Abstract

A low-angle fault exposed in tilted fault blocks beneath the Keystone thrust plate in the eastern Spring Mountains, Nevada, was designated by Longwell (1926) as the Red Spring thrust fault of pre-Keystone age. Long-well later (1960) interpreted the Red Spring thrust plate as a frontal portion of the Keystone plate that had been broken, down-dropped along high-angle faults, and overridden by continued advance of the Keystone plate at the time of thrust-faulting and synchronous strike-slip displacement along the nearby Las Vegas shear zone. Several lines of new evidence support Longwell's original interpretation of relations between the Red Spring and Keystone thrusts. Thrust faulting in the Spring Mountains area is Mesozoic in age, whereas displacement along the Las Vegas shear zone occurred in late Tertiary time. Thrust faults cut by high-angle faults beneath the Keystone plate are not restricted to the Red Spring area of the Spring Mountains. One of these thrust faults, the Contact fault of Hewett (1931), is believed to be correlative with the Red Spring thrust fault. The Red Spring-Contact thrust plate very likely overrode the land surface as evidenced by conglomeratic channel deposits beneath it, a conclusion first reached by Longwell (1926). The plate was then broken by northwest-striking high-angle faults, and an uplifted portion of it between the Cottonwood and La Madre faults was eroded away prior to emplacement of the higher Keystone plate. The Keystone plate should no longer be considered as the lowest major allochthonous unit in the Spring Mountains portion of the foreland fold-and-thrust belt.

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