Structural architecture and kinematics of the Helminthoid Flesh-Briançonnais Units coupling: A key for deciphering the tectonic evolution of the southwestern Alps

The southwestern Alps are characterized by a stack of tectonic units stemming from different paleogeographic domains (Ligure-Piemontese Ocean and European continental margin, Vanossi et al, 1986). This stack consists of the Helminthoid Flysch Unit thrusted onto the oceanic-derived Moglio-Testico Unit and the European-derived Briançonnais Units. The Late Cretaceous non-metamorphic Helminthoid Flysch Unit is detached from its original basement whose nature is debated; the Moglio-Testico Unit represent a fragment of the Ligure-Piemontese oceanic crust implicated in the oceanic subduction; whereas the Briançonnais Units are the witnesses of European continental crust progressively involved a convergent setting starting from middle Eocene age (i.e. Vanossi, 1986). Contrary, at the boundary between Maritime and Ligurian Alps the tectonic stack is characterized by apparent inverted structural relationships, i.e. with the Helminthoid Flysch located in between the Briançonnais Units. Field mapping and micro- to map-scale structural analysis outlined that each unit, and the slices also, recorded different finite strain patterns, mostly dealing with the superpositions of folding systems, developed before their coupling and at different structural levels and time-span (Sanità et al., 2020). The continental tectonic slices show similar deformation patterns to these documented in the Marguareis Unit. After their coupling the units shared the same deformation events. The boundaries between each unit and/or slices are marked by thrust surfaces represented by top-to SW high-strain cataclastic shear zones responsible of their coupling. The structural analysis allowed us to reconstruct a late Eocene-early Oligocene tectonic coupling history during which, firstly the Helminthoid Flysch thrusted onto the already exhumed Briançonnais Units and then the Marguareis Unit overthrust, with an out-of-sequence thrust, onto the Helminthoid Flysch and the lowermost Briançonnais Units, i.e. Cabanaira Unit and the continental tectonic slices. This out-of-sequence thrust is a first-order tectonic feature extending for about 70 km2 and it is responsible of the inverted structural relationships between the Helminthoid Flysch and Briançonnais Units and it played a key-role in the tectonic evolution of this sector of the southwestern Alps.