Geology of the contact area between the Internal and External Nappe Zone of the Sardinian Variscan Belt (Italy): new insights on the complex polyphase deformation occurring in the hinterland-foreland transition zone of collisional belts

Abstract

This study aims to decipher the structural evolution of a sector of the Nappe Zone in the Sardinian Variscan Belt. We investigated the Barbagia Thrust between the Meana Sardo Unit (belonging to the External Nappe Zone) in the footwall and the Barbagia Unit (belonging to the Internal Nappe Zone) in the hanging wall. Combining the geological survey with meso- and microstructural analysis, we realized a 1:10.000 scale geological map highlighting the polyphasic evolution developed under low-grade metamorphic conditions during the Variscan orogeny. Both units preserve (i) an early phase generally observed far from the tectonic contact and mainly in the Meana Sardo Unit (D1), (ii) a syn-nappe ductile deformation linked to the Barbagia Thrust activity and a top-to-the S-SW sense of shear (D2) and (iii) a large-scale nappe refolding (D3). A late extensional stage (D4), with the development of collapse folds, marks the end of the orogenic cycle.