Most of the southern McCullough Range consists of migmatitic gneiss intruded by Paleoproterozoic potassic granite and granitic orthogneiss. Bedrock foliations dip moderately westward with stretching lineations also trending westward. Shear fabrics display top-east-southeast kinematics, including discrete and distributed shear zones. Deformation and granulite-grade metamorphic conditions probably occurred at 1.6–1.7 Ga. The Proterozoic gneissic rocks are unconformably overlain by ca. 18–15 Ma volcanic and sedimentary rocks. The Miocene sequence consists of mafic to intermediate lava flows, subordinate tuffs and breccias, and associated alluvial fan conglomerates and sandstones that are likely correlative with similar age and composition volcano-sedimentary strata in the adjacent Highland Range to the east. Miocene strata are moderately tilted toward the northeast, presumably by west-side-down normal faulting on the western flank of the McCullough Range and/or by relatively minor southwest-dipping normal or normal-oblique faults in the northeast part of the map area. The major normal fault on the west flank of the McCullough Mountains displaces Tertiary syn-extensional sediments and interbedded middle Miocene basaltic andesite. The fault is buried by alluvial fan deposits, with the oldest unfaulted alluvial deposits likely early Pleistocene to possibly Pliocene in age. The Tertiary strata in the hanging wall of this fault dip gently to moderately toward the east. The displacement of the Tertiary strata implies greater than ca. 1 km of cumulative normal slip on this structure since middle Miocene time (ca. 15 Ma). The Proterozoic and Miocene rocks in the northeast part of the map area are cut by NW-striking high-angle bedrock faults that we interpret to be associated with the right-lateral Stateline fault located to the west of the map area in Ivanpah Valley. These high-angle faults exhibit complex fault striations that suggest dextral-oblique slip associated with transtension. Some of these faults are intruded by Miocene(?) basaltic dikes, thus implying Miocene slip. Where these faults cut Miocene strata, they display a maximum of several tens of meters of vertical displacement. Although the late Cenozoic timing of displacement on this fault system is poorly constrained, the map relationships suggest that most tilting and extension occurred on N- and NNE-striking (oblique?) normal faults during middle and late Miocene time, and that slip on the NW-striking dextral-oblique normal faults probably initiated during latest Miocene time.

This geologic map was funded in part by the USGS Earth Mapping Resources Initiative (Earth MRI), award number G19AC00255.