Late Cretaceous upper-crustal thermal structure of the Sevier hinterland: Implications for the geodynamics of the Nevadaplano

Abstract

Crustal temperature conditions can strongly influence the evolution of deformation during orogenesis. The Sevier hinterland plateau in Nevada and western Utah (“Nevadaplano”) experienced a Late Cretaceous episode of shallow-crustal metamorphism and granitic magmatism. Here, we investigate the thermal history of the Nevadaplano by measuring peak thermal field gradients attained in the upper 10–20 km of the crust along an east-west transect through nine ranges in eastern Nevada and western Utah, by integrating Raman spectroscopy of carbonaceous material thermometry and published conodont alteration indices with reconstructed cross sections. Thermal field gradients of 29 ± 3 °C/km were obtained in the House and Confusion Ranges in westernmost Utah. The Deep Creek, Schell Creek, and Egan Ranges in easternmost Nevada yielded elevated gradients of 49 ± 7 °C/km, 36 ± 3 °C/km, and 32 ± 6 °C/km, respectively. Moving westward, the White Pine, Butte, Pancake, and Fish Creek Ranges exhibit gradients typically between ~20–30 °C/km. The elevated thermal gradients in easternmost Nevada are interpreted to have been attained during ca. 70–90 Ma granitic magmatism and metamorphism and imply possible partial melting at ~18 km depths. Our data are compatible with published interpretations of Late Cretaceous lithospheric mantle delamination under the Sevier hinterland, which triggered lower-crustal anatexis and the resulting rise of granitic melts. The lack of evidence for structures that could have accommodated deep burial of rocks in the nearby Northern Snake Range metamorphic core complex, combined with thermal gradients from adjacent ranges that are ~1.5–3 times higher than those implied by thermobarometry in the Northern Snake Range, further highlights the debate over possible tectonic overpressure in Cordilleran core complexes. Cross-section retro-deformation defines 73.4 ± 4.6 km (76 ± 8%) of extension across eastern Nevada and 15 km of shortening in the Eastern Nevada fold belt.

The low-angle breakaway system for the Northern Snake Range décollement in the Schell Creek and Duck Creek Ranges, eastern Nevada, USA: Implications for displacement magnitude

“…The disagreement over the pre-extensional geometry of the Northern Snake Range metamorphic core complex remains unresolved, as several field relationships (summarized in Miller et al, 1999a) provide strong arguments that rocks above and below the Northern Snake Range décollement were stratigraphically contiguous prior to extension. In addition, a recent cross-section restoration across eastern Nevada and western Utah did not find evidence for large-scale contractional structures that could have accommodated deep structural burial in the region surrounding the Northern Snake Range (Blackford et al, 2022). Other factors contributing to this debate include recent studies in the Ruby-East Humboldt metamorphic core complex in northeastern Nevada, which argue that pre-extensional structural burial depths of footwall rocks were much shallower than thermobarometric estimates (Henry et al, 2011;Zuza et al, 2020), allowing for the possibility that these rocks experienced tectonic overpressure (Zuza et al, 2022).…”

“…4). Blackford et al (2022) presented a retrodeformed cross section through the Schell Creek and Duck Creek Ranges, supported by the mapping of Young (1960) and Gans et al (1985). Their study was the first to document that faults of the Schell Creek Range detachment system have low stratigraphic cutoff angles across all exposed structural levels, which requires large-magnitude (~32 km) cumulative top-down-to-the-E displacement.…”

“…Neoproterozoic to Cambrian sedimentary rocks exposed in the Snake and Schell Creek Ranges record greenschist-to amphibolite-facies metamorphism, with peak metamorphic conditions interpreted to have been attained during a Late Cretaceous magmatic episode (Fig. 1B; e.g., Miller and Gans, 1989;Cooper et al, 2010;Blackford et al, 2022). By the late stages of Cordilleran thickening in the Late Cretaceous-early Paleogene, eastern Nevada is interpreted to have been a high-elevation plateau underlain by ~50-60-km-thick crust (e.g., Coney and Harms, 1984;Cassel et al, 2014;Snell et al, 2014;Chapman et al, 2015;Long, 2019).…”

“…To the north (~20-50 km N of the latitude of cross-section A-A′), a transition in style to high-angle, domino-style normal faults that cut down to ~10-15 km depths without flattening is observed in the Egan, Cherry Creek, and Deep Creek Ranges (Fritz, 1968;Wernicke, 1981;Gans, 1982;Gans and Miller, 1983;Rodgers, 1987;Blackford et al, 2022). No extension estimates are available for the Schell Creek Range at this latitude, but Blackford et al (2022) measured 28.1 km of extension across the Deep Creek Range and 15.8 km across the Egan and Cherry Creek Ranges, indicating at least 43.9 km of extension at this latitude, which is similar to our 46.7 km approximate extension estimate for the linked Schell Creek Range detachment system-Northern Snake Range décollement.…”

Supplemental Material: The low-angle breakaway system for the Northern Snake Range décollement in the Schell Creek and Duck Creek Ranges, eastern Nevada, USA: Implications for displacement magnitude

“…Neoproterozoic to Cambrian sedimentary rocks exposed in the Snake and Schell Creek Ranges record greenschist-to amphibolite-facies metamorphism, with peak metamorphic conditions interpreted to have been attained during a Late Cretaceous magmatic episode (Fig. 1B; e.g., Miller and Gans, 1989;Cooper et al, 2010;Blackford et al, 2022). By the late stages of Cordilleran thickening in the Late Cretaceous-early Paleogene, eastern Nevada is interpreted to have been a high-elevation plateau underlain by ~50-60-km-thick crust (e.g., Coney and Harms, 1984;Cassel et al, 2014;Snell et al, 2014;Chapman et al, 2015;Long, 2019).…”