Applying newly developed luminescence dating to alluvial fans in the Anza Borrego Desert, southern California

Brittney Emmons, Seulgi Moon, Nathan D. Brown, Kimberly D. Blisniuk, & Ed J. Rhodes

Published August 24, 2016, SCEC Contribution #6949, 2016 SCEC Annual Meeting Poster #117

Over recent decades, multiple geochronologic tools have been used to date various geomorphic offsets (e.g., alluvial fans, fluvial terraces), enabling long-term slip rate studies of active faults. In this study, we show the robustness of the newly developed ‘p-IR-IRSL225’ technique as a geochronometer to date alluvial fan deposits in the Anza Borrego Desert of southern California. This new method is based on two infrared stimulated luminescence (IRSL) measurements, one at 50°C and another at 225°C (referred to as p-IR-IRSL225) and takes advantage of potassium feldspar grains that have a brightness higher than quartz used in OSL dating (Rhodes, 2015). We applied this technique to date alluvial fans that were offset by the San Jacinto fault zone and were previously dated with 10Be exposure of surface clasts and U-series disequilibria from pedogenic carbonates from subsurface clast rinds (Blisniuk et al., 2012). Initial results from the p-IR-IRSL225 method without fading correction yield dates of 5.5 +/- 0.4 (1 sigma) ka in Ash Wash, and 6.5 +/- 0.7 ka and 5.2 +/- 0.7 ka in Santa Rosa Mountains sites in the Anza Borrego Desert. These p-IR-IRSL225 dates are consistent with dates from U-series of soil carbonates, but are slightly younger than those from 10Be exposure at each location. Depositional dates from p-IR-IRSL225 indeed lay between the maximum 10Be exposure ages and the minimum U-series ages. These robust results suggest that the p-IR-IRSL225 dating technique is an excellent tool for quantifying the rate of fault movement where other dating methods cannot be applied or where dual dating techniques should be applied to extend the record of active fault deformation in southern California and elsewhere around the world.

Key Words
Luminescence, Alluvial Fan, Anza Borrego Desert, San Jacinto Fault, IRSL

Citation
Emmons, B., Moon, S., Brown, N. D., Blisniuk, K. D., & Rhodes, E. J. (2016, 08). Applying newly developed luminescence dating to alluvial fans in the Anza Borrego Desert, southern California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)