P-wave velocities in rocks at high pressure and temperature and the constitution of the central California crust

Velocities of compressional waves are determined for central California rocks at pressures up to 0.7 GPa (7 kb) and temperatures up to 450°C. These data are used to interpret the seismic velocity structure of the crust in the California Coast Ranges. The seismic data on both sides of the San Andreas fault are consistent with the following model; besides some patches of surface sediments the upper 10–15 km of the crust on the northeast side consists predominantly of sedimentary and metasedimentary rocks of the Franciscan assemblage; the lower crust, of a thickness of 15–20 km, may be composed of gabbroic or other mafic rocks. Across the fault on the south-west side, the entire crustal section is probably a granitic complex similar to that exposed on the surface. The proposed model is shown to be consistent with the observed gravity anomaly.