Tertiary extension north of the Las Vegas Valley shear zone, Sheep and Desert Ranges, Clark County, Nevada

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Detailed mapping reveals the presence of high-angle extensional faults and low-angle gravity slides on the west side of the Sheep Range. Three major high-angle faulting events each account for 20° of eastward rotation and accommodate extension between the Las Vegas Range and the Desert Range. Low-angle faults represent surficial slides in response to topography produced by extension on the high-angle faults. Faulting took place during the Miocene, synchronously with deposition of the Horse Spring Formation and with displacement on the Las Vegas Valley shear zone. The extension in the Sheep Range took place without volcanism, intrusion, or metamorphism of the Paleozoic sedimentary rocks. Offset thrust faults suggest that the area west of the Sheep Range extended almost 100% during the Miocene, while the corresponding area south of the Las Vegas Valley shear zone did not extend significantly. The shear zone bounded the extending terrane on the south, acting as a transform fault. This extension west of the Sheep Range may in part balance that mapped by Anderson (1971) in the Eldorado Mountains. The Las Vegas Valley shear zone and the Lake Mead fault system may have acted together to compensate for areas of localized extension between the Colorado Plateau and the vicinity of the Specter Range.