DISCORDANT COSMOGENIC AND U-SERIES AGES FOR A LATE-PLEISTOCENE ALLUVIAL FAN OFFSET BY THE SOUTHERN SAN ANDREAS FAULT

Long-term fault-slip rates are key observable parameters of fault behavior that must be determined by dating measurably offset landforms. Reliability of such slip-rates is in part limited by the accuracy and precision of dates for Quaternary deposits such as alluvial fans, fluvial terraces, and moraines. The T2 surface of the Biskra Palms fan near Palm Springs California (van der Woerd, 2006, JGR; hereafter, VDW), is offset hundreds of meters by the southern San Andreas Fault (SAF), and with improved measurements of age and offset we refine the late-Pleistocene slip-rate on the Coachella segment of the SAF. Herein, we contrast ages for T2 determined using: (1) ¹⁰Be on desert pavement (VDW), and (2) U-series on early-formed pedogenic carbonate (Fletcher 2006, AGU Fall Meeting).

¹⁰Be ages for T2 pavement cobbles (n = 20) range from 31 to 46 ka with median uncertainty of ~7.5 ka (all errors 2-sigma). Eliminating the oldest age (considered an inherited outlier), VDW determined a mean cobble age of 35.5 ± 5.0 ka, and considering clast and surface erosion negligible, interpreted the exposure age as the depositional age of T2.  Soil development related to the T2 surface includes dense, pure pedogenic carbonate coatings on gravel-clasts at depths of 1.7-2.0 m. Carefully selected milligram-size samples of thin (< 0.5 mm), inner carbonate coatings analyzed by TIMS yield U-series ages of 30 to 46 ka (n = 11) with median uncertainty of 0.8 ka. We rule out open U-Th systems and inherited clast-coats based on excellent reproducibility of sub-samples of 45 ka carbonate, and interpret all U-series ages as minimum estimates of the age of T2. Accordingly, the depositional age of gravels at the T2 fan surface is at least 45.3 ± 0.5 ka, ~30% older than the mean ¹⁰Be pavement age of 35.5 ka.

We infer that erosion of cobbles and exhumation of partially shielded cobbles (by relaxation of primary topography, deflation, overland flow, and bioturbation) have yielded pavement clasts whose mean ¹⁰Be age is significantly younger than the depositional age of the T2 landform, as observed for desert pavement on alluvium elsewhere.