Geologic Map of the North Half of the Tent Mountain Quadrangle, Elko County, Nevada
Series: Open-File Report 2020-03
Format: 38.5 x 28.5 inches, color; text: 16 pages with some color
Abstract: This 1:24,000-scale geologic map of the northern half 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.
This work 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 (post-29 Ma mylonitic shearing), detachment faulting, and 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 quartz diorite, and Oligocene monzogranite. New U-Pb zircon dating constraints include an Eocene quartz diorititic orthogneiss dated at ca. 40 Ma and an Oligocene monzogranite orthogneiss sample dated at ca. 31 Ma. Sparse and poorly exposed north-striking undeformed 16.85 Ma basalt dikes cut the mylonites. Geochemical analyses were conducted on all igneous rock types to constrain their petrogenesis and aid in their regional correlation. The 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 ~25% of its undeformed thickness. The Ruby Mountains–East Humboldt Range detachment fault system is exposed along parts of the western range front where undeformed Mississippian (?) clastic rocks and less deformed Ordovician (?) limestone are faulted against mylonitic rocks. Evidence for Mesozoic contractional deformation is limited, but the northernmost portion of the map area 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).
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-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.