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

Plamen N. Ganev, James F. Dolan, Sally F. McGill, & Kurt L. Frankel

Published August 2012, SCEC Contribution #1483

Measurement of a stream offset, coupled with Be-10 cosmogenic radionuclide dating of the alluvial surface (Qf2) into which the stream incised, yields a minimum latest Pleistocene-Holocene slip rate of 5.3 +1.0/-2.0 mm/yr along the central part of the Garlock fault. However, our preferred 70 ± 7 m restoration of the preserved Qf2 fan edge, which is associated with the initial incision, does not restore the deeply incised channel thalweg. Our preferred restoration for the channel thalweg is 58 ± 4 m, suggesting that deep channel incision must have occurred after ~10-15 m of sinistral offset that accrued along the Garlock fault. Climate considerations, specifically the end of the Younger Dryas period at ~11.5 ka and the onset of a summer monsoon rainfall pattern at ~8-10 ka, provide plausible time frames for the deep incision event. If the deep incision was related to one or the other of these climate events, then the resulting slip rate would be 5.1 ± 0.3 mm/yr and 6.6 ± 1.2 mm/yr, respectively. These rates are consistent with previous geologic slip rates, most of which suggest a latest Pleistocene-Holocene slip rate of ~5-7 mm/yr. These geologic slip rates, however, contrast markedly with much slower (at least twice as slow) short-term rates of elastic strain accumulation constrained by geodesy. The new Summit Range 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 that the long-term average in order to account for the relatively rapid geologic slip rates measured along the central part of the fault.

Ganev, P. N., Dolan, J. F., McGill, S. F., & Frankel, K. L. (2012). Constancy of geologic slip rate along the central Garlock fault: Implications for strain accumulation and release in southern California. Geophysical Journal International, 190(2), 745-760. doi: 10.1111/j.1365-246X.2012.05494.x.