Resolving the late Quaternary slip rate of the Santa Susana fault with in situ cosmogenic 36Cl profile dating

Reed J. Burgette, Michael P. Reed, Nathaniel Lifton, Katherine M. Scharer, Devin McPhillips, & Ronny Sholdt

Submitted August 14, 2020, SCEC Contribution #10643, 2020 SCEC Annual Meeting Poster #002

The Santa Susana fault (SSF) is a significant reverse fault in the western Transverse Ranges that connects structures bounding the Ventura and San Fernando basins. Based on published studies, the SSF is permitted a negligible to significant (0.5-10 mm/yr) slip rate in the UCERF3 model; among the largest range of any fault in the model. This work investigates the slip rate of the SSF over the late Quaternary. We identified a site along the western SSF where low relief geomorphic surfaces underlain by alluvium are vertically separated across the trace of the SSF. Exposures in both the hanging wall and footwall show that similar Quaternary sediment has been preserved on both sides of the fault, and represents a dateable, isochronous marker of fault offset. Vertical separation of this alluvial fan surface is ~25 m. We used whole-rock in situ 36Cl techniques since the source catchment of the target fan surface is developed in mudstone of the Modelo Formation, and the fine grain size is not amenable to using 10Be or 26Al in quartz. We collected and analyzed whole-rock samples of amalgamated pebbles from three depth profiles. In situ 36Cl is produced in near-surface rock and sediments via multiple production pathways, and the recent CRONUS-Earth project produced a robust, internally consistent set of in situ 36Cl production parameters for each of these pathways. We measured bulk density of the sediment using photogrammetric analysis of samples from multiple levels in the profiles. Our work in progress is adapting code from the CRONUS-Earth project to allow age estimation from the depth profile data using nonlinear least squares optimization to explore a parameter space that includes age, erosion rate, and inherited nuclide inventory. Based on preliminary analysis, the age of the offset surface is likely in the range of ~30 to 10 ka. When combined with an estimated ~43 m of dip slip since abandonment of the surface, the late Quaternary slip rate is highly likely < 5 mm/yr and likely to be in the lowest part of the previously published range, consistent with qualitative geomorphic observations and modeling of geodetic data. Our work shows the promise of 36Cl dating to yield numerical ages for late Quaternary surfaces that lack sediment with quartz grains of appropriate size, and suggests that a significant fraction of the deformation formerly accommodated by the SSF has migrated to other structures across strike.

Burgette, R. J., Reed, M. P., Lifton, N., Scharer, K. M., McPhillips, D., & Sholdt, R. (2020, 08). Resolving the late Quaternary slip rate of the Santa Susana fault with in situ cosmogenic 36Cl profile dating. Poster Presentation at 2020 SCEC Annual Meeting.

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