Tectono-metamorphic evolution of the Continental Units at the boundary between the Alpine and the Hercynian Corsica: constraints for the exhumation models in the continental collision setting

The Lower Units of Alpine Corsica (France) include a stack of tectonic units detached from the European plate margin. These units, that crop out in the central Corsica along a narrow strip at the boundary between the Hercynian Corsica (to the west) and the Alpine Corsica (to the east), are directly thrust on the Hercynian Corsica (i.e. the European plate not involved in the Alpine Orogeny) and topped by the Schistes Lustrés Complex (i.e. the oceanic and continental units buried at the deepest level during the Alpine Orogeny). Slices of the Schistes Lustrés Complex are also found within the Lower Units. In this thesis, the Lower Units cropping out in six selected areas, from Asco and Fium’Orbo valleys, have been studied, in order to provide stratigraphic, structural and metamorphic data needed to constrain their tectonic history. The Hercynian Corsica and the Schistes Lustrés Complex strictly associated to the Lower Units have been also characterized in terms of stratigraphy, structural and metamorphic features. Moreover, some geochronological data related to the Lower Units have been provided, in order to constrain the events that affected these units. The collected data indicate that the stratigraphic log of the Lower Units consists of a Paleozoic basement intruded by Permo-Carboniferous metagranitoids associated to a Permian to Middle - Late Eocene metasedimentary covers. All the formations of this succession are affected by a polyphase deformation history (D1-D3 phase) and the associated multi-step metamorphism as highlighted by the 3 generations of phyllosilicates grown along the S1 and S2 foliations. The study of the metamorphism associated the mesoand micro-structures has indicated that the Lower Units underwent to HP/LT metamorphic conditions (early D1 phase) followed by a polyphase retrograde metamorphism (late D1-D2-D3). The deformation and metamorphic history of the Lower Units is coherent with their subduction and rapid exhumation in the continental collision setting during the Late Eocene–Oligocene time span. The tectono-metamorphic study of several of these units pointed out a clear linkage between their positions in the tectonic stack and the P-T conditions reached during their subduction/exhumation paths. Although the deformation history shows common features in all the Lower Units, several differences in their metamorphism have been observed, in particular concerning the P-T values related to the geobaric and geothermic peaks. The comparison between several areas indicates that these variations can be found systematically in the Alpine Corsica, even if the maximum depth reached by the Lower Units never exceed ~50 km. Investigating the last stages of the exhumation of the Lower Units emerges that the switching from convergence to extension that affected the Western Mediterranean Sea during the Oligocene played an important role in terms of mechanics, leading to the collapse of the orogenic wedge (i.e. the last ductile event) and the activation of a strike-slip fault system (i.e. the Central Corsica Shear Zone). Extending the tectono-metamorphic study to the rim of the Hercynian Corsica at the boundary with the Alpine Corsica and to the slices of the Schistes Lustrés Complex sandwiched between the Lower Units, it made possible to describe the main tectono-metamorphic events that affected the Lower Units as (1) the subduction at depth of the continental crust, (2) its exhumation in a still convergent setting and 3) its further exhumation in an extensional setting.