Geologic Map of the Humboldt Peak Quadrangle, Elko County
By Allen J. McGrew, University of Dayton
Series: Map 186
Format: plate: 43 x 37 inches, color; text: 23 pages, color
GIS files: http://pubs.nbmg.unr.edu/CDP-Geol-Humboldt-Peak-quad-p/m186z.htm
Located in central Elko County, Nevada, the Humboldt Peak quadrangle exposes the central and highest part of the East Humboldt Range (EHR). It is flanked to the east by Clover Valley and a northerly trending line of hills. The central East Humboldt Range probably represents the structurally deepest part of the Ruby Mountains–East Humboldt Range metamorphic core complex. The high-grade core of the EHR consists of migmatitic upper amphibolite facies rocks that achieved peak P-T conditions during Late Cretaceous metamorphism up to 8 kb and 775°C. The deepest structural levels, exposed in Lizzies Basin in the west-central part of the quadrangle, form a migmatite complex with >67% leucogranitic rock. In the northwest part of the quadrangle, this migmatite complex is overlain in the cirque wall above Winchell Lake by a southward-closing, kilometer-scale recumbent fold known as the Winchell Lake fold-nappe (WLN). The WLN folds a sequence of intensely metamorphosed, migmatized and profoundly attenuated Neoproterozoic to Mississippian metasedimentary rocks. Farther north in the adjacent Welcome quadrangle the WLN is cored by Nevada’s oldest rocks—Neoarchean to early Paleoproterozoic orthogneiss and paragneiss that were thrust over the Neoproterozoic to Mississippian metasedimentary sequence before peak metamorphism and WLN-related folding. Overprinting this assemblage at higher structural levels is a WNW-directed, kilometer-scale mylonitic shear zone that diachronously exhumed this high-grade terrain during extensional tectonism bracketed between late Eocene and Miocene. Together, the mylonitic shear zone and the detachment fault that forms its roof probably accommodated much more than 15 km of extensional displacement. Cutting through the metamorphic core along the steep, eastern face of the East Humboldt Range is a younger, post-Miocene normal fault that remained active into the Quaternary. In the northern part of the quadrangle this normal fault uplifts and juxtaposes the high-grade core against moderately east-dipping Middle Miocene volcanic and hypabyssal intrusive rocks (rhyolitic quartz porphyry). A single exposure of flat-lying vitric tuff, also of Miocene age, lies to the east of the east-dipping rhyolite-bearing sequence, but it is unclear at present whether the flat-lying strata are faulted down against the rhyolitic rocks or overlie them in angular unconformity. The line of hills in the northeastern part of the quadrangle consists of Paleozoic sedimentary rock that probably represents the down-faulted “cover” of the metamorphic core complex. Finally, the late Quaternary basin fill to the east of this line of hills was faulted down against the Paleozoic sedimentary rocks along the Clover Valley fault, which strikes northward through the adjacent Welcome quadrangle toward the town of Wells where it may correlate with the fault that produced the Mw 6.0 Wells earthquake in 2008.