Preliminary paleoslip results from the Pearblossom site on the Mojave section of the San Andreas Fault

Emery Anderson-Merritt, Eric S. Cowgill, Katherine M. Scharer, & Amanda Keen-Zebert

Published August 13, 2017, SCEC Contribution #7554, 2017 SCEC Annual Meeting Poster #156

The late Quaternary slip rate along the Mojave section of the San Andreas fault (MSAF) (~30 mm/yr) [1] appears to be twice as large as geodetic estimates (~15 mm/yr) [2], although it is unclear if this difference is real or stems from assumptions in each method. Establishing a robust Holocene geologic slip rate on the MSAF is essential for addressing this problem. Here, we investigate offset landforms at the Pearblossom site along the central MSAF. The site is located where a north-flowing, ephemeral channel crosses the fault. Upstream of the fault, the ~3 m deep channel is incised along the western edge of an alluvial fan (Qf1) and has small inset terraces; at the fault, the channel cuts through a bedrock shutter ridge. Downstream of the fault and east of the active channel, a gap in the shutter ridge contains a small outwash fan (Qf2) incised by a deflected and beheaded channel. The gap in the shutter ridge and the beheaded channel are displaced right-laterally by 67 ± 7 m and 39 ± 5 m, respectively, from the inferred source channel. Deposits and landforms at the site record the following sequence of events. First, the Qf1 fan aggraded behind the shutter ridge until the active channel was able to cross the shutter ridge, triggering incision of Qf1 upstream of the fault and deposition of Qf2 downstream. Subsequent translation of the ridge blocked cross-fault sediment transport and led to aggradation in the channel upstream of the fault. The channel was eventually captured by the current outlet, triggering a second phase of incision and leaving inset fill-cut terraces along the upstream channel. We dug pits in Qf1, Qf2, and a fill-cut terrace, and radiocarbon and luminescence analyses are in progress to provide age control for the site. The age of the shutter ridge gap is bracketed by the ages of Qf1 abandonment and onset of Qf2 deposition, and the age of the beheaded channel is bracketed by Qf2 abandonment and incision of the fill-cut terraces in the active channel.
1. Matmon et al., 2005, GSAB, v. 117 p. 795-807
2. Meade et al., 2005, JGR, v. 110 iss. B3

Citation
Anderson-Merritt, E., Cowgill, E. S., Scharer, K. M., & Keen-Zebert, A. (2017, 08). Preliminary paleoslip results from the Pearblossom site on the Mojave section of the San Andreas Fault. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)