HETEROGENEOUS EXHUMATION ALONG THE SOUTHERN SAN ANDREAS FAULT AS INFERRED FROM (U-TH)/HE AGES IN THE NORTHERN SAN GABRIEL MOUNTAINS, SOUTHERN CALIFORNIA

The contribution of oblique plate motion to vertical deformation along strike-slip faults is yet to be fully understood. Although near-field mountain building is common along transpressive faults, how it relates to regional fault obliquity versus local structural complexity has not been universally established. The San Andreas fault (SAF) in southern California has an obliquity >20° and rugged topography, and locally exhibits major near-field uplift, but convergent deformation along its entire length has not yet been characterized. The northern San Gabriel Mountains (NSGM) are a narrow, wedge-shaped crystalline block that straddle ~50 km of the southern SAF, but have been overlooked by previous analyses of transpressive mountain building. This range has elongate ridges that extend parallel to the fault, suggesting that bedrock uplift has been concentrated along the fault zone as observed in pure-shear dominated transpressional settings. To quantify the distribution of transpressive mountain building, we examined topography and analyzed bedrock samples using apatite radiogenic helium dating. Despite favorable conditions for major exhumation along the SAF, apatite helium ages do not decrease towards the fault zone. Crystalline ridges adjacent to the fault zone, including Portal Ridge and Liebre Mountain, have ages >10 Ma and are topped by old erosional surfaces, suggesting that near-field bedrock uplift has been minimal. Younger ages (4-5 Ma) occur further from the fault, which suggest the bulk of the NSGM, made up of the rugged Clearwater block, experienced differential uplift in the form of tilting to the northwest prior to onset of the modern SAF. We speculate that this deformation resulted from local convergence during the transition of slip from the San Gabriel fault, an earlier strand of the San Andreas system. This implies that transpressive deformation is heterogeneous along the SAF, and that secondary structures or geometrical complexities along the fault zone may play a primary role in the near-field exhumation observed along transpressive strike-slip faults.