U-SERIES AGES FOR THE BISKRA PALMS ALLUVIAL FAN AND THEIR IMPLICATIONS FOR LONG-TERM SLIP-RATE OF THE COACHELLA VALLEY SEGMENT OF THE SAN ANDREAS FAULT, SOUTHERN CALIFORNIA

The Biskra Palms alluvial fan was deposited across the Mission Creek strand of the San Andreas Fault (SAF), and has the potential to reveal the long-term slip-rate of the SAF in the Coachella Valley region. Previous studies have attempted to estimate the age of the fan deposition by using degree of soil profile development (20-30 ka; Keller et al., 1982), and ¹⁰Be dating of pavement clasts (35.5 ± 5 ka; van der Woerd et al., 2006). We present new U-series dates for pedogenic carbonate coatings on pebbles from four soil pits on the T2 surface (van der Woerd et al, 2006 terminology), both above and below the SAF. Carefully chosen samples of dense carbonate analyzed by mass spectrometry have 3-10 ppm U and low concentrations of common Th (²³²Th), making them ideal for U-series dating. Samples of early-formed carbonate from ~1.7-2 m deep typically consist of dense coatings from 200-500 microns thick. Due to the time-lag between deposition of the fan and accumulation of datable thicknesses of soil carbonate, all U-series ages are minimum estimates for the depositional age of the fan. Apparent ages range from 29 ± 3 to 46 ± 2 ka (all errors 2-sigma). Sub-samples from an individual clast coating yields ages in good agreement (e.g., 45.0 ± 0.8, 46.0 ± 1.8, 44.8 ± 1.0, and 45.7 ± 1.0 ka; mean = 45.2 ± 0.5 ka), indicating a closed U-Th system and no inherited component in the dated coating. Maximum ages of pedogenic carbonate from different soil pits are similar, i.e., the ages from the upper fan surface (stated above) overlap with ages of 44.2 ± 0.6 and 44.1 ± 0.9 ka from the lower fan surface, which confirms our determination based on pedogenic description that the dated soil formed after abandonment of the T2 surface. The T2 fan surface must be older than the mean age of our oldest clast coating, 45.2 ± 0.5 ka. Re-evaluation of the amount of offset across the SAF is in progress, however it is likely that our older age will result in an average slip-rate for the past 45 ka that is significantly slower (on the order of ~13 mm/yr) than decadal slip-rates determined by geodesy in the Coachella Valley, suggesting either: (1) that the Biskra Palms site fails to record all of the slip accumulated on the southern SAF, (2) slip-rates depend on the time-scale of observation, or (3) geodetic slip-rates have larger epistemic uncertainties than generally acknowledged.