Pliocene Sinistral Fault Slip Across the Adobe Hills, Western Mina Deflection, Eastern California

Sarah Nagorsen-Rinke
March 2011

Abstract

The Adobe Hills region is a faulted volcanic field located within the western Mina deflection, a right-stepping zone of faults that connects the northern eastern California shear zone (ECSZ) to the south with the Walker Lane belt (WLB) to the north. New detailed geologic mapping, structural studies, and ⁴⁰Ar/³⁹Ar geochronology in the Adobe Hills constrain fault slip rates and test predictions for the kinematics of fault slip transfer into the Mina deflection. The Adobe Hills are underlain by middle Miocene latite ignimbrite that yields an ⁴⁰Ar/³⁹Ar age of 11.17 ± 0.04 Ma; Pliocene tuffaceous sandstone, intermediate basaltic lavas that yield ⁴⁰Ar/³⁹Ar ages between 3.13 ± 0.02 to 3.43 ± 0.01 Ma, and basaltic cinder cones; and Quaternary tuffaceous sands, alluvium, and lacustrine deposits. NNW-striking normal faults, WNW-striking dextral faults, and NE-striking sinistral faults cut all units. A minimum of 320 meters of ~EW-horizontal extension since the Pliocene has been accommodated by normal faults. Combining this magnitude of extension with an ⁴⁰Ar/³⁹Ar basalt age range of 3.13 ± 0.02 to 3.43 ± 0.01 Ma yields a minimum Pliocene horizontal extension rate of ~0.1 mm/yr. A single dextral fault in the southern Adobe Hills offsets a basalt ridgeline and a normal fault/hanging wall intersection by 281 ± 42 meters. This measurement, combined with the ⁴⁰Ar/³⁹Ar basalt age range, yields a minimum dextral slip rate of 0.1 mm/yr. NE-striking sinistral faults are the youngest and most continuous structures in the Adobe Hills. These faults offset basalt ridgelines, normal fault/hanging wall intersections, a channelized basalt flow, a basalt flow edge and a basalt flow contact a minimum of 2,266 ± 334 meters. This magnitude of sinistral offset combined with an ⁴⁰Ar/³⁹Ar basalt age range of 3.13 ± 0.02 to 3.43 ± 0.01 Ma yields a minimum Pliocene sinistral fault slip rate of 0.7 ± 0.1 mm/yr. This slip rate is within error of the predicted 0.8 to 0.4 mm/yr of Pliocene to Pleistocene fault slip transferred northward from the dextral White Mountains fault zone into the western Mina deflection. This result suggests that other, geomorphically distinct ENE-striking sinistral faults in the western Mina deflection southeast of the Adobe Hills either (1) accommodated very slow fault slip rates of less than 0.1 mm/yr, or (2) accommodated fault slip rates ≥0.1 mm/yr, which implies that dextral faults west of the White Mountains fault zone also transfer slip into the western Mina deflection.

2013 Update: A research paper based on this thesis is now in press (PDF) for the journal Geosphere.