Abstract
The internal zones of the Western Alps arc are derived from an oceanic and continental subduction wedge developed beneath the Adria plate during its paleogene northward drift. Exhumation of the internal zones proceded from early Oligocene onwards due to westward extrusion of the Adria plate. The prominent fold-and-thrust structures which follow the arc shape, either forward or backward verging, postdate the initial nappe stacking and overprint differently oriented older deformations which are relevant to proper retoration of this arcuate orogen to minimise overlap problems. We document this early stacking phase through outcrop-scale structural analysis at 55 sites between the Maurienne and Ubaye valleys, along with larger-scale examples of early structures. They consistently show an initial N-to NW tectonic transport, whose kinematic indicators are overprinted by either forward (W-to SW-directed) or backward (E-to NE-directed) deformation associated with post-nappe transport along the Penninic thrust. Accordingly, restoring the Briançonnais fold/thrust system must incorporate reconstruction of the nappe stack along the initial top N-NW direction of orogenic propagation, with careful consideration of their paleogeographic origin towards the S-SE. This stack was built during the Eocene Adria-Iberia collision, and overthrust the Subbriançonnais-Valaisan trough to the NW before involving the Dauphiné-Helvetic foreland. It includes different types of Paleozoic units, either Permo-Carboniferous sediments towards its base, or polymetamorphic basement above, which can be explained by inversion of a late Variscan basin and of its southern shoulder, whereas the uppermost Prepiedmont units result from inversion of the Tethyan margin toe. Mixed breccia, locally preserved close to the tectonic contact between the latter units and the overlying "Schistes Lustrés" oceanic nappes, are interpreted as olistostromes fed by both units in a very early collision stage. 39 Ar/ 40 Ar dating suggests that these shallow tectono-sedimentary formations were involved in the subduction wedge during the early Eocene, whereas younger (late Eocene) equivalent olistostromes mark the propagation of the Briançonnais stack over the external (Dauphiné/Helvetic) foreland. The Eocene orogenic wedge was rapidly exhumed during Oligocene westward indentation and radial spreading, in a markedly different tectonic context driven by extrusion around an Adriatic upper mantle indenter, which controlled development of the Western Alps arc in relation with the Ligurian sea opening.