The pre-­Alpine tectonic history of the Austroalpine continental basement in the Valpelline unit (Western Italian Alps)

The Valpelline unit is a large slice of continental crust constituting the Austroalpine Dent Blanche nappe (NW Italy). The pre-Alpine evolution of this unit holds important clues about the Palaeozoic crustal structure at the northern margin of the Adria continent, about the history of rifting in the Alpine region, and thus about the thermomechanical conditions that preceded the Alpine convergent evolution. Several stages of the deformation history and of partial re-equilibration were identified, combining meso-and micro-structural analyses with thermobarometry. Reconstructed pre-Alpine P-T-t-d paths demonstrate that the Valpelline unit experienced an early stage at pressures between 4.5 and 6.5 kbar followed by migmatite formation. A subsequent stage reached amphibolite to granulite facies conditions. This stage was associated with the development of the most penetrative fabrics affecting all of the Valpelline lithotypes. The pre-Alpine evolution ended with a weak deformation associated with a local mineral-chemical re-equilibration under greenschist facies conditions at approximate to 4 kbar and T < 450 degrees C. A Permo-Mesozoic lithospheric extension is thought to be responsible for asthenosphere upwelling, thereby causing high temperature metamorphism at medium pressure and widespread partial melting, which led to upper crustal magmatic activity.