The Lake Mead Fault System and the Las Vegas Valley Shear Zone: Strike-slip faulting and associated deformation in the Basin and Range, southeastern Nevada

Major strike-slip faults are common structural features in the late-Cenozoic tectonic framework of the Basin and Range and are the subject of this study. The structural characteristics of two major strike-slip systems in the vicinity of Las Vegas, the left-lateral Lake Mead Fault System and the right-lateral Las Vegas Valley Shear Zone, are documented using both geological and geophysical data at various scales of observations and includes detailed field mapping at 1:10,000. The strike-slip systems are shown to be composed of discrete, discontinuous fault segments. The interaction of the discontinuous strike-slip fault segments generates both regional and local deformation that is consistent with the known kinematics of strike-slip faulting. For example, along the Lake Mead Fault System, the Echo Hills are a contractional feature located within a right-stepover of a left-lateral fault. The geometry of the push-up is that of a strongly asymmetric anticline in the hanging wall of a high-angle northwest trending reverse fault that marks the southwestern boundary of the stepover region. The Overton Arm basin and the Las Vegas Valley basins are deep pull-apart basins formed by extensional stepovers along the major strike-slip systems. The geomorphic expression of the Overton Arm basin assumes the form of a rhomboid shaped topographic low clearly indicated by the shape of the Overton Arm of Lake Mead once the lake crosses into the step-over region. Detailed gravity surveys in the Overton Arm basin indicate an estimated Cenozoic basin fill of 2.5 kilometers. Along the Las Vegas Valley, three distinct basins were identified using regional gravity anomaly data. The basins are aligned in a right-stepping pattern and it is interpreted here that the basins were formed by discontinuous right-stepping strands of the Las Vegas Valley Shear Zone. The relationship of normal faults to the strike-slip faults is also consistent in that the associated normal faults are found exclusively in the extensional quadrants of the strike-slip faults and their geometry is consistent with the sense of slip across the strike-slip faults. From detailed geologic mapping of the Bitter Ridge area along the Lake Mead Fault System, it is shown that the amount of offset along the normal faults is not constant; the greatest displacement occurs at the bounding strike-slip fault and then decreases away from the strike-slip fault. A normal fault may terminate altogether before reaching the next strike-slip fault. This geometry strongly suggests that the strike-slip faulting is a significant contributor to deformation in the region and produces the normal faulting and extension in the area. Hence, many structural features in the Basin and Range, contractional as well as extensional, can be understood in terms of simple wrench tectonics