Heterogeneous ductile deformation along a mid-crustal extruding shear zone: an example from the External Hellenides (Greece)

Abstract

AbstractPetrofabric, finite strain and kinematic vorticity data were used to investigate the heterogeneous nature of ductile deformation along a 1.5–2 km thick extruding shear zone in the south Peloponnese, that formed under blueschist-facies conditions. Asymmetric quartz c-axis fabrics confirm westward thrust movements on an east-dipping shear zone and provide evidence for localized top-down-to-the-east shear sense at the front of the zone. Strain ratio (Rxz) is nearly constant (c. 3.0–4.0) along the upper structural levels of the zone but increases systematically from the middle to the bottom, approaching a value of c. 9.5 in the frontal parts, close to the basal thrust, and a value of c. 7.0 in the inner parts. The distribution of kinematic vorticity number depicts a simple-shear-dominated domain in the lower half of the shear zone and shows that the pure shear component always increases upwards in the zone, becoming dominant at the top of the inner parts of the zone. Integration of strain and vorticity data yields a shear-parallel elongation of c. 60–90% at the top and c. 40–60% at the bottom of the zone, revealing that both upper and lower surfaces of the extruding slices were ‘stretching faults’. Minimum total displacements of 25 and 41 km and slip rates of 6.5 and 10 mm/year were estimated for the basal and roof faults, respectively.