The Evolution of the Nevadaplano From Construction to Dismemberment: Investigations of Thermal Conditions, Strain Distribution, and Fabric Intensity in Eastern Nevada and Western Utah

Nolan R. Blackford

doi:10.7273/000005271

Documenting the kinematic evolution of structural systems, including the dominant mass transfer processes, the geometry and magnitude of strain, and the thermal architecture of deforming crust is fundamental for interpreting deformation processes. In eastern Nevada and western Utah the hinterland of the Sevier orogeny is interpreted to have been high-elevation orogenic plateau (Nevadaplano) that experience a punctuated tectonothermal event during the late Cretaceous followed by orogenic collapse and high-magnitude Eocene-Oligocene extension. In order to elucidate the kinematic evolution of orogenic crust and its subsequent collapse, we present peak thermal conditions of the Nevadaplano during the Late Cretaceous and investigate the magnitude of subsequent metamorphic core complex extension in the Northern Snake Range, Nevada, USA. Finally, we provide new equations that compare quartz crystallographic preferred orientation fabric intensities with finite strain magnitude calculated from the Northern Snake Range, providing a new method to quantify finite strain magnitude in quartz rich tectonites in regions where deformation markers may be absent or destroyed. At the latitude of ~39 °N, 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 ~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. Following the end of the Sevier orogeny, metamorphic core complexes locally accommodated high-magnitude extension following the collapse of the Sevier orogenic plateau. The footwall of the Eocene-Oligocene Northern Snake Range décollement (NSRD) and west-adjacent Schell Creek Range detachment system, eastern Nevada, preserves a coherent stratigraphy of ductilely deformed Neoproterozoic-Cambrian metasedimentary rocks that are exposed over a 60 km transport-parallel distance, providing an ideal opportunity to quantify ductile strain. Our data demonstrate a dramatic finite strain gradient in the transport direction, from 119% horizontal extension and 59% vertical thinning at the western flank of the Northern Snake Range to 450-1390% extension and 81-94% thinning at the eastern flank. The footwall underwent 18-20 km of cumulative ductile extension, equivalent to 38-43% of the total extension accommodated on brittle structures. Our data are compatible with a rolling hinge model of extension, where brittle displacement on a low-angle detachment was fed downward to a zone of distributed, simple shear-dominant, top-down-to-ESE ductile shearing beneath the quartz crystal-plastic transition, and footwall exhumation progressively migrated eastward. Quantifying finite strain magnitude in the Northern Snake Range is made difficult due to pervasive recrystallization that typically occurs within ductile shear zones and often destroys deformed markers from which finite strain can be measured. Here, we present equations that relate fabric intensity parameters (cylindricity and density norm) to finite strain magnitude. We measured fabric intensity parameters from 77 samples and incorporated 12 published samples across the width of the NSRD footwall and compare fabric intensity parameters to our finite strain data. Finite strain magnitude and fabric intensity increase rapidly up to tectonic strain values of ~20-25, at which point the correlative increase in fabric intensity diminishes with increasing finite strain. We present equations that express fabric intensity as a function of finite strain, and vice versa. The equations presented here can be used to provide a first-order estimate of finite strain magnitude within any ductile shear zone from which quartz fabric intensity can be measured.

The Evolution of the Nevadaplano From Construction to Dismemberment: Investigations of Thermal Conditions, Strain Distribution, and Fabric Intensity in Eastern Nevada and Western Utah

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