UPDATE ON THE TIMING, MAGNITUDE OF OFFSET, AND GEOMECHANICS OF THE SOUTHERN PART OF THE LAKE MEAD FAULT SYSTEM, NEVADA

New geologic mapping and geochronology in the northern Eldorado Mountains, southern River Mountains, and northeastern McCullough Range provides new details on the Miocene magmatic and volcanic stratigraphy, timing of onset of extension, and cumulative slip associated with the southern parts of the greater Lake Mead fault system (LMF). The bedrock stratigraphy in this area is dominated by two major middle Miocene magmatic systems: the ~15.0-13.9 Ma Railroad Pass pluton and associated volcanic pile, and the ~14.5-13.5 Ma Boulder City pluton (BCP) and associated volcanic rocks. A slightly younger, ~13.25-13.15 Ma sequence of volcanic rocks rest on the BCP along an angular unconformity, bracketing the onset of major extension in this part of the Colorado River extensional corridor at between 13.5 and 13.25 Ma. The southern part of the NE-striking, sinistral LMF consists of two kinematically linked faults: the Hemenway Wash strike-slip fault zone (HWF) that separates the Eldorado and River Mountains, and the Black Hills normal fault zone (BHF) that bounds the northwest side of Eldorado Valley. The BHF is associated with Quaternary fault scarps whereas Quaternary fault scarps have not been observed along the HWF. The HWF is bordered by a ~3 km-wide region of oroclinal flexure along either side of the fault, consisting of numerous dextral faults that curve into the primary sinistral fault zone. Based on new mapping and age dates, we infer that part of the BCP in the southern River Mountains has been offset 2-3 km left-laterally along the HWF. We also estimate that the oroclinal flexure may accommodated 2-4 km of shear, resulting in 4-7 km of total offset across the southern part of the LMF. The southern part of the LMF also marks a boundary between two structural domains. The Eldorado Mountains are dominated by N-striking normal faults that locally curve westward as they intersect the zone of oroclinal flexure adjacent to the HWF. The southern River Mountains and northeastern McCullough Range contain numerous NW-striking dextral faults associated with the Las Vegas Valley shear zone, and also numerous NE-striking sinistral faults that are probably synthetic to the LMF. The NW-striking dextral faults consistently cut the NE-striking sinistral faults in the ranges and terminating southward into the HWF and BHF.