New Holocene slip-rate sites along the Mojave San Andreas Fault near Palmdale, CA

Elaine K. Young, Eric S. Cowgill, & Katherine M. Scharer

Published August 24, 2016, SCEC Contribution #7029, 2016 SCEC Annual Meeting Poster #118

Geologic and geodetic slip rates for the Mojave segment of the San Andreas fault (MSAF) appear to be discrepant: the Quaternary geologic rate is as high as 37 mm/yr [1] while the geodetic slip rate is as low as ~15 mm/yr [2]. To better characterize the Holocene-average rate for the MSAF, we investigated several offset landforms near Palmdale, CA using detailed surficial mapping and 14C analyses of charcoal collected from hand-dug excavations. The first site, “X-12”, preserves two offset markers sourced from a north-flowing catchment that is south of the fault. The first marker is a west-facing terrace riser that was cut into older alluvium (Qoa) and offset ~75 m. The base of the riser is abutted by an alluvial fan (Qf) deposited against the riser. Progressive offset of the Qoa riser and Qf fan generated a north-facing fault-scarp that then eroded to deposit a small fan (Qff) on top of Qf north of the fault scarp. The second offset is a beheaded channel incised into Qf north of the fault that is separated 50 m from the source channel to the south. To date these landforms we opened five excavations at the site, with emphasis on the beheaded channel. Dates from within the Qf fan south/upstream of the fault and from abandoned bedload in the beheaded channel north/downstream of the fault provide maximum and minimum bounds on the age of incision of the beheaded channel of ~1500 calBP and ~600 calBP, respectively. Combining the 50 m offset with a maximum depositional age implies a slip rate of ≥ 33 mm/yr since the Qf fan was incised. Future work will date 1) post-abandonment deposits within the beheaded channel to obtain a minimum age for the beheaded channel and 2) Qff to provide an upper bound on the slip rate for the 75 m offset. At a second site, Ranch Center, a north-flowing stream cut across a shutter ridge north of the fault and was then offset and deflected ~80 m before cutting a new channel across the fault. Future work here will date the base of alluvial fans deposited by the abandoned and active channels to obtain maximum and minimum ages for the ~80 m offset, respectively. Rates from these two sites will help to better characterize the geologic Holocene slip rate on the MSAF. Future work will use rates from this study, and others, to explain the difference between longer-term geologic and more recent geodetic slip rates on the MSAF.
[1] Matmon et al., 2005, GSAB. v. 117 p. 795
[2] Becker et al., 2005, Geoph.. J. Int., v. 160 p. 634

Citation
Young, E. K., Cowgill, E. S., & Scharer, K. M. (2016, 08). New Holocene slip-rate sites along the Mojave San Andreas Fault near Palmdale, CA. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)