A new, 170 ka slip rate estimate on the Sierra Madre Fault

Nathaniel Lifton

Published August 9, 2017, SCEC Contribution #7461, 2017 SCEC Annual Meeting Poster #119

The Sierra Madre fault (SMF) system uplifts the San Gabriel Mountains along the northern Los Angeles metropolitan area. Slip rates on the Central SMF vary from ~3 mm/yr in geodetic studies to ~1 mm/yr in late Quaternary geologic studies. There have been no attempts to investigate potential variations in slip rate over time that could result from changes in fault activity in the Los Angeles re-gion. To obtain a long-term rate, we targeted a prominent geomorphic surface (Gould Mesa), preserved locally on the hanging wall of the SMF. A 3-m-thick, reddish-yellow argillic soil developed on the sur-face was previously estimated to be ca. 200-500 ka based on regional soil correlations. Our interpreta-tion of well data from the SMF footwall in the vicinity of the Jet Propulsion Laboratory constrains the vertical separation of this paleosol across the frontal strands of the fault zone to be ~ 260 m. We at-tempted to date the upper part of the Gould Mesa alluvial deposit using in situ cosmogenic nuclide isochron burial dating. Two suites of quartz-rich cobbles from natural and road-cut exposures were collected from ca. 8 m below the upper surface of Gould Mesa and analyzed for 10Be and 26Al, but low nuclide concentrations prevented reliable isochron results. Instead we used the muon production domi-nant at those depths to estimate exposure ages for the deposits from the samples with the lowest 10Be concentrations, yielding a mean exposure age estimate of 171±32 ka. This reduces age uncertainties at this site by a factor of ≥2 compared to the current value used by UCERF3, and yields a slip rate of 2.2 +1.2/-0.6 mm/yr. This rate is higher than the rate we have determined for younger terraces over the last ca. 70 ka, which could reflect intrinsic variability in the short-term rates or secular transfer of strain rate southward to other structures under the LA basin.

Key Words
cosmogenic, 10Be, 26Al, burial dating

Citation
Lifton, N. (2017, 08). A new, 170 ka slip rate estimate on the Sierra Madre Fault. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology