Quaternary Slip History of the Central Sierra Madre Fault, Southern California

Reed J. Burgette, Katherine M. Scharer, Nathaniel Lifton, Austin Hanson, Devin McPhillips, & Tammy M. Rittenour

Published August 13, 2018, SCEC Contribution #8447, 2018 SCEC Annual Meeting Poster #254

The Central Sierra Madre thrust fault (CSMF) accommodates uplift of the San Gabriel Mountains along the northern Los Angeles metropolitan area. A suite of Quaternary terrace and fan surfaces are preserved near Arroyo Seco and offset across strands of the CSMF. We estimate fault slip (with uncertainty) from offset late Quaternary alluvial fan surfaces by analyzing swath profiles from lidar and a range of fault dips from published maps and trench studies. We determined the surface ages of the offset fan terraces combining 10Be depth profile dating and IRSL dating. Two of the most prominent late Quaternary geomorphic surfaces along the CSMF have modeled ages of ~55 and ~35 ka and are offset by ~28 and ~14 m, respectively. A thick, red, argillic soil developed on the oldest terrace surface is vertically separated across the fault ~260 m, based on surface and well data. Two sets of quartz-rich cobbles were collected for 10Be/26Al isochron burial dating from ca. 8 m below the terrace and analyzed for cosmogenic nuclide abundance, but failed to yield robust isochrons due to uniformly low nuclide concentrations. Using the muon production dominant at the sampled depths and the lowest consistent 10Be concentrations from the two sites, we derived a mean exposure age estimate of ~170 ka.

In the Arroyo Seco area the calculated CSMF slip rate is ~1 mm/yr since ~55 ka and the time-averaged rate since ~170 ka is ~2 mm/yr, although the rates at the 95% confidence interval are considered equivalent. Fault dip is the dominant factor in slip rate uncertainty, and to make the slip rate constant would require that the fault dip has shallowed over time, for which we have no evidence. Given the multiple earthquake cycles over which the slower latest Quaternary rate is averaged, transfer of strain rate southward to other structures in the Los Angeles basin appears likely. The Quaternary rates provide spatial context for comparison with geodetic slip rates and late Cenozoic exhumation of the San Gabriel Mountains. The late Quaternary change in slip rate provides evidence of the ongoing evolution of plate boundary deformation.

Burgette, R. J., Scharer, K. M., Lifton, N., Hanson, A., McPhillips, D., & Rittenour, T. M. (2018, 08). Quaternary Slip History of the Central Sierra Madre Fault, Southern California. Poster Presentation at 2018 SCEC Annual Meeting.

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Earthquake Geology