Thermomechanical evolution of the high-grade core in the nappe zone of Variscan Sardinia, Italy: The role of shear deformation and granite emplacement

In the nappe zone of the Sardinian Variscan chain, the deformation and metamorphic grade increase throughout the tectonic nappe stack from lower greenschist to upper amphibolite facies conditions in the deepest nappe, the Monte Grighini Unit. A synthesis of petrological, structural and radiometric data is presented that allows us to constrain the thermal and mechanical evolution of this unit. Carboniferous subduction under a low geothermal gradient (~490-570 °C GPa-1) was followed by exhumation accompanied by heating and Late Carboniferous magma emplacement at a high apparent geothermal gradient (~1200-1450 °C GPa-1). Exhumation coeval with nappe stacking was closely followed by activity on a ductile strike-slip shear zone that accommodated magma intrusion and enabled the final exhumation of the Monte Grighini Unit to upper crustal levels. The reconstructed thermo-mechanical evolution allows a more complete understanding of the Variscan orogenic wedge in central Sardinia. As a result we are able to confirm a diachronous evolution of metamorphic and tectonic events from the inner axial zone to the outer nappe zone, with the Late Variscan low-P/high-T metamorphism and crustal anatexis as a common feature across the Sardinian portion of the Variscan orogen.