Geometry and evolution of the San Andreas Fault Zone in northern California

The evolution of the San Andreas fault system is controlled by thermal‐mechanical processes associated with the development and evolution of a narrow “slabless window” formed beneath the western edge of North America. This fault zone evolution begins after initiation of transform motion along the plate boundary with the northward migration of the Mendocino triple junction. As a consequence of initial lithospheric structure and the shallow emplacement of asthenospheric mantle, the plate boundary separating the North American and Pacific plates follows a complex three‐dimensional geometry which varies through time. Seismic velocity structure, heat flow, seismicity, surface deformation, uplift, and fault development are controlled by the evolving thermal structure in the region after triple junction passage. Thermal‐mechanical models have been used to evaluate the fault system's time‐varying three‐dimensional dynamical behavior, simulating the principal processes involved in the thermal‐mechanical evolution of the San Andreas fault system. Results from this modeling indicate that the fault system has essentially a three‐stage history. (1) In the vicinity of the Mendocino triple junction the San Andreas fault maps the eastern edge of the Pacific plate, with a broad (∼100 km) zone of asthenospheric mantle separating the Pacific and North American plates in the 25‐ to 90‐km depth range. (2) Between 37°N and ∼39°N the plate boundary (within the mantle) separating the Pacific and North American plates has developed approximately 40–60 km east of the surface trace of the San Andreas fault and lies beneath the Hayward‐Calaveras faults and associated faults. The surface trace of the plate boundary appears to be connected to the mantle segment via a lower crust subhorizontal detachment surface. This fault zone orientation produces the surface deformations observed geodetically in the region. (3) South of 37°N the surface fault again overlies the deeper plate boundary, apparently as a result of an eastward jump in the surface fault.