EVALUATING SLIP ALONG THE GARLOCK FAULT ZONE, EASTERN CALIFORNIA, AT TIMESCALES OF 104 TO 106 YEARS

The question of how dextral shear along the Eastern California Shear Zone (ECSZ) passes across the Garlock Fault Zone (GFZ) in eastern California remains uncertain. The GFZ has accumulated a total of ~65 km of left-lateral displacement at average rates of 6-7 mm/yr since the late Miocene and is presently embedded within the ECSZ, a broad zone of dextral shear. Shear along active ECSZ faults at the central GFZ has been established since at least ~4-6 Ma, yet these structures do not cross-cut the active trace of the Garlock fault. This configuration, and the recognition of significant (>10 km) displacement along inactive strands of the GFZ, suggests that the GFZ may passively rotate with distributed shear. In this work, we evaluate the timing and magnitude of slip along the central segment of the Garlock fault in the past ~2 Ma. To estimate the timing and amount of slip at 10⁴ timescales, we utilize both radiocarbon and U-series dating of lacustrine carbonate to date the 93 ± 6 m offset of a paleoshoreline at Koehn Lake to estimate slip rates of 5.5 - 7.7 mm/yr since 14.4 ± 1.5 ka. At longer timescales, ⁴⁰Ar/³⁹Ar dating of detrital sanidine populations provides maximum depositional ages (MDAs) for displaced alluvial fan and fanglomerate deposits. Near Goler Gulch, displacement of an alluvial fan terrace riser between ~800 and ~1450 meters yielded an MDA of 202 ± 19 ka and requires a minimum slip rate of 3.6 - 7.9 mm/yr. We present new soil profile development indices (PDI) at both the Koehn Lake site and near Goler Gulch. Soil PDI at these sites are minimum depositional ages and thus place lower bounds on displacement. Finally, new mapping of fanglomerate deposits with diagnostic provenance from the El Paso Mountains (Carter, 1994) allows reconstruction of displacement at timescales greater than 10⁵ years. Along with ⁴⁰Ar/³⁹Ar dating of detrital sanidine, this allows us to constrain minimum slip rates of 7.6 - 11.4 mm/yr and 4.4 - 6.2 mm/yr along the GFZ since ~770 ka and ~1.8 Ma, respectively. Collectively, our results suggest that slip rates along the Garlock fault have remained remarkably constant over time periods between the present and 2 Ma, despite accumulated shear along ECSZ faults.