TIMING AND AMOUNT OF RIGHT-LATERAL OFFSET ON THE SHEEPHEAD FAULT ZONE: EARLYDEVELOPMENT OF THE CHINA RANCH BASIN

The Sheephead Fault Zone (SFZ) bounds the China Ranch Basin (CRB) on the north. Reports on the SFZ vary in the timing, amount, and even the sense of offset. Recent studies in northern CRB show the SFZ is a major right-lateral (RL) fault with 20 km of offset. The key piercing point is the northern limit of rock avalanche deposits of 12.4 Ma granite of Kingston Peak (KPg) on the SE flank of Tecopa Peak (TP). Reversal of ~14 km of slip on the SFZ is required to restore the avalanche deposits to their inferred source at the northern limit of the avalanche path south of the Alexander Hills (AH). Further restoration of 5-6 km of slip aligns miogeoclinal strata of TP to the projected location of matching units across the SFZ on the east side of the AH. Large RL offset on the SFZ is also evident from provenance changes recorded in a well-exposed section of east-dipping fanglomerate 4-5 km SE of TP. The section consists of; 1) east-derived avalanche deposits of KPg, 2) an overlying lower fanglomerate (LF) of east-derived KPg and Pahrump Group clasts from the Kingston Range and AH, and 3) upper fanglomerate (UF) of north-derived clasts of Precambrian gneiss, miogeoclinal sedimentary rocks, and minor volcanic clasts from the southern Nopah and Resting Springs ranges (SNR and RSR). Lapilli tuff in the lower part of the UF yielded an ⁴⁰Ar/³⁹Ar age of 11.12 Ma. The new age in the context of new field data show that; a) the initiation of slip on the SFZ, b) early CRB development, and c) exhumation and collapse of the 12.4 Ma KPg pluton into CRB were essentially coincident—all occurring prior to 11.12 Ma. A prominent tuff near the top of UF where intervals become increasingly enriched in large cobbles and boulders of gneiss yielded an ⁴⁰Ar/³⁹Ar age of 8.19 Ma. Restoration of this area into alignment with paleocurrent trends and source rock in the SNR indicates most of the ~20 km of SFZ offset occurred prior to 8.19 Ma. Lastly, the SFZ bounds on the south a domain of major west-directed extension just as the Furnace Creek fault zone bounds the domain on the north. Recent studies challenge the notion that Miocene RL faulting across the region is largely accommodational in origin. RL sense-of-shear on the SFZ is not compatible with accommodating west-directed extension to the north and so we consider dextral shear on the SFZ fundamentally as a contribution to plate boundary shear.