APPLYING LUMINESCENCE THERMOCHRONOLOGY TO INVESTIGATE LANDSCAPE RESPONSE TO LATE PLEISTOCENE EXHUMATION NEAR THE MILL CREEK STRAND OF THE SAN ANDREAS FAULT (Invited Presentation)

Recent work suggests that the Mill Creek, Galena Peak, and Mission Creek strands of the southern San Andreas fault system near the southern San Bernardino Mountains may have been active in the late Pleistocene and Holocene with implication for seismic hazards in southern California. However, past efforts to resolve offset along the Mill Creek and Galena Peak faults have been limited by poor fault exposure and few datable offset units. Here we apply thermoluminescence (TL) ultra-low-temperature thermochronology to show an erosion rate contrast across these two fault strands during the late Pleistocene, consistent with dip-slip motion along these faults. Erosion rates in this region show a general deceleration of erosion from 100 to 10 ka, with the exception of samples from the western Yucaipa Ridge tectonic block, which maintain erosion rates near 10³ mm/ka during this interval. Comparison with nearby apatite (U-Th)/He ages and catchment-averaged cosmogenic ¹⁰Be erosion rates reveals general agreement among all techniques. Yet, the in situ TL erosion rates capture spatial variability obscured in the other datasets, including greater erosion rates at low elevations, a positive correlation between erosion rates and relief and transient erosional features in the landscape. Our results are consistent with a recently active western Mill Creek - Galena Peak fault configuration, a scenario that is compatible with observations of unfaulted late Pleistocene sediments observed across the eastern Mill Creek fault and with recent claims for Holocene activity on the Mission Creek fault.