PLIOCENE SINISTRAL GEOLOGIC SLIP RATES IN THE WESTERN MINA DEFLECTION: GEOLOGIC MAPPING AND AR/AR GEOCHRONOLOGY IN THE ADOBE HILLS, CALIFORNIA

New geologic mapping and ⁴⁰Ar/³⁹Ar geochronology in the Adobe Hills, western Mina deflection, provide constraints on the timing of and slip rates along sinistral faults and allow us to test predictions of fault slip transfer from the White Mountains fault zone (WMFZ), Eastern California Shear Zone (ECSZ) northward to the western Mina deflection. The Adobe Hills are underlain by Quaternary tuffaceous sands, alluvium, and lacustrine deposits, Pliocene cinder cones and phyric basalt lavas, and middle Miocene latite ignimbrite, and tuffaceous sandstone. Two well-exposed fault sets cut all units except the Quaternary. Older, discontinuous normal faults, characterized by NNW-trending curvilinear valleys yield minimum vertical displacements from 10s to ~100 meters. At least six younger 6-12 km-long, ENE-striking sinistral faults are characterized by linear valleys, alternating scarp-facing directions along strike, sinistrally offset normal faults, contacts, ridgelines, and channelized basalt flows, and left-stepping extensional and right-stepping compressional stepovers. Magnitude of sinistral offset on individual faults ranges from ~268 to 527 m. A minimum estimate for total sinistral offset across the Adobe Hills is 2,310 ± 145 m.

⁴⁰Ar/³⁹Ar geochronology on basalt lava groundmass yields a range in ages from 3.43 ± 0.01 Ma to 3.10 ± 0.02 Ma. Two basalt lavas, with ages of 3.1 ± 0.02 Ma and 3.20 ± 0.03 Ma, have been sinistrally offset 360 ± 54 and 527 ± 79, respectively, along the two most prominent sinistral faults. The combination of magnitude of offset and age yields minimum Pliocene slip rates of ~0.1 mm/yr and ~0.2 mm/yr, respectively, along these faults. Combining the range in basalt ages of 3.43 ± 0.01 Ma to 3.1 ± 0.02 Ma with our minimum estimate of total sinistral offset yields a minimum Pliocene sinistral slip rate of ~0.7–0.8 ± 0.1 mm/yr across the Adobe Hills. This slip rate is within error of the 0.4 – 0.8 mm/yr of predicted slip transfer northward from the dextral WMFZ into the western Mina deflection (Lee et al., 2009). Reconnaissance mapping in the western Mina deflection has identified additional, geomorphically prominent ENE-striking sinistral faults, suggesting either they accommodate slow slip rates of <0.1 mm/yr or fault slip transfer northward from the ECSZ occurs along faults other than the WMFZ.