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.
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New Insights into Early Cretaceous Extension in Northern Mongolia
The late Mesozoic Era was a time of widespread crustal extension in eastern Asia resulting in both rift basin and metamorphic core complex formation. Two of the more recently documented examples of this extensional phase are the Ereendavaa and Buteel metamorphic core complexes (EMCC, BMCC). Both are located in northern Mongolia proximal to the Mongol Okhotsk Suture Zone (MOSZ). The MOSZ is a profound, yet enigmatic structure that formed due to closure of the Mongol-Okhotsk Ocean, a basin that separated the Siberian and North China cratons and intervening terranes of the Central Asian Orogenic Belt. Based on published work by others, the core complexes record NW-SE extension, cooling and deformation from c. 135 to 120 Ma. We present new data as part of a collaborative research project that aims to constrain the evolution of the MOSZ more broadly and its relationship to intracontinental deformation after suturing. Our methods include analysis of satellite imagery and digital elevation models with synthesis of field, (micro)structural, and geochronologic data with published maps and studies. Based on our findings, the EMCC likely extends several 10's of km to the NE. Satellite imagery and DEMs suggest large-scale corrugations along the N-flank consistent with NW-SE extension. To the SW of the EMCC, Early Cretaceous rift basins are associated with strong NE-SW oriented lineaments. We examined the BMCC along its SW mapped extent, an area for which no data were presented in prior publications; we confirmed the presence of a top-to-the-SE detachment fault. The EMCC and BMCC, like the Yagan-Onch Hayrhan MCC in southern Mongolia, have footwall rocks previously mapped as Precambrian that are, in large part, metamorphosed Paleozoic and Mesozoic igneous and sedimentary rocks. All three MCCs exhibit evidence for structural complexity, such as NE-SW trending lineations orthogonal to the NW-SE extension direction. As in S Mongolia, we hypothesize that the NE-SW lineations in the EMCC and BMCC formed during an earlier phase of shortening. The expression of the Early Cretaceous extension (rift basin vs. MCC) appears to be controlled by the inherited structure.
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- Award ID(s):
- 1917640
- NSF-PAR ID:
- 10430742
- Date Published:
- Journal Name:
- EOS Transactions American Geophysical Union
- Volume:
- 2022
- Page Range / eLocation ID:
- T22D-0135
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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