Puzzling results in a slip rate study for the Banning Strand of the San Andreas Fault near North Palm Springs

Sally F. McGill, Paula M. Figueiredo, & Lewis A. Owen

Published August 15, 2017, SCEC Contribution #7878, 2017 SCEC Annual Meeting Poster #161

To better understand the likely rupture paths and slip distribution on the southern San Andreas Fault, additional slip rate measurements are needed from the Mission Creek, Banning and Garnet Hill fault strands in northern Coachella Valley. We present work in progress on the Banning strand of the San Andreas fault (BS-SAF) near North Palm Springs (N33.9132, W116.52671). A 1.2-m high terrace riser south of the BS-SAF appears to be an ancient western channel wall of Mission Creek that incised into a distal alluvial surface (Qal2). This terrace riser is separated by 148 ± 9 m from the western wall of Mission Creek on the north side of the BS-SAF. Several pits characterize the sedimentary units and allowed sampling for TCN 10Be, OSL and 14C dating of the displaced Qal2. Analysis of 10Be depth profiles is challenging, with one profile showing little decrease in 10Be concentration with depth and yielding a model age of 5 ka, while another profile yields a model age of 11–12 ka for Qal2. On the other hand, OSL and 14C ages are consistent with each other and suggest abandonment of Qal2 around 2 ka. These young ages are also reinforced by the absence of soil development. We infer that the 10Be ages are affected by unmodeled inheritance. These young ages of Qal2 imply an unrealistically high slip rate, if the correlation of the terrace riser with the west bank of Mission Creek is correct. To explore the possibility that our first set of ages was from overbank deposits rather than from Qal2 itself, we recently excavated several additional pits farther from the active channel. Preliminary OSL ages corroborate results from the earlier pits: the base of a new 2-m deep pit is 2.8 ka and two samples from a natural exposure of Qal2 in the eastern wall of Mission Creek, south of the fault, are also quite young (1-2 ka). Given the similar OSL and 14C ages for Qal2 it seems likely that Qal2 was indeed abandoned ~2 ka. We thus consider an alternative correlation of the truncated terrace riser south of the fault with the west edge of a gully north of the fault, which yields a right-lateral separation of 29 ± 3 m. If correct, this correlation would yield a slip rate (~14.5 mm/yr), which is more realistic, albeit still higher than previously published slip rates for the BS-SAF. However, this alternate correlation is also problematic, since it is difficult to explain the lack of preservation of the Qal2 surface south of the fault, east of the gully and west of the active channel of Mission Creek.

Key Words
slip rate, Banning, San Andreas

Citation
McGill, S. F., Figueiredo, P. M., & Owen, L. A. (2017, 08). Puzzling results in a slip rate study for the Banning Strand of the San Andreas Fault near North Palm Springs. Poster Presentation at 2017 SCEC Annual Meeting.

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