THE GEOLOGY OF THE EXTERNAL NAPPES AND FORELAND ZONES OF THE VARISCAN BASEMENT OF SARDINIA

The Variscan basement of central-southern Sardinia is characterized by rocks from Early Cambrian to Early Carboniferous, involved in a continental collision and deformed under “very low”- to “low-grade” metamorphic conditions. During the compressive phases, subhorizontal N-S and E-W shortening occurred and several tectonic units were thrust and imbricated. A Nappe Zone (NZ - located in Central and SE Sardinia) overthrust the Variscan Foreland (VF - located in SW Sardinia)during Early Carboniferous time(Carmignani et al., 1994). Deformation and metamorphism increase towards the N, moving from the VF to the NZ. Main structures in the NZ are: thrusts, marked by thick mylonitic zones, km-scale isoclinal recumbent folds, penetrative axial plane foliations and well defined stretching lineations, all deformed by late large antiforms and synforms. In the VF, shortening produced two sets of upright folds, a penetrative foliation, and discontinuous shear zones and backthrusts. Post-collisional extensional tectonics with vertical shortening occurred (perhaps related to the uplift of the orogenic wedge?), overprinting earlier contractional structures. Ductile and brittle structures developed, depending on their structural levels. Recently, a correlation between Variscan structures and tectonic evolution in the Nappe and Foreland zones of Sardinia has been proposed by Conti et al. (2001), involving a repeated change of shortening direction in both the NZ and VF. The complete nappe stack of central and southern Sardinia and structural framework of the Variscan foreland seem now well established over the last twenty years. Remaining problems include: (1) the precise ages of Variscan compressive and extensional tectonics; (2) thermo-baric conditions during collisional evolution; and (3) structural relationships between Variscan tectonics and much less deformed late Carboniferous-Permian basins.

Carmignani, L., R. Carosi, A. Di Pisa, M. Gattiglio, G. Musumeci, G. Oggiano, and P.C. Pertusati, The Hercynian chain in Sardinia (Italy), Geodinamica Acta, 7, 31-47, 1994. Conti, P., L. Carmignani, and A. Funedda, Changing of nappe transport direction during the Variscan collisional evolution of central-southern Sardinia (Italy), Tectonophysics, 332, 255-273, 2001.