Early Collisional Evolution of the Western Alps: how Important are Rift Inheritance and Paleogeography

The Western Alps, along the French-Italian border, are among the best investigated and imaged collisional belts worldwide. A major complexity of the Western Alps is their non-cylindricity and arcuate shape, as well as the occurrence of ultrahigh-pressure (UHP) metamorphic rocks. Our study shows that all these complexities are intimately linked to the interplay between the inherited rift architecture, the changing kinematics of convergence during the early stages of continental collision, and the complex 3D dynamics of the Alpine subduction system. Here we use a multi-disciplinary approach to investigate the evolution of the European/Brianonnais distal margin at the transition from subduction to early collision, which corresponds to the moment when rift inheritance and the paleogeographic configuration are the most important in controlling the orogenic structure and evolution. In a first part, we reassess the architecture of the Western Alps based on a review of field and recent geophysical studies. This allows us to define the crustal architecture as well as the along and across strike position of the different Alpine units. The use of diagnostic petrologic, stratigraphic, and structural criteria allows us to identify the rift domains of the former European/Brianonnais margin, from which the different present-day orogenic units originated. This enables us to propose a first order, synthetic rifted margin template for the Western Alps. Of particular importance is the location of the necking zone, corresponding to the limit between the thick-crusted proximal and the thin-crusted distal margin. It also separates domains with different rheology and density/buoyancy/floatability, both of which control the subduction, exhumation and accretion behavior during subduction and early collision. We find that all units containing ultrahigh-pressure rocks derive only from the thin-crusted distal hyperextended domain. In a second part, we revisit the paleogeography of the Alpine Tethys using a global kinematic restoration software (Gplates) and the new building block/rift domain concept that allows us to propose a tight fit restoration and evolution of the Atlantic Tethys junction during the Mesozoic. In this restoration, the Brianconnais corresponds to a ribbon of slightly thinned continental crust that limits, along necking zones, two overstepping en-echelon rift basins, namely the Valais domain to the northwest and the Piemonte domain to the southeast. We affirm that this uneven-margin architecture can explain most of the Western Alps complexity. In our kinematic model, convergence between Adria and Europe was mainly accommodated by strike-slip movements until the late Eocene, which corresponds to the time of formation and exhumation of UHP metamorphic rocks. Early collision was diachronous along the margin and resulted first in the reactivation of the necking zone separating the Brianconnais and Prepiemonte domains. This fundamental structure, which we name the Prepiemonte Basal Thrust, floors the units preserving ultrahigh-pressure rocks. Once the distal margin was accreted, shortening mainly stepped inboard into the European necking domain, resulting in theformation of the Penninic Basal Thrust.