ALLUVIAL FANS, A MUCH NEGLECTED DRYLAND ENVIRONMENTAL PROXY: EVIDENCE FOR LANDSCAPE EVOLUTION, PALAEOENVIRONMENTAL CHANGE AND NEOTECTONICS ALONG A TRANSFORM PLATE BOUNDARY, SOUTH-CENTRAL CALIFORNIA

Many authors have suggested that alluvial fans sensitively record environmental change (notably climate and hydrology) and tectonics. However, deciphering the potentially strong environmental record preserved within alluvial fan deposits has traditionally been severely restricted by a lack of adequate chronological control. Cosmogenic radionuclide (CRN) surface exposure and optically stimulated luminescence (OSL) dating remove this restriction by providing techniques to directly determine the timing of fan formation and/or incision. Using geomorphic and sedimentological analysis combined with CRN ¹⁰Be, ²⁶Al and ²¹Ne surface exposure and OSL dating we examined a succession of alluvial fan surfaces from Whitewater and Mission Creek to define the timing of fan formation and to help elucidate the relative roles of climate, tectonics and autocyclic processes in their formation. CRN exposure ages on surface boulders define two periods of alluvial fan growth: 70-60ka and between 8ka to present. Deformation of fan surfaces, however, are clearly also controlled by tectonics and the age of alluvial fan formation helps define rates of displacement along segments of the San Andreas fault.

The extensive dates (>50) on these surfaces show the applicability of CRN dating methods in such tectonically and geomorphically dynamic alluvial fan systems. However, the study also highlights a potential limitation to CRN exposure dating. The Mission Creek succession clearly shows younger ages on fan surfaces that are morphostratigraphically older than well-dated surfaces of 60-70 ka. These results suggest that boulder erosion and fan surface denudation potentially limit the use of surface exposure dating in this setting to deposits younger than 70ka, highlighting the need for careful field geomorphic and sedimentological evaluation in combination with CRN sampling.