OLD ROCKS AND NEW INSIGHTS FROM GEOLOGIC MAPPING OF THE NORTHERN EAST HUMBOLDT RANGE METAMORPHIC CORE COMPLEX, ELKO COUNTY, NEVADA

The Humboldt Peak, Welcome and part of the Wells Quadrangles, Elko County, Nevada cover the northern part of the Ruby-East Humboldt metamorphic core complex and record a geologic history ranging from Neoarchean to Holocene. A post-Miocene east-dipping normal fault system bounds the East Humboldt Range exposing a natural cross-section through ~2 km of high-grade metamorphic and igneous infrastructure overprinted by >500 m of WNW-directed mylonites. Along its east flank, the range-bounding fault drops down the Clover Hill block, which exposes the mylonitic zone and overlying detachment fault and a brittlely extended cover of progressively lower grade, younger-on-older, Paleozoic fault slices and an unconformably overlying 5 km-thick sequence of middle Eocene to late Miocene volcanic, volcaniclastic and sedimentary rock.

The oldest rocks in Nevada, the Neoarchean to Neoproterozoic gneiss complex of Angel Lake, form the core of a multi-kilometer scale, southward-closing recumbent fold, the Winchell Lake nappe (WLN). The Angel Lake gneiss complex forms a thrust allochthon that overrode a ductilely attenuated but essentially complete sequence of Neoproterozoic to Mississippian metasedimentary rock before folding by the WLN, migmatization and peak metamorphism at PT conditions up to 10 kb, >750˚C. The WLN overlies the Lizzies Basin block, a sequence of high-grade, intensely migmatized Neoproterozoic to lower Paleozoic paragneiss and marble. Overprinting the metamorphic core are the normal-sense shear zone and detachment fault that together accommodated tens of kilometers of extension in mid- to late Cenozoic time. Cooling of the terrain through ~200^oC occurred by the early Miocene, but final exhumation did not occur until after 9 Ma.

The Cenozoic cover sequence includes middle Eocene to Oligocene volcaniclastic rocks, lower Miocene sedimentary rock, a middle Miocene rhyolite complex, and younger Miocene sedimentary rocks and vitric tuffs, all strongly tilted and faulted down against the detachment. The onset of extension may be recorded by the sedimentary sequence of Clover Creek, a sequence of coarse clastics, megabreccia (rock avalanche deposits), and intercalated lacustrine strata bracketed between the 29-Ma tuff of Campbell Creek and ~15.5-Ma tuffaceous sandstone in the basal Humboldt Formation.