HOW STEADY IS THE GARLOCK FAULT

Geologic, geomorphic, paleoseismic and geodetic data reveal significant temporal and spatial variability in slip rates and strain on the Garlock fault (GF) in southern California. With the aim of better understanding its mechanical role in facilitating active strain and slip on the North American-Pacific plate margin, we synthesize: (1) existing long- and short-term slip rate data, with new data herein; (2) mapped strain patterns; (3) existing paleoseismic data; and (4) geodetic data for the GF.

The 250-km-long GF cuts orthogonally across the Eastern California shear zone and is the second-largest active fault in California. New geologic mapping of a 50-km section of the GF in Searles Valley (SV) and Pilot Knob Valley (PKV) give intriguing spatial and temporal complexities to the post-5 Ma mechanical history of this fault. New geologic slip-rate data include estimated 30-20 k.y. and 4-3 k.y. terraces with incised drainages offset ~230 and 47 m, respectively. Known slip-rate estimates of 4-7 mm/yr were derived from 13.8-10 k.y. (¹⁴C-age) offset shorelines in SE SV^§. Modern seismicity on the GF includes a Mw 4.4 (January 2009) at Goler Canyon in NW Fremont Valley and a Mw 3.2 (Spring 2008) in SE PKV. Pre-historical seismicity over a ~7000 k.y. period is highly irregular with 4 of 6 dated events within the past 2 k.y.^¶ Significant N-S shortening, uplift and inversion of the Plio-Pleistocene Christmas Canyon basin adjacent to the Slate Range, and potentially uplift of the Slate Range too, suggests the loci of strain and deformation is not constant in space and time. Geodetic data does not discern sinistral slip on the GF and suggests the structure is locked or currently inactive.

A synthesis of these seemingly conflicting data may be reasoned if the GF is to behave differently over varying timescales in terms of slip, strain and seismicity. It may be that the GF shares a genetic link between the San Andreas fault system and the ECSZ, whereby transient strain gradients intermittently affect the three structures. The difference between late-Holocene geologic and geodetic slip-rate estimates as well as the lack of geodetically documented strain may indicate that the GF is late in its earthquake cycle.

^§ McGill and Sieh, 1993, JGR, v. 98

^¶ Dawson et al., 2003, JGR, v. 108