Back arc extension and denudation of Mediterranean eclogites

Eclogite-facies rocks exposed in Mediterranean back arcs are delimited from above by low-angle normal faults and detachments. Nevertheless, our work demonstrates that these extensional structures associated with back arc extension played only a limited role in removing the overburden from above the eclogites. Extension in Mediterranean back arcs began in the late Oligocene or early Miocene, but the pressure - temperature - time (P-T-t) paths of eclogite-facies rocks exposed in these areas indicate that a major part of the overburden, several tens of kilometers, has been removed from above these rocks prior to the Oligo-Miocene. We show that the time period bracketed between the peak of eclogite metamorphism (Eocene in the central Aegean, probably Upper Cretaceous in Corsica and the Betics) and the onset of back are extension in the Oligo-Miocene was characterized by thrust faulting. In the central Aegean, Corsica, and the Betics, eclogite-bearing units were partly unroofed and then overthrusted lower-pressure units. We emphasize that, with one exception (Tinos island, Greece), the entire inventory of extensional contacts operated subsequently to the overthrusting of the eclogites above the lower-grade sequences. Thus Mediterranean back arc extension lags behind a major part of the denudation process, and is superposed on orogenic wedges that contain eclogite-facies rocks at relatively shallow structural levels. We emphasize that the mode of occurrence of eclogites in Mediterranean back arc regions involves a continuum of in - situ crustal accretion below the eclogites, widespread P-T paths that show cooling or isothermal decompression, and lower-grade rocks at the bottom of the structural pile. Thus instead of reflecting whole - crust back arc extension, the tectonic style associated with the denudation of Mediterranean eclogites better fits an active accretionary-wedge setting. This is similar to the mode of occurrence of eclogite-facies rocks in mountain belts, such as the western Alps, where decompression was synorogenic and back arc extension played no role.