AGES, GEOLOGIC CONTROLS, AND LANDSCAPE EFFECTS OF ROCK AVALANCHES IN SOUTHERN CALIFORNIA

We present new mapping and age constraints on rock avalanche (RA) deposits in two regions of southern California. (1) San Gabriel Mountains (SGM): Detailed lidar and field mapping within the 74 km² San Antonio Canyon (SAC) catchment in the eastern SGM reveals that SAC is 25 % landslide deposits by area (three times the number and twice the area previously mapped). Six RA deposits have been dated through cosmogenic surface exposure (¹⁰Be) and sedimentary constraints (¹⁴C, pIR-IRSL). All deposits with preserved RA surface morphology (four) yield late Holocene ages despite some being previously mapped as Early Quaternary. A c. 15 ka burial age for sediment beneath a RA only identifiable in a roadcut, indicates that geomorphic evidence of landslides in the SGM can be erased on Holocene timescales. The oldest dated deposit suggests that there may be a ~50 kyr limit on landslide deposit preservation in the steep, actively uplifting eastern SGM. Many of the long runout RA deposits have dammed SAC, driving formation of new bedrock gorges (i.e. epigenetic gorges) rather than regaining the buried paleovalley.; this appears to be an important process throughout the SGM. (2) Salton Trough: On the northeast side of the Santa Rosa Mountains numerous RA deposits are associated with the low-angle West Salton Detachment Fault (WSDF). Cosmogenic exposure (¹⁰Be) of boulders on three RA deposit surfaces all yield Late Pleistocene ages. Notably, the ~8 km-runout Martinez Mountain RA (previously suggested as early Holocene) appears to have occurred c. 45 ka. The failures have clear geologic control; all RA deposits examined contain only hangingwall-derived material and detailed mapping indicates the WSDF is the primary failure surface in all cases. We suggest the geologic framework for the Salton Trough RAs are a Late Pleistocene analog for many extensive megabreccia deposits mapped above Miocene-active detachment fault planes in Death Valley and throughout the Basin & Range province.