Late Miocene to Holocene Faulting Along the Southwestern Inyo Mountains Fault Zone, Eastern California

John Casteel
December 2005

Abstract

New geologic mapping and geochronology have documented the first slip rate estimates along the southwestern Inyo Mountains (SWIM) fault zone, eastern California. The SWIM fault zone is located along the southwestern flank of the Inyo Mountains, east of Owens Lake, and records evidence for fault slip from the late Miocene to the Holocene. The fault zone is composed mainly of east and west-dipping normal and dextral oblique-slip faults which strike ~NW in the northern segment and ~NNE in the southern segment. Faults offset Miocene to Pliocene alluvial fan sediments and basalt lavas, and Quaternary alluvial fan surfaces, Qf1, Qf2a and Qf2b. Offset basalt lava ages are constrained from ⁴⁰Ar/³⁹Ar geochronology to be between 5.22 ± 0.17 Ma and 6.22 ± 0.05 Ma. The age of an offset Qf1 fan surface is younger than an underlying 6–9 Ma tephra and older than a cosmogenic radionuclide model surface-exposure age of 88 ± 17 ka. A Qf2a fan surface is constrained by a cosmogenic radionuclide model surface-exposure age of 141 ± 28.2 ka, and Qf2b fan surfaces are younger than Qf2a fan surfaces and older than ~50 ka Owens Lake paleoshorelines which truncate Qf2b fan surfaces. The magnitude of maximum vertical offset for basalt lavas and Qf1 fan surfaces is 600 ± 60 m and 32.7 ± 6.5 m respectively. The magnitude of maximum dextral offset for basalt lavas and Quaternary fan surfaces is 1100 ± 100 m, and 170 ± 25 m respectively. For Miocene to Pliocene basalt lavas, we estimate a maximum vertical slip rate of 0.1 ± 0.1 mm/yr and a net oblique slip rate vector of 0.2 ± 0.1 mm/yr at 27º N52W. Because our Qf1 surface age is not well constrained, we do not report a slip rate for that surface, however, we do provide a net oblique slip vector for offset Qf1 surfaces of 171 m at 11º S56E. For Qf2a fan surfaces we estimate a vertical slip rate of 0.1 ± 0.1 mm/yr, and for Qf2b surfaces we estimate a vertical slip rate of < 0.1 mm/yr. Our vertical slip rate estimates of 0.1 ± 0.1 mm/yr obtained from both Miocene to Pliocene basalt lavas and Qf2a alluvial fan surfaces indicate that the rate of extension across southern Owens Valley has remained constant since the inception of extensional faulting just prior to ~6.2 Ma. Combining SWIM dextral fault slip rates with those previously estimated along the Owens Valley and Lone Pine fault zones, yields 2.8 ± 0.5 mm/yr of dextral slip across Owens Valley. This slip rate estimate is in agreement, within uncertainty, of slip rate estimates obtained from an improved kinematic fault slip model of geodetic data across Owens Valley. Our results suggest that a geodetic model which accounts for earthquake cycle effects in the viscoelastic lower crust is more realistic than simpler elastic half-space models when estimating fault slip across Owens Valley.