CENOZOIC EXTENSION IN THE RIVER MOUNTAINS, SOUTHERN NEVADA: INTERACTION OF THE SADDLE ISLAND DETACHMENT WITH THE LAS VEGAS VALLEY SHEAR ZONE AND LAKE MEAD FAULT SYSTEM

The Miocene volcanic rocks and sediments of the River Mountains east of Las Vegas, Nevada comprise the upper plate to the west-dipping Saddle Island detachment (SID) and record two stages of extension in the Central Basin and Range. This study focuses on the initial 13.5-9 Ma period of east-west extension. It is herein suggested that motion on the SID was synchronous with dextral slip on the Las Vegas Valley shear zone and sinistral slip on the Lake Mead fault system. Up to 20 km of net slip is indicated for the SID by restoring the 13.5 Ma volcanic rocks of the River Mountains on top of equivalent plutonic rocks of the Wilson Ridge Pluton in Mojave County, Arizona. Both conjugate and orthorhombic faults are present in the study area and indicate plain (2-D) and orthorhombic (3-D) strain, respectively. Orthorhombic faulting is interpreted as a consequence of corrugations on the SID. Late-Pliocene to Holocene west-northwest extension on high-angle faults overprints this earlier stage and is interpreted to cut the SID.