Orogen-parallel tectonic transport in the Variscan belt of northeastern Sardinia (Italy): implications for the exhumation of medium-pressure metamorphic rocks

Abstract

A transpressive crustal-scale dextral shear zone is documented in the Variscan Basement of northeastern Sardinia. It indicates the presence of a shear deformation parallel to the belt overprinting previous D1 structures related to nappe stacking and top-to-the-S and -SW thrusting. The L2 stretching lineation points to an orogen-parallel stretching and to a general change in the tectonic transport from D1 to D2. Phase D1 developed during initial frontal collision, whereas the D2 deformation was characterized by dextral shearing during the increasing curvature of the Ibero-Armorican arc. Transpressional deformation developed in a regime of decreasing pressure. It caused telescoping of the Barrovian isograds and the exhumation of the low- to medium-grade metamorphic rocks. In this sector of the Variscan belt, exhumation is due to continuing compression with an increasing component of horizontal displacement. The overall change of the shortening direction in a large sector of an orogenic belt, with the occurrence of increasing orogen-parallel displacement, may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse.