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Cenozoic extension in the River Mountains and Frenchman Mountain, Southern Nevada

Cenozoic Extension in the River Mountains and Frenchman Mountain, Southern Nevada by William M. Rittase Dr. Wanda J. Taylor, Examination Committee Chair Professor o f Geoscience University o f Nevada, Las Vegas The River Mountains are the eroded remnants o f a mid-Miocene stratovolcano complex located between Henderson, Nevada and western Lake Mead. This study addresses the tectonic and societal significance of (1) a 13.5-9 Ma initial stage and (2) a late-Pliocene(?)-present stage o f extension. Multiple slip histories were recorded on NW-, N-, NE-, and E-striking conjugate fault sets, in addition to numerous orthorhombic faults. Multiple corrugations on the Saddle Island detachment (SID) during 13.5-9 Ma extension are interpreted to have produced localized zones o f triaxial strain and orthorhombic faulting. Conjugate faults occurred where the SID was planar. The multiple kinematics suggest that 13.5-9 Ma tectonism in the River Mountains was controlled by transient slip gradients on the Las Vegas Valley shear zone and the Lake Mead fault system. Active faults o f the second period o f extension may impact present day Las Vegas. An earthquake rupturing the combined 38 km length o f the Ithaca Avenue Fault-Frenchman Mountain Fault could produce a M^ 6.9 ± 0.3 earthquake.