Longwell (1960) has postulated about 25 mi. of right-lateral displacement along a major shear zone in Las Vegas Valley, extending from Frenchman Mountain northwest past Mercury, Nevada. Because of their areal distribution, several Paleozoic stratigraphic units in

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Clark County, Nevada, provide convincing evidence for right-lateral displacement that is in close accord with that postulated by Longwell.

The Rhipidomella nevadensis zone (=lower Indian Springs Member of Longwell and Dunbar, 1936) of (?) Early Pennsylvanian age is at the base of the Bird Spring Formation in the northwestern Spring Mountains on the southern side of the shear zone, but is present 26 mi. farther southeast (Dry Lake and Arrow Canyon Ranges) on the northern side of the shear zone. This fossil zone is missing, probably by facies change, from sections only a few miles farther southeast on both sides of Las Vegas Valley. The Arrowhead Limestone Member of the Monte Cristo Formation (Mississippian) is present in the southeastern Spring Mountains, the Goodsprings district east of the Keystone thrust (Hewett, 1931), and in the upper plate of the Keystone thrust in the central Spring Mountains, but is absent farther northwest. North of the shear zone this member is present in the central Muddy Mountains but is absent west of California Wash, indicating about 25 mi. of southeasterly displacement of rocks on the northern side of Las Vegas Valley. The Eureka Quartzite (Ordovician) is present in the northwestern Spring Mountains, and the Sheep, Las Vegas, and Arrow Canyon Ranges, but is absent east of California Wash and in the central Spring Mountains southeast of Mt. Charleston. The Kaibab Limestone (Middle Permian) is widely distributed in the Spring and Muddy Mountains, but is absent west of California Wash where thick fusulinid-bearing Permian limestones in the Las Vegas and Arrow Canyon Ranges presumably are chronologic equivalents.

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