Seismic Anisotropy of the Pelona-Orocopia-Rand Schist beneath the Mojave Block, Southern California

Abstract:

The POR schist is thought to comprise much of the lower crust of southern California, and is one of the archetypes for understanding shallow slab subduction and subduction erosion. Porter et al. (2011) attributed anisotropy in the lower crust of southern California to the POR schist, and interpreted the orientation of anisotropy beneath the Mojave block to result from fossil fabric related to initiation of the San Andreas Fault. Most seismic methods for analyzing anisotropy employ the assumption of transverse isotropic (TI) symmetry, where velocities are symmetric about 1 unique axis. The effects of this simplification have not been critically evaluated using measurements of the full symmetry of rock samples. We measured mineral crystal preferred orientations (CPO), and calculated elastic tensors for samples of lower crustal rocks of the Mojave block, including POR schist and plutonic rocks. This method captures the full elastic tensor with no enforced symmetry, and thus allows evaluation of simplified symmetry assumptions.

Velocity anisotropy in individual samples of the POR schist ranges from ~2–12% in V_P and ~3–15% in V_S, which is consistent with results of Porter et al. (2011). When all schist samples are averaged together to approximate the bulk schist, the anisotropy is significantly reduced to ~6% in V_P and ~8% in V_S, which is lower than the average values inferred by Porter et al. (2011). Felsic schist samples are fairly well approximated by TI symmetry, whereas mafic schist, and mylonitic samples have a strong orthorhombic component. Felsic plutonic rocks have low anisotropy, and complicated low-order symmetry, while plutonic rocks with significant hornblende content have orthorhombic symmetry. The symmetry of the average lower crust beneath the Mojave block is not likely to be purely TI, and this contributes to differences between seismic interpretations of anisotropy and the intrinsic elasticity inferred from laboratory measurements.