Latest Quaternary slip rates of the San Bernardino strand of the San Andreas fault, southern California, from Cajon Creek to Badger Canyon

Abstract

Four new latest Pleistocene slip rates from two sites along the northwestern half of the San Bernardino strand of the San Andreas fault suggest the slip rate decreases southeastward as slip transfers from the Mojave section of the San Andreas fault onto the northern San Jacinto fault zone. At Badger Canyon, offsets coupled with radiocarbon and optically stimulated luminescence (OSL) ages provide three independent slip rates (with 95% confidence intervals): (1) the apex of the oldest dated alluvial fan (ca. 30-28 ka) is right-laterally offset ⁓300–400 m yielding a slip rate of 13.5 +2.2/−25 mm/yr; (2) a terrace riser incised into the northwestern side of this alluvial fan is offset ⁓280–290 m and was abandoned ca. 23 ka, yielding a slip rate of 11.9 +0.9/−12 mm/yr; and (3) a younger alluvial fan (13–15 ka) has been offset 120–200 m from the same source canyon, yielding a slip rate of 11.8 +4.2/−35 mm/yr. These rates are all consistent and result in a preferred, time-averaged rate for the past ⁓28 k.y. of 12.8 +5.3/−47 mm/yr (95% confidence interval), with an 84% confidence interval of 10–16 mm/yr. At Matthews Ranch, in Pitman Canyon, ⁓13 km northwest of Badger Canyon, a landslide offset ⁓650 m with a 10Be age of ca. 47 ka yields a slip rate of 14.5 +9.9/−62 mm/yr (95% confidence interval). All of these slip rates for the San Bernardino strand are significantly slower than a previously published rate of 24.5 ± 3.5 mm/yr at the southern end of the Mojave section of the San Andreas fault (Weldon and Sieh, 1985), suggesting that ⁓12 mm/yr of slip transfers from the Mojave section of the San Andreas fault to the northern San Jacinto fault zone (and other faults) between Lone Pine Canyon and Badger Canyon, with most (if not all) of this slip transfer happening near Cajon Creek. This has been a consistent behavior of the fault for at least the past ⁓47 k.y.