Jurassic–Cenozoic tectonics of the Pequop Mountains, NE Nevada, in the North American Cordillera hinterland

Abstract

The Ruby Mountains–East Humboldt Range–Wood Hills–Pequop Mountains (REWP) metamorphic core complex, northeast Nevada, exposes a record of Mesozoic contraction and Cenozoic extension in the hinterland of the North American Cordillera. The timing, magnitude, and style of crustal thickening and succeeding crustal thinning have long been debated. The Pequop Mountains, comprising Neoproterozoic through Triassic strata, are the least deformed part of this composite metamorphic core complex, compared to the migmatitic and mylonitized ranges to the west, and provide the clearest field relationships for the Mesozoic–Cenozoic tectonic evolution. New field, structural, geochronologic, and thermochronological observations based on 1:24,000-scale geologic mapping of the northern Pequop Mountains provide insights into the multi-stage tectonic history of the REWP. Polyphase cooling and reheating of the middle-upper crust was tracked over the range of <100 °C to 450 °C via novel 40Ar/39Ar multi-diffusion domain modeling of muscovite and K-feldspar and apatite fission-track dating. Important new observations and interpretations include: (1) crosscutting field relationships show that most of the contractional deformation in this region occurred just prior to, or during, the Middle-Late Jurassic Elko orogeny (ca. 170–157 Ma), with negligible Cretaceous shortening; (2) temperature-depth data rule out deep burial of Paleozoic stratigraphy, thus refuting models that incorporate large cryptic overthrust sheets; (3) Jurassic, Cretaceous, and Eocene intrusions and associated thermal pulses metamorphosed the lower Paleozoic–Proterozoic rocks, and various thermochronometers record conductive cooling near original stratigraphic depths; (4) east-draining paleovalleys with ~1–1.5 km relief incised the region before ca. 41 Ma and were filled by 41–39.5 Ma volcanic rocks; and (5) low-angle normal faulting initiated after the Eocene, possibly as early as the late Oligocene, although basin-generating extension from high-angle normal faulting began in the middle Miocene. Observed Jurassic shortening is coeval with structures in the Luning-Fencemaker thrust belt to the west, and other strain documented across central-east Nevada and Utah, suggesting ~100 km Middle-Late Jurassic shortening across the Sierra Nevada retroarc. This phase of deformation correlates with terrane accretion in the Sierran forearc, increased North American–Farallon convergence rates, and enhanced Jurassic Sierran arc magmatism. Although spatially variable, the Cordilleran hinterland and the high plateau that developed across it (i.e., the hypothesized Nevadaplano) involved a dynamic pulsed evolution with significant phases of both Middle-Late Jurassic and Late Cretaceous contractional deformation. Collapse long postdated all of this contraction. This complex geologic history set the stage for the Carlin-type gold deposit at Long Canyon, located along the eastern flank of the Pequop Mountains, and may provide important clues for future exploration.

Supplemental Material: Multi-stage construction of the Little Cottonwood stock, Utah, USA: Origin, intrusion, venting, mineralization, and mass movement

“…Nonetheless, the age of onset of post-orogenic extension in the western North American Cordillera is debated (e.g., Best and Christiansen, 1991;Best et al, 2009;Canada et al, 2019;Zuza et al, 2021;Lund Snee and Miller, 2022). Although there may have been local extension during the emplacement of granitoids ca.…”

Hot Cordilleran hinterland promoted lower crust mobility and decoupling of Laramide deformation

“…The Cordilleran hinterland broadly records Middle Jurassic to Cretaceous plutonism, metamorphism, and distributed lowmagnitude shortening 19,[31][32][33][34][35] , which was followed by a widespread phase of Late Cretaceous to early Cenozoic (ca. 90-60 Ma) regional metamorphism, crustal anatexis with peraluminous plutonism originating from partial melting of pelites and orthogneisses, synconvergent extension, and negligible contractional deformation 15,32,33,[36][37][38][39][40][41] Snake Range metamorphic core complexes (Fig.…”

“…The Cordilleran hinterland broadly records Middle Jurassic to Cretaceous plutonism, metamorphism, and distributed lowmagnitude shortening 19,[31][32][33][34][35] , which was followed by a widespread phase of Late Cretaceous to early Cenozoic (ca. 90-60 Ma) regional metamorphism, crustal anatexis with peraluminous plutonism originating from partial melting of pelites and orthogneisses, synconvergent extension, and negligible contractional deformation 15,32,33,[36][37][38][39][40][41] Snake Range metamorphic core complexes (Fig. 1) show that peak metamorphic conditions were attained in the Late Cretaceous to early Cenozoic (Supplementary File), contemporaneous with the postulated shallowing of the Farallon plate and start of Laramide orogeny within the continental interior.…”