Seismic anisotropy of the Pelona Schist from electron backscatter diffraction (EBSD) measurements of crystal preferred orientations (CPOs)

Seismic Anisotropy of the mid to lower crust is difficult to measure in the field by seismologists. Using a scanning electron microscope (SEM) to obtain crystal preferred orientations (CPO's) of mineral grains from samples taken from this region allows us to calculate seismic anisotropy theoretically using published mineral elastic constants. Calculations are made according to methods described in Mainprice et. al. (1990). Polished thin sections from the Pelona Schist were analyzed using electron back scatter diffraction (EBSD) in an SEM. The slide was tilted 70 degrees from horizontal. The samples included: 4 mica schists that were cut parallel to foliation, 3 corresponding mica schists cut perpendicular to foliation, and 2 xenoliths with high amphibole content. The 4 mica schists had max Vp's that ranged from 6.44 to 7.26 km/s; Vp anisotropies that ranged from 4.8% to 29.2%; and max Vs anisotropies from 7.0% to 42.6%. The 3 corresponding mica schists had max Vp's that ranged from 6.26 to 6.61 km/s; Vp anisotropies from 3.1% to 7.9%; and Vs anisotropies from 4.9% to 6.61%. In samples that were cut orthogonal to foliation biotite is difficult to index using the automated indexing routine and had to be found manually to improve calculations. This may account for significant differences in anisotropy between corresponding samples. The xenoliths had max Vp's of 6.14 and 6.48 km/s; their Vp anisotropies were both 6.5%; and their Vs anisotropies were 8.35 and 8.37. Our calculations give slightly higher maximum Vp than the average Vp reported in Pellerin and Christensen (1997) for similar rock types from the area. Calculated anisotropies are comparable to Pellerin and Christensen (1997). Samples with large amounts of mica had the highest anisotropy. On average, the calculated velocities have approximately hexagonal symmetry with a unique slow axis. In contrast, one sample contained significant plagioclase that caused velocities to be fast perpendicular to foliation. An assumption of hexagonal symmetry with a unique slow axis is a valid assumption for the Pelona Schist.