New interpretations at the Ranch Center and Key Slide slip rate sites, Mojave section of the San Andreas Fault, Palmdale, CA

Elaine K. Young, Katherine M. Scharer, Amanda Keen-Zebert, Emery Anderson-Merritt, Eric S. Cowgill, & Ray J. Weldon

Published August 15, 2019, SCEC Contribution #9794, 2019 SCEC Annual Meeting Poster #130

The Holocene slip rate on the ~100km long Mojave section of the San Andreas Fault (MSAF) is not well known, despite its importance for understanding apparent discrepancies between geologic and geodetic slip rates along this fault section and potential long-term fault interactions in southern California. Here we build on our previous work at the Ranch Center site by presenting a new luminescence data set and a reinterpreted offset magnitude. We also introduce a new interpretation of the Key Slide site (also known as Ritter Ranch [1]). At Ranch Center, a channel is deflected 47.7-95.5m behind a shutter ridge. To bracket the age of the offset landform, we date deposits emplaced before and after the channel formed. The channel is incised into alluvium, Qa, which has a radiocarbon age of ~4200calBP and a luminescence age between ~4000-2000yrBP. The post-channel deposit, Qf1a, has a radiocarbon age of ~3700-2000calBP and a luminescence age between ~2800-1700yrBP. The Ranch Center slip rate permitted by the luminescence ages is ~12-56mm/yr, and the rate determined by radiocarbon ages is 16.1-26.2mm/yr. The Key Slide site consists of an offset landslide. Archeological investigation and full excavation of the western edge of the landslide provide timing and displacement used for the slip rate at this site. The landslide extended down a north-facing slope and across the MSAF onto the floodplain of Amargosa Creek (Qal). The 60.4m offset of the excavated western margin of the slide provides a single displacement estimate [2]. The eastern margin of the landslide was not excavated and its exact location is unknown. Samples were collected from hearths on the Qal surface that were covered by the landslide [2]. The youngest radiocarbon samples from the hearths provide the maximum age, 2850-2755calBP, of the landslide. Combining the maximum age with the offset of the western margin of the landslide yields a maximum slip rate of 21.9mm/yr for the Key Slide site. These results suggest consistent slip histories between the Ranch Center and Key Slide site. The two sites span <500m and provide similar and independently determined slip rates. The X12 site [3], ~20km SE along the MSAF, has a slip rate of ~28-108mm/yr since ~1700calBP. This suggests that the MSAF was slower prior to ~2ka.

[1] Weldon, 2009, USGS/NEHRP #G09AP00012 (UO #238610) Report
[2] Swiatek et al., 2008, Progress Report for 2008, SCEC Award #08109
[3] Young et al., 2018, SCEC Annual Meeting 2018

Key Words
San Andreas Fault, Slip Rate, Holocene, Radiocarbon, Luminescence

Citation
Young, E. K., Scharer, K. M., Keen-Zebert, A., Anderson-Merritt, E., Cowgill, E. S., & Weldon, R. J. (2019, 08). New interpretations at the Ranch Center and Key Slide slip rate sites, Mojave section of the San Andreas Fault, Palmdale, CA. Poster Presentation at 2019 SCEC Annual Meeting.

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