CENOZOIC EXTENSION IN THE RIVER MOUNTAINS, SOUTHERN NEVADA

Deformation in the River Mountains, located SE of Las Vegas, NV offers insight into the Neogene tectonic evolution of southern Nevada and the Central Basin and Range. Comprised mostly of 13.5-12 Ma dacite, the structures here record the interplay between normal and oblique-slip faulting. This study (1) characterizes tectonism in the Lake Mead tectonic domain during the last 13.5-12 m.y., (2) relates extension in the River Mountains to the development of the Las Vegas Valley, and (3) examines the seismic risk to the Las Vegas community posed by Quaternary faults.

New 1:12,000 scale geologic mapping reveals four fault sets striking NW, N (2 sets) and ENE. NW-striking faults both cut all other fault sets and terminate at ENE-striking faults, which suggests that motion was transferred onto the ENE-striking faults. Kinematic data on the ENE-striking faults suggest multiple slip directions ranging from normal to oblique, consistent with slip transfer. Based on geometric and spatial relationships, two age relationships exist between N- and ENE-striking faults. (1) Most N-striking faults end at, but are not repeated across ENE-striking faults. Thus, motion was transferred onto the ENE-striking faults as supported by three differing slickenline rakes. (2) A few N-striking faults cut, and thus, postdate ENE-striking faults. All four fault sets cut the 9-5 Ma Muddy Creek Formation. In addition, at least two NW-striking faults cut Quaternary sediments. Post- 9-5 Ma faulting (1) probably is associated with development of Las Vegas basin, (2) may be linked regionally to the formation of and activity on the Eastern California shear zone and Walker Lane Belt to the west and (3) is younger than activity on the Las Vegas Valley shear zone and Saddle Island detachment. Consequently, much of this deformation is younger than the major regional Miocene deformation. Evidence of Quaternary faulting suggest some faults pose a significant seismic threat to the Las Vegas community.