Long-Term Slip Rate on the Southern San Andreas Fault Determined by Th-230/U Dating of Pedogenic Carbonate.

Determinations of long-term slip rates are limited, in part, by our ability to accurately estimate the age of offset landforms. U-series dating on pedogenic carbonate provides a relatively novel way of dating landforms, with strengths that complement more widely applied cosmogenic (CRN) techniques. We present new Th-230/U dates for pedogenic carbonate coatings on pebbles of the Biskra Palms fan, near Indio, California, which is offset by the southern San Andreas fault. Small, carefully chosen samples of dense pedogenic carbonate analyzed by mass spectrometry, have 3-10 ppm uranium and low common thorium (Th-232), making them highly favorable for U-series dating. Only minor corrections for initial Th-230 are necessary, and are made using Th-232 as an index isotope with propagation of uncertainties. Samples of early-formed carbonate collected from depths of about 2 m in fan soils typically consist of dense coatings 200-500 microns thick. Such coatings from 6 pebbles from 3 different locations within the fan yield apparent ages between 30 ± 2 ka and 46 ± 2 ka (all errors 2 sigma), with a median age of 38.4 ka (n= 11). Each age averages over the sampled interval of coating growth, hence the spread of ages reflects clast-to-clast variation in coating accumulation rates. All ages are therefore minimum ages for the stabilization of the fan because the time lag between stabilization and carbonate pedogenesis at Biskra Palms is, as yet, unknown. Sub-samples of individual clast-coatings yield ages in good agreement-- e.g., 45.0 ± 0.8 ka, 46.0 ± 1.8 ka, 44.8 ± 1.0 ka, 45.7 ± 0.9 ka (MSWD = 0.88), demonstrating closed U-Th systems. Van der Woerd et al (2006) reported an average CRN age of 35.5 ± 2.5 ka for the fan surface and an offset of 565 ± 80 m, for a slip rate of 15.9 ± 3.4 mm/a. Using their offset, and our oldest mean age of 45.3 ± 0.5 Ma (n=4, ages above) as the minimum landform age, we estimate a maximum slip rate of 12.5 ± 1.8 mm/a. This maximum average slip rate for the past 45 ka is distinct from both modern rates for the southern San Andreas determined by geodesy (e.g. 23 ± 2 mm/a; Bennett et al., 2006), and the rate at Cajon Pass (24.5 ± 3.5 mm/a) determined for the past 15 ka (Weldon and Sieh, 1985). As noted by van der Woerd et al, such discrepancies suggest that: 1) large changes in slip-rate occurred prior to 15 ka, 2) the Biskra Palms site fails to record all of the slip on the southern San Andreas, or 3) that southward of Cajon Pass significant slip is transferred to other faults. More U-series dating is needed to understand the patterns of pedogenic carbonate accumulation on the scale of the fan, in selected soil profiles, and within individual coatings. In particular, sampling of the coatings at higher spatial resolution will better determine the age of onset of carbonate accumulation, thereby providing a closer estimate of the time of fan stabilization and abandonment.