The complexity of shallow levels tectonics during continental collision: evidence from the boundary between Maritime and Ligurian Alps

The boundary between Maritime and Ligurian Alps, southwestern Alps, is classically regarded as formed by a pile of continental- (Briançonnais Units) and oceanic-derived tectonic units (Moglio-Testico Unit and Helminthoid Flysch) stacked toward southwest. The continental units represent the thinned portion of the Europe continental margin, whereas those oceanic are considered as the remnants of the Ligure Piemontese ocean; the latter interposed between the Europe and Adria Plate in the Middle-Late Jurassic. Starting from Late Cretaceous, the convergence between Europe and Adria Plate progressively led to the consumption of the Ligure Piemontese ocean, until to the involvement of the Europe continental margin into the alpine subduction zone during the Eocene time. However, the tectonic units cropping out in this sector of the Alps, are thought to record their deformation patterns under different Pressure (P) and Temperature (T) conditions related to shallow crustal levels into the Alpine wedge without evidence of deeper-related processes, i.e., subduction-related deformation and/or metamorphism. In this Thesis, the tectonic units cropping out at the boundary between Maritime and Ligurian Alps were investigated in order to provide a new stratigraphic, structural and metamorphic dataset needed to better constrain their tectonic history. All of the tectonic units are characterized in terms of stratigraphy, structural evolution and metamorphic imprint following a multidisciplinary approach including: fieldwork, stratigraphic analysis, provenance, structural analysis and P–T estimation. The collected datasets indicate that, in this sector of the southwestern Alps the tectonic pile is composed by, from the top to the bottom: the Marguareis Unit (Briançonnais Domain); the Moglio-testico Unit (Ligure Piemontese ocean sedimentary cover); the Helminthoid Flsych Unit (Ligurian Domain, ocean-continent transition zone); several continent-derived tectonic slices (Cima del Becco and Chambeuil Slices) with Briançonnais affinity; and the Cabanaira Unit (Briançonnais Domain). The Briançonnais Units succession is coherent with the sedimentary evolution of the Europe continental margin approaching toward the subduction zone as proposed by previous authors; The Moglio-Testico Unit succession shows a transition from pelagic to basin plain deposits reflecting the sedimentary evolution of a portion of the Ligure Piemontese ocean approaching toward the subduction zone; The Helminthoid Flysch Unit sedimentary evolution is coherent with that proposed for the External Ligurian Units cropping out in the Northern Apennine. Structural analysis outlined that each unit recorded a polyphase deformation history dealing with the superposition of pre- syn- and post-coupling tectonic events developed under different P and T conditions. The pre-Oligocene 5 pre-coupling structures are represented by superposed folding systems, associated with metamorphic re-crystallization, and confined to each unit. The syn-coupling structures are represented by thrusting and folding cutting the previous structures and developed at shallower crustal levels. These structures are considered as responsible for the coupling of the units and thought to be developed during the late Eocene-early Oligocene. The last deformation history is shared by all of the units and it is represented by flat-lying open folding system with no evidence of metamorphic re-crystallization and high-angles normal to transtensive faulting system which is not able to profoundly modify the previous architecture of the tectonic pile. Thermobarometric estimation indicates that the tectonic units reached their metamorphic peak conditions spanning from anchizone/epizone to blueschists field, during the pre-coupling events (i.e., before their juxtaposition), and, subsequently, they have been exhumed following different P–T paths into the Alpine wedge. The exhumation up to the shallower structural levels is represented by the syn- and post-coupling structures. The whole datasets outlined that the syn-coupling thrusting event played a first order role in the tectonic evolution of this sector of the southwestern Alps.