FAULT KINEMATICS IN THE WESTERN MINA DEFLECTION: FIELD STUDIES IN THE ADOBE HILLS, CALIFORNIA

The Adobe Hills, CA are located within the western Mina deflection, a right-stepping zone of faults that connect the northern eastern California Shear zone (ECSZ) to the south with the Walker Lane Belt (WLB) to the north. The Pliocene fault slip history in this region has not been studied in detail, but is critical for understanding the kinematics of fault slip transfer. To address this issue, we completed detailed mapping and kinematic studies in the Adobe Hills to test a fault slip kinematic model whereby <1.0 mm/yr of dextral fault slip from the northern ECSZ was transferred into the western Mina deflection. The Adobe Hills are underlain by 11.1-11.7 Ma (Gilbert et al., 1968) welded and unwelded latite ignimbrite, tuffaceous sandstone, a number of 2.7-4.2 Ma (Gilbert et al., 1968) phyric basalt flows, and basalt cinder cones. Sinistral and normal faults cut and offset all units except the cinder cones. The cinder cones exhibit either no evidence for offset or evidence for a few meters of sinistral offset, suggesting they were emplaced during the waning stages of faulting. ENE-striking sinistral faults dominate the area and either cut older normal faults or curve, via a left-step, into a normal fault geometry. Evidence for sinistral faulting includes ENE-trending linear valleys; left-stepping extensional and right-stepping compressional stepovers; sinistrally offset normal faults, ridgelines, and contacts; and alternating scarp-facing directions along strike. A minimum estimate for sinistral offset across the Adobe Hills is 2100 ± 100 m. Evidence for normal faulting includes NNW-trending linear valleys and vertically offset basalt flows. Because of the absence of vertically offset marker beds, scarp height, ranging from 10s of meters to ~100 meters, yields a minimum vertical displacement across normal faults. If basalt lavas in the Adobe Hills are the same age as those to the east (2.7-4.2 Ma), these ages combined with our sinistral offset measurements indicate a minimum sinistral slip rate of 0.5-0.8 mm/yr across this part of the western Mina deflection. This rate is within error of the 0.8-0.4 mm/yr fault slip rate predicted to have been transferred northward from the dextral White Mountains fault zone, northern ECSZ, into the western Mina deflection (Lee et al., 2009).