Widespread middle to late Eocene calc‐alkalic volcanism, which formed the Northeast Nevada Volcanic Field, marks the earliest Tertiary volcanism in the northern Basin and Range. The central part of this major field in northeast Nevada and adjacent Utah is herein defined by 23 40Ar/39Ar ages that range from 42.6 to 39.0 Ma, rock chemistry from 12 localities, stratigraphic position of the volcanic rocks above a regional middle Eocene unconformity, volcanic setting, and lithology. The type area is at Nanny Creek, in the northern Pequop Mountains, Nevada, where rhyolite ash flow tuffs are overlain by a thick section of intercalated andesitic to dacitic flows and flow breccias and rhyolite ash flow tuffs. The intermediate composition rocks are locally derived throughout the volcanic field, whereas the sources for rhyolite ash flow tuffs are unknown. The uniform and widespread occurrence of the andesitic and dacitic flows and flow breccias strongly suggests that the upper crust was perforated by intermediate composition magma across the entire region. In the central part of the field the middle Eocene volcanic rocks rest with depositional angular discordance on deformed middle Paleozoic to Triassic strata; ostracode‐bearing limestone, probably of early Eocene age, is locally present below the volcanic rocks. In the western and southeastern parts of the field these middle Eocene volcanic rocks rest with depositional angular discordance on lower Eocene lacustrine strata of the Elko and White Sage Formations, respectively. This angular discordance documents a middle Eocene deformational event previously unrecognized in the region.

Abstract Mesozoic crustal shortening in the North American Cordillera’s hinterland was related to the construction of the Nevadaplano orogenic plateau. Petrologic and geochemical proxies in Cordilleran core complexes suggest substantial Late Cretaceous crustal thickening during plateau construction. In eastern Nevada, geobarometry from the Snake Range and Ruby Mountains-East Humboldt Range-Wood Hills-Pequop Mountains (REWP) core complexes suggests that the ~10–12 km thick Neoproterozoic-Triassic passive-margin sequence was buried to great depths (>30 km) during Mesozoic shortening and was later exhumed to the surface via high-magnitude Cenozoic extension. Deep regional burial is commonly reconciled with structural models involving cryptic thrust sheets, such as the hypothesized Windermere thrust in the REWP. We test the viability of deep thrust burial by examining the least-deformed part of the REWP in the Pequop Mountains. Observations include a compilation of new and published peak temperature estimates (n=60) spanning the Neoproterozoic-Triassic strata, documentation of critical field relationships that constrain deformation style and timing, and new 40Ar/39Ar ages. This evidence refutes models of deep regional thrust burial, including (1) recognition that most contractional structures in the Pequop Mountains formed in the Jurassic, not Cretaceous, and (2) peak temperature constraints and field relationships are inconsistent with deep burial. Jurassic deformation recorded here correlates with coeval structures spanning western Nevada to central Utah, which highlights that Middle-Late Jurassic shortening was significant in the Cordilleran hinterland. These observations challenge commonly held views for the Mesozoic-early Cenozoic evolution of the REWP and Cordilleran hinterland, including the timing of contractional strain, temporal evolution of plateau growth, and initial conditions for high-magnitude Cenozoic extension. The long-standing differences between peak-pressure estimates and field relationships in Nevadan core complexes may reflect tectonic overpressure.

No abstract

No abstract

No abstract