LATEST PLEISTOCENE SLIP RATES OF THE SAN BERNARDINO STRAND OF THE SAN ANDREAS FAULT AT BADGER AND PITMAN CANYONS, SOUTHERN CALIFORNIA

We mapped and dated several generations of latest Pleistocene alluvial fans that are offset from their source in Badger Canyon due to slip along the San Bernardino strand of the San Andreas Fault (SAF). Offset estimates coupled with calibrated radiocarbon (14C) and optically stimulated luminescence (OSL) ages provide three independent slip rate estimates from two different alluvial fans. (1) The right-lateral offset of the apex of the older fan is represented by a trapezoidal probability density function (pdf) with a flat top from 300-400 m, tapering to zero at 280 m and 470 m. A charcoal sample from sediments below the apex of the fan yielded a 14C age of 28.4 ± 0.3 ka, providing a slip rate of 10-16 mm/yr. (All reported dates are ± 2σ and slip rate ranges are 95% confidence intervals). (2) A terrace riser incised into the northwestern side of this fan is offset about 280 m, with a triangular pdf tapering to zero at 236 m and 290 m. Deposition on the part of the fan below this riser ceased with the emplacement of a landslide at 23.1 ± 0.35 ka, yielding a slip rate of 11.6 ± 1 mm/yr. (3) The offset of the younger alluvial fan is represented by a trapezoidal pdf with a flat top from 110-210 m, tapering to zero at 85 m and 280 m. Samples collected from sediments of this fan provide ages of 13.3 ± 2.2 ka (OSL) and 14.86 ± 0.27 ka (14C), yielding a slip rate of 7-20 mm/yr.

In the vicinity of Pitman Canyon, an offset landslide with a 10Be age of 34.1 ± 3.7 ka (using the Lal[1991]/Stone[2000] time-dependent model) yields a slip rate of 20 ± 6 mm/yr. A similar rate is suggested by the poorly defined ~ 18 m offset of a ~1000 ka channel.

The slip rates from Badger Canyon are similar to the published SAF slip rate of 7-16 mm/yr at Plunge Creek, located 18 km to the southeast, yet they are only half as large as the published slip rate of 24.5 ± 3.5 mm/yr at Cajon Creek, 16 km to the northwest. The rates from Pitman Canyon (2.5 km southeast of Cajon Creek) are intermediate between those at Cajon Creek and Badger Canyon. These data suggest that slip transfers from the Mojave section of the SAF to the northern San Jacinto fault zone between Cajon Creek and Badger Canyon. The slip transfer most likely occurs within the 14-km long section (from 3 km northwest to 11 km southeast of Cajon Creek) where the two faults are parallel and only 1.8-3.2 km apart.