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Seismic Properties and CPO Relationships of a Crustal Scale Shear Zone in Granodiorite, Serifos, Greece


IPGP - Îlot Cuvier


Séminaires communs Géomagnétisme-Paléomagnétisme

Salle 310


Université d'Ottawa

Serifos island is the southern-eastern terminus of the West Cyclades Detachment System, where a late-tectonic granodiorite pluton exhibits ductile to brittle structures due to ongoing extension accommodated by the shear zone. The crystallographic preferred orientations (CPO) of samples collected across a strain gradient were measured using electron backscatter diffraction and seismic properties were calculated using Voigt-Reuss-Hill averaging. All samples show bulging quartz grains with brittle fractures and the weakly deformed granodiorite exhibits feldspars with straight, sharp boundaries. Quartz in the moderately deformed samples is recrystallized, feldspar grains have serrated boundaries and mica crystals are bent. In the mylonite, the magmatic quartz grains recrystallize by a combination of dislocation glide and bulging resulting in a smaller grain size, suggesting a significant component of shear strain. Quartz CPO is very strong in the weakly deformed sample recording prism slip and is very weak in the mylonite. Orthoclase and anorthite possess a weak CPO in all samples suggesting that temperatures were too low for feldspar to deform by dynamic recrystallization (450-500°C for Fsp, compared to 250°C for Qz). All but the less deformed samples have similar directions for fast P-wave velocity and slow P-wave velocity for anorthite. Orthoclase P-wave and S-wave velocities are similar for only the highly deformed samples. Also in highly deformed samples, there appears to be a relation between the maximum P-wave velocity and the CPO of anorthite. The S-wave anisotropy of the weakly deformed granodiorite is the highest of all samples (maximum anisotropy: 8%), and the anisotropy of the mylonite is the weakest of all samples (maximum: <3%). Texture analysis provides information on the seismic anisotropy of variably deformed granodiorite, which can be considered as a good approximation of bulk crustal material.