the basis of the distribution of focal mechanisms 5 zones with different stress regime: the Western Seismic Arc (Briançonnaise-Arc), the Eastern Seismic Arc (Piedmont-Arc), the area SW from Turin in the Po Plain, the French/Italian Coastline (also Mediterranean Alps) and the E-W trending arm of the Ligurian Sea. The Briançonnaise-Arc is characterized by shallow crustal seismicity up to 15 km of depths, which affects especially the brittle sedimentary sequences. The focal mechanisms show a transtensive stress field for the Western Seismic Arc. The seismic events in the Piedmont-Arc occur from shallow to middle crust up to 30 km of depth and seem to follow the western edge of the “Ivrea Body” (a fragment of deeper crust). The stress field suggested by focal mechanisms is transpressive at shallow level with an increasing compressional component with depth. In between those two seismic arcs is a zone with considerably less seismic activity. In the area SW from Turin in the Po Plain is a cluster of subcrustal events with up to 70 km of depth. According to literature, this zone inhabited the rotation pole of the Adrian microplate and thus represents a place in which the European, Adrian and Ligurian domains meet. The French/Italian Coastline is represented by many seismic events up to middle crust, of mainly transtensive and strike-slip character. The E-W trending Ligurian Sea instead is dominated by strike-slip, thrust and transpressive events, which confirm the closure of the Ligurian Sea, as already observed by various authors. In literature is described that in the inner arc of a bent mountain belt the strain ellipses display arc-.parallel shortening and thickening of the mantle lithosphere, whereas the outer arc is dominated by arc-parallel stretching accompained by crustal thinning. In between is a neutral zone, in which the forces annihilate each other. This is in agreement with the observations in the arcuate-shaped Western Alps: the External Alps are prone to oblique extension and shallow seismicity, whereas the Internal Alps submit to transpressional forces and mid-crustal seismicity. The neutral surface is in agreement with the position of the “Ivrea Body”, using the neutral zone for intrusion. As in the case of the Western-Central Alps the Apenninic indenter is advancing, part of the accumulated stress gets dissipated along oblique normal and strike-slip faults leading to lateral extrusion of the French/Italian Mediterranean Alps towards SE. This explains the closure of the E-W trending arm of the Ligurian Sea and is in agreement with the observed principle horizontal stress vector in NW±SE direction in literature. 30 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Structural analysis of the nappe stack in the Urtier Valley, Cogne (Western Alps) Andrea Loprieno(1), Alessandro Ellero(2) (1) Viale Cavagnet 45, Cogne, Aosta, Italy (2) CNR-IGG, Pisa, Italy In this contribution we summarize the main results of structural investigations originally performed during diploma theses in the Urtier Valley, an area located at the northern flank of the Gran Paradiso Massif. Five different tectonic units were mapped in the area, from the geometrically lowermost to the uppermost they are: 1) the Gran Paradiso basement unit mainly composed of late Variscan granitoids and metapelites; 2) the Péne Blanche unit (part of the so-called “Fasceaux de Cogne”) which comprises Triassic quartzites, dolomites and carbonates and Liassic calcschists; 3) the oceanic Broillot unit including ophiolites and their metasedimentary cover; 4) the Bardonney mélange unit (BM) with ophiolites only represented in clasts and/or slices from centimeter to pluridecameter in size within an abundant carbonate/quarzitic and/or metabasic matrix; 5) the continental basement slices of Tour Ponton and Acque Rosse units. The structural evolution of the Urtier Valley is characterized by three major deformation phases and related structures affecting all tectonic units (Loprieno et al., 2009). D1 structures are represented by relict microfabric associated with eclogite-facies assemblages and small-scale isoclinal folds. It is also responsible for early stage of nappe stack formation. More problematic is the recognition of the D1 metamorphic grade in the BU. Overall it shows a blueschist facies metamorphism, similarly to what described for this unit outside of the study area (e.g. Colle della Rosa Dondena Unit by Battiston et al., 1984). However, high pressure assemblages have been recently recognized within garnet and chloritoid micaschists (Beltrando et al., 2008), suggesting an eclogitic facies metamorphism also for the BU. D2 is the dominant deformation stage, refolding the tectonic contacts between the units, showing D2 isoclinal folds on all scales, associated with S2 greenschist facies pervasive mylonitic fabric that represents the main foliation at regional scale. D2 folds axes and L2 lineation show a common orientation with E-W trend. D3 is instead characterized by open to close folds with NE or SW gently dipping axial plane that refold the composite main foliation S1/S2. At the scale of the study area, the reconstruction of the general architecture of the stacked units is facilitated by strati- graphic markers within the ophiolitic units (Ellero & Loprieno, 2014), showing how the nappe pile as a whole is deformed by a D2-D3 interference structure, with a large scale D2 synform refolded by a large scale D3 Urtier antiform. We suggest that this structure can be interpreted as the eastern closure of the Valsavarenche mega-fold, the so-called “Valsavaranche backfold” of Argand (1911) within the schistes lustres units, representing an extension of the structures described in adjacent areas by Bucher et al. (2003). REFERENCES - Argand E. (1911). Sur les plissements en retour et la structure en éventail dans les Alpes occidentals. Bull. Soc. Vaud: Sc. Nat., XLVII. - Battiston P., Benciolini L., Dal Piaz G.V., De Vecchi G., Marchi G., Martin S., Polino R. & Tartarotti P. (1984). Geologia di una traversa dal Gran Paradiso alla Zona Sesia-Lanzo in alta Val Soana, Piemonte. Mem. Soc. Geol. It., 29, 209-232. - Beltrando M., Lister G., Hermann J., Forster M. & Compagnoni R. (2008). Deformation mode switches in the Penninic units of the Urtier Valley (Western Alps): evidence for a dynamic orogen. J. Struct. Geol., 30, 194-219. - Bucher S., Schmid S.M., Bousquet R. & Fügenschuh B. (2003). Late-stage deformation in a collisional orogen (Western alps): nappe refolding, back-thrusting or normal faulting? Terra Nova, 15, 109-117. - Loprieno A., Ellero A., Elter P. & Molli G. (2009). Structural geometries and tectonic evolution of ophiolite and continental units in the Urtier Valley, Cogne (Western Alps). 9th Alpine Workshop “AlpShop 2009”, Cogne, Italy. - Ellero A. & Loprieno A. (2014). Ophiolitic units of the Northern Appenines as a tool to unravel Schistes Lustres stratigraphy of the Western Alps and their geodynamic context: the Urtier valley example, Cogne, Aosta Valley. Meeting in memory of Piero Elter. The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” Pisa, June 26-27, 2014. 31 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Zircon (U-Th)/He dating of the Penninic basal thrust Matteo Maino(1), Finlay M. Stuart(2), Andrea Ceriani(1,3), Alessandro Decarlis(4), Andrea Di Giulio(1), Silvio Seno(1) and Massimo Setti(1) (1) Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia, Italy (2) Isotope Geosciences Unit, SUERC, Scottish Enterprise and Technology Park, East Kilbride,UK (3) The Petroleum Institute, Abu Dhabi (4) IPGS/EOST, Strasbourg, France We presents new zircons (U–Th)/He (ZHe) ages from the thrust fault damage zone and surrounding wall rocks to determine the emplacement age of the southernmost segment of the Penninic basal thrust. Thermocronometry is integrated with X-ray diffraction analysis of clay minerals (illite crystallinity) and fluid inclusion microthermometry on vein-filling minerals to constrain the temperature conditions of the damage zone and the wall rocks during thrusting. This approach has been applied to the basal thrust of the Helminthoid Flysch, which represents the tectonic front of the European Alps accretionary wedge (Maino et al., 2013). Zircons selected for He dating are from Early Cretaceous arkosic sandstones the base of which has been involved into the thrust zone. ZHe ages of the fault rocks are completely reset (28.8±3.4 to 33.8±4.0 Ma) while the wall rock samples show pre-depositional inherited ages (117.4±14 to 158.7±18.9 Ma). Thermal constraints support warmer conditions (~220-300°C) within the fault damage zone respect to the wall rocks (< ~200°C), with the presence of a transition zone corresponding to a gradual increase of ZHe ages at progressive distance from the top of the fault damage zone. These results are in excellent agreement with the available independent geological and thermochronometric constraints that support early Oligocene movement of the basal thrust of the Helminthoid Flysch (Ford et al., 1999; Maino et al., 2012; Decarlis et al., 2013). Our results underscore, for the first time, the validity of ZHe tecnhique as a reliable thermochronometer to dating brittle faults developed into shallow crustal levels (< 6-7 km) on geological timescales. REFERENCES - Decarlis, A., Dallagiovanna, G., Lualdi, A., Maino, M. and Seno, S. (2013). Stratigraphic evolution in the Ligurian Alps between Variscan heritages and the Alpine Tethys opening: a review. Earth-Science Reviews, 125, 43-68. - Ford, M., Lickorish, W.H. and Kusznir, N.J. (1999). Tertiary Foreland sedimentation in the Southern Subalpine Chains, SE France: a geodynamic appraisal. Basin Research, 11, 315-336. - Maino, M., Dallagiovanna, G., Dobson, K., Gaggero, L., Persano, C., Seno, S. and Stuart, F.M. (2012). Testing models of orogen exhumation using zircon (U–Th)/He thermochronology: insight from the Ligurian Alps, Northern Italy. Tectonophysics, 560-561, 84-93, doi:10.1016/j.tecto.2012.06.045. - Maino, M., Decarlis, A., Felletti, F., Seno, S. (2013). Tectono-sedimentary evolution of the Tertiary Piedmont Basin (NW Italy) within the Oligo–Miocene central Mediterranean geodynamics. Tectonics, 32, 3, 593-619, DOI: 10.1002/tect.20047. 32 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Impact of surface processes and structural heritage on the structure and dynamics of orogenic wedges : Insights from Taiwan Jacques Malavieille Université Montpellier 2, CNRS UMR 5243, Lab. Géosciences Montpellier, France and LIA-ADEPT 536 CNRS-NSC, France-Taiwan As mountain belts are long lived structures, their evolution involves numerous processes that interact since the early history, beginning during oceanic subduction and ending during the late orogenic evolution, which leads to erosion and the ultimate destruction of topography. Most orogens form in subduction settings due to plate convergence involving large horizontal shortening and strong deformation of the crust developing into an overall wedge shape during their evolution. I will focus on the Taiwan orogen caused by the subduction of a continental margin lower-plate under an oceanic upper-plate bearing a volcanic arc following intra-oceanic subduc- Figure 1 tion, a process commonly known as arc-continent collision. Thus, after the development of a sedimentary accretionary prism and closure of the oceanic domain, continuous subduction of the lithospheric mantle induces deformation of the continental crust and controls the structural asymmetry of the growing mountain belt. Since the pioneer works by Dahlen, Davis and Suppe in the Eighties, mountain belts have been often considered by geologists as crustal scale accretionary wedges whose deformation mechanisms can be satisfactorily described by a Coulomb behavior. The theory offers a simple mechanical framework allowing a division into different tectonic regimes depending on wedge stability : critical, undercritical, overcritical. Since then, it has been shown that orogens commonly adopt a distinct geometry with a low-tapered pro-wedge facing the subducting plate, and a high-tapered retro-wedge on the internal side. Erosion has rapidly been added as a significant parameter because the impact of material transfer on the mechanics and structural evolution of sub-aerial wedges relative to submarine ones is major. In the active Taiwan orogen major questions can be addressed concerning; mechanisms of lithospheric deformation in convergent settings, subsequent active deformation involving large seismogenic faults, processes of mountain building (from oceanic subduction to continental subduction and late orogenic extension) and impact of surface processes. In this area, the obliquity of the plate convergence involves the progressive subduction of the continental margin of China inducing the fast growth of the mountain belt. Today, the orogen culminates at about 4000 meters rising from sea-level a few million years ago. Impact of climate and surface processes are thus particularly well expressed allowing to study their interaction with tectonic processes in the island which is particularly vulnerable to natural hazards (one of the highest population densities in the world). Because of the high convergence rate (~ 9 cm/ yr) and the complex interaction (doubly-verging oceanic and continental subduction) between converging Eurasia and Philippine Sea plates (PSP), deformation 33 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Figure 2 rates and erosion rates are extreme (horizontal shortening > 2 cm/yr on major faults, vertical motions up to 3 cm/yr). In the domain of active continental subduction, where the orogenic wedge grows today, estimates of middle-term shortening rates on active faults as well as deformation mechanisms in the wedge show a surprising behavior. Most of the convergence is accounted for by just a few major faults, on the western side of the wedge in the foreland and on its backside against the Philippine sea upper-plate onland along the Longitudinal Valley and offshore the Coastal Range, the accreted part of the Luzon Volcanic arc. Thus most of the bulk shortening occurs today on foreland faults and along the backside of the mountain belt, leaving little (if any) horizontal shortening within the body of the wedge (Fig. 1). Together with new constraints on the thermal evolution and exhumation of the metamorphic Central Range (CR), this kinematic has been analyzed by numerical thermo-kinematic and analog models involving erosion, in which underplating at depth sustains the growth of the orogenic wedge. Results of models show that interactions between tectonics and surface processes play a major role in controlling relief morphology, fault evolution, uplift inside the orogen and localization of exhumation. The main processes of wedge growth in Taiwan can be described by accretion in the frontal part of the thrust wedge and underplating of tectonic units at depth under the Central Range, involving strong uplift and thickening of the internal domain and backthrusting on the PSP upper-plate. Thus, intermediate-term deformation is strongly partitioned in Taiwan suggesting important insights on the present-day behavior and growth mechanisms for large faults that are responsible for great earthquakes (Fig. 2). Following this regional approach, I will address major open questions regarding the global and local responses (i.e., at orogenic scale and at the scale of faults or ridges) of an orogenic wedge under the impact of tectonic or climatic forcing at different time scales. Insights from analog models are used to; - illustrate the impact of first order parameters such as the initial geodynamic subduction setting, material transfer in the wedge, structural inheritance (OCT and inherited extensional structures), - show how the interactions between surface processes and tectonics influence the structures and the behavior of orogenic wedges (kinematics of deformation, exhumation mechanisms, and long-term evolution). Finally, analogies between the evolution of Taiwan orogen and the evolution of the Alps-Apennine geodynamic system will be outlined, foregrounding the key role of continental subduction and subduction reversal. 34 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The basal part Modino Unit Succession under the belt-foredeep system of the Northern Apennines (Italy) Alessandra Marchi(1), Luca Pandolfi (1, 2), Rita Catanzariti(2) (1) Earth Science Department, Pisa University, Italy (2) IGG-CNR Pisa, Italy The Modino Unit turbidite system of the Northern Apennines foreland basin provides an excellent opportunity to study the sedimentary and structural variations within the context of spatial and temporal distribuition of source rocks during the evolution of foreland basins. The substrate of this Unit was interpreted as a stratigraphic-structural mélange (Plesi et al., 2000), the expression of polyphase tectonic phases of Cretaceous-Eocene accretion that have affected the external part of Ligurian and Subligurian domain. In this article we present the preliminary data of different stratigraphic sections outcropping in Tuscan and Emilian regions, with particulary attention to the lower part of Modino Unit Succession, of pre-Ligurian stage, unconformably deposited at the top of the Ligurian and Subligurian substrate, composed by Fiumalbo Shale Fm. and Marmoreto Marl Fm. The tectonic setting of this Unit is complex and necessitated the use of stratigraphical, biostratigraphical and petrographical studies to achieve this goals. The Modino Unit succession is composed by three different formations: The Fiumalbo Shale Fm. followed in some sections of coarse breccia deposits that cover the substrate (Riccovolto Breccia) are made up of mostly red and green shales with intercalations of limestone and turbidite-like sandstones beds more or less extensive (Rio Acquicciola Sandstones Auctt. or M. Sassolera Sandstones Auctt.). The Marmoreto Marl Fm. are characterized by fine emipelagic sediments, have a massive structure (with rare thin layers of fine sandstones). The Monte Modino Sandstone Fm. are constituted by one or more sequences of turbidite facies with quite variable vertically and laterally . Their deposition occurs preferentially in the middle and front al part of the prism and is quickly interrupted, on its southwestern margin, by the thrust belt materials. From this reason the axis of sedimentation moving outwards. A petrographical study on turbidite-like sandstone beds in Fiumalbo Shale Fm., show a petrofacies character- ized by a modal composition of Q48F27L+CE25, according with the composition of Monte Modino Sandstone of this study, while shows different composition in the Fine-Grained Rock Fragments Compositional Mode (Lm-Lv-Ls plot). The sandstones in Fiumalbo Shale Fm., are composed by different tipology of fine grained lithic fragments, and its composition changes strata-strata in the same stratigraphic sections. The fine grained lithic fragments are composed by dominating metamorphic origin clasts and ophiolithic rock fragment associated with unmetamorphic radiolaritic fragments. The biostratigraphical analysis indicate that the age of these formations is comprised between Lutetian and Chattian ages. These formations reflect a slope environment, with quite deep and with a strong affinity to Epiligurian sections, which resemble the coeval succession Monte Piano-Ranzano and their sedimentation environment reflects a time and an area of major physiographic expression of the prism. The composition of this arenites is interpreted as being controlled mainly by synsedimentary tectonics connected with the evolution of an accretionary prism east vergent. This composition reflects the different stages of the process of accretion and is the expression of the different sedimentary environments that were gradually generating. The lower part of Modino Unit succession seems to be supplied by two different source areas, the classic Alpine source area and a more proximal “Liguride derived” source, maybe located in the proto-apenninic wedge. REFERENCES - Plesi G., Chicchi S., Daniele G. & Palandri S. (2000). La Struttura dell’Appennino reggiano-parmense fra Valditacca, il Passo di Pradarena e il M. Ventasso. Boll. Soc. Geol. It., 119, 267-296. 35 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The late Carboniferous-Permian successions of the Northern Apennines: new data from the contact between San Lorenzo Schists and Asciano Breccias and Conglomerates in the Pisani Mts. inlier (Tuscany, Italy) Marini F.(1), Pandeli E. (1,2) & Tongiorgi M.(3) (1) Earth Sciences Department, University of Florence, Italy (2) CNR-Institute of Geosciences and Georesources, Section of Florence, Italy (3) Natural History Museum, University of Pisa, Italy The Pisani Mts is tectonic window allowing the exposure of the deepest metamorphic tectonic units of the Northern Apennines chain. The Pisani Mts. metamorphic inlier is made up of two tectonically superimposed tectonic sub-units (i.e the M.Serra and S.Maria del Giudice sub-Units) and it is laterally bounded by high-angle normal faults. As in the other Tuscan inliers, the Pisani Mts. successions includes a Paleozoic “basement” here are represented by the Buti banded phyllites and quartzites (Upper Ordovician?), San Lorenzo schists (SLs, Upper Carboniferous-Early Permian) and Asciano breccias and conglomerates (Abc, Permian?) that are separated by erosional angular unconformity contacts at map scale (Rau & Tongiorgi, 1974; Pandeli et al., 1994). According to its sedimentological features and to the abundant fossil floristic assemblage of Wephalian?/ Stephanian to Autunian time interval, previous Authors suggested that the essentially grey to black siliciclastics of SLs were deposited in a mostly fluvio-lacustrine environment characterized by reducing conditions, but neritic faunal associations has been recently discovered in the middle-lower part of the formation (Rau & Tongiorgi, 1974; Landi Degl’Innocenti et alii, 2008). The unfossiliferous Abc is represented by dominant purplish, polymictic rudites. These poorly-mature and -sorted sediments has been associated to fluvial-fan deposits in a semi-arid environment (Rau & Tongiorgi, 1974). The sharp and erosional stratigraphic contact between SLs and Abc had been previously interpretated as the angular unconformity of the Permian extensional rejuvenation event of the Variscan Chain in Europe (Saalian event). For improving the knowledge of this important Permian event in Tuscany from a paleogeographic, paleoenvironmental and paleotectonic point of view, the Authors carried out new studies about the SLs and Abc in the Pisani Mts.. In this frame, the Authors analized a well-exposed, continuous section containing the SLsAbc contact along the road to Pian della Conserva lo- cality (see Fig. 1). In this place, We measured a detailed lithostratigraphic section (132 m of total lenght) and sampled the lithotypes for geochemical (main elements through X-ray fluorescence technique) and microscopic (textural and modal) analyses. The studies reveal that the SLs/Abc contact in the Pian della Conserva Section is gradual and it is characterized by passage for alternations of the lithotypes of both formations. In particular, the top of SLs consists of an alternance of thick grey to black metapelitic beds with quartzitic metasandstone intercalations and rare quartz-rich microconglomeratic beds: the latter become more frequent, coarser and thicker towards the top of SLs, where thin green phyllitic levels and a volcanoclastic layer also occur. The transition is represented by an about 20 m-thick alternation of varicoloured (mostly green) phyllitic lithotypes with greenish-grey metasand- Figure 1 - Structural sketch of the Pisani Mts. (modified from Rau & Tongiorgi, 1974). The asterisk indicates the location of the studied outcrop, i.e. Pian della Conserva section. 36 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 stone and fine-grained metarudites. The Abc basal part is made up of the same greenish metasiliciclastics but with decimetric to metric intercalations of violet and purple polymictic metabreccias and low-mature metaconglomerates and metapelites. Upward these two latter lithotypes largely prevail. The passage between the two formations is outlined not only by the cromatic change, but also by petrographic-geochemical data that demonstrate a change from a reducting to a oxiding environment (e.g. the clear variation of LOI content and of Fe2+/Fe3+ ratio). The following results of the study can be pointed out: 1) Because of the gradual stratigraphic passage, the unfossiliferous Abc can be likely attributed to Permian (late Early Permian?); 2) The lithological-sedimentological data confirm that the SLs/Abc contact define the tectonic rejuvanation of the Variscan landscape during the Saalian Event, but also show a gradual change in the climatic (from humid to semi-arid) conditions. This can be related to the modification of the landscape (onset of new tectonic barriers) and of the drainage (from fluvial valleys characterized by more or less wide, often marshy flood plains to very active alluvial fans set close to tectonic highs due to the Saalian strong rejuvenation of the relief); 3) The presence of volcanic layers in the Late Carboniferous-Autunian Sls has been pointed out for the first time and show similarities with the metavolcanites (Iano porphiritic schists) present at the top and interbedded within the Torri breccia and conglomerate (that corresponds to Abc) in the Iano Carboniferous-Permian succession (Pandeli, 1998). REFERENCES - Landi Degl’Innocenti V., Pandeli E., Mariotti Lippi M. & Cioppi E. (2008) - The Carboniferous-Permian succession of the Pisani Mountains (Tuscany, Italy): preliminary data from the De Stefani collection (Natural History Museum of Florence). Boll. Soc. Geol. It., vol. 127 n° 3, 548-558 - Pandeli E. (1998) - Permo-Triassic siliciclastic sedimentation in the Northern Apennines: new data from the Iano metamorphic inlier (Florence). Mem. Soc. Geol. It., 53, 185-206. - Pandeli E., Gianelli G., Puxeddu M. and Elter F. M. (1994) - The Paleozoic Basement of the Northern Apennines: stratigraphy, tectono-metamorphic evolution and alpine hydrothermal processes. Memorie Società Geologica Italiana, 48, 627-654. - Rau A. & Tongiorgi M. (1974) - Geologia dei Monti Pisani e SE della Valle del Guappero, Mem. Soc. Geol. It., 13, 227-408 37 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Thermo-mechanical numerical model of the transition from continental rifting to oceanization: the transition from Permian-Triassic thinning to oceanisation in the alpine chain Anna Maria Marotta(1), Katya Conte(1), Manuel Roda(2), Maria Iole Spalla(1, 3) (1) Department of Earth Sciences “Ardito Desio”, Università degli Studi di Milano, Italy (2) Universiteit Utrecht, Fac. of Geosciences, Utrecht, Italy (3) CNR-IDPA, Milano, Italy The transition from continental rifting to oceanization has been investigated by mean of a 2D thermo-mechanical numerical model in which the formation of oceanic crust and serpentinite, due to the hydration of the uprising peridotite, as been implemented. Model predictions have been compared with natural data related to the Permian-Triassic thinning affecting the continental lithosphere of the Alpine domain, in order to identify which portions of the present Alpine belt, preserving the imprints of Permian-Triassic high temperature (HT) metamorphism, is compatible, in terms of lithostratigraphy and tectono-metamorphic evolution, with a lithospheric extension preceding the opening of the Ligure-Piemontese oceanic basin. The HT Permian-Triassic metamorphic re-equilibration overprints an inherited tectonic and metamorphic setting consequent to the Variscan subduction and collision, making the Alps a key case history to explore mechanisms responsible for the re-activation of orogenic scars. Our comparative analysis supports the thesis that the lithospheric extension preceding the opening of the Alpine Tethys did not start on a stable continental lithosphere, but developed by recycling part of the old Variscan collisional suture. 38 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Lower Miocene contractional shearing in the Massa Unit (Northern Apennines, Italy): in situ U-Th-Pb dating of monazite Chiara Montomoli (1), Rodolfo Carosi (2), Antonio Langone (3), Angelo Borsani (4), Salvatore Iaccarino (1) (1) Dipartimento di Scienze della Terra, University of Pisa, Italy (2) Dipartimento di Scienze della Terra, University of Torino, Italy (3) C.N.R. Istituto di Geoscienze e Georisorse, U.O.S. Pavia, Italy (4) C.G.T., Università di Siena, San Giovanni Valdarno, Arezzo, Italy The occurrence of metamorphic units with different metamorphic imprints in the metamorphic core of the Northern Apennines has been recognized since a long time (Elter, 1975; Carmignani et al., 1978; Franceschelli et al., 1986; Kligfield et al., 1986). However, the timing of peak metamorphism is debated as few data are available (Kligfield et al., 1986). Structural analysis performed on the Massa Unit on the western side of the Apuane Alps (Northern Appennines) revealed that the prominent fabric is an S2 penetrative foliation transposing an earlier S1 foliation. L2 stretching lineation trends SW-NE and kinematic indicators point to a top-to-the NE sense of shear. S2 foliation affected most of the metamorphic porphyroblasts (e.g. chloritoid, kyanite) developed during and after the metamorphic peak. D1 deformation phase, related to the thickening stage of the belt, has been transposed by D2 so that D1 relics are observed only in D2 microlithons. The nappe pile has been later affected by large scale folds nearly parallel and perpendicular to the belt and extensional tectonics at upper structural levels (Carosi et al., 2002, 2004). We conducted for the first time U-(Th)-Pb in-situ analysis on metamorphic monazites along both S1 and S2 foliations. Monazite on S2 gave concordant ages at ~ 1816 Ma Ma. Older ages (~ 20-24 Ma) have been obtained for monazite along S1, within the S2 microlithons, grown during prograde metamorphism. The latter ages shift the metamorphic peak of several Myr with respect to the ~ 27 Ma age proposed by Kligfield et al. (1986) by K/Ar on white micas in the Apuane Unit. The latter age is in conflict with the age of the youngest sediments of the AU (Pesudomacigno) undergoing metamorphism and deformation. These conflicting data led some Authors to shift the metamorphic peak towards even younger ages (11-12 Ma, Patacca et al., 2013). This age is unlikely because at 13-10 Ma AU has already reached upper structural levels (< 9 km) after metamorphic peak as suggested by zircon fission tracks (Balestrieri et al., 2011 with references), being the annealing temperature of zircons (~ 250°C) much lower with respect to the peak metamorphic temperature (T ~ 400-500°C, Franceschelli & Memmi, 1999, Molli et al., 2000). Anyway we must be aware that the different metamorphic units in the metamorphic core could have registered (different) peak metamorphism in different times. The new data allow to better constrain the timing of superposition of the different tectonic units of the metamorphic core of the Northern Apennines. The occurrence of clasts from the MU with two foliations in the cataclasites (“brecce poligeniche”) at the contact between MU and the overlying Tuscan Nappe (Carosi et al., 2002, 2004) implies that tectonic coupling happened after ~ 18-16 Ma. Structural data suggest that the tectonic coupling between MU and Apuane Unit is achieved post D1 after the metamorphic peak and by a reverse shear zone during contractional tectonics during the thickening stage of the Northern Appennines. Extensional tectonics occurred later, possibly at 13-10 Ma by the so called “collapse folds” and low-angle and high-angle normal faults at higher structural levels (depth < 9 km; Fellin et al., 2007) when the metamorphic units have been already exhumed reaching the upper part of the crust and deforming under brittle conditions (Carosi et al., 2002, 2004, Balestrieri et al., 2011). REFERENCES - Balestrieri, M. L., Pandeli, E., Bigazzi, G., R. Carosi, R., Montomoli, C., 2011. Age and temperature constraints on metamorphism and exhumation of the syn-orogenic metamorphic complexes of Northern Apennines, Italy. Tectonophysics 509, 254–271. 39 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 - Carmignani, L., Giglia, G., Kligfield, R., 1978. Structural evolution of the Apuan Alps: an example of continental margin deformation in the Northern Apennines, Italy. Journal Geology 8, 487–504. - Carosi, R., Montomoli, C., Bertuccelli, N., Profeti, M., 2002. The structural evolution of the southern Apuan Alps: new constraints on the tectonic evolution of the Northern Apennines (Italy). Comptes Rendus Geosciences 334, 339–346. - Carosi, R., Montomoli, C., Pertusati, P.C., 2004. Late tectonic evolution of the Northern Apennines: the role of contractional tectonics in the exhumation of the Tuscan units. Geodinamica Acta 17, 253–273 - Elter P. (1975) Introduction à la géologie de l’Apennin Septentrional. Bull. Soc. Geol. France, 17: 956-962. - Fellin, M.G., Reiners, P.W., Brandon, M.T., Wüthrich, E., Balestrieri, M.L., Molli, G., 2007.Thermochronologic evidence for the exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy. Tectonics 26, TC6015. doi:10.1029/2006TC002085. - Franceschelli, M., Leoni, L., Memmi, I., Puxeddu, M., 1986. Regional destribution of Al-silicates and metamorphic zonation in the low-grade Verrucano metasediments from the Northern Apennines, Italy. Journal of Metamorphic Geology, 4, 309–321. - Franceschelli M, Memmi I. 1999. Zoning of chloritoid from kyanite-facies metapsammites, Alpi Apuane, Italy. Mineralogical Magazine 63, 105-110. - Kligfield, R., Hunziker, J., Dallmeyer, R.D., Schamel, S., 1986. Dating of deformation phases using K–Ar and 40Ar/39Ar techniques: results from the Norhern Apennines. Journal of Structural Geology 8 (7), 781–798. - Molli, G., Giorgetti, G., Meccheri, M., 2000. Structural and petrological constraints on the tectono-metamorphic evolution of the Massa Unit (Alpi Apuane, NW Tuscany, Italy). Geological Journal 35, 251–264. - Patacca, E., Scandone, P., Conti, P., Mancini, S., Massa, G., 2013. Ligurian-derived olistostrome in the Pseudomacigno Formation of the Stazzema Zone (Alpi Apuane, Italy). Geological implications at regional scale. Italian Journal of Geosciences 132, 463-476. 40 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Out of sequence thrust in the inner zone of northern Apennines: insight from Elba Island nappe stack Musumeci G.(1), Massa G.(2), Pieruccioni D.(2, 3), Mazzarini F.(4) (1) Dipartimento di Scienze della Terra, Università di Pisa, Italy (2) CGT, Centro di GeoTecnologie, Università di Siena, San Giovanni Valdarno, Arezzo, Italy (3) Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Italy (4) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Italy In the northern Tyrrhenian Sea, the Elba Island is one of the westernmost portions of the northern Inner Apennine belt and represents a key area to deciphering the structural evolution of the belt inner zone. The central-eastern Elba Island is made up by nappe stacking of several sedimentary and metamorphic tectonic units, derived from both continental and oceanic domains (Trevisan, 1950; Bortolotti et al., 2001). Since the Late Miocene, nappe stack was affected by emplacement of magmatic rocks rocks (M. Capanne and Porto Azzurro plutons ~ 6.8-5.9 Ma), followed by postcollisional extension, with development of a low angle normal fault (i.e. Zuccale fault; Keller and Pialli, 1990; Pertusati et al., 1993). A noteworthy feature of Elba island nappe stack is a anomalous tectonic repetition of Tuscan and Ligurian units thrust sheets due to a slice of Tuscan nappe sequence (Complex III) tectonically sandwiched between two thrust sheets of Ligurian units. The Complex III is made up of the Rio Marina Unit (Deschamps et al., 1983) and the tectonically reduced Tuscan Nappe sequence. The former consists of Rio Marina Fm. (Late Paleozoic metasandstones and phyllites) and Verrucano Fm. (Middle Trias metasandstone). The overlying Tuscan nappe sequence is made up of Mesozoic carbonate formations, tectonically overlain by Scaglia Fm. which at the map scale discordantly rests above the Jurassic carbonate formations, and Rio Marina Fm. phyllites. This structural setting is consistent with the ‘Serie ridotta”, interpreted as the result of Middle Miocene extension of the chain (Carmignani et al. 2004). The nappe stack is folded in a NW-SE striking kilometre-scale antiformal tight fold continuously exposed along the eastern Elba coastline from Ortano to the north of Rio Marina. The antiform is characterized by a gently (5° - 30°) N-NW plunging axis and an eastward vergence with a shallowly westward dipping axial plane surface, which refolded the previous foliation (S1) and bedding (S0). The axial plane cleavage is well developed in phyllites and metasandstones and locally appears as a penetrative main foliation at mesoscale (S2), wrapping lenses of massive quartz - conglomerates (“Anagenites” Auct.). Locally S-C structures show an eastward sense of shear. The antiform hinge zone and the normal limb are well exposed along the coastline to the north of Rio Marina. Phyllites and metasandstones of the Rio Marina Fm show vertical lying bedding (S0) and subparallel cleavage (S1). Locally (Vigneria beach) decametric isoclinal folds, associated with S1, are recognized. Southward Rio Marina the antiform inverted thinned limb is cross cut by a tectonic contact. The serpentinites slice at the top of the Acquadolce Unit appear under this tectonic contact. Field evidences and borehole data show that the antiform is in turn cross-cut by the Zuccale Fault. Then, by high angle normal and strike-slip faults characterized by Fe-hydroxides veins. The eastward vergent antiform, exposed along the eastern coast, refolded the early nappe stack leading to the unusual tectonic interfingering of the Tuscan and Ligurian Units. Thus the lower serpentinite slice, cropping out below the Tuscan derived metamorphic and sedimentary units of Complex III, corresponds to the laminated inverted limb of antiform, successively affected by the contact metamorphism related to Porto Azzuro pluton emplacement. Age of post nappe folding deformation is bracketed between the age of the folded “Serie Ridotta” (Late Langhian, 13 Ma) and the emplacement of intrusive rocks dated in the central-eastern Elba as being from the Messinian (7-6 Ma). Therefore, the anomalous tectonic repetition of Tuscan and Ligurian units thrust sheets in the eastern Elba nappe stack gives evidence of Middle-Late Miocene out-of-sequence thrusting postdating nappe stack and predating granite emplacement and upper crustal extension (Late Miocene-Pliocene). We suggest that collisional tectonics in the inner zone of northern Apennines was polyphased 41 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 with early stacking of tectonic units followed by out of sequence thrust predating the eastward shifting of the thrust front towards the external zone during late Miocene-Pliocene. REFERENCES - Bortolotti, V., Fazzuoli, M., Pandeli, E., Principi, G., Babbini, A., & Corti S. (2001). Geology of Central and Eastern Elba Island, Italy. Ofioliti, 26 (2a), 97 – 150. - Carmignani, L., Conti, P., Cornamusini, G. & Meccheri, M. (2004). The Internal Northern Apennines, the Northern Tyrrehenian sea and the Sardinia – Corsica block. Special Volume of the Geological Society for the IGC 32 Florence-2004, 59-77. - Deschamps, Y., Dagallier, G., Macaudière, J., Marignac, C., Moine B. & Saupé, F. (1983). Le gisment de pyrite-hématite de valle Giove (Rio Marina, Ile d’Elbe, Italie), Partie 1. Schweiz. Mineral. Petrogr. Mitt. 63, 149-165. - Keller, J.V.A. & Pialli G. (1990). Tectonics of the island of Elba: a reappraisal. Boll. Soc. Geol. It., 109, 413-425. - Pertusati, P.C., Raggi, G., Ricci, C.A., Duranti, S. & Palmeri, R. (1993). Evoluzione post-collisionale dell’Elba centro-orientale. Mem. Soc. Geol. It., 49, 297-312. - Trevisan, L. (1950). L’Elba orientale e la sua tettonica di scivolamento per gravità. Mem. Ist. Geol. Univ. Padova, 16, 5-39. 42 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Paleogeographic problems of the Eocene foreland basin of the south-central Pyrenees Emiliano Mutti University of Parma, Italy In the Eocene foreland basin of the south-central Pyrenees thick and extensive accumulations of basinal turbidites, the Hecho Group turbidites, have long been considered as sourced from the east, i.e. from the thick alluvial, fluvio-deltaic and shelfal strata outcropping in the Tremp-Graus syncline, through feeder channels crossing the lateral ramp of the Cotiella thrust sheet. For this reason and because of the very good exposures, this basin and its turbidites have become very popular worldwide. Field work I carried out with my co-workers over the last 25 years (after 20 years spent studying the Hecho Group turbidites) shows that (1) the Tremp-Graus basin is entirely preserved and does not contain channels funneling sediment to the west; (2) paleocurrent directions, stratigraphic relationships and facies distribution patterns clearly indicates that the Tremp-Graus basin, a piggyback basin on top of the Tremp Unit, was separated since the Castissent time (early Eocene) from the turbidite Hecho Group basin by a structurally-controlled ridge, herein termed the Foradada Ridge. The ridge was the expression of the oblique transpressive belt associated with the southward and westward translation of the Tremp Unit. The Foradada Fault is the most prominent feature of the ridge separating two basically different domains. Along this fault, the northern and southern margins of the basin came into contact through time as a result of the dextral slip motion of the fault. The main right-lateral motion occurred during the late-middle Eocene. The northern margin, i.e. the Tremp-Graus basin, displays spectacular northerly-derived deltas and fandeltas (Castissent and Santa Liestra time); at the same time, the southern margin shows a large southerly-fed river delta system (La Fueva delta system). The mudstone-dominated delta-slope facies of this delta interfinger with the Hecho Group turbidites. The source of these turbidites is thus represented by southerly-fed delta systems that migrated westward as a result of thrust propagation. Much of the La Fueva delta system must now be buried below the Montec and the Sierra Marginales thrust sheets. The location of the main source area of the Hecho Group turbidites raises several problems. The large volume of these turbidites and the lateral continuity of many sandstone beds favour a source from relatively large and mature fluvio-deltaic systems with drainage basins probably located in the Iberian and Catalan chains. A similar paleogeographic setting is that of the sandy flysches of the northern Apennines that had an extra-orogenic source area being mainly fed from the Alps. The lesson that can be learnt from the south-central Pyrenean basin is that a reasonable paleogeographic understanding of the fill of a tectonically-active basin can only be achieved through an approach that includes sedimentologic, stratigraphic, and structural studies at local and regional scale. 43 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Microstructural evidence of weakening mechanisms developed along incipient Low-Angle Normal Faults in pelagic limestones (Southern Apennine, Italy) Novellino R.(1), Prosser G.(2), Viti C.(1), Spiess R.(3), Agosta F.(2), Tavarnelli E.(1), Bucci F.(4) (1) Dipartimento di Scienze Fisiche, della Terra e Ambientali, Università degli Studi di Siena, Italy (2) Dipartimento di Scienze Geologiche, Università della Basilicata, Potenza, Italy (3) Dipartimento di Geoscienze, Università degli studi di Padova, Italy (4) CNR-IRPI, Perugia, Italy Low-Angle Normal Faults (LANFs) consist of shallowly-dipping extensional tectonic structures, whose origin relates to a mechanical paradox currently debated by a number of researches. The easy slip along these faults suggests a strain-weakening process active during fault nucleation and growth. Weakening mechanisms may include: i) presence of weak minerals; ii) high fluid pressure which, causing a drastic reduction of the effective stress, and iii) dynamic fault weakening during coseismic rupture. In the Basilicata portion of Southern Apennines, LANFs have been extensively studied by geological mapping and field structural analysis. Differently, a detailed microstructural observations are not hitherto available in the geological literature. For this reason, in this note, we summarize the results of microstructural analysis carried out on fault rock samples collected from a well-exposed mesoscopic LANFs. The present work is aimed at analyzing the weakening mechanisms that took place along the study faults. The incipient study LANFs are characterized by a narrow and discontinuous damage zone surrounding a very thin fault core that include a discrete slip-surface. The offset is in the range of tens of centimeters to few meters. At the microscope scale, the sampled rocks reveal the coexistence of different structural features such as: i) pervasive shape preferred orientation defined by elongated grains of calcite, producing a distinct foliation; ii) Crush Microbreccia (CM), formed of angular clasts locally in contact with each other; iii) several Ultracataclastic Veins (UV), departing from the slip-surfaces and cutting across the slip-zone. TEM investigation reveal the presence of ultrafine to calcite-nanoparticles (<200 nm) aggregate within UV, and iv) decarbonation features, where calcite grains exhibit irregular boundaries, vacuum and vesicles, most likely related to degassing processes. Thermal decomposition results in formation of a calcite aggregate made of lenticular calcite grains, 1 to 10 micron long. This honeycomb like structure of calcite grains shows well-defined shape- (SPO) and crystallographic-preferred orientations (CPO), consistent with the shear sense. These latter microstructural features, testify that in localized area, next to the slip-surface, an abrupt increase of temperature was accompanied by dynamic recrystallization. At least, two generation of calcite veins, approximately orthogonal to the slip-surface, show systematic cross-cutting relationships with the above illustrated structural features. Overall, the investigated features form a complex structural network resulting from ductile to brittle deformation mechanisms, and microstructural analysis provides significant insight in the static and dynamic fault weakening mechanisms acting along the shear zone. 44 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The role of strike-slip tectonics in the early to late formation of the southern Calabrian Alpine-Apennine chain system (Calabria, Southern Italy) Gaetano Ortolano(1), Vincenzo Tripodi(2), Rosolino Cirrincione(1), Salvatore Critelli(2), Francesco Muto(2), Roberto Visalli(1) 1 Dipartimento di Scienze Biologiche Geologiche ed Ambientali, Università di Catania, Italy 2 Dipartimento di Biologia, Ecologia e Scienze della Terra, Università della Calabria, Italy The kinematic and the rheological features of some earlyto late-Alpine strike-slip tectonic structures have been here investigated with the aim to contribute to better constrain the former as well as the recent stages of the arc shaped formation of the southern Calabrian orogenic segment. The development of these strike-slip tectonics have been always widely invoked by several authors to explain the drift of the Calabrian terrane as a consequence and/or as a precursor of the Alpine back-arc basin expansion. Nevertheless, clear evidences of these deep seated junction bands are resulted rarely recognizable due to the pervasive activity of the Oligo-Miocene thin skinned thrusting accompanied by Plio-Pleistocene normal tectonics. In this tectonic framework, the Calabrian orogenic segment can be interpreted as the result of the amalgamation of remnants of an original segment of the southern European Hercynian chain (Fig.1a) reworked during the Alpine-Apennine orogeny (Fig.1b). This evolution led to the formation of a composite terrane (i.e. Calabride Composite Terrane - CCT) (Fig.1a) constituted by basement rocks presently merged in several Hercynian or possibly older sub-terranes. These rocks were locally overprinted during the different stages of the Alpine metamorphic cycle, which also affected part of the Mesozoic oceanic-derived units and sedimentary sequences, and were definitively stacked during the Alpine-Apennine thin skinned thrusting event. In this geodynamic scenario, results clear as the reconstruction of the evolution of the strike-slip tectonics from its embryonic (Late Cretaceous-Early Paleocene) to the present-day activity plays a crucial role in the not yet fully unraveled distribution of plate and/or microplate that crowd the geodynamic puzzle of the western Mediterranean realm. In this framework, in order to constrain the evolution of the strike-slip tectonics of the southern CCT, three different strike-slip shear zones, aligned along the junction between the Aspromonte and Serre Massifs, have been structurally investigated, contributing also to explain the lateral jux- taposition of differently evolved crystalline basement domains. The first one, the Palmi Shear Zone (PSZ) represents the most ancient relic of transcurrent activity recorded in this sector of the southern Alpine chain. It is characterized by a 400 m wide tabular structure with subvertical foliation, involving skarn, migmatite and tonalite belonging to an original southern European Hercynian high grade crustal section. The present-day average attitude is WNW-ESE, and can be followed in outcrop for about 1200 m inland up to disappear below a Tortoanian age silicic-clastic formation (Ortolano et al., 2013). Field evidences accompanied by Rb-Sr age data on whole mica (Prosser et al., 2003), brackets the mylonitic shearing activity between 56 to 51 Ma. Structural analysis of mylonitic rocks (Fig. 1c’,c’’) highlighted as the average attitude of the subvertical foliation as well as the stretching lineation ranging from W-E, to NW-SE with a transport direction top-to-E, SE in the present-day geographic coordinates (Ortolano et al., 2013) (Fig.1c’). The second and the third ones, the Nicotera-Gioiosa Shear Zone (NGSZ) and the Molochio-Antonimina Shear Zone (MASZ), respectively, represent the most prominent morphological evidences of the NW–SE fault system in the southern CCT. The (MASZ) bordering the southern end of the Gerace– Antonimina graben, and can be considered as the natural brittle evolution of the deepest-rooted Palmi Shear Zone (PSZ), outcropping in the southern portion of the Gioia Tauro basin (Fig.1b, d). The NGFZ is a relevant regional structure of the strike–slip fault system that borders the Strait of Siderno to the north. In oblique-slip fault systems, displacement generates important topographic gradients with associated uplift and subsidence. The activity of these two last shearing zones can be framed from the Serravallian–Tortonian stages up to now. The present-day average attitude is WNW-ESE and in 45 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Figure 1 a) Geological sketch map of Alpine and pre-Alpine chains in the Western Mediterranean area; b) Calabrian Peloritani Orogen schematic map (after Angì et al. 2010) with study areas location; c) Evidences of mylonitic related structures of the Palmi shear zone: c’) sheat-fold in mylonitic leucogneiss with E-W vertical attitude, c’’) altered feldspar porphyroclast within mylonitic skarns; d) Overview of the Molochio-Antonimina tectonic aligment. 46 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 their evolution have exhibited both strike–slip and normal kinematics (Tripodi et al., 2013). From this brief review, results clear as the present-day tectonic framework of southern CCT scenario, cannot be explained invoking just the main juxtaposition of a northern (central and northern Calabria) and a southern sector (southern Calabria and north-eastern Sicily) approximately separated in correspondence of the Catanzaro trough (Fig.1b). This subdivision has to be indeed completed by other minor ones, based on one hand by evidences of transcurent tectonic alignment testified by formerly developed mylonitic deep-seated shearing activity (i.e. PSZ), replaced by a well testified Tortonian age strike-slip tectonics (MASZ and NGSZ) actually operating within an overall extensional geodynamics as also supported by the structural data and for the contribution of the Miocene–Quaternary tectono-stratigraphic history of the Siderno basin. In this view, the protracted strike-slip tectonic activity passing from deep seated mylonitic shearing to brittle ones has likely played a crucial role in the approaching the different crustal sectors forming the actual southern CCT, by means of the activation or re-activation of transcurrent crustal-scale shear zones, representing lithospheric structures that act as highly localized deformative weakening bands that facilitated the movement between more rigid crustal blocks. REFERENCES: - Angì G. Cirrincione R. Fazio E. Fiannacca P. Ortolano G. & Pezzino A. (2010). Metamorphic evolution of preserved Hercynian crustal section in the Serre Massif (Calabria-Peloritani Orogen, southern Italy). Lithos 115:237–262. - Cirrincione R. De Vuono E. Fazio E. Fiannacca P. Ortolano G.Pezzino A. & Punturo R. (2010). The composite framework of the southern sector of the Calabria Peloritani Orogen. Rendiconti Online Societa Geologica Italiana 11, 1, 93-94. - Ortolano, G., Cirrincione, R., Pezzino, A., Puliatti, G. (2013). Geo-Petro-Structural study of the Palmi shear zone: Kinematic and rheological implications Rendiconti Online Societa Geologica Italiana, 29, pp. 126-129. - Prosser G. Caggianelli A. Rottura A. & Del Moro A. (2003). Strain localization driven by marble layers: the Palmi shear zone (Calabria-Peloritani terrane, Southern Italy). GeoActa, vol. 2, pp. 155-166. - Tripodi V. , Muto F. & Critelli S.(2013). Structural style and tectono-stratigraphic evolution of the Neogene–Quaternary Siderno Basin, southern Calabrian Arc, Italy, International Geology Review, 55:4,468-481. 47 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Relationships between the Kirşehir massif and the Sakarya zone (Turkey): geological and petrographical data from the Ankara-Erzincan suture and surroundings Pandeli E.(1, 2), Elter F.M.(3), Toksoy Köksal F.(4), Principi G.(1) (1) Department of Earth Sciences, University of Florence, Italy (2) CNR-Institute of Geosciences and Georesources, Section of Florence, Italy (3) DISTAV, University of Genoa, Italy (4) Geological Engineering Department, Middle East Technical University of Ankara, Turkey Anatolia Peninsula is a key area to reconstruct a coherent geodynamic model for the Eastern Mediterranean. In particular, the geological framework of Eastern Mediterranean is the result of a series of geodynamic events which, following the closure of Paleotethys in late Paleozoic (Cimmerian orogeny), led to ?Permian-Triassic rifting and ocean spreading in the equatorial part of Pangea generating the Neotethys. The resulting ocean, the Neotethys, was subdivided in various branches, separating a series of continental blocks between Gondwana and Laurasia. The Anatolian Peninsula (i.e. Anatolian Plate) is geologically subdivided in three main zones, from N to S, Pontides, Anatolides and Taurides. They are divided by two main spectacular ophiolitic sutures: the Izmir-Ankara-Erzincan suture zone (IAESZ), between the Pontides (Sakarya Zone p.p.) and the Anatolides, due to the subduction of the northern Neotethys branch; the Southern Taurides suture (TSZ) due to the subduction of the southern Neotethys branch. Our study was performed in the areas to the north-east of Ankara (W-SW of Çankırı, i.e. Kalecik and Şabanözü, NW and SW of Çorum and SE of Amasya), where the Sakarya Zone units crop out in the surroundings the IAESZ, where the oceanic Ankara Ophiolitic Melange (Sakarya Zone ) is in tectonic contact with the continental Anatolian Units (Kırşehir Massif). In the above said localities, we studied the relationships between the continental successions of the Sakarya Zone and the Ophiolitic Melange. In particular, geological and structural surveys were made in typical sites and samples of rocks were collected for petrographic and microstructural analyses. The aim of the study is to define the lithological-stratigraphical features of the successions, their compositional features and structural evolution both of the Ankara Ophiolitic Melange and of the more or less metamorphic continental successions of the Sakarya Zone in the above said areas. The Ankara Ophiolitic Melange is made up of Mesozoic brecciated ophiolitic rocks; in particular it is mostly a serpentinite breccia including olistoliths of serpentinite, gabbro, basalt, chert and pelagic limestone in a serpentinite-shaly foliated matrix. Locally olistoliths of Mesozoic platform carbonates (likely coming from the Kırşehir succession) are also present. The size of the olistoliths varies from centimetric to hectometric; in this latter case, portions of the pristine stratigraphic succession (e.g. stratified pelagic limestones, pillow basalts with cherts at the top) are preserved. Generally the biggest ophiolitic bodies are at the top of the melange and their contact is locally underlined through shear zones that are also present at different levels of the melange. The structural data collected in the different sites show that the shear zones are characterized by a prevalent topto-the NE sense of shear. The continental units of the Sakarya Zone succession are made up of prevalent high diagenetic to low-grade metamorphic rocks, Paleozoic-Triassic in age and Mesozoic to Tertiary mostly carbonate formations. In the Çankırı area, the former are represented by metasiliciclastics (phyllite and metasandstone with rare carbonate interbeds), locally intruded by intermediate-basic magmatic dykes, passing vertically to stratified to massive carbonate succession with horizons of pillow basalts. In Çorum area, the metasiliciclastic rocks overlie middle grade regional polymetamorphic rocks (garnet-bearing micaschist and gneiss s.l., marble and amphibolites), locally characterized by a blueschist facies event. Instead, vario-coloured phyllites and green metavolcanites prevail in the Amasya area below the Mesozoic carbonate successions. These successions are generally deformed by three events: 1) D1 that produced a fine-grained continuous foliation (quartz+sericite+chlorite+calcite+oxides+organic matter±albite) with mineral lineations locally associated to isoclinal/tight folds; 48 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 2) D2 folding which is characterized by spaced zonal to discrete crenulation cleavage (generally underlined by alignment of oxides+organic matter ± sericite) and by mostly NE-SW striking axes (except the areas west and south-west of Çankırı, where SE dipping axes are generally present); 3) local D3 weak folding without blastesis that is defined by kinks with NW-SE striking axes. In the Çorum micaschist, gneiss and amphibolites, the blue amphibole appears nematoblastic to diablastic re- spect to the main (D1? or pre-D1?) schistosity and is retrogressed into Greenschist Facies mineral assemblages. The Ophiolitic Melange tectonically underlies the the Sakarya Zone continental successions, but locally (e.g. along the road Çorum-Alaca) their geometrical relationships are reversed. This can be due to the activity of main transcurrent structures that also produced the variation of the axial strike of the D2 folds within the Sakarya continental units in the different areas. 49 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Seep carbonates and chemosynthetic coral communities in the Bocco Shale (Internal Ligurides, Northern Apennine, Italy) as record of trenchslope limit in the Early Paleocene alpine accretionary wedge Luca Pandolfi (1,2), Chiara Boschi (2), Edoardo Luvisi (1), Alessandro Ellero (2), Michele Marroni (1, 2), Francesca Meneghini (1) (1) Dipartimento di Scienze della Terra, Università di Pisa, Italy (2) Istituto di Geoscienze e Georisorse, CNR, Pisa, Italy In Northern Apennines, the Internal Liguride units are characterized by an ophiolite sequence that represents the stratigraphic base of a Late Jurassic-Early Paleocene sedimentary cover. The Bocco Shale represents the youngest deposit recognized in the sedimentary cover of the ophiolite sequence, sedimented just before the inception of subduction-related deformation history. The Bocco Shale has been interpreted as a fossil example of deposits related to the frontal tectonic erosion of the alpine accretionary wedge slope. The frontal tectonic erosion resulted in a large removal of material from the accretionary wedge front reworked as debris flows and slide deposits sedimented on the lower plate above the trench deposits. These trench-slope deposits may have been successively deformed and metamorphosed during the following accretion processes. The frontal tectonic erosion can be envisaged as a common process during the convergence-related evolution of the Ligure-Piemontese oceanic basin in the Late Cretaceous-Early Tertiary time span. In the uppermost Internal Liguride tectonic unit (Portello Unit of Pandolfi and Marroni. 1997), that crops-out in Trebbia Valley, several isolated blocks of authigenic carbonates, unidentificated corals and intrabasinal carbonatic arenites have been recognized inside the finegrained sediments that dominate the Early Paleocene Lavagnola Fm. (cfr. Bocco Shale Auctt.). The preliminary data on stable isotopes from blocks of authigenic carbonates (up to 1 m thick and 3 m across) and associated corals archive a methane signatures in their depleted carbon isotope pattern (up to δ13C –30‰ PDB) and suggest the presence of chemosynthetic paleocommunities. The seep-carbonates recognized at the top of Internal Liguride succession (cfr. Bocco Shale Auctt.) occur predominantly as blocks in very thick mudstone-dominated deposits and probably developed in an environment dominated by the expulsion of large volume of cold methane-bearing fluids focused in the frontal part of the Early Paleocene alpine accretionary wedge. 50 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Explosion breccias Peacock D.C.P. Statoil, Bergen, Norway Breccias with clear cements occur in carbonate rocks in transpressional thrust sheets in Wyoming (USA) and elsewhere. They are not cataclasites because they have not been created by friction along faults, but appear to have been produced by explosion of the host rocks, so are here defined as explosion breccias. CO2 is generated by metamorphism of carbonates or by hydrocarbon maturation. The CO2 migrates up the thrust sheet because it is less dense than water, and is trapped in the hanging-wall anticline. The top seal is breached, either by the large column of CO2 or by strike-slip faulting. This causes rapid decompression of pore fluids, with more CO2 coming out of solution as pressure and temperature drop, with explosive brecciation caused as escaping CO2 elevates pore pressure as it migrates upwards. This is analogous to the role of degassing in explosive volcanic eruptions. The loss of pore pressure and decrease in acidity as the CO2 escapes causes cementation of the brecciated rock. 51 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Stresses, overpressure and transient strike-slip during tectonic inversion Peacock D.C.P. (1), Casini G. (1), Anderson M.W. (2), Tavarnelli E. (3) (1) Statoil, Bergen, Norway (2) School of Earth, Ocean and Environmental Sciences, University of Plymouth, UK (3) Dipartimento di Scienze della Terra, Università di Siena, Italy Changes in the orientations and relative magnitudes of stresses during the tectonic cycle will cause a transient phase in which the intermediate compressive stress is vertical, tending to promote strike-slip faulting. This can occur between regional extensional (maximum compressive stress vertical) and regional contractional (minimum compressive stress vertical), either at the start of basin inversion or after the main phase of orogenic contraction. Such variations in the relative magnitudes of the stress axes can be promoted by variations in overburden or by variations in tectonic forces. The reduction of overburden during uplift and erosion can cause overpressure to develop in units with a top seal, manifested by vertical or horizontal veins, strike-slip faults, thrusts and inverted normal faults. The development of such uplift-induced overpressure may explain the localisation of deformation during basin inversion, such as the patchy occurrence of Alpine inversion in parts of southern England. It is also possible that transient phases of strike-slip faulting during inversion events may breach seal units, thereby promoting hydrocarbon migration and reducing fluid pressure. 52 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The Epiligurian basin in the middle Enza valley (Northern Apennines, Italy): evidences of a Priabonian-Serravallian syn-depositional polyphased deformation zone Alberto Piazza(1), Andrea Artoni(1) (1) Università degli Studi di Parma, Dipartimento di Fisica e Scienze della Terra “Macedonio Melloni”, Parma, Italy The Epiligurian succession cropping out in the middle Enza Valley belongs to one of the widest Eocene-Miocene wedge-top basin of Northern Apennines of Italy (Fig. 1a). New stratigraphic and structural data, collected through a detailed geological survey (Fig. 1b) and integrated with literature data, allow a tectono-sedimentary evolutionary model to be proposed. The model highlights that the sedimentation is controlled by the activity of a diffused and polyphased deformation zone since Priabonian to Serravallian. This zone, here named Enza Valley Deformation Zone (EVDZ), underwent two main tectonic phases (Fig. 2): 1) a low angle extensional phase (Priabonian – Early Rupelian), 2) a left lateral transpressive phase (Rupelian-Serravallian). Both tectonic phases are syn-sedimentary as evidenced by: unconformities, lateral variations in thicknesses of stratigraphic units and the presence of chaotic deposits within the sedimentary succession. The first tectonic phase (Priabonian- Early Rupelian) is evidenced by the arenaceous-conglomeratic lithofacies of Val Pessola Member (Ranzano Formation, Ran 2a) that fills a narrow, localized and asymmetric basin whose depocenter is located toward south in the study area. On the southern margin of the basin (now overturned), the Ran2a lithofacies unconformably overlays the lowermost stratigraphic units of the Epiligurian succession (Baiso clay breccias and Monte Piano formations – Cerina Feroni et al., 2002) which, on their turn, lays on the Cassio Unit. The latter is the uppermost tectonic unit of the Ligurian tectonic stack which, from bottom to top, is given by Caio, Groppallo and Cassio units (Fig. 1b). Instead, toward north-northwest, the Ran 2a onlaps on the Caio tectonic units (the lowermost Ligurian unit) and onto normal faulted blocks of Monte Piano-Ran 2a succession. The above stratigraphic and structural relationships suggest that the Ran 2a was deposit- ed in a basin formed during the development of a south–dipping, low angle extensional faults system which exhumed the lowermost Ligurian tectonic unit (Caio Unit) and created a narrow, localized depocenter which is about 500 m deep to the south (fig. 2, stages 1-3) The second tectonic phase (Rupelian-Serravallian) is characterized by transpressive tectonics. It is testified by the development of a complex structural association of faults and folds that affect the Epiligurian succession up to the Contignaco Formation (Burdigalian), the underlaying Ligurian units and the earlier formed low angle extensional faults system. The overall geometry of the structure formed during this second phase is an overturned syncline having WNW-ESE axis and a NNE-SSW axial trace: the Enza Valley syncline of Ottria et al. (2001). In detail, it is a very complex structure, characterized by two en-echelon, overturned polyphased synclines that are cross-cut by both transcurrent fault systems, striking between N-S and NNE-SSW, and low-angle embricate thrust systems (Fig. 1b). The association of all these structures, their orientations as well as the mesostructural data can be framed in a SW-NE trending left lateral transpressive zone. The syn-depositional and transpressive activity of the EVDZ is evidenced by the occurrence of a depocenter located in the NW portion of the study area and filled by the Lagrimone sandstone member (Late Rupelian) (Cerrina Feroni et al., 2002). The Lagrimone sandstone can be considered the marker of the inception of the transpressive phase of the EVDZ. The end of this phase can be defined by the units sealing the EVDZ toward north where NNESSW trending structures (Enza, the Rio Maillo and the Monte Faiedolo anticlines) are unconformably buried under the Langhian-Serravallian deposits of the Bismantova Group (De Nardo et al., 1991). Thus, the EVDZ transpressive phase is developing since Late Rupelian to Serravallian. 53 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Figure 1 - Location (a) and structural map (b) of the studied area. 54 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Figure 2 - Sketches illustrating the stratigraphic and tectonic evolution of the Epiligurian wedge-top basin from Priabonian to Serravallian. 55 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Looking at the EVDZ from a regional point of view, it can be prosecuted toward both north and south. Northward, as mentioned above, it is traceable into the NNE-SSW trending structures which preserve a Miocene syn-depositional activity as already suggested by De Nardo et al. (1991). Southward it can be associated to the Monte Ventasso structure developed during lower-middle Miocene (Vescovi 2005). Therefore, the EVDZ can be part of a wider and longer deformation zone crossing obliquely the main NW-SE oriented compressive structures of the Northern Apennines. The results of this study give new insights on the geometries and the behavior of Apenninic transversal structures and reveal how these structures control the tectonic-sedimentary evolution of the Epiligurian wedge-top basins since the inception of their formation during Eocene and Oligocene. REFERENCES - Cerrina Feroni A., Ottria G., Vescovi P. (2002). Carta Geologica d’Italia alla scala 1:50.000, Foglio 217 “Neviano degli Arduini”. Servizio Geologico d’Italia - Regione Emilia Romagna, S.E.L.C.A., Firenze. - De Nardo M.T., Iaccarino S., Martelli L., Papani G., Tellini C., Torelli L, Vernia L. (1991). Osservazioni sull’evoluzione del bacino satellite epiligure Vetto-Carpineti-Canossa (Appennino Settentrionale). Mem. Descr. Carta Geol. d’It., XLVI 209-220, Roma. - Ottria G., Catanzariti R., Cerrina Feroni A. (2001). The Ranzano unit boundaries in the type area: Lower Oligocene events in the epi-Ligurian Succession (northern Apennines,Italy). Eclogae geol. Helv., 94, 185-196. - Vescovi P. (2005). The Middle Miocene Mt. Ventasso Mt. Cimone arcuate structure of the Emilia Apennines. Boll. Soc. Geol. It., 124, 53-67. 56 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Evolution of the Alps-Apennine boundary: EU/IB/AD transcurrence leading to a crustal vortex Gianfranco Principi(1), Benedetta Treves(2) (1) Dipartimento di Scienze della Terra, Università di Firenze, Italy (2) Istituto Geoscienze e Georisorse, CNR, Sezione di Firenze, Italy The present junction between the Alpine and Apenninic chains is an amazing crustal hinge, shaped like a vortex (‘Ligurian Knot’, auctt.), that links two orogenic belts with opposite polarity (Western Alps/Northern Apennines) and is cut through by a N-S transcurrent fault (Sestri-Voltaggio Line). This boundary got localized at the junction of the European, Iberian and Adriatic plates, along important inherited lithospheric discontinuities that played a major tectonic role since at least Triassic and certainly Jurassic and Cretaceous times. The kinematics of this junction represents a key point to unravel the geodynamics of the Western Mediterranean area, and has been puzzling geologists for over 50 years, but is still not understood and a matter of debate. We review the structures that have been proposed in the many tectonic reconstructions of the area, trying to model the EU/IB/AD kinematics. We try to address in particular the following unsolved points: 1) What kind of deformation happens at depth under the junction of the two chains with opposite vergence? What are the deep lithospheric structures that controlled the crustal deformation and superficial structures? 2) How is the rotation of Adria about a pivot point located at its edge structurally realized? What are the faults and geologic processes that accomplish what kinematically appear as EU/IB and EU/AD rotations? 3) Is there a major partitioning of strain and/or decoupling in the crust and lithosphere? Can the lithosphere rotate around a vertical axis? 4) The Corsica “Faces Game”: those who look from the West see it as Alpine, those who look from the East see it as Apenninic…. it may be instead correct one who looks at it from the South, and sees a major role of transcurrence. We discuss the differences and similitudes between the Corsica/Apennines and Ligurian Alps/Apennines boundaries. We propose that the ‘Ligurian Vortex’ could be kinematically explained through a sequence of at least three transcurrent stages active from Late Cretaceous up to recent with different directions: - The first one was active as a sinistral transpression during the Late Cretaceous-Eocene along the SWNE original IB/AD transtensional plate boundary, that also continued many older (Triassic and Jurassic) rift structures still recognizable along the Iberian margin, in Provence, in the Adriatic basement of the Northern Apennines (La Spezia basin, isopic zones of the Tuscan sequence), in the Sudalpine margin (Belluno basin) and in the Southern Apennines (Lagonegro basin). The NE-SW trend is still recognizable along the Canavese Line, the Pamplona Fault (dividing the Pyrenees from the Cantabrian Chain) and the External massifs of the Western Alps (Pelvoux, Aar) probably reflecting their exhumation along this same SW-NE fault system. The Sestri-Voltaggio Lineament also likely belonged to it, but it subsequently (Oligocene to Pliocene) suffered a 60-70 degrees rotation along with the Corso-Sardinian Block and Adriatic basement. - The second movement occurred from Eocene on, along a set of N-S left-lateral strike-slip faults that took the northward displacement of Adria+Africa towards stable Europe and allowed a transform scissor/cut between a portion of Adria subducting westwards under Iberia (Apennines) and a portion of Adria overriding the European crust along the Alpine arc. The Late Eocene age of calcalkaline magmatism in western Sardinia (38.3Ma), and Provence (about 35Ma), related to the already ongoing Apenninic subduction was either contemporary or even older than the blueschist metamorphism in Corsica, affecting also European margin units (comprising Paleozoic granitoids), and together with the presence of non metamorphic ophiolitic units above the “Alpine Corsica” tectonic pile support the hypothesis that the Corsican Schistes Lustrèes represent metamorphosed fragments of the subducted Apenninic (Ligurian) ocean. - The third kinematic stage, which includes the Late Tertiary anticlockwise rotations at the surface, could have been accomplished at a lithospheric level through an ENE-WSW directed transcurrent drag system that increased in displacement from north to south as a rota- 57 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 tion of Adria around the ‘Ligurian hinge’. This probably caused the strong curvature in the Western Alps, extension in the Tyrrhenian Sea and compression along the Apenninic front. We stress the need to recognize a major transform cut within Adria, probably already present in the Cretaceous, but especially important during this stage, that could allow this plate to split into two parts: one being subducted along the Apennines and the other overriding the European crust along the Alpine belt. 58 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Walking through the Tethys in the Monviso Ophiolite (Piemonte, Italy) Franco Rolfo(1, 2), Gianni Balestro(1), Alessandro Borghi(1), Daniele Castelli(1, 2), Simona Ferrando(1), Chiara Groppo(1), Pietro Mosca(2), Roberto Compagnoni(1) (1) Dipartimento di Scienze della Terra, Università di Torino, Italy (2) CNR Istituto di Geoscienze e Georisorse, unità di Torino, Italy The Monviso Ophiolite (MO) in the Italian Western Alps is one of the best known relics of oceanic crust in the orogen. Moreover, it is one of the strategic areas chosen to represent the geodiversity of Piemonte Region (Rolfo et al. 2014). The MO gives the chance to see and appreciate different portions of the ancient ocean along a relatively short mountain trail; from the Po river springs at Pian del Re, a path from 2000 m up to about 2350 m a.s.l. shows all different ophiolitic lithologies – modified after the Alpine evolution - within few kilometers. GEOLOGICAL SETTING The MO is a N-S trending body, 35 km long and up to 8 km wide, tectonically emplaced between the un- derlying continental-derived Dora-Maira Massif and the ocean-derived Piemonte Zone metasedimentary units. The MO formed during the Mesozoic opening of the western Alpine Tethys and underwent eclogitic metamorphism during Alpine subduction (e.g. Lombardo et al. 1978). The MO encompasses the whole lithological spectrum of the Piemonte-Ligurian ophiolites: peridotite, gabbro, dolerite, basalt, as well as cover sediments (Fig. 1). The MO comprises two major tectonometamorphic units, trending north-south and dipping to the west, separated by a major shear zone: the “Monviso Upper Unit” to the west and the “Lago Superiore Lower Unit” to the east (Castelli et al. 2014). Figure 1 – Geological sketch map of the Monviso area, with the excursion route and location of Stops 1 to 8 described in the text (modified from Castelli et al. 2014). 59 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Recent advances in petrology, geochemistry and geochronology suggest a short duration of igneous activity in the MO and a short time span (from ca 170 to ca 150 Ma) for the entire Piemonte-Ligurian Tethys, with an embryonic ocean (max 380 km wide; Piccardo et al. 2001) experiencing first oceanic hydrothermal alteration (Nadeau et al. 1993) and then, during the Alpine subduction, eclogite-facies metamorphism (e.g. Lombardo et al. 1978). Later exhumation produced re-equilibration under progressively lower pressure conditions at blueschist- and greenschist-facies. A GEOLOGICAL TRAIL ACROSS THE TETHYS OCEAN A general view of the MO internal structure is evident from stop 1 between Pian Melzè and Pian del Re (Fig. 1), by looking southward to the ridge from Monte Grané (2314 m) to Viso Mozzo (3019 m) and Monviso (3841 m). Along this section are exposed, from left to right and from bottom to top: the serpentinite at Monte Grané and in the lower ridge above Pian Melzè; the metagabbro at the foot of Viso Mozzo and the low ridge above Pian del Re; the Viso Mozzo metabasite and the Colle di Viso serpentinite, which crops out in the low ridge between Viso Mozzo and Monviso. Along the path from Pian del Re to Lago Fiorenza, the first outcrop of serpentinite (Fig. 1, stop 2) deriving from primary spinel-lherzolite of the upper mantle underlying the oceanic crust, is either massive or sheared (Fig. 2a). It mainly consists of antigorite ± clinopyroxene, brucite, Mg-rich chlorite, Ti-clinohumite, metamorphic olivine and chrysotile; ore minerals are magnetite, FeNi-alloys and sulphides. Different generations of metamorphic veins crosscut the serpentinite. About 500 m SSE of Lago Fiorenza (Fig. 1, stop 3), a folded elongated body of metasediments – originally covering the former oceanic crust – is tectonically embedded within serpentinite. This sliver, about 50 m-thick, belongs to an eclogite-facies shear zone which passes through Colletto Fiorenza. The metasediments consist of impure marbles, calcschists and micaschists grading to quartzite (Fig. 2b). Rare cm-thick layers of metabasite are interbedded with metasediments. At Colletto Fiorenza (Fig.1, stop 4), a tectonic contact between calcschists and the overlying metagabbro (Fig. 2c) is marked by a zone from a few m to a few tens of m thick of strongly sheared serpentinite and talc-carbon- Figure 2 – (a) Stop 2: outcrop of serpentinite of the Basal Serpentinite Unit above Pian del Re; (b) Stop 3: folded calcschists and impure marbles of the Basal Serpentinite Unit; (c) Stop 4: contact between serpentinite and metagabbro at Colletto Fiorenza; (d) Stop 5: smaragdite metagabbro at Lago Chiaretto; (e) Stop 8: eclogite-facies omphacite-bearing veins cross-cutting the eclogitic FeTi-oxide metagabbro foliation. 60 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 ate-amphibole-schist, with tectonic inclusions of banded eclogitic rocks. Between Colletto Fiorenza and Lago Chiaretto (Fig. 1, stop 5), beautiful smaragdite metagabbro – former Mg-Al-rich plutons in the lower oceanic crust - locally includes transposed cm- to dm-thick dykes of metabasalts. The metagabbro shows a well developed foliation and consists of a whitish matrix and emerald-green porphyroclasts of “smaragdite”, the local name for Cr-omphacite (Fig. 2d). The metabasaltic dykes, now eclogite, are very fine-grained and locally preserve remnants of a porphyritic structure. Few boulders preserve igneous layering textures, with emerald-green bands (enriched in Cr-omphacite) corresponding to layers with high modal clinopyroxene. West of the smaragdite metagabbro, along the path leading to Lago Lausetto and Rifugio Giacoletti, sheepback rocks (Fig. 1, stop 6) consist of alternating layers of eclogite and metagabbro from a few cm- to several dmthick. Eclogite largely prevails over metagabbro. The melanocratic layers (now eclogite) derive from primary FeTi-oxide gabbro, whereas leucocratic layers derive from Mg-Al gabbro protoliths. South of Lago Lausetto, along the path leading to Rifugio Giacoletti (Fig. 1, stop 7), a narrow alluvial plane hides a shear zone separating Fe-Ti metagabbro mylonites (to the E) from foliated prasinite (to the W). No primary features, such as pillow structure or porphyritic texture, are preserved within the prasinite; nice pillows can be observed in the nearby Vallone dei Duc. Prasinite is fine-grained and mainly composed of albite, clinozoisite/epidote, chlorite, amphibole, and accessory rutile, titanite, apatite and opaque ores. Finally, at the western side of Lago Superiore (Fig. 1, stop 8), a layered eclogitic sequence contains small (10x10 cm) lenses of low-strain domains still preserving the igneous microstructure and surrounded by large volumes of eclogite-facies mylonites. Cigar-shaped boudins of eclogite within the metagabbro outline a complex interference pattern of folds. Eclogite-facies veins are spectacular – and indeed very rare worldwide - and witness fluid migration during subduction (Fig. 2e). They contain mostly Na-pyroxene and minor garnet, rutile and apatite. In tension gashes, most pyroxene is fibrous. REFERENCES - Castelli, D., Compagnoni, R., Lombardo, B., Angiboust, S., Balestro, G., Ferrando, S., Groppo, C. & Rolfo, F. (2014). The Monviso meta-ophiolite Complex: HP meta-morphism of oceanic crust & interactions with ultramafics. GFT – Geological Field Trips, in press. ISSN: 2038-4947. 8-35. - Lombardo, B., Nervo, R., Compagnoni, R., Messiga, B., Kienast, J.R., Mevel, C., Fiora, L., Piccardo, G.B. & Lanza, R. (1978). Osservazioni preliminari sulle ofioliti metamorfiche del Monviso (Alpi Occidentali). Rendiconti della Società Italiana di Mineralogia e Petrologia, 34, 253-305. - Nadeau, S., Philippot, P. & Pineau, F. (1993). Fluid inclusion and mineral isotopic compositions (H-O-C) in eclogitic rocks as tracers of local fluid migration during high-pressure metamorphism. Earth and Planetary Sciences Letters, 114, 431-448. - Piccardo, G.B., Rampone, E., Romairone, A., Scambelluri, M., Tribuzio, R. & Beretta, C. (2001). Evolution of the Ligurian Tethys: inference from petrology and geo-chemistry of the Ligurian Ophiolites. Periodico di Mineralogia, 70, 147-192. - Rolfo F., Benna P., Cadoppi P., Castelli, D., Favero-Longo, S.E., Giardino, M., Balestro, G., Belluso, E., Borghi, A, Cámara, F., Compagnoni, R., Ferrando, S., Festa, A., Forno, M.G., Giacometti, F., Gianotti, F., Groppo, C., Lombardo, B, Mosca, P., Perrone, G., Piervittori, R., Rebay, G. & Rossetti, P. (2014). The Monviso massif and the Cottian Alps as symbols of the Alpine chain and geological heritage in Piemonte, Italy. Geoheritage, in press. DOI: 10.1007/s12371-014-0097-9. 61 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Alps-Dinarides vs. Alps-Apennines transition: changes in subduction polarity in space and time Stefan M. Schmid Institut für Geophysik, ETH Zürich, Switzerland There are substantial differences between the Alps-Dinarides and the Alps-Apennines transitions. The Alps-Dinarides transition has to do with a long-lived change in polarity that already initiated during the opening of Alpine Tethys, contemporaneous with obduction of the West Vardar ophiolites onto Adria. Spreading in Alpine Tethys triggered late Jurassic obduction of Neotethys whereby Adria represented the lower plate. This configuration was maintained until collision with Europe (Tisza-Dacia), Adria remaining in a lower plate position in respect to Europe until Paleogene times. During the “Miocene revolution” the geometrical configuration of the Alps- Dinarides transition was most severely modified: Tisza and Dacia, continental fragments of Europe, invaded the Carpathian embayment, thereby destroying the original along-strike New- Zealand-type subduction polarity change. This former transform linking two opposite subduction polarities along strike is located along the present-day Mid-Hungarian shear zone. We propose that the situation is fundamentally different in the Apennines. As recognized early on by Piero Elter, the northern Apennines underwent a change in subduction polarity, which is not a primary along-strike change but one in time affecting a volume of rocks within the northern Apennines (Internal Ligurides) that originally was part of the Alps (e.g. Elter 1993: “… the Internal Ligurides can be considered an integral part of the alpine tectonic building.”), and that was later, sine Late Oligocene time, affected by the Apennines orogeny. Conversely, the Ligurian Alps, located in the hinterland of the Monferrato, became part of the Apennines. The Bacino Terziario Piemontese, sealing Western Alps and Internal Ligurides, allows for dating the onset of Apennines orogeny in a former part of the Alps (Western Alps and Internal Ligurides). However, there is a controversy as to whether there was an earlier along strike change in subduction polarity located further south, somewhere between Northern Apennines-Corsica, the latter being an integral part of the Alps, and Sardinia-Southern Apennines that, according to the majority of authors (e.g. La- combe & Jolivet 2005), had opposite polarities since at least the Eocene due to an along-strike change in subduction polarity. On the other hand, Michard et al. (2006), Molli (2008) and Handy et al. (2010), consider, or even favor, the possibility of a reversal in subduction polarity that affected the entire Western Mediterranean realm, going all the way to Calabria and the Betic Cordillera in late Eocene to early Oligocene times. From an Alpine perspective it is argued that the Sesia-Dent Blanche system is not part of the Austroalpine in a tectonic sense. The Alpine subduction zone essentially becomes intra-oceanic, the Sesia- Dent Blanche merely representing an extensional allochthon detached from Adria and floating within the Piemonte-Liguria Ocean Adria continent transition zone (Froitzheim et al. 1996). A lack of true continent-continent (Europe-Adria) collision in the Western Alps, as opposed to eastern Switzerland and Austria, would leave the east Piemonte-Ligurian basin open until Oligocene times when it started to be obducted over the internal Ligurides due to a change in subduction polarity, possibly affecting the entire Western Mediterranean realm (Handy et al. (2008). REFERENCES - Elter, P. (1993). Detritismo ofiolitico e subduzione: riflessioni sui rapport Alpi e Appennino. Mem. Soc. Geol. It. 49: 205-215. - Froitzheim, N., Schmid, S.M., and Frey, M. (1996). Mesozoic paleogeography and the timing of eclogite-facies metamorphism in the Alps: A working hypothesis. Eclogae geol. Helv. 89: 81-110. - Handy, M. R., Schmid, S.M., Bousquet, R., Kissling, E. & Bernoulli, D. (2010). Reconciling plate-tectonic reconstructions of Alpine Tethys with the geological– geophysical record of spreading and subduction in the Alps. Earth-Science Reviews 102: 121–158. - Lacombe, O. & Jolivet, L. (2005). Structural and kinematic relationships between Corsica and the Pyrenees-Provence domain at the time of the Pyrenean orogeny. Tectonics 24: TC1003. 62 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 - Michard, A., Negro, F., Saddiqi, O., Bouybaouene, M.L., Chalouan, A., Montigny, R., Goffé, B., (2006). Pressure–temperature–time constraints on the Maghrebide mountain building: evidence from the Rif-Betic transect (Marocco, Spain), Algerian correlations, and geodynamic implications. Comptes Rendus Geoscience 338: 92–114. - Molli, G. (2008). Northern Apennine–Corsica orogenic system: an updated view. In: Siegesmund, S., Fügenschuh, B., Froitzheim, N. (Eds.), Tectonic Aspects of the Alpine–Dinaride–Carpathian System: Geological Society, London, Special Publication 298: 413–442. 63 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Low-angle extensional fault systems in the Northern Apennines thrust wedge: similarities and differences between the Tellaro and Val di Lima detachments Fabrizio Storti(1), Fabrizio Balsamo(1), Luca Clemenzi(1), Giancarlo Molli(1, 2), Philippe Muchez(3), Rudy Swennen(3) 1 NEXT - Natural and Experimental Tectonics Research Group - Department of Physics and Eart Sciences “Macedonio Melloni”, University of Parma, Italy 2 Department of Earth Sciences, University of Pisa, Italy 3 Department of Earth and Environmental Sciences, Heverlee, Belgium Despite representing somehow still mechanically puzzling deformation structures, low-angle extensional fault systems are well known in the Apenninic thrust wedge, from the Tyrrhenian side (e.g. the Zuccale Fault) to the external sectors (e.g. the Alto Tiberina Fault). A common feature of these fault systems, regardless of their age, is the eastward tectonic transport direction. In this contribution, we describe the Tellaro and Val di Lima detachments: two exhumed low-angle extensional fault systems that affected the carbonate-dominated multilayer of the Tuscan basinal succession (Upper Triassic to Oligocene in age) and are now exposed in the Northern Apennines mountain belt. We characterize the structural architecture of the fault systems and constrain the P-T condition of the main kinematic activity, and the paleofluid evolution of the fault zones, by means of petrographic, mineralogical, geochemical, and fluid inclusion analyses on fault rocks and fault-related dolomite and calcite veins. The Tellaro Detachment developed in the internal portion of the Northern Apennines wedge and is well exposed along the Tyrrhenian coastline, between Lerici and Tellaro villages. It consists of an array of anastomosed flat-lying fault zones, onto which subsidiary high-angle faults sole down, and caused a remarkable thinning of the Tuscan succession. The extensional deformation was mostly accommodated by the misoriented low-angle faults, with a average tectonic transport direction toward NE and local values ranging from N010E to N080E. Synkinematic dolomitization, brecciation, dissolution, and veining are widespread along the near horizontal brittle shear zones. This fault system was characterized by long-lasting kinematic activity during progressive exhumation from ~ 5 to 2 km depths, associated with a complex paleofluid evolution. Multidisciplinary analysis of the different infill generations precipitated in fault breccias and in syn-tectonic veins indicate mixing of local- and far-sourced parent fluids, and precipitation of calcite and dolomite cements at different temperature and depths. The Val di Lima Detachment developed in the central portion of the Northern Apennines wedge, and is exposed in the Lima valley, east of the Alpi Apuane metamorphic complex. This fault system affects the right-side-up limb of a kilometric-scale recumbent isoclinal anticline and is, in turn, affected by superimposed folding and late-tectonic high-angle extensional faulting. Our data indicate that the low-angle fault system was active at about 180°C and 5 km depth and acted as a combined conduit-barrier system which affected fluid circulation within the upper crust in two ways: i) it allowed the migration of low-salinity fluids, due to the increased permeability along the fault zone; ii) it favored footwall fluid overpressures, localized where an impermeable lithology (e.g. foliated cataclasite) developed in the fault core and acted as an efficient hydraulic barrier. Low-salinity fluid circulation in fault damage zones also characterized the late-stage evolution of the low-angle fault system, and was associated with the post-kinematic recrystallization of calcite veins at shallower conditions (~ 4 km). Despite both fault systems were active with very shallow eastward dip and at comparable crustal levels, the major difference between them is their post-kinematic evolution. The Val di Lima detachment resulted from a single extensional pulse that affected the shallow portion of the wedge within its orogenic contractional evolution, as confirmed by the late-stage folding associated with out-ofsequence thrusting. Such extensional tectonics was triggered by the gravitational disequilibrium resulted from thick-skinned thrusting and antiformal stacking at depth. Fault system geometry was controlled by the favorable orientation of weak discontinuities within the upper part of the Tuscan succession. On the other hand, the Tellaro Detachment is located in an area where tectonic thinning of the shallower portion of the wedge is widespread. The Tellaro Detachment, in particular, likely cuts through the Punta Bianca Anticline and is dissected by the Pliocene high-angle extensional fault system bounding to the west the Lower Magra Valley basin. This suggests that the Tellaro Detachment formed in response to the late-orogenic tectonic thinning of the inner sector of the Northern Apennines tectonic wedge. 64 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 In search of Adria’s rails: the mid-Tyrrhenian western Adria’s rail and its pivoting ligurian tip Benedetta Treves(1) and Giuseppe Nirta(2) (1) Istituto di Geoscienze e Georisorse, CNR, Florence Unit, Italy (2) Department of Earth Sciences, University of Florence, Italy Transcurrent motion goes very often undetected in the geologic record because of the lack of clear and permanent markers that instead characterize extension (volcanism, magnetic anomalies) and convergence (compression, orogenic deformation). Its effect on the upper crust is also extremely transient and easily erased by minor components of vertical (extensional or compressional) movements. Moreover, surface crustal deformations do not necessarily reflect directly subsurface kinematics and can be different from the underlying deeper lithospheric strain. Strain partitioning and vertical decoupling in the lithosphere play a crucial role in many geodynamic systems. In the geodynamic super-puzzle of the Western Tethys/ Western Mediterranean realm, a dominant role of transcurrent tectonics throughout the Mesozoic and Cenozoic can explain many unsolved problems. Occurrence of major lateral displacements among the EU, AF, IB and AD plates helps to solve the following kinematic problems and inconsistencies that arise from a cylindrical, two-dimensional view of the Western Mediterranean tectonics: 1) The tectonics, orientation and geometry of the Apennine orogenic belt are very difficult to reconcile with the EU/AF Tertiary displacement path and plate motion directions.2) The Tyrrhenian Sea is a curious extensional basin between two converging plates. 3) Did the Alps-Apennine junction migrate in time? Can it be analysed through a triple point migration process of the three plates meeting at the junction, Adria, Europe and Iberia?4) What kind of deformation actually occurs at depth in the lithosphere under the Alps-Apennine hinge? In particular, we propose the presence of a major crustal discontinuity, that presently runs with NS orientation along the eastern margin of the Corsica-Sardinia Massif. This must have acted as a left-lateral strike-slip rail with a major role in Adria’s 700 km northern displacement in the Tertiary. In fact, Adria’s convergence toward the European Plate, that started in the Late Cretaceous, must have been accomplished along a major rail along its western mar- gin. This rail (the ‘mid-Tyrrhenian rail) has a present N-S orientation, probably resulting from counterclockwise rotation of an original NNE-SSW oriented discontinuity, and is located along the eastern margin of the Corsica-Sardinia block = Western margin of the Tyrrhenian Sea. Regarding the odd location and direction of Western Mediterranean basin extension with respect to the northerly EU/AF motion, rotation of smaller crustal blocks related to a deeper, decoupled, lithospheric transcurrent drag seems to be able to explain the inconsistencies. Adopting the kinematic solution by Viti et al., (2011), the E-W and later NE-SW left-lateral shear of the African Plate with respect to Europe possibly caused the rotation of several smaller crustal blocks located in between: Balearic, Corso-Sardinian, Adriatic. A ‘locked’ corner of the Adriatic block (the NW one) acted as pivot and Euler pole for Adria’s rotation, splitting this plate into two portions: to the north it overthrust the European crust, to the SW it underthrust the Iberian margin. A transform cut between the two portions is a necessary tectonic requirement to accomplish this dynamics, but and it’s difficult to locate at the surface and so far went unrecognized. A triple point analysis of the motion of EU/AD/IB, meeting at the locked corner, helps unravel the older geodynamic history, but for the Tertiary evolution of the Alps-Apennine junction, the amount of crustal rotation and deformation prevents a simple vector analysis of the plate boundaries. The curved arms of the Alpine and Apenninic belts, joining above the Gulf of Genova in a sort of spiral torsion, strongly suggest a vortex-style deformation related to the anticlockwise rotation of Adria about the ‘Ligurian Knot’ (Laubscher et al., 1992). We present a filmstrip of many original solutions that have been proposed in the past for solving the puzzling geodynamic junction between Alps and Apennines. Among them, still stands the model by Elter and Pertusati (1973) that gave us a powerful graphic solution on which to work and think. 65 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 REFERENCES - Elter, P. & Pertusati, P.C. (1973). Considerazioni sul limite Alpi-Appennino e sulle relazioni con l’arco delle Alpi occidentali. Mem. Soc. Geol. It., 12, pp. 359-375. - Laubscher, H., G. C. Biella, R. Cassinis, R. Gelati, A. Lozej, S. Scarascia, & I. Tabacco, (1992). The collisional knot in Liguria. Geologische Rundschau 06/1992; 81(2), pp. 275-289. - Viti, M. , Mantovani, E. , Babbucci, D. & Tamburelli, C., (2011). Plate kinematics and geodynamics in the Central Mediterranean. Journal of Geodynamics, Vol.51(2), pp.190-204 . 66 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Evidences of late Miocene low-angle extensional tectonics affecting the western Northern Apennines orogenic wedge Paolo Vescovi, Mirko Carlini, Andrea Artoni, Luca Clemenzi, Luigi Torelli Physics and Earth Sciences Department “Macedonio Melloni”, University of Parma, Italy The present work investigates the Neogene structural evolution of the western portion of the Northern Apennines from the Tyrrhenian coast of eastern Liguria to the Po Plain foothills, including the key areas of Magra and Taro river valleys (Fig. 1). Within this area, the internal portion of the mountain belt shows a complex structural architecture with remarkable lateral variability. To the SE, well-developed Plio-Pleistocene high-angle extensional faulting (Bernini e Papani, 2002) is associated with wide exposures of Paleogene-Neogene foredeep deposits and the underlying Mesozoic carbonate-dominated sedimentary sequences of the Tuscan paleogeographic domain. On the contrary, to the NW and NE, where the extensional fault system gradually decreases in offset and length, the Paleogene-Neogene Tuscan foredeep deposits are exposed only within the M.te Zuccone tectonic window. Figure 1 - structural scheme of the western Northern Apennines. Modified after Bernini et al. (1997). 67 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The analysis of two regional-scale seismic lines integrated with field geological and structural data (Fig. 1) allowed to recognize evidences of low-angle extensional tectonics which affected the western Northern Apennines orogenic wedge at different structural levels and predates the Plio-Pleistocene high-angle extensional faults. At shallow levels (< 1 km), low-angle normal faulting is located at the base of the Epiligurian wedge-top basin deposits (Artoni et al., 2006). At slightly deeper structural levels (~ 1 to 4 km), low-angle extensional faults occur in the Ligurian and Subligurian allochthonous units. These faults downthrown the uppermost Ligurian unit (i.e. Gottero Unit, Figs. 1, 2) directly onto the deeper foredeep units (Elter and Schwab, 1959) and they realize a tectonic thinning estimated in ~1-4 km (LANF in Carlini et al, 2013). At deeper structural levels (~5 km), low-angle extensional fault systems have thinned the carbonate-dominated Mesozoic sequence of the Tuscan succession (Clemenzi et al., submitted; Storti, 1995). In all these cases, the low-angle extensional tectonics indicates top-to-NE tectonic transport direction. At shallow structural levels (<1 km down to ~4 km), low-angle tectonic thinning has been constrained to an age interval comprised between middle Miocene (age of the youngest foredeep units buried underneath the allochthonous units) and late Miocene (age of the oldest deposits sealing the allochthonous units at the Po Plain foothills). At deeper structural levels, the low-angle extensional faulting postdates the middle-late Miocene thermal peak associated with the maximum tectonic burial of the Tuscan succession and is post-dated by Plio-Pleistocene high-angle extensional faulting (Fellin et al., 2007 and references therein). All the previously and newly recognized cases of low-angle extensional tectonics presented in this study have been framed in a consistent regional framework and interpreted as episodes of tectonic thinning which affected the western Northern Apennines during the late Miocene. This late Miocene low-angle extension would have been triggered by the mechanical and gravitationally instability resulting from orogenic wedge over-thickening related to the deep contractional deformations. During the Plio-Pleistocene times, thick-skinned tectonics migrated toward NE as testified by seismic “basement” units involved in the deformations (Argnani et al, 2003); this younger growth of the orogenic wedge enhanced the high-angle extensional faulting (Bernini e Papani, 2002) and dismembered the earlier low-angle extensional faults. Figure 2 - schematic sections across the western Northern Apennine showing the late Miocene low-angle extensional fault affecting the Ligurian and Epiligurian units. See Fig. 1 for the location of the schematic sections. 68 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The late Miocene low-angle extensional tectonics represent a key feature of the Northern Apennines: it played a fundamental role in the development of the structural architecture of the mountain belt. REFERENCES - Argnani, A., Barbacini, G., Bernini,M., Camurri, F., Ghielmi,M., Papani, G., Rizzini, F., Rogledi, S., Torelli, L., (2003). Gravity tectonics driven by Quaternary uplift in the Northern Apennines: insights from the La Spezia– Reggio Emilia geo-transect. Quat. Int. 101–102, 13–26. - Artoni, A., Bernini, M., Vescovi, P., Lorenzi, U., Missorini, E., (2006). Estensione alla sommità del cuneo orogenico appenninico: contatti tettonici elisionali nella Successione epiligure di M. Barigazzo (Appennino settentrionale, prov. di Parma). Rend. Soc. Ital., 2, 69–72. - Bernini, M., Papani, G., (2002). La distensione della fossa tettonica della Lunigiana nord-occidentale (con Carta Geologica alla scala 1:50000). Boll. Soc. Geol. It., 121, 313-341. - Bernini, M., Vescovi, P., Zanzucchi, G., (1997). Schema strutturale dell’Appennino Nord-Occidentale. L’Ateneo Parmense Acta Naturalia, 33, 43-54. - Carlini, M., Artoni, A., Aldega, L., Balestrieri M. L., Corrado S., Vescovi P., Bernini M., Torelli, L., (2013). Exhumation and reshaping of far-travelled/ allochthonous tectonic units in mountain belts. New insights for the relationships between shortening and coeval extension in the western Northern Apennines (Italy). Tectonophysics, 608, 267–287. - Clemenzi, L., Molli, G., Storti, F., Muchez, P., Swennen, R., Torelli, L. (submitted). Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines, Italy). Journal of Structural Geology - Elter, P., Schwab, K., (1959). Note illustrative alla carta geologica all’1-50000 della regione Carro-Zeri-Pontremoli. Boll. Soc. Geol. Ital. 78, 157– 187. - Fellin, M.G., Reiners, P.W., Brandon, M.T., Wüthrich, E., Balestrieri, M.L., Molli, G., (2007). Thermochronologic evidence for the exhumational history of the Alpi Apuane metamorphic core complex, northern Apennines, Italy. Tectonics 26. - Storti, F., (1995). Tectonics of the Punta Bianca Promontory: Insights for the evolution of the Northern Apennines, Northern Tyrrhenian Sea basin. Tectonics, 14, 832-847. 69 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Structural analysis of out-of-sequence thrust faults in western Sorrento Peninsula (southern Apennines, Italy) Stefano Vitale(1), Federico Borreca(2), Sabatino Ciarcia(1), Bilal El Ouaragli(3), Fabio Laiena(1), Francesco D’Assisi Tramparulo(1) (1) Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Università di Napoli Federico II, Italy (2) Agenzia Regionale Campana Difesa Suolo (ARCADIS), Napoli, Italy (3) Department of Earth Sciences, F.S.T.-Tangier, University of Abdelmalek Essaadi, Morocco INTRODUCTION This work is aimed to analyze the out-of-sequence thrust faul crosscutting the Apennine Platform Unit and the unconformable upper Tortonian wedge-top-basin deposits cropping out in the western sector of the Sorrento Peninsula (Fig. 1). These thrusts can be correlated to other outof-sequence structures spread out in the whole southern Apennine chain. According to their wide diffusion and the constant vergence toward the northern sectors, it is possible to ascribe these structures to a well-defined tectonic stage during the Apennine orogenic evolution, occurred in the Upper Messinian-Lower Pliocene interval. GEOLOGICAL SETTING The backbone of the southern Apennines (Fig.1) is made of Apennine Platform carbonates (Mostardini and Merlini, 1986) including a large variety of sedimentary facies ranging from basin to slope, margin and platform (e.g. Vitale & Ciarcia, 2013 and references therein), tectonically sandwiched between the basin successions of the Ligurian Accretionary Complex (LAC; Ciarcia et al., 2012; Vitale et al., 2013a,b), on the top, and the Lagonegro-Molise Basin deposits (Mostardini & Merlini, 1986) on the bottom, the latter overlying the buried Apulian Platform carbonates (Fig. 1). Figure 1. Tectonic map of the southern Apennines (modified after Vitale & Ciarcia, 2013) and cross section (modified after Mazzoli et al., 2008). 70 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 The southern Apennines formed as consequence of the closure of the Ligurian Basin, characterized by the transition from oceanic to thinned continental lithosphere, with the subduction of its westernmost sector under the European Plate starting from the Eocene time (Vitale & Ciarcia, 2013) and the successive frontal accretion of the easternmost sector in the Aquitanian-Burdigalian interval with the detachment of the Nord-Calabrese, Parasicilidi and Sicilidi Units (Ciarcia et al., 2012 and references therein) from their pre-Eocene basements. Currently the latter successions together with the VLT-HP Frido Unit (Vitale et al., 2013a) form the LAC widely spread out in the whole southern Apennines (Fig. 1). The Apennine Platform was included in the tectonic wedge from the Middle-Late Miocene by means of a dominant thick-skinned tectonics (Vitale & Ciarcia, 2013) and finally overthrust onto the successions of the Lagonegro-Molise Basin. The latter was definitively closed before the upper part of the Early Pliocene with the activation of several thrust sheet imbrications by means of a thin-skinned tectonics and together the whole orogenic chain overthrust onto the Apulian Platform carbonates (Ciarcia & Vitale, 2013). The latter realm was deformed by ramp-dominated thrust faults from the Early Pliocene to Middle Pleistocene, probably as reactivation of previous normal structures (Shiner et al., 2004). WESTERN SORRENTO PENINSULA The western Sorrento Peninsula (Fig. 2) mainly consists of Cretaceous shallow water limestones covered by Burdigalian-Serravallian foredeep calcarenites and sandstones (Nerano Fm.). The succession is unconformably sealed by wedge-top basin deposits of the upper Tortonian Punta Lagno Fm. (De Blasio et al., 1981) made of calcareous conglomerates, calcareous and varicoloured clay olistostromes and olistoliths (amongst others from the LAC units), marls and sandstones. The structural survey marked a complex geometry characterized by a main flat-lying thrust fault running from the Cala di Mitigliano to Capo di Sorrento (Fig. 2) with a displacement of at least 5 km joining to a ramp at base of the Mt. San Costanzo characterized by a small displacement. The main thrust is well exposed in the Cala di Mitigliano where a main sub-horizontal plane outcrops with pervasive slickensides indicating an N-tectonic transport and several secondary thrust and back-thrust Figure 2 - (a) Geological map of Western Sorrento Peninsula (modified after ISPRA, 2014). (b) Cross section. 71 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 planes and rare folds. The main thrust superposes the Cretaceous limestones onto the Miocene sandstones and calcarenites, the latter pervasively deformed by Riedle shear planes with calcite fibres indicating a northward tectonic transport. This main thrust fault crops out also in the Punta Lagno and Marina di Puolo forming small klippen. In these areas several secondary thrust and back-thrust faults occur. In the Marina di Puolo and Capo di Sorrento areas this thrust deformed the foredeep sandstones and the unconformable upper Tortonian Punta Lagno Fm. with the setup of thrust faults, related folds and rarely with an associated crenulation cleavage. The main thrust at base of Mt. San Costanzo is well exposed in the southernmost sector of the Cala di Mitigliano (Fig. 2). Here this structure shows a high dip angle and overturned strata in the footwall. Thrust faults and Riedle shear planes indicate a northward tectonic transport. REFERENCES - Ciarcia, S. & Vitale, S., (2013). Sedimentology, stratigraphy and tectonics of evolving wedge-top depozone: Ariano Basin, southern Apennines, Italy. Sedimentary Geology, 290, 27-46. - Ciarcia, S., Mazzoli, S., Vitale, S. & Zattin, M., (2012). On the tectonic evolution of the Ligurian accretionary complex in southern Italy. Geological Society of America Bulletin 124, 463–483. - De Blasio, I., Lima, A., Perrone, V. & Russo, M., (1981). Nuove vedute sui depositi miocenici della Penisola Sorrentina. Bollettino della Societa Geologica Italiana 100, 57–70. - ISPRA, 2014. Carta Geologica d’Italia alla scala 1:50.000, Foglio 466 ‘Sorrento’. - Mazzoli, S., D’Errico, M., Aldega, L., Corrado, S., Invernizzi ,C., Shiner, P. & Zattin, M. (2008). Tectonic burial and ‘young’ (< 10 Ma) exhumation in the southern Apennines fold and thrust belt (Italy). Geology 36, 243-246. - Mostardini, F. & Merlini, S., (1986). Appennino centro-meridionale: sezioni Geologiche e proposta dimodello strutturale.Memorie della Società Geologica Italiana 35, 177–202. - Shiner, P., Beccacini, A. & Mazzoli, S., (2004). Thin-skinned versus thick-skinned structural models for Apulian Carbonate Reservoirs: constraints from the Val D’Agri Fields. Marine and Petroleum Geology 21, 805–827. - Vitale, S. & Ciarcia, S., (2013). Tectono-stratigraphic and kinematic evolution of the southern Apennines/Calabria-Peloritani Terrane system (Italy). Tectonophysics, 583, 164–182. - Vitale, S., Fedele, L., Tramparulo, F. d’A., Ciarcia, S., Mazzoli, S. & Novellino, A., (2013a). Structural and petrological analysis of the Frido Unit (southern Italy): new insights into the early tectonic evolution of the southern Apennines- Calabrian Arc system. Lithos, 168-169, 219-235. - Vitale, S., Ciarcia, S. & Tramparulo, F. d’A., (2013b). Deformation and stratigraphic evolution of the Ligurian Accretionary Complex in the southern Apennines (Italy). Journal of Geodynamics, 66, 120-133. 72 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Multiscale structural analysis supporting petrologic modelling to infer HP/UHP mineral assemblages of subducted rodingites from the upper Valtournanche, Western Alps, Italy Davide Zanoni(1), Gisella Rebay(2), Maria Iole Spalla(1, 3) (1) Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Italy (2) Dipartimento di Scienze della Terra e dell’Ambiente, Università degli Studi di Pavia, Italy (3) C.N.R.-I.D.P.A. sezione di Milano, Italy Rodingites associated with serpentinite massifs are generally considered the products of a metasomatic ocean floor process (e.g. O’Hanley et al., 1992; Palandri & Reed, 2004; Panseri et al., 2008; Bach & Klein, 2009). Even if rodingitisation can take place in subduction environments, as in the case of the Othrys massif (Tsikouras et al., 2009), re-structuration and metamorphic transformations involving rodingites within a convergent system are generally neglected. Former studies on rodingites from the Alpine belt (Weinschenk, 1894; Cornelius, 1912; Staub, 1915; Rondolino, 1937; Amstutz, 1962), described rodingitic mineral assemblages as metasomatic relicts escaping the Alpine regional metamorphism until when Dal Piaz (1967) pointed out that rodingites were widely re-equilibrated during Alpine HP metamorphic evolution together with their country rocks. Successively rodingites affected by HP re-equilibration have been described in other portions of the Zermatt-Saas Zone (Pfulwe and M. Avic regions), in the Ligurian Alps (Voltri Massif) and in southern Spain (Piccardo et al., 1980; Messiga et al., 1983; Puga et al., 1999; Li et al., 2008; Ferrando et al., 2010). In order to define the HP mineral assemblages in the boudinaged rodingites of the upper Valtournanche, embodied in HP/UHP partly de-hydrated serpentinites of Zermatt-Saas Zone (Rebay et al., 2012), we undertook a multiscale combined structural and metamorphic analysis. On the basis of different associations of minerals and their modal amount, three types of rodingite are distinguished: epidote-, garnet-chlorite-clinopyroxene-, and vesuvianite-bearing rodingites (Zanoni et al., 2012). Up to cm-sized relict clinopyroxene porphyroclasts point out that these rodingites derive from ocean floor metasomatism of former gabbro dykes intruded in mantle rocks. The deformation history recorded in the serpentinites affects also rodingites, as proven by continuous struc- tural correlation in the field. All rocks record, after the ocean floor history, four stages of ductile deformation and the first three groups of structures are associated with new mineral growth. D1 and D2 developed under HP to UHP conditions and D3 under lower pressure conditions. The following syn-D2 assemblages in rodingites are interpreted as developed under the same HP/UHP conditions already inferred in serpentinites (2.5 ± 0.3GPa and 600 ± 20°C, Rebay et al., 2012), for the Alpine subduction: epidoteII, clinopyroxeneII, Mg-chloriteII, titaniteI, ± garnetII, ± tremoliteI in epidote-bearing rodingites; Mg-chloriteII, garnetII, clinopyroxeneII, ± vesuvianiteII, ± opaque minerals, in garnet-chlorite-clinopyroxene-bearing rodingites; vesuvianiteII, Mg-chloriteII, clinopyroxeneII, garnetII, ± titaniteI, ± epidoteI in vesuvianite-bearing rodingites. Thermodynamic modelling in epidoteand garnet-chlorite-clinopyroxene-bearing rodingites quantitatively confirms the P-T range for climax conditions inferred in the associated serpentinites. Pre-D1 mineral relicts such as Cr-rich spinel and cmsized clinopyroxene porphyroclasts are interpreted as magmatic vestiges of the rodingitised gabbro dykes, whereas Cr-rich garnet and Cr-Ti-Ca-rich vesuvianite as developed during oceanic metasomatism. Distinct chemical compositions characterise the same minerals recrystallised during the Alpine subduction. Multiscale structural analysis shows that the dominant structural and metamorphic imprint recorded by the Valtournanche eclogitised rodingites developed during the Alpine subduction and that, in the favourable bulk composition (Ca-rich), vesuvianite is stable up to the estimated P-climax metamorphic conditions. REFERENCES - Amstutz, A. (1962). Notice puor une carte géologique de la Vallée de Cogne et de quelches autres espaces au Sud d’Aoste. Archives des Sciences de Genève, 15, 1-104. 73 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 - Bach, W. & Klein, F. (2009). The petrology of seafloor rodingites: Insights from geochemical reaction path modeling. Lithos, 112, 103-117. - Cornelius, H.P. (1912). Petrographische Untersuchengen in den Bergen zwischen Septimer und Julierpass. Neues Jahrbuch für Mineralogie, Gelogie und Paläontologie, 35, 374-498. - Dal Piaz, G.V. (1967). Le “granatiti” (rodingiti L. S.) nelle serpentine delle Alpi occidentali italiane. Memorie della Società Geologica Italiana, 6, 267-313. - Ferrando, S., Frezzotti, M.L., Orione, P., Conte, R.C. & Compagnoni, R. (2010). Late-Alpine rodingitization in the Bellecombe meta-ophiolites (Aosta Valley, Italian Western Alps): evidence from mineral assemblages and serpentinization-derived H2-bearing brine. International Geology Review, 52(10-12), 1220-1243. - Li, X.P., Rahn, M. & Bucher, K. (2008). Eclogite facies metarodingites – phase relations in the system SiO2-Al2O3-Fe2O3-FeO-MgO-CaO-CO2-H2O: an example from the Zermatt-Saas ophiolite. Journal of Metamorphic Geology, 26, 347-364. - Messiga, B., Piccardo, G.B. & Ernst, W.G. (1983). High- pressure eo-alpine parageneses developed in magnesian metagabbros, Gruppo di Voltri, Western Liguria, Italy. Contributions to Mineralogy and Petrology, 83, 1-15. - O’Hanley, D.S., Schandl, E.S. & Wicks, F.J. (1992). The origin of rodingites from Cassiar, British Columbia, and their use to estimate T and P(H2O) during serpentinization. Geochimica et Cosmochimica Acta, 56, 97-108. - Palandri, J.L. & Reed, M.H. (2004). Geochemical models of metasomatism in ultramafic systems: Serpentinization, rodingitization, and sea floor carbonate chimney precipitation. Geochimica et Cosmochimica Acta, 68(5), 1115-1133. - Panseri, M., Fontana, E. & Tartarotti, P. (2008), Evolution of rodingitic dykes: metasomatism and metamorphism in the Mount Avic serpentinites (Alpine ophiolites, Southern Aosta Valley). Ofioliti, 33(2), 165-185. - Piccardo, G.B., Messiga, B. & Cimmino, F. (1980). Antigoritic serpentinites and rodingites of the Voltri Massif; some petrological evidences for their evolutive history. Ofioliti, 5(1), 111-114. - Puga, E., Nieto, J.M., Díaz de Federico, A., Bodinier, J.L. & Morten, L. (1999). Petrology and metamorphic evolution of ultramafic rocks and dolerite dykes of the Betic Ophiolitic Association (Mulhacén Complex, SE Spain): evidence of eo-Alpine subduction following an ocean-floor metasomatic process. Lithos, 49, 23-56. - Rebay, G., Spalla, M.I. & Zanoni, D. (2012). Interaction of deformation and metamorphism during subduction and exhumation of hydrated oceanic mantle: Insights from the Western Alps. Journal of Metamorphic Geology, 30, 687-702. - Rondolino, R. (1937). Sopra alcuni minerali della Valtournanche (Valle d’Aosta). Periodico di Mineralogia, 8, 53-56. - Staub, R. (1915). Petrographische Untersuchungen im westlichen Berninagebirge. Zürcher & Furrer, Zürich. Tsikouras, B., Karipi, S., Rigopoulos, I., Perraki, M., Pomonis, P. & Hatzipanagiotou, K. (2009). Geochemical processes and petrogenetic evolution of rodingite dykes in the ophiolite complex of Othrys (Central Greece). Lithos, 113: 540-554. - Weinschenk, E. (1894). Beiträge zur Petrographie der östlischen zentralalpen, speziell des Gross-Venedigerstokes. I. Über die Peridotite und die aus ihnen hervorgegangenen Serpentingesteine. Genetischer Zusammenhang derselben mit den sie begleitenden Minerallagerstätten. Abhandlung bayerische Akademie Wisseschaften, 18, 651-713. - Zanoni, D. Rebay, G., Bernardoni, J. & Spalla, M.I. (2012). Using multiscale structural analysis to infer high-/ultrahigh-pressure assemblages in subducted rodingites of the Zermatt-Saas Zone at Valtournanche, Italy. In: M. Zucali, M.I. Spalla, & G. Gosso (Eds.), Multiscale structures and tectonic trajectories in active margins. Journal of the Virtual Explorer, Electronic Edition, 41, paper 6. 74 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Strain and reaction-rate partitioning mapping in the Mombarone Mt. Mucrone area (Sesia Lanzo Zone, Italian Western Alps) Michele Zucali(1, 2), Francesco Delleani(1), M. Iole Spalla(1, 2), Guido Gosso(1, 2) (1) Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Italy (2) CNR-IDPA, Milano, Italy Permian intrusive bodies and metasediments of the Mt. Mucrone – Mt. Mars – Mombarone area locally display granulite to amphibolite pre-Alpine metamorphic imprint, preserved within the widely re-equilibrated volumes, under eclogite-facies conditions during the Cretaceous. They are now part of the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone in the Alpine HP metamorphic belt of the Western Alps. The purpose of this contribution is to illustrate the technique for mapping the strain and reaction rate partitioning in a polymetamorphic and polydeformed terrain, as that surfacing along the Mt. Mucrone – Mt. Mars – Mombarone divide. Results have been synthesised in terms of deformation history and associated metamorphic evolution on a solid-type of map at original scale at 1:10.000 and 1:5.000, with the purpose to quantitatively evaluate the structural and metamorphic memory of rocks forged during a continental thinning and successively deeply involved in the Alpine subduction system. Seven successive stages of deformation have been recognized (Delleani et al., 2013; Zucali, 2002) showing associated metamorphic imprints evolving from eclogite (D1 to D3) and blueschist facies (D4), developed during the active subduction of the Alpine oceanic crust, to greenschist facies assemblages (D5 to D7) consequent to the Alpine continental collision. The combined structural and petrologic studies made possible to correlate the degree of fabric evolution and metamorphic transformation at the map scale and highlight that strain-partitioning is a critical factor controlling reaction progress. More in detail, relationships between Fabric Evolution Degree (FED) and reaction progress in different lithotypes suggests that differences in mineral compositions of protoliths and original textures can exert an influence on the reaction progress when FED remains low. The mosaic of successive deformation imprints visualised in the maps indicate that the High Deformation stage represents a threshold after which deformational and metamorphic effects proportionally increase up to the total replacement of pre-existing minerals where new fabrics evolved up to the stage of a continuous foliation, in agreement with similar observations in other Alpine areas (e.g. Salvi et al., 2010). These results focus on the role of strain energy in catalysing metamorphic reactions. In addition, our observations indicate that where the fabric evolution remains lower than HD the thermal regime, as well original mineral rock composition and fabric evolution, can significantly influence the degree of metamorphic transformation: for the same FED, the metamorphic reaction progress is more evolved during D2 (eclogite facies) or during D5 (greenschist facies) than during D4 (blueschist facies). REFERENCES - Delleani, F., Spalla, M.I., Castelli, D. & Gosso, G. (2013). A new petrostructural map of Monte Mucrone metagranitoids (Sesia-Lanzo Zone, Western Alps). Journal of Maps, 9, 410-424. - Salvi, F., Spalla, M.I., Zucali, M. & Gosso, G. (2010). Three- dimensional evaluation of fabric evolution and metamorphic reaction progress in polycyclic and polymetamorphic terrains: a case from the Central Italian Alps. Geological Society of London Special Publications, 332, 173-187. - Zucali, M. (2002). Foliation map of the “Eclogitic Micaschists Complex” (M. Mucrone-M.Mars-Mombarone, Sesia-Lanzo Zone, Italy). Memorie di Scienze Geologiche, Padova, 54, 87-100. 75 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 Index - ARGNANI A. - Collision, collapse and delamination: the making of the Alps-Apennine connection ..................................................................................................................... ...... 3 - BALBI P., ELTER F.M., ARMADILLO E., PIAZZA M. - A new stratigraphic and tectonic model of the south eastern Monte Antola unit and its relationship with the Monte Gottero Unit (Eastern Liguria) ................................................................................................. 5 - BALESTRO G., FESTA A., TARTAROTTI P. - Broken and dismembered formations in the Monviso Meta-ophiolite Complex (Western Alps) .................................................. ...... 6 - BARALE L., BATTAGLIA S., BERTOK C., D’ATRI A., ELLERO A., LEONI L., MARTIRE L., PIANA F. - Geological setting of the southern termination of the western Alps arc (Maritime and western Ligurian Alps, NW Italy) ............................................. ...... 8 - BARRECA G., MONACO C.- Geological and geophysical evidences for diapirism in South-Eastern Sicily (Italy) and implications on paleo-tectonics of the Western Ionian Domain ..................................................................................................................................... 10 - BERNOULLI D. - The concept of ophiolites from Alexandre Brogniart to Piero Elter ....... 12 - BETTELLI G., PANINI F., REMITTI F., VANNUCCHI P. - Reconciling the geology of the Emilia Apennines and Tuscany across the Livorno-Sillaro lineament, Northern Apennines, Italy ............................................................................................................................... 13 - BONATTI E. - Piero Elter: from the Apennines to the Atlantic ............................................. 15 - CERRINA FERONI A., PUCCINELLI A. - The geological sheet n° 261 – Lucca ( CARG): main results and some suggestion .................................................................................................. 16 - CORSI B., ELTER F.M., PIAZZA M. - The Portofino conglomerate (Eastern Liguria): stratigraphic notes ............................................................................................................... ...... 18 - DAL PIAZ G.V. - Struttura ed evoluzione della catena alpina: dallo Structural Model of Italy al Progetto Crop ed ai loro sviluppi .................................................................................. 19 - DECARLIS A., MANATSCHAL G., HAUPERT I, MASINI E. - The tectono-stratigraphic evolution of distal magma-poor rifted margins: examples from the Alpine Tethys, the E-Indian and Newfoundland-iberia rifted margins .............................................................. 21 - ELLERO A., LOPRIENO A. - Ophiolitic units of the Northern Apennines as a tool to unravel schistes lustres stratigraphy of the western Alps and their geodynamic context: the Urtier Valley example, Cogne, Aosta Valley ....................................................................... 22 - ELTER F.M., GAGGERO L., PANDELI E. - The late-Variscan tectonic barriers: the beginning of alpine Wilson cicle? ............................................................................................. 24 76 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 - FESTA A., PINI G.A., DILEK Y. - Olistostromes and mélanges in the External Ligurian Units in Monferrato (NW Italy) .......................................................................................... ...... 25 - GIUSTI R., PANDELI E., ELTER F.M. - Ductile shear zone in the termo-metamorphic aureole of Monte Capanne: data from Cavoli-Colle Palombaia area (south-western Elba Island, Italy) ......... 26 - HOBBS N., LUVISI E., MARRONI M., MENEGHINI F., PANDOLFI L. - The role of the Ottone-Levanto Line in the geodynamic evolution of the Northern Apennine: evidences from the high Sturla Valley, Liguria, Italy ..................................................................... 28 - KRAUS R.K., ELTER F.M., ELENA E., SOLARINO S. - Distribution of differing stress regimes in the western central Alps controlled by inner arc bending? ............................. 30 - LOPRIENO A., ELLERO A. - Structural analysis of the nappe stack in the Urtier Valley, Cogne (Western Alps) ................................................................................................................ 31 - MAINO M., STUART F.M., CERIANI A., DECARLIS A., di GIULIO A., SENO S., SETTI M. - Zircon (U-Th)/He dating of the penninic basal thrust ............................................. 32 - MALAVIEILLE J.- Impact of surface processes and structural heritage on the structure and dynamics of orogenic wedges : insights from Taiwan ........................................................ 33 - MARCHI A., PANDOLFI L., CATANZARITI R. - The basal part Modino unit succession under the belt-foredeep system of the Northern Apennines (Italy) ..................................... 35 - MARINI F., PANDELI E., TONGIORGI M. - The late Carboniferous-Permian successions of the Northern Apennines: new data from the contact between San Lorenzo Schists and Asciano Breccias and conglomerates in the Pisani Mts. inlier (Tuscany, Italy) ................ 36 - MAROTTA A.M., CONTE K., RODA M., SPALLA M.I. - Thermo-mechanical numerical model of the transition from continental rifting to oceanization: the transition from Permian-triassic thinning to oceanisation in the Alpine chain ................................................. 38 - MONTOMOLI C., CAROSI R., LANGONE A., BORSANI A., IACCARINO S. - Lower Miocene contractional shearing in the Massa Unit (Northern Apennines, Italy): in situ U-Th-Pb dating of monazite ...................................................................................................... 39 - MUSUMECI G., MASSA G., PIERUCCIONI D., MAZZARINI F. - Out of sequence thrust in the inner zone of Northern Apennines: insight from Elba Island nappe stack ........... 41 - MUTTI E. - Paleogeographic problems of the Eocene foreland basin of the South-central Pyrenees .............................................................................................................................. 43 - NOVELLINO R., PROSSER G., VITI C., SPIESS R., AGOSTA F., TAVARNELLI E., BUCCI F. - Microstructural evidence of weakening mechanisms developed along incipient low-angle normal faults in pelagic limestones (Southern Apennine, Italy) ................ ...... 44 - ORTOLANO G., TRIPODI V., CIRRINCIONE R., CRITELLI S., MUTO F., VISALLI R. - The role of strike-slip tectonics in the early to late formation of the southern Calabrian Alpine-Apennine chain system. (Calabria, Southern Italy) ....................................................... 45 77 Meeting in memory of Piero Elter - The relationships between Northern Apennine and western Alps: state of the art fifty years after the “Ruga del Bracco” - Pisa, June 26-27, 2014 - PANDELI E., ELTER F.M., TOKSOYKÖKSAL F., PRINCIPI G. - Relationships between the Kirşehir Massif and the Sakarya Zone (Turkey): geological and petrographical data from the Ankara-Erzincan suture and surroundings .......................................................... 48 - PANDOLFI L., BOSCHI C., LUVISI E., ELLERO A., MARRONI M., MENEGHINI F. - Seep carbonates and chemosynthetic coral communities in the Bocco Shale (Internal Ligurides, Northern Apennine, Italy) as record of trench-slope limit in the early Paleocene alpine accretionary wedge................................................................................. ...... 50 - PEACOCK D.C.P. - Explosion breccias .................................................................................. 51 - PEACOCK D.C.P., CASINI G., ANDERSON M.W., TAVARNELLI E. - Stresses, overpressure and transient strike-slip during tectonic inversion ..................................................... 52 - PIAZZA A., ARTONI A. - The epiligurian basin in the middle Enza Valley (Northern Apennines, Italy): evidences of a Priabonian-Serravallian syn-depositional polyphased deformation zone ....................................................................................................................... 53 - PRINCIPI G., TREVES B. - Evolution of the Alps-Apennine boundary: EU/IB/AD transcurrence leading to a crustal vortex .................................................................................... 57 - ROLFO F., BALESTRO G., BORGHI A., CASTELLI D., FERRANDO S., GROPPO C., MOSCA P., COMPAGNONI R. - Walking through the Tethys in the Monviso ophiolite (Piemonte, Italy) .................................................................................................................. 59 - SCHMID S.M. - Alps-Dinarides vs. Alps-Apennine transition: changes in subduction polarity in space and time .......................................................................................................... 62 - STORTI F., BALSAMO F., CLEMENZI C., MOLLI G., MUCHEZ P., SWENNEN R. Low-angle extensional fault systems in the Northern Apennines thrust wedge: similarities and differences between the Tellaro and Val di Lima detachments .......................................... 64 - TREVES B., NIRTA G. - In search of Adria’s rails: the mid Tyrrhenian western Adria’s rail and its pivoting ligurian tip ................................................................................................. 65 - VESCOVI P., CARLINI M., ARTONI A., CLEMENZI L., TORELLI L. - Evidences of late Miocene low-angle extensional tectonics affecting the western Northern Apennines orogenic wedge ................................................................................................................... ...... 67 - VITALE S., BORRECA F., CIARCIA S., el OUARAGLI B., LAIENA F., D’ASSISI TRAMPARULO F. - Structural analysis of out-of-sequence thrust faults in western Sorrento Peninsula (Southern Apennines, Italy) ............................................................................. 70 - ZANONI D., REBAY G., SPALLA M.I. - Multiscale structural analysis supporting petrologic modelling to infer hp/uhp mineral assemblages of subducted rodingites from the upper Valtournanche, western Alps, Italy ........................................................................... ...... 73 - ZUCALI M., DELLEANI F., SPALLA M.I., GOSSO G. - Strain and reaction-rate partitioning mapping in the Mombarone - Mt. Mucrone area (Sesia Lanzo zone, italian western Alps) .................................................................................................................................... 75 78 Pisa, giugno 2014
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