NEW INSIGHTS INTO THE QUATERNARY BLACK HILLS FAULT, SOUTHERN NEVADA

The Black Hills fault (BHF) is a complex fault zone located in the northern McCullough Range along the western edge of Eldorado Valley, southern Nevada. Previous studies suggested that the BHF is a NE-striking fault with a single, continuous scarp that resulted from one Holocene seismic event. However, this study has documented nine strands, suggesting the fault is composite. In addition, seismic refraction data suggest that there may be as much as 4 m of Quaternary offset along one strand, although only 2.8 m of offset is observed at the surface. This difference also suggests that the fault had multiple Quaternary slip events. Most strands are sinuous and strike N to NE, while some are linear, discontinuous, and oblique to the main strands. Stereonet plots of fractures in bedrock reflect two dominant orientations of the fault zone: N-NE and NW.

Regressions based on maximum scarp height (2.8 m) from other workers indicate that the BHF fault is capable of generating a magnitude 7.0 ± 0.4 earthquake with a 49 km surface rupture length. In this study, only 4.5 km of surface rupture was observed along the fault zone. It was previously suggested that the fault extends ~9 km S of the mapped trace, but here we suggest that the fault terminates at a NW-striking, SW-dipping normal fault along the southwestern edge of the Black Hills of the McCullough Range. We suggest that multiple faulting events and/or simultaneous movement on multiple strands occurred over a 4.5 km rupture length.

Lateral displacement along the fault zone was determined by drainage deflection and offset analysis. Active drainage patterns along the range front show equal amounts of left- and right-lateral deflections suggesting these drainages are younger than the last faulting event, while older drainages show a slight left-lateral component of slip. This analysis suggests that the BHF exhibits sinistral-normal oblique-slip motion. A larger left-lateral component of motion is observed on strands near the southern terminus of the fault than on the strands to the north. Along strike to the N, the strands diverge and splay to the N through the Black Hills and NE into the River Mountains, where the BHF may link to the left-lateral Lake Mead fault system (LMFS). Currently, work is focusing on the possible relationship between the BHF and LMFS. Trenching will also be done to determine the kinematics and history of the fault.