Reverse Faulting Within a Continental Plate Boundary Transform System

Contractional deformation is common along transform plate margins where plate motion is oblique to the plate boundary. While faults that accommodate this deformation are often inferred to be subsidiary to the main plate boundary fault, we typically lack direct geometric or kinematic information. Here we investigate the timing of faulting relative to changes in the orientation of the North American‐Pacific plate boundary. Coeval with development of an oblique plate boundary segment (i.e., the “Big Bend” of the San Andreas fault), active shortening is inferred to have initiated at ∼5 Ma in the Western Transverse Ranges (WTR). However, new low‐temperature thermochronometric data yield Miocene to Pleistocene apatite (U‐Th‐Sm)/He cooling ages and partially reset zircon (U‐Th)/He ages. Inverse thermal modeling indicate that reverse faulting initiated as early as 10 Ma, several million years prior to our current understanding of the timing of the Big Bend. Reverse faults in the WTR also propagate from west to east, toward the San Andreas Fault, rather than outwards from it. New and existing thermochronometry data delineate the WTR as the locus of rapid post‐Miocene exhumation, and demonstrate that similar exhumation is not present in the broader region surrounding the Big Bend. We posit that reverse faulting is localized in the WTR because of a weak underlying lithosphere and predates the more recent geometric anomaly of the restraining bend in the transform margin. Apatite and zircon helium thermochronometry data from the Western Transverse Ranges (WTR) yield late‐Miocene through Pleistocene cooling agesReverse faulting within the block initiated prior to, and propagates toward, the Big Bend restraining bend in the San Andreas faultRapid post‐late‐Miocene tectonic exhumation in the region surrounding the Big Bend is localized within the WTR Apatite and zircon helium thermochronometry data from the Western Transverse Ranges (WTR) yield late‐Miocene through Pleistocene cooling ages Reverse faulting within the block initiated prior to, and propagates toward, the Big Bend restraining bend in the San Andreas fault Rapid post‐late‐Miocene tectonic exhumation in the region surrounding the Big Bend is localized within the WTR