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.

Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

“…This exhumation is recorded within the lower Oligocene molasse basins on both sides of the renewed relief, sourced from high-pressure metamorphic rocks and oceanic mélanges of various metamorphic grades. The curvature of the arc was progressively acquired, producing radial spreading of transport lineations (Choukroune et al, 1986;Platt et al, 1989b;Lickorish et al, 2002) and southward increasing counter-clockwise rotations of internal units (Collombet et al, 2002). The southern part of the arc now cross-cuts perpendicularly the palaeo-accretionary wedge.…”

Fold interference pattern at the top of basement domes and apparent vertical extrusion of HP rocks (Ambin and South Vanoise massifs, Western Alps)

“…The lower part of this slice is unknown. (ii) A medium slice in which are mixed, from west to east, basement units, Triassic cover units and Liassic units belonging to the 'Dent Parrachée nappe' (Platt et al, 1989). This latter consists of Norian to Eocene sediments (Deville, 1987).…”

“…The 'La Valette' shear zone corresponds to the 'thrust' (Platt et al, 1989) of La Pointe de l'Echelle-Rateau d'Aussois (upper slice exclusively constituted by conclomeratic Permian) at the top of the medium slice. This 'thrust' is accommodated by a thick level (1w3 m) of chlorito-albitic schists (corresponding probably to mylonites of basement) that can be followed from the Col de la Masse in the south, to the Col de la Valette-Col du Tambour (Fig.…”

“…The orogen is commonly regarded as a classic collisional belt representing the closure of one or more ocean basins from the Late Cretaceous to Late Eocene (w90-35 Ma) and the subsequent progressive indentation of Adria into Europe (e.g. Platt et al, 1989). The internal part of the orogen (the 'Penninic Alps') consists of a series of continental terranes (the Austroalpine-Sesia zone, the Briançonnais zone and the Internal Massifs), which are aligned as arcuate ribbons parallel to the strike of the orogen (Fig.…”

“…In a more general way, the dilemma between supporters of vertical or horizontal related exhumation models result for a large part from the difficulty to infer the geometry of the deep-seated structures at the time of their formation. Structural and microstructural characteristics of the exhumation of the Briançonnais, Piemont and Internal Massifs high pressure rocks, during a complex deformation event globally called D 1 , are still disputed (Caron, 1974(Caron, , 1977Platt et al, 1989;Philippot, 1990;Agard et al, 2001). The kinematic of early deformation in these high pressure domains is poorly known and previous observations are often contradictory (Choukroune et al (1986) or Platt et al (1989) for a different interpretation), due to the post-D 1 intense transposition features (Lefèvre and Michard, 1976;Philippot, 1990;Henry et al, 1993;Rolland et al, 2000;Agard et al, 2001;Wheeler et al, 2001;Reddy et al, 2003).…”