Abstract

This paper addresses the contribution of extension in the upper crust during syncollisional exhumation of subducted material. We present structural data based on the analysis of ductile, brittle-ductile, and brittle deformation features in metapelitic schists and meta-basalts from the Schistes lustrés nappe stack in the Piedmont zone of the western Alps. To link the structures with synkinematic pressure and temperature estimates, geothermobarometry was performed using fluid inclusion investigation in quartz together with temperature estimations based on metamorphic chlorite composition. Samples used for geothermobarometry were collected taking into account the structural framework within veins associated with extensional structures observed at the regional scale. The early generation of veins associated with boudinage occurred at 415–345 °C and 6–2.5 kbar, and subsequent incipient normal faulting occurred at temperatures of 315–235 °C and pressures <1.7 kbar. The transition from ductile to brittle extension occurred between ca. 27 Ma and ca. 10 Ma. These results indicate a continuous extension in the upper crust during exhumation and continuous deformation associated with regional-scale tilting of the exhumed Schistes lustrés tectonic pile. The extensional tectonic regime in greenschistfacies conditions remains active today in most of the internal Alpine zones as the Alpine collision continues.

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