Dating offset fans along the Mojave section of the San Andreas fault using cosmogenic 26Al and 10Be

Abstract

Analysis of cosmogenic 1 0 Be and 2 6 Al in samples collected from exposed boulders (n = 20) and from buried sediment (n = 3) from offset fans along the San Andreas fault near Little Rock, California, yielded ages, ranging from 16 to 413 ka, which increase with dis-Lance from their source at the mouth of Little Rock Creek. In order to determine the age of the relatively younger fans, the erosion rate of the boulders and the cosmogenic nuclide inheritance from exposure prior to deposition in the fan were established. Cosmogenic nuclide inheritance values that range between 8.5 x 10 3 and 196 × 10 3 atoms 1 0 Be g - 1 quartz were determined by measuring the concentrations and ratios of 1 0 Be and 2 6 Al in boulders (n = 10) and fine sediment (n = 7) at the outlet of the present active stream. Boulder erosion rate, ranging between 17 and 160 mm k.y. - 1 , was estimated by measuring 1 0 Be and 2 6 Al concentrations in nearby bedrock outcrops (n = 8). Since the boulders on the fans represent the most resistant rocks in this environment, we used the lowest rate for the age calculations. Monte Carlo simulations were used to determine ages of 16 ′ 5 and 29 ′ 7 ka for the two younger fan surfaces. Older fans (older than 100 ka) were dated by analyzing 1 0 Be and 2 6 Al concentrations in buried sand samples. The ages of the three oldest fans range between 227 ′ 242 and 413 ′ 185 ka. Although fan age determinations are accompanied by large uncertainties, the results of this study show a clear trend of increasing fan ages with increasing distance from the source near Little Rock Creek and provide a long-term slip rate along this section of the San Andreas fault. Slip rate along the Mojave section of the San Andreas fault for the past 413 k.y. can be determined in several ways. The average slip rate calculated from the individual fan ages is 4.2 ′ 0.9 cm yr - 1 . A linear regression through the data points implies a slip rate of 3.7 ′ 1.0 cm yr - 1 . A most probable slip rate of 3.0 ′ 1.0 cm yr - 1 is determined by using a Χ 2 test. These rates suggest that the average slip along the Mojave section of the San Andreas fault has been relatively constant over this time period. The slip rate along the Mojave section of the San Andreas fault, determined in this study, agrees well with previous slip rate calculations for the Quaternary.