PRECARIOUSLY BALANCED ROCK (PBR) EXHUMATION AND EXPOSURE AGE CONSTRAINTS FOR SEISMIC HAZARD IN SOUTHERN CALIFORNIA: PROPOSING A CLIMATE CHANGE MODEL

Precariously balanced rocks (PBRs), also known as pedestal rocks, tors, and perched rocks, are naturally occurring landforms that are easily disturbed by earthquakes. In seismically active regions, PBRs effectively act as seismoscopes in residence for thousands of years, constraining the ground motions that have occurred during this time period (the PBR age). Thus, the age of PBRs and their rate of renewal through exhumation are important for validating seismic hazard models and understanding rates of soil formation and landscape evolution in tectonically active regions. Cosmogenic radionuclide (CRN) surface exposure dating can be applied to constrain the age of formation of PBRs, but to interpret nuclide abundance requires knowledge of the geomorphic history of the rock. Initial efforts by Bell et al. (1998) indicated that rocks were precariously balanced for greater than 10,000 years in southern California. In a pilot study, we collected 10 samples from 6 PBRs at four sites spanning a 110-km section of the San Andreas fault for CRN exposure dating using ¹⁰Be. Pedestal samples consistently yielded younger apparent ages than the associated PBR, indicating that exhumation history is important in developing exposure ages. Preliminary exposure times (“PBR ages”) assuming a fast exhumation model and simplified shielding corrections range from 16 kyr to 23 kyr, suggesting that formation of PBRs in southern California is linked to late Pleistocene pluvial conditions at the end of the last glacial, or climate change and exhumation by rapid erosion at the late Pleistocene / Holocene transition.