A 2-MY HISTORY OF SLIP ALONG THE GARLOCK FAULT ZONE, EASTERN CALIFORNIA

The Garlock fault zone, in eastern California, is an intracontinental transform that initiated in the late Miocene and subsequently accommodated ~65 km of left-lateral displacement at average rates of 6-7 mm/yr. The fault zone is presently embedded within a broad zone of dextral shear, the Eastern California Shear Zone (ECSZ), that passes across the Mojave province and extends northward along the western margin of the Basin and Range province. Finite displacement across structures in the central ECSZ totals at least ~18 km (Andrew and Walker, 2017), yet individual faults terminate against the Garlock fault zone without truncating it. To evaluate slip along the Garlock fault during the past ~4 Ma of deformation along the ECSZ, we assess slip rates along the central segment of the Garlock fault over timescales ranging from ~10 ka to ~2 Ma. At Koehn Lake, new radiocarbon and U-series dating of lacustrine carbonate associated with a paleoshoreline of Koehn Lake displaced by 93 ± 6 m suggests slip rates of 5.5 - 7.7 mm/yr since 14.4 ± 1.5 ka. ⁴⁰Ar/³⁹Ar dating of detrital sanidine from alluvial fan and fanglomerate deposits provides maximum depositional ages (MDAs) which place bounds on displacement along the Garlock during the Pleistocene. 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. Unique lithologic provenance allow reconstruction of fanglomerate deposits to distinct watersheds within the El Paso Mountains (Carter, 1994). Fanglomerates with an MDA of 767 ± 23 ka have been displaced by 6-8.5 km, requiring a minimum slip rate of 7.6 - 11.4 mm/yr. Similarly, deposits with provenance from Mesquite Canyon require 8.3 - 11.3 km displacement in the past 1.84 ± 0.033 Ma and minimum slip rates between 4.4 - 6.2 mm/yr. Our results imply that slip rates along the Garlock fault have remained relatively constant over the past ~2 Ma, during accumulated shear along ECSZ faults. Interval slip rates may have been elevated in the past 0.8 Ma, but require additional constraints from deposits of this age. Overall, the application of ⁴⁰Ar/³⁹Ar dating of detrital sanidine can be a powerful tool to place new constraints on the age of ancient sedimentary deposits, particularly in the presence of a source of young silicic volcanism.