The age of the Keystone Thrust: Laser‐fusion 40Ar/39Ar dating of Foreland Basin Deposits, southern Spring Mountains, Nevada

Abstract

Nonmarine sedimentary and volcaniclastic foreland‐basin deposits in the Spring Mountains of southern Nevada are cut by the Contact and Keystone thrusts. These synorogenic deposits, informally designated the Lavinia Wash sequence by Carr (1980), previously were assigned a Late Jurassic to Early Cretaceous(?) age. New 40Ar/39Ar laserfusion and incremental‐heating studies of a tuff bed in the Lavinia Wash sequence support a best estimate age of 99.0±0.4 Ma, indicating that the Lavinia Wash sequence is actually late Early Cretaceous in age and establishing a maximum age for final emplacement of the Contact and Keystone thrust plates consistent with the remainder of the Mesozoic foreland thrust belt. The reinterpretation of the age of the Lavinia Wash sequence and the consequent limit on the age of foreland thrust faults in the southern Spring Mountains strengthen widely accepted correlations of the Contact‐Red Springs and Keystone thrusts with the Cretaceous Summit‐Willow Tank and Muddy Mountain‐Glendale thrusts in the Muddy Mountains. These correlations were partly in conflict with previously reported ages and geologic interpretations of structures in the Spring Mountains.