Controls and implications of anisotropy across a strain gradient within granodiorite, Serifos, Greece

Abstract

Microtextural analyses including electron backscatter diffraction (EBSD), anisotropy of magnetic susceptibility (AMS) and the anisotropy of anhysteretic remanent magnetization (AARM) have been applied across a shear zone within a Miocene granodioritic intrusion on Serifos island, Western Cyclades, Greece. The shear zone records progressive deformation through a ductile to brittle transition. One of the consequences of anisotropic crust is the variation in seismic wave velocity with the direction of propagation, which is largely controlled by the crystallographic preferred orientation (CPO) of anisotropic minerals such as micas and feldspars. CPO data was collected using EBSD and seismic properties were calculated using Voigt-Reuss-Hill averaging of the single minerals' elastic stiffness tensor. The magnetic fabric is used to characterize weakly defined tectonic fabric, thus complementing the CPO data. The structure of rocks exhibiting weak magmatic fabric, as defined by magnetic anisotropy and CPO, appears to reflect the elliptical shape of the pluton. The SW-directed kinematics of the Western Cyclades produced an ENE-striking, SSE dipping low-angle shear zone in the granodiorite that imparted a signature onto the magnetic data of foliated and mylonitic samples, creating a composite fabric from the magmatic emplacement fabric and the top-to-SW directed shear. As for the seismic properties, P-wave anisotropy reaches 11.8% and is consistently controlled by the CPO of plagioclase feldspar, with the direction of fast P-wave propagation subperpendicular to foliation and the direction of slow P-wave propagation subparallel to foliation. The magnetic fabric, coupled with the tectonic fabric inferred from CPO data, gives unique insight into the character of anisotropic minerals in a syntectonic intrusion. This is especially useful for weakly strained rocks where the lineation and foliation (given by the magnetic fabric) can be related to the seismic anisotropy derived from the CPO data. Given the anisotropy caused by the preferred orientation fabrics of major rock-forming minerals, the microstructural analysis of anisotropic minerals is fundamental for incorporating seismic anisotropy into large-scale geophysical models.