Thrusting and backthrusting in the Briançonnais domain of the western Alps

Abstract

Summary The Briançonnais domain comprises the deformed remnants of a semi-independent microcontinental strip that lay close to the European margin of the Mesozoic Tethys. A Triassic carbonate platform of south-Alpine type was broken up by extension in Early to Middle Jurassic time, producing a syn-rift sequence characterized by small basins with marginal breccia wedges, internal unconformities, and local terrestrial sediments. The pre-Triassic basement was probably reduced to about half its original thickness during this extensional phase. Late Jurassic and Cretaceous subsidence was accompanied by pelagic sedimentation. Flysch sedimentation began shortly before the onset of compressional tectonics in late Palaeocene or Eocene time. Underthrusting and accretion of the Briançonnais domain to the developing Alpine thrust wedge caused an early phase of thrusting, producing antiformal stacks of kilometrescale thrust sheets in parts of the external Briançonnais. The internal margin of the domain was underplated at depths of up to 40 km, and was affected by high-P low-T metamorphism and regional ductile deformation. Stretching lineations and slip-vectors related to this stage trend NW in the northern French Alps, but swing to WSW in the S. Backthrusting in Oligocene time was probably associated with the start of collision with the European margin proper. Families of back thrusts and associated recumbent folds cut, deform or reactivate older structures. The members of each family appear to have been activated in sequence towards the internal margin of the Alps, forming large-scale duplexes, the floor and roof thrusts of which are in part reactivated early thrusts. The trend of stretching lineations and slip-vectors in the Briançonnais domain vary in both space (NW in the N, SW in the S), and time (later trends lie consistently anticlockwise of early ones). These trends and their changes are incompatible with calculated Africa/Europe plate motions for the Tertiary. Independent NW to WNW motion of Adria relative to Europe is probably required. Anticlockwise rotation and bending of the internal Alps during collision could explain the radial pattern of the early vectors. The radial pattern of the later backthrusting matches the radial pattern of coeval forethrusting in the external zones, however; this is therefore unlikely to be a result of later rotations, and may reflect the dominance of gravitational body forces during the later stages of mountain-building.