LATE PLEISTOCENE SLIP RATE ON THE COACHELLA VALLEY SEGMENT OF THE SAN ANDREAS FAULT AND IMPLICATIONS FOR REGIONAL SLIP PARTITIONING

Partitioning of slip between the southern San Andreas (SAF) and San Jacinto (SJF) faults is significant for understanding earthquake behavior, seismic hazards, fault zone evolution, and crustal interactions associated with ongoing reorganization of the plate boundary. However, long-term (>10 kyr) slip rates on these faults are poorly known due to limited data and difficulty of estimating ages of alluvial surfaces using soil chronology. Even at sites where surface ages are well known, erosion and deformation of Pleistocene alluvial features can complicate the calculation of fault offset and slip rates. This problem has influenced previous interpretations of an offset late Pleistocene alluvial fan at Biskra Palms, Coachella Valley segment of the SAF. Strike-slip offset of the fan was estimated at ~700 m by Keller et al. (1982). Van der Woerd et al. (2001) obtained an exposure age of 30.1 +/- 2.4 ka from Be and Al isotopes, and used the 700 m offset to calculate a slip rate of 23.3 +/- 3.5 mm/yr. This result is suspect because the original measurement of offset assumes a linear fan margin oriented perpendicular to two strike-slip fault strands, but low-sun-angle aerial photography shows that the original fan margin is curved and trends at an oblique angle to the faults. A detailed reconstruction of the fan margin indicates total offset of ~450 +/- 40 m, and the revised slip rate is ~15 +/- 3 mm/y, significantly slower than previous estimates. This is a maximum rate because (1) in the absence of inheritance complications, exposure ages provide a minimum estimate of surface age, and (2) cross-fault shortening may have produced some apparent right-lateral separation. A rate of 15 +/- 3 mm/yr at Biskra Palms is consistent with measured rates of Pleistocene shortening in San Gorgonio Pass and San Bernadino Mts, and 15-20 mm/yr slip rate on the SJF, as presented in other studies. Modern seismicity and tectonic geomorphology provide further evidence that slip is transferred southward from the SAF to the SJF across the San Bernadino basin, a large transtensional basin bounded on the SE by active normal faults. These observations support a model in which the SJF was initiated at ~1.5 Ma in response to crustal shortening in the San Gorgonio restraining bend (Morton and Matti, 1993), but large gaps in existing data highlight the need for new work in this region.