GEOLOGY AND GEOPHYSICS ILLUMINATE LATE QUATERNARY OFFSET ALONG THE CADY FAULT WITHIN THE EASTERN CALIFORNIA SHEAR ZONE, SOUTHERN CALIFORNIA

We interpreted geologic mapping, magnetic anomalies, and paleomagnetic vertical axis rotations to constrain offsets along the east-striking sinistral Cady fault within the Mojave block of the eastern California shear zone. Field mapping of Quaternary deposits and aeromagnetic data indicate that the Cady fault forms a significant structural boundary separating NW-striking faults to the south, such as the dextral Rodman, Pisgah, and Lavic Lake faults, from ENE-striking faults to the north, such as the Manix fault. Both methods indicate that Cady fault strands truncate NW-striking dextral faults and hence sinistral offset in this region is likely younger in age and more dominant kinematically. Oblique sinistral offset rates, estimated from field mapping of displaced Quaternary alluvial fan deposits and regional age constraints obtained through luminescence dating techniques, decrease with older deposit age such that late Pleistocene/Holocene deposits yield 0.8 mm/yr rates; whereas, minimum rates for middle to early Pleistocene deposits are as low as 0.002 mm/yr, assuming the time-averaged fault offset ensued immediately following deposit formation. We estimate a total sinistral offset of ~5-6 km along the Cady fault based upon reconstruction of displaced magnetic anomalies and assuming extensional strain in the region began ~10 Ma, and a long-term offset rate of ~0.6 mm/yr. The mapped surface trace of the Cady fault is ~23-km long and is concealed at its western end by Holocene eolian and fluvial deposits south of the Mojave River. Aeromagnetic data support the interpretation that the fault bends to a more WNW-strike extending westward for a total length of ~32 km, where it possibly connects with the Manix fault. Kinematic reconstructions incorporating both slip along the Cady and Manix faults and differential vertical axis rotation of blocks across the Cady fault are consistent with the present topographic low of the Newberry Springs basin, which also coincides with a prominent gravity low.