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

Abstract

Petrofabric, 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.

Moine Thrust zone mylonites at the Stack of Glencoul: I – microstructures, strain and influence of recrystallization on quartz crystal fabric development

“…Ramsay & Huber 1987, pp. 611-613;Law et al 2004;Grasemann et al 2006;Williams et al 2006;Xypolias & Kokkalas 2006;Xypolias et al 2010).…”

Ductile extrusion in continental collision zones: ambiguities in the definition of channel flow and its identification in ancient orogens

“…the Kaoko Belt in Namibia; Goscombe et al 2003aGoscombe et al , b, 2005, and the reversal in the sense of vorticity across the region of deformation (e.g. southwestern Hellenides; Xypolias & Doutsos 2000;Xypolias & Koukouvelas 2001;Xypolias & Kokkalas 2006).…”

The Nature of Ductile Deformation in the Phyllite-Quartzite Unit (External Hellenides)

“…According to these studies, middle Miocene−Pleistocene extension controlled both the exhumation of HP-rocks from ~10 km to the surface and the initiation of basin formation (Papanikolaou and Vassilakis 2010 and references therein). However, these models seem to underestimate the importance of contraction-related structures deforming Middle Miocene−Early Pleistocene sediments of Crete (Kokkalas and Doutsos, 2001;Kokkalas et al, 2006;Chatzaras et al, 2006;Klein et al, 2008;Tortorici et al, 2010), suggesting that the role of extension is overestimated.…”

“…This, in addition to the presence of strong quartz c-axes preferred orientation patterns recorded in Peloponnese and Kythira suggest that dislocation creep was the dominant deformation mechanism during D 2 shearing (Xypolias and Kokkalas, 2006). Evidence for deformation by dissolution precipitation creep is mainly restricted to metasiltstones, where quartz clasts are embedded in a phyllosilicate-rich matrix and occasionally show pressure shadows on both sides of grains (Schwarz and Stockhert, 1996).…”

“…Panagos et al, 1979, Seidel et al ,1982. The PQ unit is tectonically emplaced on the Plattenkalk unit by a major ductile thrust (Greiling, 1982;Doutsos et al, 2000;Zulauf et al, 2002;Xypolias and Kokkalas, 2006), the "Basal thrust".…”