AN INTEGRATED STUDY OF UPPER MANTLE XENOLITHS FROM THE SAN ANDREAS FAULT, CALIFORNIA

The Coyote-Anderson alkali basalt (CAB) erupted at ~3 Ma along the Calaveras Fault, a splay of the San Andreas Fault near San Francisco, California. This eruption took place ~ 6 m.y. after the passage of the Mendicino triple junction. Small, equigranular upper mantle xenoliths are common within the CAB and provide a unique opportunity to study the North American lithospheric mantle beneath the San Andreas Fault.

Numerical methods were used to calculate a geotherm for the CAB site at 3 Ma. The results of two-pyroxene geothermometry, combined with the calculated geotherm, indicate that the mantle xenoliths were derived from depths of 36.3 to 42.8 km at temperatures of 980 to 1100°C. A positive correlation between ¹⁴³Nd/¹⁴⁴Nd and ¹⁴⁷Sm/¹⁴⁴Nd is interpreted to reflect a metasomatic event in the mantle source at ~129 Ma, coincident with major Cretaceous arc activity but not with the slab window trailing the Mendicino triple junction. Furthermore, the values of e_Nd (0) and ⁸⁷Sr/⁸⁶Sr for clinopyroxene indicate that the mantle is relatively primitive and MORB-like.

The lattice preferred orientation (LPO) of olivine from five upper mantle xenoliths was measured using a 5-axis universal stage microscope. For all samples, the olivine a-axes tend to form point distributions while the b- and c- axes form point or girdle distributions. The LPO patterns are consistent with dislocation creep in olivine at relatively high temperature conditions. Shear wave splitting data from this region in California permits restoration of the mantle fabrics to their position prior to entrainment in the basalt flows. This reconstruction allows us to directly compare the magnitude of shear wave splitting measured in seismic studies to that inferred from the LPO of olivine in the xenoliths.