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Ital.J.Geosci. (Boll.Soc.Geol.It.), Vol. 128, No. 2 (2009), pp. 605-613, 5 figs. (DOI: 10.3301/IJG.2009.128.2.605)
Plio-Quaternary tectonic evolution of the Northern Apennines thrust fronts
(Bologna-Ferrara section, Italy): seismotectonic implications
G. TOSCANI (*), P. BURRATO (**), D. DI BUCCI (***), S. SENO (*) & G. VALENSISE (**)
ABSTRACT
The outermost, NE-verging fronts of the Northern Apennines
(Italy) are overlain by a thick syntectonic sedimentary wedge filling
up the basin beneath the Po Plain. Due to fast sedimentation rates
and comparatively low tectonic rates, the fronts are generally buried.
Evidence for their activity includes scattered historical and instrumental earthquakes and drainage anomalies controlled by growing
buried anticlines. The largest earthquakes, up to Mw 5.8, are associated with active compression, with a GPS-documented shortening
rate <1 mm/a.
We used geological, structural and morphotectonic data to draw
a N-S-striking section between Bologna and Ferrara, aimed at analyzing whether and how the deformation is partitioned among the
frontal thrusts of the Northern Apennines and identifying the potential sources of damaging earthquakes. We pointed out active anticlines based on the correspondence among drainage anomalies, historical seismicity and buried ramps. We also analyzed the evolution
of the Plio-Quaternary deformation by modeling in a sandbox the
geometry, kinematics and growth patterns of the thrust fronts.
Our results (i) confirm that some of the main Quaternary
thrusts are still active and (ii) highlight the partitioning of deformation in the overlap zones. We note that the extent and location of
some of the active thrusts are compatible with the location and size
of the main historical earthquakes and discuss the hypothesis that
they may correspond to their causative seismogenic faults.
KEY WORDS: Fold-and-thrust belt, active tectonics, seismogenic
sources, Po Plain.
RIASSUNTO
Evoluzione tettonica plio-quaternaria dei fronti di accavallamento nord-appenninici (transetto Bologna-Ferrara, Italia):
implicazioni sismotettoniche.
I fronti di accavallamento più esterni della catena Nord-Appenninica sono sepolti da una spessa coltre di sedimenti clastici che
colma l’intera Pianura Padana e che è stata studiata in particolare
tramite pozzi profondi e sezioni sismiche a riflessione, realizzate
prevalentemente per scopi esplorativi (idrocarburi). Questi studi
mostrano un sistema di pieghe e accavallamenti sepolti Nord-Est
vergenti, che hanno influenzato e controllato la deposizione dei
cunei di sedimenti silicoclastici sintettonici; per la parte plio-quaternaria, questi cunei mostrano spessori fino a 9 km.
In Pianura Padana, in virtù della rapida sedimentazione clastica
e dei limitati ratei di deformazione che caratterizzano l’area, le evidenze di tettonica attiva sono estremamente scarse e di difficile lettura. Alcune di queste evidenze, seppur deboli, si riscontrano nelle
anomalie del reticolo idrografico, sotto forma sia di deviazioni fluviali che di repentine variazioni dell’attività erosiva del corso
(*) Dipartimento di Scienze della Terra, Università di Pavia.
Via Ferrata, 1 - 27100 Pavia, Italy. E-mail: [email protected].
(**) Istituto Nazionale di Geofisica e Vulcanologia. Via di Vigna
Murata, 605 - 00143 Roma, Italy.
(***) Dipartimento della Protezione Civile. Via Vitorchiano, 2 00189 Roma, Italy.
d’acqua, variazioni che sono risultate essere controllate in prevalenza dalla crescita delle anticlinali sepolte al di sotto di tali anomalie. Inoltre, i cataloghi della sismicità storica e strumentale mostrano
che la Pianura Padana meridionale è interessata da una sismicità da
bassa a moderata (fino a Mw 5,8), caratterizzata da meccanismi
focali compressivi. I breakout di pozzi profondi e i meccanismi focali
mostrano entrambi un Shmax orientato circa perpendicolarmente
all’andamento dei fronti di accavallamento sepolti. I dati GPS,
infine, evidenziano un debole raccorciamento in direzione N-S (velocità inferiore a 1mm/a).
In questo contesto, il lavoro punta a verificare: 1) il grado di
attività dei diversi accavallamenti sepolti dell’Appennino Settentrionale, in particolare nell’arco delle pieghe ferraresi; 2) se e come la
deformazione si ripartisca tra essi e 3) quali degli accavallamenti
individuati possano essere interpretati come sorgenti di terremoti
potenzialmente dannosi. Abbiamo quindi integrato dati geologici,
strutturali e morfotettonici e, sulla base dell’interpretazione di dati
di pozzo e linee sismiche a riflessione, abbiamo realizzato una
sezione a scala regionale orientata circa SSW-NNE.
Per investigare l’attività degli accavallamenti sepolti e/o delle
anticlinali di rampa ad essi associate, abbiamo analizzato l’assetto
morfotettonico dell’area investigata, confrontando la posizione delle
anomalie nel reticolo del drenaggio con quella delle strutture
sepolte. Abbiamo quindi proiettato la sismicità storica e strumentale
dell’area sulla sezione geologica, per confrontare la sua distribuzione
rispetto all’assetto strutturale e alle deformazioni rilevate, in particolare nei sedimenti quaternari. Infine abbiamo realizzato alcuni
modelli analogici volti a riprodurre l’evoluzione della deformazione
lungo il transetto investigato, in particolare quella plio-quaternaria,
con l’obiettivo di studiare la cinematica e l’evoluzione dei fronti di
accavallamento e le interazioni tra attività tettonica e sedimentazione nelle fasi finali della strutturazione di tali fronti.
I risultati mostrano che i sovrascorrimenti principali:
– sono stati attivi durante il Quaternario e in parte lo sono
attualmente;
– presentano una partizione della deformazione nelle zone di
sovrapposizione;
– hanno ubicazione e geometrie compatibili con i principali
terremoti storici dell’area, dei quali potrebbero perciò costituire
le faglie-sorgente.
TERMINI CHIAVE: Catena a pieghe e accavallamenti, tettonica
attiva, sorgenti sismogenetiche, Pianura Padana.
1. INTRODUCTION
The Po Plain is a densely populated region of Northern Italy, hosting significant productive activities as well
as important historical centers. Infrequent reverse-faulting earthquakes (PONDRELLI et alii, 2006) with Mw up to
5.8 (CPTI Working Group, 2004) pose a limited hazard
but a comparatively high risk to most of these cities.
In the decades immediately following World War II,
the subsurface geology of the region was widely investigated for hydrocarbon exploration purposes. It is well
established that a thick clastic sequence fills the Po Plain
flexural basin, burying the outermost thrust sheets of the
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Northern Apennines fold-and-thrust belt and the southernmost thrust sheets of the Southern Alps. The recent-tocurrent activity of these thrusts and their seismogenic
potential, however, is still not fully appreciated and is
hence in need of further investigations. Due to the combination of fast sedimentation rates and low deformation
rates, the thrusts are buried and the surface evidence of
their activity is faint and elusive.
We used a multidisciplinary approach along a transect between Bologna and Ferrara to verify the extent of
current activity of the thrusts buried beneath the Po Plain
and the seismotectonic implications of such activity. We
compared information on the main historical earthquakes of the area and geomorphological clues of growing buried anticlines with information on the geometry of
thrust sheets obtained by the integrated analysis of subsurface data, in particular seismic reflection lines and
deep well logs. To evaluate the behavior of the analyzed
thrusts through time and the partitioning of deformation
among them, we compared our transect with a parallel
cross-section drawn for the same structural arc by previous investigators. Finally, we simulated the evolution of
thrusting through a set of scaled analogue models.
The preliminary results of this approach are discussed in this note. They suggest that the main thrusts
analyzed have been all active during the Quaternary, that
some of them are still active, and that based on their size
and location they may have acted as the causative faults
of the main historical earthquakes.
2. GEOLOGICAL AND SEISMOTECTONIC SETTING
2.1. GEOLOGY AND GEOMORPHOLOGY
The tectonic setting of the buried Northern Apennines
is characterized by three adjoining arcs: Monferrato,
Emilia and Ferrara-Romagna, respectively from west to
east (fig. 1). The Ferrara-Romagna arc is further subdivided into three second order structures: Ferrara,
Romagna and Adriatic (BIGI et alii, 1990).
Our SSW-NNE transect runs from Bologna to Ferrara, crossing the Ferrara structure. It starts close to the
northeastern topographic margin of the Northern Apennines, that corresponds to the «Pedeapenninic Thrust
Front» (PTF) as defined by BOCCALETTI et alii (1985). The
PTF separates the uplifted and exposed part of the accretionary wedge from the part buried under the Po Plain
(figs. 1 and 2). To the NNE our transect runs through the
most external, buried thrust fronts of the Northern Apennines and terminates near Ferrara, just south of the current location of the Po River. The outermost thrust sheets
are covered by a thick Plio-Quaternary clastic sequence
forming the depositional wedge-top. This sequence can be
up to 9 km-thick at the core of the synclines (ORI et alii,
1986; BIGI et alii, 1990; DOGLIONI, 1993; BARTOLINI et alii,
1996; FORD, 2004). As a result, the buried thrusts and
related fault-propagation folds have been studied almost
exclusively by means of seismic reflection lines and deep
well logs acquired for hydrocarbon exploration (e.g. PIERI
& GROPPI, 1981; ORI & FRIEND, 1984; MASSOLI et alii,
2006). These data show a system of NE-verging blind
thrusts and folds that controlled the syntectonic deposition of sedimentary wedges. The Northern Apennines
fold-and-thrust belt and the related foredeeps migrated
northeastwards during the Late Oligocene-Quaternary
(e.g. MALINVERNO & RYAN, 1986; ROYDEN et alii, 1987;
DOGLIONI, 1991; SCROCCA et alii, 2007). The thrust fronts
propagated at rates ~9.0 mm/a (BASILI & BARBA, 2007,
and references therein).
2.2. SUBSURFACE GEOLOGY
Information on the Po Plain subsurface comes from
extensive hydrocarbon exploration, which unveiled its
deep structural and stratigraphic setting (e.g. BIGI et alii,
1990), and from shallow water wells, which describe the
architecture of the younger deposits (REGIONE EMILIA
ROMAGNA & ENI-AGIP, 1998). This large dataset allows
several marker surfaces to be recognized and mapped.
These buried surfaces develop over large areas and are
deformed in synclines and anticlines; these latter located
above the ramps of the blind thrusts. From the deepest
(oldest) to the shallowest (youngest), the most continuous
surfaces are: 1) the bottom of the Pliocene marine
deposits, a first order stratigraphic marker (fig. 3) overlain by sediments varying in age within the Pliocene succession at the scale of the entire Po Plain; 2) the bottom
of the Quaternary continental deposits, which coincides
with the bottom of the lower alluvial unit (ca. 0.65 Ma);
and 3) the bottom of the upper alluvial unit (traditionally
dated ca. 0.35-0.45 Ma). Tighter constraints on the age of
these buried surfaces are available in literature; for
instance, MUTTONI et alii (2003) assign the surface corresponding to the first prominent Pleistocene glacio-eustatic lowstand to the marine isotope stage (MIS) 22, dated
0.87 Ma.
The overall sedimentary evolution of the Po Plain follows a regressive trend, from an open marine environment in the Pliocene to shallow marine, paralic and continental environments throughout the Quaternary. The
Upper Quaternary sedimentary bodies are separated by
major unconformities related to glacio-eustatic cycles
(AMOROSI et alii, 2004). In cross-section, the sedimentary
bodies defined by the regional marker surfaces exhibit
growth strata on the limbs of the anticlines, thus testifying the synsedimentary activity of these folds. Growth
strata have been effectively used to quantify deformation
rates after careful removal of differential compaction
effects (SCROCCA et alii, 2007).
2.3. RECENT DEFORMATION, SEISMICITY AND SEISMOGENIC
SOURCES
The southern Po Plain is a low relief region. In our
study area, the only surface expression of the growing
buried anticlines is the control they exert on the drainage
pattern, which consists of river diversions and channel
pattern changes (e.g.: CASTIGLIONI & PELLEGRINI, 2001;
BURRATO et alii, 2003). In particular, BURRATO et alii
(2003) mapped drainage anomalies all over the Po Plain
and correlated them to existing buried faults to verify
whether these anomalies may have a tectonic origin. For
the study area these investigators highlighted anomalies
in the Po, Idice, Reno and Panaro Rivers (see their table 1
and fig. 5) as induced by the current activity of the
Ferrara structure (fig. 2).
Concerning the subsurface, an analysis of growth
strata across the Mirandola anticline, a few kilometers
west of the studied transect, yielded an uplift rate
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Fig. 1 - Simplified tectonic map of the Po Plain and surrounding regions, showing the Northern Apennines and Southern Alps main thrusts
and faults as red lines. Yellow and orange polygons: individual seismogenic sources and seismogenic areas, respectively, from DISS database
(DISS WORKING GROUP, 2007; BASILI et alii, 2008). Structures: Monferrato Arc, MA; Emilian Arc, EA; Ferrara-Romagna Arc, FRA; Pedeapenninic Thrust Front, PTF; Western Southern Alps buried thrust, SABT; Schio-Vicenza line, SV; Thiene-Conegliano thrust front, TC; CansiglioManiago thrust front, CAM. GPS vectors in the inset are from SERPELLONI et alii (2005).
– Mappa tettonica semplificata della Pianura Padana e aree circostanti, che mostra in rosso i maggiori thrust e le faglie dell’Appennino Settentrionale e del Subalpino. I poligoni gialli ed arancioni rappresentano rispettivamente sorgenti sismogenetiche ed aree sismogenetiche, dal database
DISS (DISS WORKING GROUP, 2007; BASILI et alii, 2008). Strutture: arco del Monferrato, MA; arco delle pieghe Emiliane, EA; arco FerrareseRomagnolo, FRA; fronte del thrust pedeappenninico, PTF; thrust sepolto del Sudalpino Occidentale, SABT; linea Schio-Vicenza, SV; fronte del
thrust Tiene-Conegliano, TC; fronte del thrust Consiglio-Maniago, CAM. I vettori GPS nel riquadro sono tratti da SERPELLONI et alii (2005).
decreasing from ca. 0.5 mm/a to 0.2 mm/a in the past
1.4 Ma (SCROCCA et alii, 2007). Moving to the southwest,
activity of the PTF is testified near Bologna by deformation and faulting of Middle-Upper Pleistocene deposits,
uplifted river terraces correlated with alluvial fan
sequences buried in the plain, and tectonic landforms of
the mountain front (AMOROSI et alii, 1996; BOCCALETTI et
alii, 2004; PICOTTI & PAZZAGLIA, 2008).
In agreement with the geological evidence for active
compression and shortening, borehole breakouts show
Shmax oriented ca. N-S (MARIUCCI et alii, 1999; MONTONE & MARIUCCI, 1999; MONTONE et alii, 2004; PONDRELLI et alii, 2006), while GPS data indicate shortening
at a rate of less than 1 mm/a roughly in the same direction (SERPELLONI et alii, 2005; DEVOTI et alii, 2008). Italian earthquake catalogues show that the southern Po
Plain is characterized by earthquakes having contractional focal mechanisms (CASTELLO et alii, 2006; PON-
et alii, 2006) and that have reached Mw 5.8 in
historical times (CPTI WORKING GROUP, 2004).
According to the Database of Individual Seismogenic
Sources (DISS; DISS WORKING GROUP, 2007; BURRATO et
alii, 2008), the Po Plain is bordered by active thrust fronts
belonging to the Northern Apennines and Southern Alps
chains, represented in DISS as discrete individual potential earthquake sources. In the study area, these sources
are N-verging blind thrusts belonging to the PTF and to
the outermost buried fronts of the Northern Apennines
(fig. 2). Available individual sources are only those so far
interpreted as responsible for the largest historical and
instrumental events. However, DISS hypothesized also
the activity and earthquake potential of the remainder of
the same thrust fronts, listing them in the database as
«Seismogenic Areas» encompassing an unspecified number of individual sources (see BASILI et alii, 2008, for the
meaning of this definition).
DRELLI
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Fig. 2 - Simplified map of the study area, showing the buried Northern Apennines thrust fronts (gray lines), historical and instrumental seismicity from CPTI and CSI catalogues (black squares and small diamonds), drainage anomalies (white circles) from BURRATO et alii (2003),
and Shmin directions (white bars) from MONTONE et alii (2004). Seismogenic sources (yellow rectangles; DISS WORKING GROUP, 2007): 1)
Casalecchio di Reno; 2) Ferrara; 3) Bagnacavallo; 4) Faenza; 5) Crespellano; 6) Loiano. Earthquakes associated with sources: 1) 3 Jan 1505
Bologna (Mw 5.5); 2) 17 Nov 1570 Ferrara (Mw 5.5); 3) 11 Apr 1688 Romagna (Mw 5.9); 4) 4 Apr 1781 Romagna (Mw 5.8); 5) 20 Apr 1929
Bolognese (Mw 5.6); 6) 14 Sep 2003 Monghidoro (Mw 5.3). Earthquakes discussed in the text: A) 22 Oct 1796 Emilia Orientale (Mw 5.6); B) 13
Jan 1909 Bassa Padana (Mw 5.5); C) 3 Mar 1624 Argenta (Mw 5.4); D) 30 Dec 1967 Bassa Padana (Mw 5.4). Cities: BO, Bologna; FO, Forlì;
RA, Ravenna. Gray lines indicate the trace of the sections of fig. 3.
– Mappa semplificata dell’area oggetto dello studio che mostra i fronti sepolti dei thrust Nord-Appenninici (linee grigie), la sismicità storica e
strumentale dai cataloghi CPTI e CSI (quadrati neri), anomalie nel drenaggio (pallini bianchi) da BURRATO et alii (2003), e direzioni di Shmin
(segmenti bianchi) da MONTONE et alii (2004). Sorgenti sismogenetiche (rettangoli gialli; DISS WORKING GROUP, 2007): 1) Casalecchio di Reno;
2) Ferrara; 3) Bagnacavallo; 4) Faenza; 5) Crespellano; 6) Loiano. Terremoti associati alle sorgenti: 1) 3 Gen 1505 Bologna (Mw 5.5); 2) 17 Nov
1570 Ferrara (Mw 5.5); 3) 11 Apr 1688 Romagna (Mw 5.9); 4) 4 Apr 1781 Romagna (Mw 5.8); 5) 20 Apr 1929 Bolognese (Mw 5.6); 6) 14 Set 2003
Monghidoro (Mw 5.3). Terremoti citati nel testo: A) 22 Ott 1796 Emilia Orientale (Mw 5.6); B) 13 Gen 1909 Bassa Padana (Mw 5.5); C) 3 Mar
1624 Argenta (Mw 5.4); D) 30 Dic 1967 Bassa Padana (Mw 5.4). Città: BO, Bologna; FO, Forlì; RA, Ravenna. Le linee grigie indicano la traccia
delle sezioni in fig. 3.
3. THE BOLOGNA-FERRARA CROSS-SECTION
To check whether, where and how tectonic strain is
partitioned within the Ferrara-Romagna arc, and
whether tectonic activity variations along the arc are
responsible for the current structural setting, we considered two regional cross sections. Fig. 3 shows two parallel SW-NE sections crossing the Ferrara structure (see
fig. 2 for location). Section A is the result of an integrated analysis of seismic reflection lines and deep well
logs, while section B, located a few kilometers to the
east, is redrawn after MASSOLI et alii (2006). Both sections show clearly the folds and faults that form the
architecture of the Ferrara structure. In the innermost
part of both sections (SW; left side) the main thrusts
exhibit a complex geometry: section A shows multiple
detachments at different structural levels, while section
B shows mostly deep-seated detachments (group G1 in
fig. 3). A second group of ramp anticlines is seen in the
middle portion of the Ferrara structure and of the sections (group G2, fig. 3). Both sections show a similar
structural style and a main deep-seated thrust with shallower splays, all associated with minor backthrusts. No
faults involve the bottom of the Quaternary deposits,
which appear to seal the brittle structures that were
active up to the end of the Pliocene. All the observed
structural features join each other to form a main and
wider structural arc, as highlighted also by the third
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Fig. 3 - Schematic structural sections across the study area. Section A (Bologna-Ferrara, see trace in fig. 2) has been interpreted and depthconverted from seismic data; section B was redrawn after MASSOLI et alii (2006). Labels: Pls=Pleistocene; uPl=Upper Pliocene; mPl=Middle
Pliocene; lPl=Lower Pliocene; uMe=Upper Messinian; Ol-lMe=Oligocene-Lower Messinian; Ol=Oligocene; Ap-uCr=Aptian-Upper Cretaceous;
mLs-uLs=Middle-Upper Lias; uCa-No=Upper Carnian-Norian; uPe-lCa=Upper Permian-Lower Carboniferous; pCa=pre-Carboniferous.
G1=group 1 of structures; G2=group 2; G3=group 3.
– Sezioni geologiche dell’area di studio. La sezione A (Bologna-Ferrara, vedasi traccia in fig. 2) è stata interpretata e convertita in profondità da
dati di sismica a riflessione; la sezione B è ridisegnata da MASSOLI et alii (2006). Sigle: Pls=Pleistocene; uPl=Pliocene superiore; mPl=Pliocene
medio; lPl=Pliocene inferiore; uMe=Messiniano superiore; Ol-lMe=Oligocene-Messiniano inferiore; Ol=Oligocene; Ap-uCr=Aptiano-Cretaceo superiore; mLs-uLs=Lias medio-superiore; uCa-No=Carnico superiore-Norico; uPe-lCa=Permiano superiore-Carbonifero inferiore; pCa=pre-Carbonifero. G1=gruppo 1 di strutture; G2=gruppo 2; G3=gruppo 3.
group of ramp anticlines (group G3, fig. 3). This group
includes the outermost thrusts, which again show the
same structural style on both sections, with a deeper
thrust and associated secondary structures. In this case,
however, the bottom of the Quaternary sediments is cut
by brittle structures, testifying a progressive transfer of
tectonic activity toward the outermost portions of the arc
through time. A comparison of the structural elevation of
the buried anticlines shows considerable differences
among the different groups of faults and folds: the innermost group (G1) is comparable in sections A and B; in
the middle part of the structure (G2), the shallowest and
more elevated anticlines are seen in section A; the shallowest thrust anticlines of G3, i.e. the outermost thrust
sheets, are seen in section B (fig. 3). These differences in
the elevation of anticlines correspond to differences in
the amount of tectonic displacement; in particular, the
uneven distribution of elevations and displacements
within the same regional structure suggests that the current setting of the arc originated from a complex series
of tectonic events which resulted in partitioning of
strain, both along the arc and through time.
4. ANALOGUE MODELS
As stated earlier, this work is mainly devoted to
detecting and understanding the kinematic evolution of
the Ferrara arc, in particular during the Plio-Quaternary.
Therefore, we carried out a set of analogue models aimed
at analyzing the deformation through a section suitable
for a comparison with our Bologna-Ferrara transect
(fig. 4). The models were prepared with quartz sand and
glass microbead layers in a 30 cm-wide and 80 cm-long
sandbox (at a scale between 1:200,000 and 1:250,000).
We first reproduced the initial natural stratigraphic
succession. In the southern Po Plain, the sedimentary
succession starts with Triassic shallow-water and partly
evaporitic carbonate rocks, and goes on with Jurassic and
Cretaceous carbonate rocks overlain by Middle Eocene
marly limestone. The latter formation is topped by Tertiary and Quaternary sediments that can be roughly subdivided into two principal cycles. The first cycle, mainly
Oligocene-Miocene in age, is formed by deep-water sediments (until Langhian) and turbidite deposits (LanghianTortonian). The second cycle starts with Messinian sediments lying above a major regressive erosional surface
detectable all over the Po Plain, and is formed by siliciclastic sediments and evaporite deposits, mainly at the
edges of the basin. Sedimentation of pelite and sand
deposits prevailed during the Pliocene and Pleistocene
(DONDI et alii, 1982; DONDI & D’ANDREA, 1987).
We reproduced this succession using mainly quartz
sand. The basal detachment below this simplified succession was created by introducing a layer of glass
microbeads; the elevation of this layer from the bottom of
the sandbox was changed in each experiment to allow the
lowermost thrust to detach at different depths within the
corresponding Paleozoic basement. A further décollement,
corresponding to Oligocene marls with low shear-strength,
was modeled by another layer of microbeads located
2.5 cm above the basal detachment. Finally, to account for
the dip of the foreland monocline during deformation
(MARIOTTI & DOGLIONI, 2000), the sandbox baseplate was
tilted of 5° towards the mobile backstop.
The restoration of the geological cross sections of fig. 3
indicates that a number of tectonic, sedimentary and
erosional events followed one another through time, as
suggested by the age of the resulting tectonic structures.
A set of sandbox experiments can hardly model such a
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Fig. 4 - Comparison between section A (from fig. 3) and one of the internal sections of the analogue models carried out in this work, showing
similarities between the main faults of the intermediate and outermost structures (thick black lines). Model sections can be found in the
lower part of the picture (not to scale), which shows the undeformed stage and the stage following the deposition of syn-tectonic sediments.
The synthetic stratigraphy adopted in the models is sketched on the right. Sh=shortening in centimeters.
– Confronto tra la sezione A (da fig. 3) ed una delle sezioni interne dei modelli analogici realizzati per questo studio, che mostra similitudini tra le
faglie principali delle strutture intermedie e più esterne (linee nere). Le sezioni del modello sono visibili nella parte inferiore della figura (non in
scala) e mostrano lo stato indeformato e lo stadio di deformazione seguente alla deposizione dei sedimenti sintettonici. La stratigrafia sintetica
adottata per i modelli è schematizzata sulla destra. Sh=raccorciamento in centimetri.
complex geological setting. Therefore, we tried to reproduce only a few major structures in order to highlight the
evolution of the most recent tectonic events that affect
Plio-Pleistocene sediments. After the emplacement of the
tectonic wedge (innermost part of the experiment) and
the formation of the first thrusts affecting the foreland
(roughly corresponding with the Late Miocene) we intro-
duced a single depositional event. This event represents
the sedimentation of the Upper Messinian, Pliocene and
Pleistocene foredeep deposits, in accordance with evidence from the geological cross sections. It was reproduced by overlying a 1.5 cm-thick layer of quartz sand at
the front of the outermost thrusts, after 23 cm of shortening. We went on shortening the models by moving the
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Fig. 5 - Seismogenic sources associated with the largest earthquakes of the study area (DISS WORKING GROUP, 2007) interpreted in the light
of the results obtained in the present study.
– Sorgenti simogenetiche associate ai maggiori terremoti dell’area di studio (gruppo di lavoro DISS, 2007) interpretati alla luce dei risultati
emersi in questo lavoro.
sliding backstop up to the end of the experiment (i.e. until
the sand started to fall outside the unconfined wall of the
sandbox). This occurred after 45 cm of backstop shifting;
the final shortening hence exceeds 50%.
The sandbox was provided with a lateral glass wall.
This allowed the advancing of deformation through time
to be fully appreciated over the entire depth of the
models. It also allowed pictures of sections of the
models to be taken while moving the sliding backstop.
In the meanwhile, pictures of the model’s surface were
taken as well.
The comparison between the regional cross sections
and the analogue models shows interesting similarities in
terms of structural style. For instance, the number of the
main structures shown in the model of fig. 4 is the same
as observed in the geological section. The innermost
structure corresponds to the PTF, whereas the intermediate and outermost ones correspond to the fronts buried
under the Po Plain sediments. The intermediate structure
shows deep-seated thrust faults with associated backthrusts, whereas the outermost structure is formed by a
deep thrust with a forward splay detached at a shallower
level. In the initial deformation stages, the model’s kinematics shows that thrusts begin and develop in-sequence.
After sedimentation in the foredeep and due to the resulting extra-load, however, a strong reactivation of the
innermost structure takes place and the later thrusts
affecting the overlying sediments show an increase in
wavelength. Without syn-tectonic sedimentation, the
activity of the innermost thrust of the model would have
been limited in time and followed by the inception of a
new thrust ahead of it, in the undeformed parts of the
sandbox. This circumstance was verified by carrying out
a reference experiment with no syntectonic sedimentation; after 23-24 cm shortening a new thrust formed in
the outer parts of the model and the previously active
thrust stopped. The sediments added at this stage of
deformation effectively extended the duration of activity
of the innermost thrust.
We conclude that structural style and evolution of the
models are strongly influenced by syntectonic sedimentation. The evolving pattern of our models also reveals that
thrust activity can skip backward from the outermost
fronts, buried beneath the Po Plain, to the inner ones
(that can be compared to the PTF, currently the most
active structure in the study area), and that it can be partitioned among them depending on the occurrence, number and size of sedimentary events through time.
5. DISCUSSION AND CONCLUSIONS
Recent and historical seismicity, drainage anomalies
and syntectonic Plio-Quaternary growth strata highlight
the ongoing tectonic activity of the Northern Apennines
thrust fronts. Significant seismicity can be correlated
both with the outer buried fronts and with the inner PTF
(figs. 2 and 5).
The comparison between sections A and B (fig. 3)
shows that both the displacement and structural elevation
of the different groups of thrusts are not constant along
strike, i.e. there is a strain partitioning along the fronts
and inside the whole Ferrara structure. The cumulative
long-term displacement of individual thrusts seems to
match the extent of seismic release. In other words, the
largest historical earthquakes appear to have been generated by those individual thrusts or group of structures
that are characterized by the highest displacement values
(fig. 5).
In fig. 5 we projected the geometry of the seismogenic
sources associated with the largest earthquakes of the
study area according to the DISS database (DISS WORKING GROUP, 2007) and suggested a possible interpretation
of the main faults in the light of the results obtained in
the present study. Some of the proposed individual seismogenic sources segment the PTF to the SSW of our
sections (sources 1, 4 and 5 in fig. 2). Moving outwards,
the source of the 11 April 1688, Bagnacavallo earthquake
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G. TOSCANI ET ALII
(Mw 5.9) can be ascribed to group G1, that shows a comparable structural elevation in both sections. No earthquakes
correspond to any of the thrusts of this group in the zone
represented in section A, probably due to their shallower
décollement (whereas in section B thrusts are deepseated). The 3 March 1624, Mw 5.4, 22 October 1796,
Mw 5.6, 13 January 1909, Mw 5.5, and 30 December 1967,
Mw 5.4 earthquakes can be tentatively associated with
group G2 (A, B, C and D, respectively, in fig. 2). The longterm displacement of this group of structures decreases
eastward, suggesting that there may be little or no seismogenic potential at all in coincidence with section B. If
this were the case, the sources of the 1624 and 1967
earthquakes should be located along this group of structures somewhere west of section B. Group G2 defines a
minor arc within the larger Ferrara structure and the epicenters of the earthquakes discussed above concentrate in
its central part, where the thrusts presumably attained
the largest displacement (fig. 2). Finally, we ascribe the
source of the 17 November 1570, Mw 5.5 Ferrara earthquake to the outermost group of structures (G3), as
shown in section A. Section B indicates that, further east,
group G3 exhibits a larger long-term displacement, but
the lack of historical and instrumental earthquakes precludes any inference on the seismogenic potential of this
part of the thrusts.
ACKNOWLEDGMENTS
This research has benefited from funding provided by the Italian Presidenza del Consiglio dei Ministri - Dipartimento della Protezione Civile (DPC) and by PRIN 2005 grant (University of Pavia).
Scientifics papers funded by DPC do not represent its official opinion and policies. The two referees, Adriano Zanferrari and Sergio
Rogledi, are kindly acknowledged for their suggestions and advice
that greatly improved the first version of the manuscript. The cross
sections, both geological and coming from analogue models, have
been treated with 2DMove© of Midland Valley Exploration Ltd.
(Glasgow, UK).
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Received 31 March 2008; revised version accepted 22 October 2008.