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3D Modeling and Kinematics of the External Zone of the French Western Alps (Belledonne and Grand Châtelard Massifs, Maurienne Valley, Savoie)

2006; Swiss Geological Society; Volume: 99; Issue: 2 Linguagem: Inglês

10.1007/s00015-006-1183-z

1420-9128

Didier Marquer, Philippe Calcagno, J C Barfety, Thierry Baudin,

Geological and Geochemical Analysis

Three dimensional geometric modeling was undertaken on the external crystalline massifs (Belledonne and Grand Châtelard) of the Western Alps. This external zone corresponds to the internal part of the European passive margin developed during Liassic rifting. It was thrust towards the north-west during Oligocene-Miocene compression resulting in tectonic inversion of the European continental margin. The contact between basement and cover rocks was modeled in three dimensions using 30 cross-sections oriented NW-SE and NE-SW, together with geological outcrops located onto the Digital Elevation Model of the area. Three interfaces in the model correspond to the main geological features: (i) the unconformity between the Triassic series and the metamorphic Palaeozoic basement (generally assumed to have been horizontal at the beginning of the Alpine cycle), (ii) ENE-WSW extensional faults associated with the main stage of passive margin development, and (iii) steep NNE-SSW trending faults that developed during the main compressive stage of Alpine collision. The Miocene deformation is very heterogeneous and results in networks of anastomosing brittle-ductile shear zones. This deformation corresponds to a NW-SE compression associated with an important vertical stretching in the basement massifs. The eastern border of the Belledonne massif is locally backthrust towards the south-east and shows a bulk dextral shear component. 3D geometric modeling revealed the overall anastomosing shape of the main fault zones, which undulate around vertically stretched crustal-scale basement boudins. Former structures, such as Liassic extensional faults, are preserved within low-strain domains located in the core of these boudins. At lower scale, the 3D model pointed out characteristic locations where the investigated shear zone patterns reveal the bulk Miocene deformation.