Dynamics of San Andreas Fault Formation: Capture of a Microplate during the Demise of Subduction

The capture of microplates by the Pacific Plate drives the transition from subduction to intracontinental, strike-slip motion along the San Andreas Fault (SAF). However, the underlying mechanics behind microplate capture and formation of intracontinental strike-slip faults remain unclear. Through 3D thermo-mechanical models with fluid migration, we find that northwestward Pacific Plate motion transitions from being accommodated at the Pacific-Farallon ridge to the megathrust between the Farallon slab and North America, and finally to an emergent, fluid-weakened intracontinental strike-slip fault. This transition occurs during slab detachment, triggered by decaying subduction convergence, strengthening of the megathrust with slowing water release, and eventual subduction termination. With the detached Monterey slab paleogeographically restored, forward large-scale convection models show that the paleo slab corresponds to the prominent, high-seismic velocity anomaly in the mantle transition zone below Nevada and Utah. The extension of the overriding American Plate facilitates the formation of the strike-slip fault. The computations suggest the connection between the low viscosity and high permeability subducted plate interface and the North American lower crust may lead to shearing, fluid transfer, and serpentinization and eventual SAF formation, offering insights into the spatial variations of volcanism, fault creeping, and seismicity along the SAF.