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New!  Geologic Map of the Ruby Mountains– East Humboldt Range–Wood Hills–Pequop Mountains Metamorphic Core Complex Region, Elko County, Nevada
Ruby Mountains compilation
 
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Title Geologic Map of the Ruby Mountains– East Humboldt Range–Wood Hills–Pequop Mountains Metamorphic Core Complex Region, Elko County, Nevada

Author: Andrew V. Zuza, Seth Dee, and Christopher D. Henry
Year: 2025
Series: Open-file Reports
Version: 2025-08
Format: 2 plates, 38" x 61" & 26.25" x 46.25", text, 29 pages
Scale: 1:62,500

This 1:62,500-scale geologic map encompasses much of the Ruby Mountains–East Humboldt Range–Wood Hills Pequop Mountains (REWP) metamorphic core complex (MCC) in Elko County, northeast Nevada. The map area covers the northernmost Ruby Mountains, most of the East Humboldt Range, the Wood Hills, the northern Pequop Mountains, and adjacent valleys (e.g., from west to east, Starr Valley, Secret Valley, Clover Valley, Independence Valley, and Goshute Valley). This compilation map reveals important field relationships that bracket the Mesozoic Cenozoic evolution of the REWP MCC, associated with Mesozoic contractional deformation and crustal thickening and followed by Cenozoic metamorphic core complex formation and extension. Bedrock units consist of Archean(?)–Proterozoic para- and orthogneiss, Neoproterozoic–Triassic passive margin stratigraphy, and Mesozoic–Cenozoic intrusions and volcanic rocks. The pre Cenozoic stratigraphy is variably metamorphosed, with higher metamorphic grade rocks found in the west in the core of the REWP MCC; upper greenschist-lower amphibolite grade rocks are exposed in the Ruby Mountains–East Humboldt Range, whereas unmetamorphosed upper Paleozoic strata are observed in the Pequop Mountains. Mesozoic contractional deformation is best evidenced by the southeast-directed Independence thrust fault in the Pequop Mountains, which is interpreted to have been active in the Late Jurassic based on a crosscutting Late Jurassic lamprophyre sill. Other possible Mesozoic contractional deformation includes observed older-over-young fault relationships within the East Humboldt Range and complex fold nappes observed within the core of the REWP MCC. Scattered Late Jurassic intrusions are observed across the study area. Late Cretaceous intrusions are predominately peraluminous granites associated with voluminous crustal melting that are very common in the core of the REWP, but they are also found in the Wood Hills and Pequop Mountains as minor dikes. In the Cenozoic, relatively primitive Eocene quartz diorite and gabbro sills intruded the Ruby Mountains–East Humboldt Range, which greatly heated the mid-crust and was followed by the emplacement of more evolved Oligocene monzogranite bodies. A ~1 km thick mylonitic shear zone that deformed Oligocene and older rocks along the western flank of the East Humboldt Range but is crosscut by ca. 17 Ma basalt dikes led to the domal exhumation of the core of REWP MCC. We interpret that this Oligocene doming was driven by heating and melting of the lower-plate rocks, as evidenced by the spatial correspondence of Oligocene plutonism and REWP MCC shear zones. This exhumation did not generate any observed syn-kinematic sediments. A smaller subdome may have developed beneath the Wood Hills to generate the ~1.5 km wavelength recumbent folds within the Wood Hills.
Later, Miocene regional extension first generated a series of apparent low-angle normal faults followed by high angle normal faults. The apparently low-angle normal faults are subparallel to bedding, such as the Pequop fault or the Ruby Mountains–East Humboldt Range detachment fault, but these faults reactivated existing inclined structures, including the hanging wall of the Independence thrust in the Pequop Mountains and the margins of the domal REWP MCC in the East Humboldt Range, respectively. Therefore, these early normal faults actually initiated at moderate dips (40–50°) and tilted the major mountain ranges in their footwalls. In the East Humboldt Range, the detachment fault cuts ca. 17 Ma basalt dikes, which implies activity in the middle Miocene. Miocene extension led to the formation of the syn-kinematic sequence of Clover Creek and Humboldt Formation, which are observed across the compilation area. Later high-angle normal faulting has continued to impact the study area, including active fault scarps observed along the western flanks of the East Humboldt Range and Pequop Mountains.
The Quaternary deposits in the compilation map area record two major glaciations, late Pleistocene Lake Clover, and active normal faulting. In the Ruby Mountains–East Humboldt Range, glacial deposits associated with the Angel Lake and Lamoille glaciations are well preserved. Fault scarps cut glacial outwash surfaces associated with these glaciations, with 7 m and 30 m scarps respectively, yielding vertical separation rates of ~0.21–0.35 mm/yr in the latest Pleistocene and ~0.09–0.17 mm/yr in the late Pleistocene. The normal faults that bound the western flank of the Pequop Mountains likely involved similar slip rates. In the Independence and Goshute valleys lacustrine gravels are deposited on shorelines, beach bars, and spits recording the highstand and recessional stages of latest Pleistocene Lake Clover.

Suggested Citation: Zuza, A.V., Dee, S., and Henry, C.D., 2025, Geologic map of the Ruby Mountains–East Humboldt Range–Wood Hills–Pequop Mountains metamorphic core complex region, Elko County, Nevada: Nevada Bureau of Mines and Geology Open-File Report 2025-08, scale 1:62,500, 29 p.

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Original Product Code: OF2025-08