Paleostress magnitude in a fold-thrust belt (External Hellenides, Greece): Evidence from twinning in calcareous rocks

Abstract

Based on twinned calcite in carbonate samples collected from the External Hellenides, we calculated paleostress magnitude and its across-strike variation into the Pindos Zone (PZ) and the Gavrovo Zone (GZ). These two zones are located west of the suture zone between the External and the Internal Hellenides. Paleostress magnitude is in the order of 110 MPa close to the suture, dropping toward the foreland and reaching a value of 50 MPa 80 km from the suture. The obtained distribution of paleostress levels in the External Hellenides resembles that recorded in other orogens, such as the Appalachian and Northern Pyrenean. The paleodepths of sampled carbonates during the initial stage of thrusting, which were estimated using the differential stress values, are structurally sensible. Comparing our results to available information on the evolution of paleostress magnitude with depth in compressional settings at the upper crust, we suggest that the mechanical behaviour of the cover thrust sheets of the External Hellenides is governed by the law of friction under nearly hydrostatic conditions. Based on this finding, as well as on published rheological data from rocks exposed at the deeper structural levels of the External Hellenides, a rheological profile was proposed. On this profile, brittle-ductile transition occurs at a depth of around 9-10 km, slightly deeper than the basal decollement of the External Hellenides.