HR: 1340h
AN: T43A-1096    [Abstracts]
TI: Synchronous Late Quaternary slip rate variability on two strands of the San Jacinto fault, California
AU: * Le, K N
EM: kle@email.unc.edu
AF: University of North Carolina, Chapel Hill, Mitchell Hall 104 South Road Campus Box #3315, Chapel Hill, NC 27599-3315,
AU: Oskin, M
EM: oskin@email.unc.edu
AF: University of North Carolina, Chapel Hill, Mitchell Hall 104 South Road Campus Box #3315, Chapel Hill, NC 27599-3315,
AB: We present new results that show slip rates varied synchronously by a factor of two over the past 35 kyr along two parallel strands of the San Jacinto fault. Our results combine high-resolution LiDAR digital topography, field mapping and ¹⁰Be exposure-age dating from two of the most active strands of the southern San Jacinto fault: the Clark fault (CLF) and Coyote Creek fault (CCF). These faults form numerous NW-striking scarps that offset three generations of Quaternary alluvial fan surfaces, Q2b, Q3a, and Q3b. We dated alluvial fans along both the CCF and CLF using ¹⁰Be sampling methods adapted for available material and degradation of the surface. For younger surfaces with well-preserved bar and swale morphology, we used a new sampling method where 12 to 20 chips from quartz-bearing boulders lodged within a bar were amalgamated into a single sample. For older surfaces we either sampled individual meter-sized boulders or collected samples from a 2 m-deep depth profile. Surface ages are consistent between CCF and CLF sites: 40 ± 12 ka and 31 ± 6 ka for Q2b, 7.1 ± 1.6 ka and 4.6 ± 1.6 ka for Q3a, respectively. Samples from Q3b near the CLF yielded ages of 1.0 ± 0.2 ka and 2.1 ± 0.3 ka. CCF samples have not yet been corrected 10Be inheritance, thus we use the CLF ages to calculate preliminary slip rates. Late Pleistocene to present rates are CLF: 2.2 ± 0.5 mm/yr, CCF: 3.4 ± 0.9 mm/yr, and 5.6 ± 1.4 mm/yr combined. Mid-Holocene to present rates are CLF: 4.1 ± 1.5 mm/yr, CCF: 6.7 ± 2.8 mm/yr, and 10.9 ± 4.3 mm/yr combined. Latest Holocene CLF slip rate exceeds 3 mm/yr. The combined Late Pleistocene to present slip rate for the southern San Jacinto fault is less than one third the rate deduced from the onset of faulting ca. 1 Ma. Mid-Holocene to present slip rates for both the CLF and CCF are about double their ca. 35 kyr rates, but are less than the 16 - 20 mm/yr geodetic loading rates and the >16 mm/yr slip rate since 1 ka at Hog Lake. We conclude that (1) overall slip rate of the San Jacinto fault has probably decreased significantly since fault inception, (2) slip rates vary over 1 to 5 kyr time scales and may be presently elevated due to a cluster of activity, and (3) other structures such as normal faults and folds could be accommodating up to one half of the deformation across the fault zone.
DE: 8106 Continental margins: transform
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: 2007 Fall Meeting