OPTICAL DATING OF FLUVIAL SEDIMENTS IN THE MOJAVE DESERT, CALIFORNIA, USED TO RECONSTRUCT FAULT SLIP RATES

Faults in California accommodate most of the relative motion between the Pacific and North American tectonic plates, along either one main strike-slip fault, – the San Andreas fault – or a network of sub-parallel faults (e.g., the San Jacinto, Elsinore and San Andreas faults to the south of Cajon Pass). Slip is also accommodated along many other associated faults and folds. The contemporary movements of different fault-bounded blocks are relatively well established on decadal timescales using remote sensing and GPS, and on timescales of millions to tens of millions of years, dating offset geologic features with radiometric methods. However, on timescales of decades to several hundred thousand years, determining total fault offset and mean slip rate is harder. Critical questions for understanding fault dynamics and improving earthquake risk assessment include the degree to which slip on faults is clustered into episodes of more rapid movement, and whether slip is accommodated by different sub-parallel faults. In many cases, streams with offset courses can be recognised, and in some cases offset terrace surfaces can be located, especially when using LiDAR data to complement field mapping. Radiocarbon and terrestrial cosmogenic nuclides have been used to date these features, but both have limitations of age range and sample suitability. OSL (optically stimulated luminescence) and IRSL (infra-red stimulated luminescence) have great potential to complement these techniques, though the characteristics of quartz in some parts of southern California are suboptimal, displaying low sensitivity and other limitations. We explore approaches to date mid- to late Holocene small terrace units within alluvial fans that have been offset laterally by the Garlock fault, a major left-lateral strike-slip system in the Mojave Desert, California. We investigate OSL and IRSL characteristics of quartz and feldspar fractions, and explore the severity of incomplete bleaching.