LITHOSPHERIC MANTLE DEFORMATION IN THE CENTRAL SAN ANDREAS FAULT SYSTEM

A major question in deforming zones is the extent to which crustal deformation is coupled to deformation at lower lithospheric levels. Mantle xenoliths from central California provide unique samples of fabrics beneath the San Andreas fault system that can be compared with seismic anisotropy data at the surface and numerical simulations in order to test the amount of coupling in this plate boundary system.

The Coyote Lake basalt, located on the Calaveras fault, contains spinel peridotite xenoliths from the mantle beneath the fault system. Unlike many Tertiary volcanic fields in California, the basalt erupted well after passage of the Mendocino triple junction (6 m.y.) allowing xenoliths to record plate boundary-related deformation before entrainment. Geothermometry combined with thermal modeling suggests that xenoliths originated from depths of 37.8-42.5 km. The lattice preferred orientation of olivine in the xenoliths shows strong point distributions, suggesting high-temperature conditions, and high calculated shear wave anisotropies (6%). The strong fabric and xenolith depth estimates demonstrate that a mantle shear zone exists beneath the crustal Calaveras fault to a depth of at least ~40 km. Shear wave splitting observations throughout central California are often interpreted in terms of two anisotropic layers: a lower asthenospheric layer with EW fabrics unrelated to the fault system and an upper lithospheric mantle layer where fabrics rotate into parallelism with the San Andreas fault. The upper layer is estimated to be ~100 km wide, indicating that mantle deformation is not focused exclusively beneath the crustal faults. Numerical simulations are used to interpret lithospheric-scale deformation patterns. A critical aspect of the simulations is that different boundary conditions (e.g., side-driven vs. top driven models) result in significantly different outcomes.

Mantle xenoliths from the Coyote Lake basalt record the strong fabrics developed in the mantle directly beneath the San Andreas fault system. Shear wave splitting data demonstrate that the fault system affects a wide swath of lithospheric mantle. These datasets together suggest that crustal deformation is coupled to mantle deformation.