HR: 1330h
AN: T22B-0514    [PDF]
TI: Constant Long-Term Slip Rates Along the Mojave Section, San Andreas Fault Determined From Cosmogenic $^{10}$Be and $^{26}$Al Analysis in Boulders on Displaced Alluvial Fans
AU: * Matmon, A
EM: amatmon@usgs.gov
AF: U.G. Geological survey, 345 Middlefield Rd., Menlo Park, CA 94025
AU: Schwartz, D
EM: dschwartz@usgs.gov
AF: U.G. Geological survey, 345 Middlefield Rd., Menlo Park, CA 94025
AU: Hanks, T
EM: thanks@usgs.gov
AF: U.G. Geological survey, 345 Middlefield Rd., Menlo Park, CA 94025
AU: Finkel, R
EM: finkel1@llnl.gov
AF: Lawrence Livermore National Laboratory, South Vasco Rd., Livermore, CA 94550
AU: Clemmens, S
EM: clemmens1@llnl.gov
AF: Lawrence Livermore National Laboratory, South Vasco Rd., Livermore, CA 94550
AU: Mushkin, A
EM: mushkin@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, 63 Johnson Hall, Box 351310, Seattle, WA 98195
AB: Remnants of 6 alluvial fans are displaced 0.6-16.5 km southeastward from their original location at the mouth of Little Rock Creek (LRC) along the Mojave section of the San Andreas fault (SAF). The surfaces of the two closest fans, displaced 0.6 and 1.75 km, are rough with numerous exposed boulders, whereas the most distant fan (~16.5km) has a smooth surface with essentially no exposed boulders. To determine the ages of fan surfaces from boulders at the surface, it is necessary to estimate the boulder erosion rate and the cosmogenic nuclide inheritance from prior exposure in the LRC drainage system. Inherited values, measured in sediments at the mouth of the river, range between 8500 and 71500 $^{10}$Be atoms g$^{-1}$quartz. Boulder erosion rate was determined from measurements of in-situ $^{10}$Be in bedrock pinnacles. The amount of offset is determined by measuring the distance along the fault from the northwestern most point of each fan and the piercing point within the LRC. The determination of the piercing point is somewhat uncertain and depends on the understanding of the historical development of the LRC outlet. Preliminary results of in-situ cosmogenic $^{10}$Be and $^{26}$Al analyses from boulders located on the two closest displaced fan surfaces suggests that their minimum ages range between 22.5$\pm$2.4 $^{}$ and 44.0$\pm$4.7 $^{}$ ka using a boulder erosion rate of 10 m My$^{-1}$and inheritance value of 8500 $^{10}$Be atoms g$^{-1}$quartz. These ages imply an average slip rate of 3.8$\pm$ 0.5 $^{}$ cm yr$^{-1}$. If an inheritance value of 71500 $^{10}$Be atoms g$^{-1}$ quartz is used, calculated ages range between 14.3$\pm$1.1 $^{}$ and 32.5$\pm$3.4 $^{}$ ka and the average slip rate is 6.7$\pm$0.6 $^{}$ cm yr$^{-1}$. Considering plate tectonic kinematics in this region, we think that the higher slip rate is unreasonable. Significant erosion of boulders and erosion of the fan surfaces of the more distant fan remnants, displaced 6.5-16.5 km from the mouth of the LRC, prevent direct calculation of fan ages based on cosmogenic measurements of boulders at the surface. Ages of these fans were determined by modeling the $^{26}$Al/$^{10}$Be ratio for burial age dating. Preliminary results of in-situ cosmogenic $^{10}$Be and $^{26}$Al analysis from buried samples indicate burial ages that range between 291$\pm$30 $^{}$ and 466$\pm$49 $^{}$ ka. These ages imply an average slip rate of 3.4$\pm$0.2 $^{}$ cm yr$^{-1}$. Several estimates of the Holocene and late Pleistocene slip rate between the Carrizo Plain and Cajon Pass have been previously calculated using $^{14}$C dating of offset morphologic features. Most of these estimates range between 2 to 3.5 cm yr$^{-1}$. Our results suggest that slip rate has been relatively constant for at least the past 450 ky and lies within the range of previous late Pleistocene and Holocene estimates.
DE: 7221 Paleoseismology
DE: 8110 Continental tectonics--general (0905)
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting