South Variscan terrane accretion: Sardinian constraints on the intra-Alpine Variscides

Structural data are presented that show that the Sardinian Variscides form the boundary of the Variscan intra-Alpine terrane, thrust onto a Gondwana foreland. This model replaces the currently held hypothesis that the south Armorican suture zone continues into the Sardinian Variscides. The orogenic activity propagated toward the foreland in two distinct waves: (1) thrusting, burial and Barrovian type metamorphism that reached the internal orogen in Late Devonian times and lasted in the external orogen until Early Carboniferous times. Transpressional strain partitioning is suggested by frontal thrusting in the external orogen and lateral displacement in the internal orogen. (2) Normal faulting, exhumation and local Buchan type metamorphism were superimposed on the internal orogen probably since Early Carboniferous times and reached the external orogen contemporaneously with the Late Carboniferous emplacement of voluminous granitic intrusions. Extensional doming and lateral displacement indicate a transtensional regime. The sense of orogen-parallel displacement during both stages was left-lateral on inward dipping movement planes and right-lateral on outward dipping movement planes. The resulting left-lateral displacement between the intra-Alpine terrane and the Gondwana foreland is due to the bending of the Variscan belt, during which the Sardinian segment underwent a bookshelf type movement within an overall right-lateral displacement regime.