The Lucy Gray Mountains primarily consist of Paleoproterozoic deformed potassic granite that intruded older generations of Paleoproterozoic orthogneiss and migmatite. The distinctive 1.66-Ga Beer Bottle Pass pluton occupies the northern part of the map area. Metamorphic foliations dip steeply to moderately toward the west, and stretching lineations also plunge toward the west. Shear fabrics display top-ESE kinematics in both discrete and distributed shear zones. In one locality, the Beer Bottle Pass pluton is emplaced over orthogneiss along a discrete top-southeast ultramylonite-mylonite shear zone that is several meters wide. Deformation and granulite-grade metamorphic conditions probably occurred during and soon after the emplacement of the potassic granites, around 1.6–1.7 Ga. The Proterozoic crystalline rocks are unconformably overlain by a >700 m-thick sequence of mostly ca. 24–15 volcanic and sedimentary rocks. The oldest Tertiary unit is a distinctive and regionally recognized basal conglomerate that reaches a maximum thickness of 60 m in discontinuous exposures along the east flank of the mountain range. The basal conglomerate is poorly sorted and composed of a diverse assemblage of well-rounded clasts of gneiss, granite, limestone, quartzite, and eolian sandstone. The overlying volcanic strata begin with a series of olivine-phyric basanite lavas interbedded with the 18.6-Ma Peach Spring Tuff, a distinctive regional ignimbrite marker unit that occurs as thin (<12 m), laterally discontinuous lenses (up to 600 m wide) in the east-central part of the map area. These strata are overlain by a sequence of basaltic andesite and dacitic lava flows and tuff breccias that are in turn overlain by a second regionally recognized ignimbrite, the 15.2-Ma tuff of Bridge Spring. The entire Tertiary sequence is correlative with very similar age and composition volcano-sedimentary strata in the adjacent southern McCullough Range and Highland Range to the east.

Miocene strata are moderately tilted toward the ENE in the northeastern map area, and variably toward the SE and SW in the southeastern map area; variations in dip direction in the southeastern area seem to be controlled locally by differential tilting on conjugate SW- and NE-dipping normal and normal-oblique faults. A steeply dipping, ESE-striking fault in the east-central map area is responsible for at least ~650 m of right-lateral separation of the Tertiary volcanic sequence, though it is unclear whether it reflects right-lateral slip, southwest-side-down normal slip, or a combination of both. Several SW-dipping normal faults displace early Pleistocene to Pliocene(?) deposits in the central part of the range and along the western range front. Although the late Cenozoic timing of displacement on this fault system is poorly constrained, the map relationships suggest that most tilting and extension probably occurred on N- to NW-striking normal and/or dextral-oblique normal faults during middle and late Miocene time, followed by more recent down-to-west normal faulting during Pliocene(?) and early Pleistocene time.

Research for this publication was supported by the U.S. Geological Survey Earth MRI initiative (Cooperative Agreement No. G19AC00255).