Kinematic vorticity and strain rate patterns associated with ductile extrusion in the Chelmos Shear Zone (External Hellenides, Greece)

Kinematic vorticity, strain rate, finite strain and structural data have been used to describe the heterogeneous nature of ductile deformation in an extruding crustal-scale shear zone, the Chelmos Shear Zone, (CSZ) of the External Hellenides. The CSZ has a curved shape, which is marked by a frontal-flat and a steep zone. The spatial variation in the kinematic vorticity number is strongly correlated with the geometry and the structural depth of the shear zone. The pure shear component of deformation decreases from back to front and from top to bottom. The ductile thinning normal to the shear zone's boundaries is higher in the steep zone and decreases westwards. Quartz grain size and strain rate in the CSZ is distributed into two approximately homogeneous domains. The higher strain rate is recorded in the steep zone and the base of the frontal-flat zone with lower rates in the upper parts of the frontal-flat zone. Based on mesoscopic structural data and the spatial distribution of the shear strain rate, the existence of a west-dipping back-flow zone in the boarder zone between these two domains is inferred. The overall pattern of deformation in the CSZ implies that tectonic extrusion from the steep zone occurred at the final stages of ductile deformation and was associated with the formation of the backflow zone in the front of the shear zone. Ductile deformation of rocks probably started close to simple shear on a planar low-angle shear zone. progressively became penetrative, and more general shear occurred as the CSZ steepened up. This progressive increase in pure shearing may have occurred in a heterogeneous manner and was higher close to both the eastern margin of the steep zone and the top of the frontal-flat zone. Solid-state extrusion in the CSZ was also associated with a heterogeneous and progressive increase in strain rate. (C) 2001 Elsevier Science B.V. All rights reserved.