Vorticity of flow in ductile thrust zones: examples from the Attico-Cycladic Massif (Internal Hellenides, Greece)

Abstract

AbstractMicrostructural, petrofabric, strain and vorticity data from quartz-rich tectonites were used to investigate the kinematics of rock flow in the Evia and Ochi ductile thrust zones, formed during exhumation of the high-pressure nappes of the Attico-Cycladic Massif. The Evia thrust zone defines the base of the Styra nappe while the Ochi thrust zone defines the contact between the Styra and the overlying Ochi nappe. A dominant top-to-the-ENE sense of shearing along both thrust zones is indicated by several shear sense criteria. Deformation in the structurally deeper Evia thrust zone occurred under approximately plane strain conditions and was characterized by a RXZ strain ratio varying from 3 to 6. The vorticity profile above the thrust plane shows a slight down-section increase in the kinematic vorticity number (Wm) from 0.8 to 0.9, as well as the presence of local thin domains with a higher pure shear component of deformation. In the overlying Ochi thrust zone, a downward increase in Wm values from 0.6 to 0.9 is detected both above and below the thrust plane. Here, rocks have been deformed in the general constrictional field with RXZ values ranging between 5 and 8. A transport-parallel elongation of 30–90% and 50–160% has been estimated for the Evia and Ochi thrust zones, respectively, implying that ENE-directed extrusive flow controlled the formation, stacking and exhumation of the Styra and Ochi nappes.