Seismic anisotropy under extended crust: evidence from upper mantle xenoliths, Cima Volcanic Field, California

Abstract

A model of P-wave velocity anisotropy and shear-wave splitting for upper mantle under extended crust is derived from comparisons of seismic field measurements and observed xenolith anisotropies. Seismic velocity measurements and anisotropies calculated from petrofabric measurements of 15 upper mantle xenoliths from the Cima Volcanic Field, Eastern Mojave Desert, California show average maximum anisotropies of 6.9% for V p and 4.5% for V s. Xenolith compressional wave velocities corrected for upper mantle temperature and pressure agree well with field observations in the vicinity of the Cima Volcanic Field. The velocities calculated from the olivine fabrics suggest that upper mantle foliations beneath the region are horizontal, and lineations trend ENE. The fabrics are believed to originate from early Cenozoic extension in the area, which had the same trend as the lineations. The unusually high upper mantle temperatures in this region apparently have not significantly affected upper mantle fabrics.