Plutonic Piercing Points as Recorders of Continental Transform Fault Evolution: Chronologic Insights From the Transpressive Southern San Andreas Fault

Abstract The San Andreas Fault (SAF) zone is a prototypical continental transform fault system used to understand the rates and timing of plate‐boundary‐scale deformation. Extensive studies of the SAF document internal complexities, including how transpressive uplift occurs and varies spatially in strike‐slip systems. We utilize geochronology and low‐temperature thermochronology to re‐evaluate a plutonic piercing point and to identify heterogeneities in exhumation along the SAF restraining bend, known as the “Big Bend.” The Triassic K‐feldspar megaporphyritic monzogranite piercing point at Liebre Mountain and Mill Creek represents ∼160 km of right‐lateral fault motion. Here we present new zircon U‐Pb, bulk rock geochemistry, apatite and zircon (U‐Th)/He, and apatite fission track chronometry on a high‐resolution fault‐perpendicular sample transect from 0.25 to 4 km away from the SAF. Our results, coupled with inverse thermal history modeling, confirm that the Liebre Mountain and Mill Creek bodies broadly share a thermal history until the initiation of SAF‐related separation at ∼5 Ma. Age relationships along fault‐perpendicular transects reveal asymmetrical uplift recorded in the lowest temperature chronometers (<120°C) closest to the fault. Our results show that Mill Creek has undergone vertical exhumation in the last 5 million years as evidenced by young ∼2 Ma apatite (U‐Th)/He ages. We propose that recent differential uplift along the Big Bend could be tied to localized subsurface fault geometry and dip angle. We confirm that the Triassic megaporphyritic monzogranite is a robust piercing point that can be used in holistic reconstructions of the SAF and provides insight into mountain‐building kinematics in strike‐slip systems.