LONG TERM SAN ANDREAS FAULT HISTORY RE-EXAMINED THROUGH A MULTICHRONOMETER STUDY OF A PIERCING POINT

The San Andreas Fault (SAF) has shaped western North America to its modern-day landscape and provides scientists an opportunity to study an active continental transform fault, giving insight into the timing and rates of plate boundary scale processes. Long term paleogeographic and palinspastic reconstructions of the SAF use knowledge of offsets along individual fault segments at various points in time. Geologic piercing points are critical to understanding total displacement along faults segments, but current proposed SAF piercing points yield discrepant offset estimates ranging from 160-240 km. In this study we explore the time-temperature history of the Triassic megaporphyritic monzogranite piercing point of Liebre Mountain and Mill Creek proposed to represent 160 km of offset along the SAF restraining bend known as the “Big Bend”.

We utilize zircon U-Pb and whole rock geochemistry to confirm that the Liebre Mountain and Mill Creek megaporphyritic monzogranites originated from the same pluton. Zircon U-Pb record ~240 Ma crystallization at both sites with ~1.6 Ga inherited cores. Apatite and zircon (U-Th)/He, and apatite fission-track paired with inverse thermal history modeling indicates that the two sites broadly share a thermal history through SAF related exhumation at around ~5 Ma and thus, the offset observed today could be entirely attributed to the SAF. Fault-perpendicular transects reveal asymmetrical uplift recorded in our apatite (U-Th)/He and apatite fission-track chronometers (<120°C). We couple our results with regional geologic data and knowledge of subsurface fault geometry as illuminated by the SCEC community fault model. We propose that the differential transpressional uplift displayed in our low temperature data is tied to subsurface fault geometry and dip angle and that uplift may be accommodated along a positive flower structure. Our results demonstrate the power of plutonic piercing points that can provide insight into mountain-building kinematics in strike-slip systems.