The Nevada Test Site and adjacent Nellis Air Force Base Bombing and Gunnery Range lie north and east of the Las Vegas Valley-Walker Lane shear zone, near the thickest part of the Paleozoic Cordilleran miogeosyncline, and east of an area characterized by large plutons of Mesozoic age. Ranges, other than volcano-tectonic mountains, trend north except in the southern part where they have been rotated to northeasterly strikes near the Las Vegas Valley shear zone. Alluvium-filled basins comprise about 30 percent of the area. Paleozoic and uppermost Precambrian sedimentary rocks total about 40,000 feet in thickness and form approximately 30 percent of the outcrops. The remainder consists primarily of volcanic rocks and related intrusive masses of Tertiary age.
Two major thrust fault systems of Mesozoic age, each with displacements of several tens of miles occur in the eastern and southern parts of the area where most of the pre-Tertiary rocks are exposed. Normal faults are abundant throughout, and those that flank the major basins have throws of several thousand feet. Several left-lateral transcurrent faults have been recognized in the southern ranges. These appear to be satellitic to the right-lateral Las Vegas Valley shear zone.
The western and central parts are characterized by numerous volcanic centers, at least five of which gave rise to calderas. The principal volcanic rocks are ash-flow tuffs of rhyolitic and quartz-latitic composition. Several of these have lateral extents of more than 100 miles. The ash-flow tuffs have radiometric ages ranging from 26 m.y. to 7 m.y.