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Title: Geologic map of the Tent Mountain quadrangle, Elko County, Nevada

Author: Andrew V. Zuza, Seth Dee, Hugh A. Hurlow, Arthur W. Snoke, and Benjamin J.C. Laabs
Year: 2021
Series: Open-File Report 2021-03
Version: supersedes Open-File Report 2020-03
Format: sheet: 38.5 x 28.5 inches, color; text: 25 pages, color
Scale: 1:24,000; inset 1:12,000 (Dorsey Creek area)

This 1:24,000-scale geologic map of the Tent Mountain 7.5-minute quadrangle covers part of the western flank of the East Humboldt Range in Elko County. The range consists of high-relief glacially carved bedrock valleys and fault-bounded piedmonts of Miocene basin deposits covered by Quaternary fans. The western margin of the map area is Starr Valley and the eastern extent of the quadrangle is the north-trending ridgeline defined by Hole in the Mountain Peak and Humboldt Peak.

This work was built on earlier M.S. thesis mapping of Hurlow (1987). The map covers part of the Ruby Mountains–East Humboldt Range metamorphic core complex, which exhumed upper greenschist-lower amphibolite grade metamorphosed and highly attenuated Neoproterozoic through Mississippian marble and siliciclastic strata and voluminous intrusions. A combination of Cenozoic Oligocene mylonitic shearing, middle Miocene detachment faulting, and Miocene-present range tilting exhumed these rocks from depth. Igneous rocks make up most of the bedrock—commonly more than two-thirds—including Cretaceous–Tertiary leucogranite; Eocene granodiorite, granite, and quartz diorite; and Oligocene monzogranite. New U-Pb zircon dating constraints include two Eocene quartz diorititic orthogneiss samples dated at ca. 40–39 Ma, undeformed Eocene granodiorite dated at ca. 39 Ma, and two Oligocene monzogranite orthogneiss samples dated at ca. 32–31 Ma. Sparse and poorly exposed north-striking undeformed 16.85 Ma basalt dikes cut the mylonites, and these dikes are cut by later Miocene detachment faulting. Geochemical analyses were conducted on all igneous rock types to constrain their petrogenesis and aid in their regional correlation. The sub-detachment bedrock units are all strongly mylonitized and lineated (WNW trend) with kinematic indicators showing predominately top-west shear. The structurally deepest rocks show weaker fabrics. The stratigraphic section from Ordovician Eureka Quartzite to Neoproterozoic McCoy Creek Group has been attenuated to less than ~20% of its undeformed thickness. The Ruby Mountains–East Humboldt Range detachment fault system is well exposed in the western and southern parts of the map area, where it is focused in Mississippian Chainman Shale, placing unmetamorphosed and non-mylonitic Mississippian through Permian rocks over mylonitic lower Paleozoic and Neoproterozoic paragneiss. The ages of upper-plate limestone units were aided by fossil identification. The detachment fault is primarily located at the base of the Mississippian Diamond Peak Formation, but several hanging-wall ramp geometries cutting up through the Permian strata probably reveal several fault slivers that root into the master detachment fault. The upper plate rocks variably show brecciation and silica alteration, and the map pattern of these features suggests they are related to high-angle normal faulting parallel to the range front, rather than earlier detachment faulting.

There is some evidence for pre-extensional, Mesozoic contractional deformation. This includes the northernmost portion of the map area that may expose a southern limb of the Winchell Lake nappe and an inferred thrust fault that emplaces this structure over the rest of the map area. These structures correlate with those mapped in the Humboldt Peak quadrangle to the east (McGrew, 2018). Older-over-younger relationships, including Ordovician rocks over Mississippian strata in the western map area, imply some remnant phases of contractional deformation that was subsequently deformed by Cenozoic normal-sense shearing.

The west side of the East Humboldt Range is bound by the active, west-dipping Ruby Mountains frontal fault zone, which extends for more than 60 km to the southwest. In the northwest corner of the map the fault makes a west step resulting in a broad, hanging wall uplift underlain by Miocene to Pliocene Humboldt Formation conglomerate, sandstone, shale, tephra, and tuffaceous sediments with gentle and variable, but generally north to northeast dips. Age constraints from 40Ar/39Ar dating include detrital feldspars ages that require the sediments to be younger than 23 Ma, and ages from four tephra samples that range from 15.77 Ma to as young as 5.15 Ma. Miocene sediments are interbedded with 15.3 Ma Jarbidge-type rhyolite.

Repeated late Quaternary surface-rupturing earthquakes along active traces of the frontal fault are recorded by increased uplift and dissection of Quaternary surfaces as a function of relative age. Fault scarps in Holocene deposits have up to 2.5 m of vertical separation while glacial outwash surfaces, Qgo­­­­­­­­­­­­­­­­­­­­­­­­­­­­­1 (younger) and Qgo2 (older), are faulted with scarps up to 7 m high in Qgo­­­­­­­­­­­­­­­­­­­­­­­­­­­­­1 and 30 m high in Qgo2. Cosmogenic 10Be exposure ages of boulders atop the outwash surfaces yield preferred ages of 14–27 ka for Qgo­­­­­­­­­­­­­­­­­­­­­­­­­­­­­1 and 120–130 ka for Qgo2, which correspond with the ages of the Angel Lake and Lamoille glaciations, respectively. The upper reaches of several drainages have well-preserved glacial moraine deposits that are also correlated to the two late Pleistocene glacial advances. A slip rate of the frontal fault system was calculated using lidar-derived topographic profiles and preferred glacial outwash ages, 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.

This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G19AC00383 and G20AC00390.

Suggested citation:
Zuza, A.V., Dee, S., Hurlow, H.A., Snoke, A.W., and Laabs, B.J.C., 2021, Geologic map of the Tent Mountain quadrangle, Elko County, Nevada: Nevada Bureau of Mines and Geology Open-File Report 2021-03, scale 1:24,000, 25 p.

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