NEW HOLOCENE SLIP-RATE SITE ALONG THE CENTRAL GARLOCK FAULT, SEARLES VALLEY, SOUTHEASTERN CALIFORNIA

The Garlock fault is a left-lateral fault about 250 km long that extends northeastward and eastward along the northern margin of the Mojave Desert. Geodetic studies imply that present-day elastic strain accumulation is primarily occurring along northwest-striking, right-lateral faults both north and south of the Garlock fault and that very little left-lateral strain is accumulating across the Garlock fault. Nonetheless, the Garlock fault is a through-going structure with abundant evidence for Holocene slip. Along the stretch of fault east of Trona Road, in Searles Valley, numerous incised channels and alluvial fans of apparent Holocene or latest Pleistocene age are offset tens of meters. For this study we present mapping of an offset alluvial fan located about 4.2 km east of Trona Road. The offset fan is best reconstructed by restoring 58 ± 8 meters of left-lateral slip. The slightly incised east edge of the fan provides the sharpest offset marker, but the crest of the fan and the tapered west edge of the fan appear to be offset the same amount. We dug a 2-m deep pit on the center of the fan a few tens of meters north of the fault and collected samples for Be-10 and optically stimulated luminescence (OSL)dating. Because the measured offset applies to the constructional crest and depositional west edge of the fan, the surface exposure age of the fan from the Be-10 depth profile and the burial age of the uppermost fan deposits from OSL samples 65 and 80 cm deep, will provide a close (rather than a maximum) estimate of the time required to accomplish the 58 meters of offset. This site may thus provide an important new Holocene slip rate for the central Garlock fault.