2003 Seattle Annual Meeting (November 2–5, 2003)
Paper No. 255-3
Presentation Time: 1:30 PM-5:30 PM

COSMOGENIC 10BE AND 26AL ANALYSIS IN BOULDERS ON DISPLACED ALLUVIAL FANS: IMPLICATIONS FOR LONG-TERM SLIP RATES ALONG THE MOJAVE SECTION, SAN ANDREAS FAULT

MATMON, Ari, U.S. Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025, amatmon@usgs.gov, SCHWARTZ, David, U.S. Geol Survey, MS 977, 345 Middlefield Rd, Menlo Park, CA 94025, HANKS, Tom, U.S. Geol Survey, 345 Middlefield Rd, Menlo Park, 94025, FINKEL, Robert, Lawrence Livermore National Laboratories, Livermore, 94550, CLEMMENS, Samuel, USGS/LLNL, Lawerence Livermore National Lab, Livermore, CA 94550, and MUSHKIN, Amit, Department of Earth and Space Sciences, Univ of Washington, 63 Johnson Hall, Box 351310, Seattle, 98195

Remnants of alluvial fans are displaced 1-14 km from their original location at the mouth of Little Rock Creek (LRC) along the Mojave section of the San Andreas fault (SAF). Preliminary results of in-situ cosmogenic 10Be and 26Al analysis from boulders located on these fans suggests that their minimum ages, ranging from 14.3± 1.1 to 291± 31.0 Ky, increase with distance from the LRC.

The displaced fans consist of rounded to sub-rounded boulders (up to 1 meter) embedded in grus and sandy matrix. The main lithologies of the boulders are granite, quartz diorite, granodiorite, gneiss, gabro and amphibolite. All of these lithologies outcrop along the slopes of LRC.

The density of boulders at the surface of the fans decreases with distance from the mouth of the LRC. The closest fan surface (~1km) is rough with many exposed boulders whereas the most distant fan (~14km) has a smooth surface with hardly any exposed boulders. We suggest that as the fan surface matures, boulders erode into grus, which is transported and accumulates in depressions on the fan surface. With time depressions are filled to the point where boulders are not exposed and the original morphology of the fan’s surface is smoothed.

Preliminary ages of the offset fans imply an average long-term slip rate of 33± 6 mm/yr. The age calculation based on the cosmogenic analysis is sensitive to erosion rate and cosmogenic nuclide inheritance assigned to each sampled boulder. Additional analysis will help constrain these parameters and the long-term slip rate.

Several estimates of the slip rate between the Carrizo Plains and Cajon Pass have been previously calculated using 14C dating of offset morphologic features. In the Wallace Creek area, Sieh and Jahns (1984) calculated slip rates of 34± 3 mm/yr for the past 3700 years and 36± 5 mm/yr for the past 13250 years. A rate of 24± 3 mm/yr for the past 14400 years was calculated in the Cajon Creek area (Weldon and Sieh, 1985). Slip rates that range between 16 and 38 mm/yr for the last 3500 years were calculated in the LRC area (Schwartz and Weldon, 1987). Our results suggest that slip rate has been relatively constant for at least the past 300ky and lies within the range of previous late Pleistocene and Holocene estimates.

2003 Seattle Annual Meeting (November 2–5, 2003)
Session No. 255--Booth# 54
Neotectonics/Paleoseismology (Posters)
Washington State Convention and Trade Center: Hall 4-F
1:30 PM-5:30 PM, Wednesday, November 5, 2003

Geological Society of America Abstracts with Programs, Vol. 35, No. 6, September 2003, p. 580
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