Constancy of geologic slip rate along the central Garlock fault: implications for strain accumulation and release in southern California

Abstract

SUMMARY We present new slip rate estimates for the central Garlock fault. The incised edge of an alluvial fan deposit with a 10Be cosmogenic radionuclide age of 13.3 +5.9/−1.1 ka is offset 70 ± 7 m yielding a slip rate of 5.3 +1.0/−2.5 mm yr−1. A stream thalweg that is deeply incised into this surface is offset a lesser amount (58 ± 4 m) providing the lower bound for this rate. Climate considerations, however, suggest that the deep incision of this thalweg may have occurred at a younger date, specifically at the end of the Younger Dryas period at ∼11.5 ka or at the onset of a summer monsoon rainfall pattern at ∼8–10 ka. If the deep incision was related to one of these climate events, then the resulting slip rate would be 5.1 ± 0.3 and 6.6 ± 1.2 mm yr−1, respectively. All of the slip rates are consistent with previous geologic slip rates, most of which suggest a latest Pleistocene–Holocene slip rate of ∼5–7 mm yr−1. These geologic slip rates, however, contrast markedly with much slower (at most half as fast) short-term rates of elastic strain accumulation constrained by geodesy. The new slip rate thus adds to a growing body of evidence indicating that the Garlock fault is experiencing a period of transient lack of strain accumulation, in which the lower crust (and mantle?) beneath the fault is deforming at a rate that is much slower than its long-term average rate. These observations suggest that the Garlock fault experiences two modes of strain accumulation: the current ‘slow’ mode, in which strain accumulates very slowly along the fault, and ‘fast’ mode, during which the fault must store and release elastic strain energy at much faster rates than the long-term average to account for the relatively rapid geologic slip rates measured along the central part of the fault.