How the geo-boundary conditions control the continental crust's exhumation: three examples from the Lower Units (Alpine Corsica, France)

The Lower Units (Alpine Corsica, France) are a stack of tectonic units that represents the portion of the European plate margin involved in the continental subduction since the Late Eocene time (Molli, 2008). 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 for the P-T values related to the baric and thermic peaks. The comparison between several sectors indicates that these variations are common in all the areas of the Alpine Corsica. These variations seems to be connected with the tectonic position in the stack of the Lower Units, i.e. if the Lower Units are in contact with the Variscan Corsica (i.e. the European plate not involved in the Alpine orogeny), with the Schistes Lustrés Complex (i.e. the oceanic and continental units buried at the deepest level during the Alpine orogeny) or with the Upper Units (i.e. the tectonic units belonging to the Alpine Corsica affected by a weak deformation and metamorphism). The structural analysis on the units surrounding the Lower Units becomes therefore crucial, in order to understand how and how much the Variscan Corsica and the Schistes Lustrés Complex interacted with the Lower Units during their subduction and subsequent exhumation. Following this goal, we have studied three areas in which the Lower Units interplay with the Variscan Corsica, the Schistes Lustrés and the Upper Units, in order to provide the structural and tectono-metamorphic data needed to constrain the behaviour of the continental crust in three different geological settings. The Cima Pedani area, where three units of the Lower Units group occur in a tectonic window under the Schistes Lustrés Complex and the Upper Units, provides the opportunity to study the relationships between the different units of the Alpine Corsica. Also in the Corte area three tectonic units belonging to the Lower Units have been recognized. Among them, the westernmost one directly overthrust the Variscan Corsica whereas the other Lower Units are characterized along their boundaries by thin slices of the Schistes Lustrés Complex (Di Rosa et al., 2017). In the Ghisoni area, the Lower Units are instead represented by a single unit that is interposed between the Variscan Corsica and the Schistes Lustrés Complex