Change of nappe transport direction during the Variscan collisional evolution of central-southern Sardinia (Italy)

Abstract

The collisional Variscan (Early Carboniferous) evolution of the greenschist facies metamorphic basement of Sardinia is characterized by three deformation phases and two 90° reorientations of shortening direction. During the first phase of deformation N–S shortening occurred, with south-facing isoclinal folding, south-directed nappe emplacement and mylonitization in central and southeastern Sardinia, and large-scale E–W striking upright folds in southwestern Sardinia. Emplacement of the Barbagia, Meana Sardo, Gerrei and Riu Gruppa units took place during this event. This deformation phase was followed by E–W shortening, with W-directed emplacement of the Sarrabus and Arburese units along the Villasalto thrust and upright to E-dipping large-scale N–S striking upright folds in southwestern Sardinia. During the third deformation event, the shortening direction again rotated 90°, and N–S shortening produced kilometer-scale antiforms and synforms, the most prominent features in the metamorphic basement of Sardinia. The limbs of the kilometer-scale antiforms and synforms are vertically shortened during gravitational collapse, with contemporaneous faulting and conjugate shear zone development. Presence of large-scale E–W oriented antiforms was therefore critical for the formation of late Variscan extensional features, such as E–W striking, low-angle normal faulting.