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Testing for slip rate changes on the Sierra Madre fault: Progress on dating an offset terrace surface of possible middle Pleistocene age

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

Published August 15, 2016, SCEC Contribution #6946, 2016 SCEC Annual Meeting Poster #099

The Sierra Madre fault (SMF) system juxtaposes the San Gabriel Mountains against a series of basins along the northern margin of the Los Angeles metropolitan area. Previous studies have suggested that deformation rates on this fault vary spatially as part of the broader plate boundary system, or that the locus of deformation has migrated southward into the Los Angeles basin over the Quaternary. A well-preserved flight of fan terraces in the Arroyo Seco area of Altadena and Pasadena, CA provides a location to assess whether there is a long-term reduction in strain on the SMF. A complementary slip-rate study (Hanson et al., this meeting) focused on lower fan terraces that are offset vertically ~3-20 m, providing late Pleistocene slip rates for this fault. This project targets the prominent Gould Mesa geomorphic surface thought to be middle Pleistocene in age, locally preserved across the Central SMF. The Gould Mesa surface is capped by a thick, deep red soil, and is thought to be ~200-500 ka based on published regional soil correlations. Nearby wells in the footwall of the SMF contain a thick red clay, interpreted to be correlative with this surface soil, and constrain the vertical separation of the top of the Gould Mesa surface across the frontal strands of the fault zone to be ~250 m. We are applying cosmogenic nuclide isochron burial dating to determine the age of the upper part of the Gould Mesa alluvial deposit. Burial dating analyzes pairs of radionuclides (10Be and 26Al in this case) produced by high-energy cosmic rays in clasts at the surface (and potentially during transport) that have subsequently been deeply buried. Decay of the radionuclides at different rates makes their ratio sensitive to duration of burial, and analysis with an isochron technique yields ages since burial. We collected quartz-rich cobbles from natural and roadcut exposures from 8-10 m below the upper surface of Gould Mesa at two locations. Each set of samples was collected from a layer <50 cm thick to ensure the clasts have had a common burial history. Physical sample processing is complete, and the samples are now undergoing chemical processing and analysis. Based on synthetic modeling of the burial depth and inheritance values from other cosmogenic studies on the SMF, we anticipate age uncertainties for each of the two isochron burial dating sites to be ~70 ka or better, which we hope will reduce the uncertainties on the long-term Quaternary slip rate at this site by a factor of ≥2 compared to the current level used by UCERF3.

Burgette, R. J., Lifton, N., Scharer, K. M., McPhillips, D., & Hanson, A. (2016, 08). Testing for slip rate changes on the Sierra Madre fault: Progress on dating an offset terrace surface of possible middle Pleistocene age. Poster Presentation at 2016 SCEC Annual Meeting.

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Earthquake Geology