New kinematic models for Pacific-North America motion from 3 ma to present, II: Evidence for a "Baja California shear zone"

We use new models for present-day Pacific-North America motion to evaluate the tectonics of offshore regions west of the Californias. Vandenburg in co astal Alta California moves at the Pacific plate velocity within uncertaint ies (similar to1 mm/yr) after correcting for strain accumulation on the San Andreas and San Gregorio-Hosgri faults with a model that includes a viscoe lastic lower crust. Modeled and measured velocities at coastal sites in Baj a California south of the Agua Blanca fault, a region that most previous mo dels consider Pacific plate, differ by 3-8 mm/yr, with coastal sites moving slower that the Pacific plate. We interpret these discrepancies in terms o f strain accumulation on known on-shore faults, combined with right lateral slip at a rate of 3-4 mm/yr on additional faults offshore peninsular Baja California in the Pacific. Offshore seismicity, offset Quaternary features along the west coast of Baja California, and a discrepancy between the magn etically determined spreading rate in the Gulf Rise and the total plate rat e from a geological model provide independent evidence for a "Baja Californ ia shear zone.".