CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-1 ANATOMY OF A COMPOSITE MOUNTAIN BELT Batholiths Back Arc Metamorphic nappes Ophiolite Allochthon Crustal accretionary prism Foreland Miogeocline Parautochthon Foredeep Autochthon Hinterland CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS The description of the internal structure of mountain belt consists in the characterization of the different structural units that compose the orogen, their limits and the history of their relatif displacements. Relatif displacements result from mechanical instabilities. Four types of mechanical instabilities can be considered : (1) thrust faults, (2) strike slip faults, (3) anatectic domes, (4) extensional detachments and decollements. Granitic plutons Remiremont Normal fault Brézouard Granite Crest Granite Bilstein Orthogneiss Gneiss Permo-triassic Bassin 10 km Lalaye-Lubine strike slip fault NORD CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS VII-2-1 NAPPES AND THRUSTS The superimposition of nappes can be described following fou r elementary models characterized by a particular strain field: * Rigid translation (no internal deformation) Uppermost part of the crust or foreland of mountain belts * Ductile gliding Ductile deformation localized along the fault zone l 1 l 2 trajectories * Gravitational collapse The base of the pile increases whereas its thickness decreases l 1 l 2 trajectories * Compression The strain increases from the front of the pile toward the back l 1 l 2 trajectories CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS VII-2-2 STRIKE SLIP FAULTS Thickness Displacement 10 km -> 500 to 1000 km Discontinuous c Depth Heterogeneous s Continuous l 3l 2 l 1l 2 CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS VII-2-3 DIAPIRIC DOMES AND FORCED INTRUSIONS Partially melted (anatexite and migmatite) or completely mel ted rocks (magma) have the tendency to move upward. Diapir model The density contrast that exists between the melted rocks an d the surrounding rocks is the driving force responsible for the upward displa cement. At depth the viscosity of the surrounding rock is low enough to allow the displacement of the diapir. Forced intrusion At shallow depth the viscosity of the surrounding rock is hi gher and the diapir is stopped. To continue its upward displacement the m agam has to be forced to flow into dikes. Forced intrusion Brittle Ductile Diapir Dr gh Mafic intrusion CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS VII-2-3 DIAPIRIC DOMES AND FORCED INTRUSIONS Diapir 90 Strike l 1 l 2 Dip l 1 l 2 0 0 90 0 90 90 Pitch l 1 0 Strike l 1 l 2 l 1l 2 trajectories thrust thrust Strain Regime Forced intrusion Lateral and vertical expansion CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS VII-2-3 DIAPIRIC DOMES AND FORCED INTRUSIONS Forced intrusion => contact metamorphism Lateral and vertical expansion is responsible for the strain field l 1l 2 trajectories: In cross section and in map view CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS VII-2-4 EXTENSIONAL SYSTEMS 30 km Intra-continental rift 100 km Passive margin 15 km Oceanic rift 200 Post-orogenic crustal thinning to 1000 km CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS VII-2-4 EXTENSIONAL SYSTEMS Continuous Heterogeneous Heterogeneous with detachments merging with a decollement Heterogeneous with detachment dominant Discontinuous Discontinuous with volume change CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS VII-2-4 EXTENSIONAL SYSTEMS l 1l 2 trajectories CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-2 MAJOR STRUCTURES OF MOUNTAIN BELTS VII-2-4 EXTENSIONAL SYSTEMS Development of a Metamorphic Core Complex a P 0 brittle crust 20 km (ca. 350°C) ductile crust 60 km half graben 0 block rotation 20 50 km 0 20 40 km gneiss - migmatite dome 0 20 30 km CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-3 STRAIN FIELD INTERFERENCES VII-3-1 DECOLLEMENT + DETACHMENT Décollement Weak layer Competent layer Detachment CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-3 STRAIN FIELD INTERFERENCES VII-3-2 INTRUSIONS + COAXIAL DEFORMATION A B A B CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-3 STRAIN FIELD INTERFERENCES VII-3-3 INTRUSIONS + NON-COAXIAL DEFORMATION Homogeneous Heterogeneous CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-3 STRAIN FIELD INTERFERENCES VII-3-4 EXTENSION-DIAPIR Structural sketch map of the extension system in the eastern French Massif Central a A N AM BM GM MC b 300 km b Lyon B N B' VF 5 km Stephanian basin B Anatectic dome B' km 0 20 40 Lithospheric mantle b CHAPTER VII : LARGE SCALE STRUCTURES IN OROGENS VII-3 STRAIN FIELD INTERFERENCES VII-3-5 EXTENSION-DIAPIR-STRIKE SLIP FAULTS-NAPPES Structural sketch map of the extension system in the southern French Massif Central Permian sediments Coal-bearing Stephanian Epizonal Anchizonal A 500 km Saint-Affrique basin Monts de Lacaune thrusts B Lodève basin Albigeois nappes M on ta e gn N re oi Gne i ss Do me So uth Axial Zone ern thr u B' ol df n a st elt db N 20 km Southern Montagne Noire B N B' Stephanian Basin micaschistes Metamorphic Core granite gnessic and migmatitic rocks S Recumbent Fold Nappe Paleozoic sediments 5 km
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