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1. Ductile strain in the footwall of the Northern Snake Range metamorphic core complex, eastern Nevada, USA: implications for its structural evolution

   Blackford, NR; Long, SP; Lee, J; Stevens, J (October 2022, Geological Society of America)

   The geometry and magnitude of finite strain in the ductile footwalls of metamorphic core complexes are important parameters for testing the predictions of models of extension, yet are often difficult to quantify due to the rare preservation of deformed markers. The footwall of the Northern Snake Range core complex in eastern Nevada preserves a coherent stratigraphy of ductilely thinned Neoproterozoic-Cambrian metasedimentary rocks that are exposed over a 30 km transport-parallel distance, and thus provides an important opportunity to quantify footwall strain. We measured strain ellipsoids from stretched detrital quartz grains and ribbons in 45 samples that span the full exposed distance of the ductilely sheared footwall of the master detachment fault (the Northern Snake Range décollement), and we combined our data with 11 published strain ellipsoids. On the eastern side of the range, where recrystallization limits the preservation of detrital quartz grains, we estimated finite strain by comparing the attenuated thicknesses of Neoproterozoic-Cambrian rock units to their regional stratigraphic thicknesses. Our data demonstrate a dramatic gradient in ductile strain in the transport direction, from 39% subhorizontal extension and 32% subvertical thinning at the western flank of the Northern Snake Range to 450-1440% extension and 81-94% thinning at the eastern flank. The footwall underwent 18-20 km of cumulative ductile extension, which is equivalent to 38-43% of the 47 km of total extension accommodated on brittle structures. Kinematic vorticity estimates from published quartz petrofabrics define an eastward-increasing component of top-to-the-ESE simple shear. Our data are compatible with a rolling hinge model of extension, where displacement on a low-angle, upper-crustal, brittle detachment fault system was fed downward to a zone of distributed, simple shear-dominant, top-down-to-ESE ductile shearing beneath the quartz crystal-plastic transition. The progressive eastward translation and brittle thinning of the hanging wall resulted in the eastward migration of exhumation of footwall rocks. Migrating exhumation may in part be responsible for the eastward-increasing finite strain gradient, as footwall rocks in the eastern part of the range experienced a longer strain history. 

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2. Extreme ductile thinning of Cambrian marbles in the footwall of the Northern Snake Range metamorphic core complex, Nevada: implications for extension magnitude and structural evolution

   Long, SP; Lee, J; Blackford, NR (May 2024, Geological Society of America)

   NA (Ed.)

   Documenting the geometry, magnitude and kinematics of ductile deformation provides important insights into the structural and rheological evolution of continental lithosphere. The Northern Snake Range metamorphic core complex in eastern Nevada provides an exceptional opportunity to investigate the geometry and magnitude of ductile strain during high-magnitude continental extension. Decades of mapping-based research has provided exceptional stratigraphic context for the footwall of the low-angle, top-down-to-ESE, normal-sense Northern Snake Range dècollement (NSRD). In the northern part of the range, Middle-Late Cambrian marble units in the NSRD footwall, which have a cumulative stratigraphic thickness of 1107 ± 107 m in adjacent ranges, were ductilely thinned during Late Eocene-Late Oligocene extension. From west to east across the range, these rocks have been thinned from 869-935-m-thick (15-21% structural thinning) to 54-88 m-thick (92-95% structural thinning) across a 12 km lineation-parallel distance. Ductile extensional strain was accompanied by the development of pervasive linear-planar fabrics and produced megaboudins of calcareous schist units that are ~100-500-m-long, ~15-25-m-thick, and separated by as much as ~1000 m. The magnitude of subhorizontal, ESE-directed, lineation-parallel ductile extension increases eastward across the range from 24 ± 21% to 1226 ± 256%, and total ductile extension across the range is 12.1 ± 2.2 km (167 ± 31%). Quartz recrystallization microstructures and published calcite-dolomite thermometry indicate deformation temperatures of ~400-550 °C during initial Late Eocene-Late Oligocene ductile extensional shearing. NSRD footwall rocks in the eastern part of the range experienced a longer ductile extensional strain history and a prolonged residence time at higher temperatures compared to the western part of the range. This was facilitated by the progressive eastward migration of denudation-related cooling and was likely enhanced by shear heating that scaled eastward with strain magnitude, and/or a possible eastward increase in burial depth. These factors promoted the development of the extreme ductile strain gradient in the NSRD footwall across the Northern Snake Range. 

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3. Crustal Thermal Architecture, Structural Reconstructions, Field Relationships, and Geophysical Data Rule Out Deep Structural Burial of the Footwall of the Northern Snake Range Metamorphic Core Complex (Nevada, USA)

   https://doi.org/10.1029/2024TC008368  

   Long, Sean P; Blackford, Nolan R; Lee, Jeffrey; Soignard, Emmanuel (October 2024, Tectonics)

   Abstract Thermobarometry in the Northern Snake Range metamorphic core complex (Nevada, USA) implies pre‐extensional burial of footwall rocks to 21–30 km depths, while geologic field relationships support 7–13 km pre‐extensional depths. This has fueled a 40‐year‐long debate, which has far‐reaching implications for how pressure data are interpreted in orogenic settings. Here, we test published models for deep burial by integrating regional cross‐section reconstructions with new (n = 95) and published (n = 132) peak temperature measurements, field relationships and published geophysical data. Burial of Neoproterozoic‐Cambrian metasedimentary footwall rocks to 21–30 km depths is incompatible with a regional seismic reflection cross‐section that interprets the top of Precambrian crystalline basement at 17–20 km depths. Two reconstructed cross‐sections define 42 km and 50–65 km of displacement on the master detachment fault and demonstrate that the higher displacement ranges (>66–94 km and >76–102 km, respectively) necessary to exhume rocks from 21 to 30 km depths are not possible without spatially overlapping Cambrian rocks preserved in its footwall and hanging wall. The 22°C/km average Late Cretaceous thermal gradient predicted by thermobarometry is incompatible with the 46 ± 10°C/km Late Cretaceous peak thermal gradient that we calculate down to 15–20 km pre‐extensional depths. Field relationships that rule out large‐magnitude shortening invalidate models for deep footwall burial via thrust or reverse faulting. We conclude that there is no scenario for deep burial that is compatible with structural/geophysical constraints, crustal thermal architecture, and field relationships. This necessitates a non‐lithostatic interpretation for pressures from the Northern Snake Range, similar to recent interpretations for other Cordilleran metamorphic core complexes. 

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4. Stratigraphic and geochronologic investigation of the Muddy Creek Basin: Implications for the Eocene tectonic evolution of southwest Montana, USA

   https://doi.org/10.1130/B37268.1  

   Thoresen, Haley E; Cassel, Elizabeth J; Smith, M Elliot; Stockli, Daniel F; Jicha, Brian R (February 2024, Geological Society of America Bulletin)

   Sedimentary basins record crustal-scale tectonic processes related to the construction and demise of orogenic belts, making them an invaluable archive for the reconstruction of the evolution of the North American Cordillera. In southwest Montana, USA, the Renova Formation, considered to locally represent the earliest accumulation following Mesozoic−Cenozoic compressional deformation, is widespread but remains poorly dated, and its origin is debated. Herein, we employed detrital zircon U-Pb and (U-Th)/He double dating and sanidine 40Ar/39Ar geochronology in the context of decimeter-scale measured stratigraphic sections in the Renova Formation of the Muddy Creek Basin to determine basin evolution and sediment provenance and place the basin-scale record within a regional context to illuminate the lithospheric processes driving extension and subsidence. The Muddy Creek Basin is an extensional half graben in southwest Montana that is ∼22 km long and ∼7 km wide, with a >800-m-thick sedimentary package. Basin deposition began ca. 49 Ma, as marked by multiple ignimbrites sourced from the Challis volcanic field, which are overlain by a tuffaceous fluvial section. Fluvial strata are capped by a 46.8 Ma Challis ignimbrite constrained by sanidine 40Ar/39Ar dating. An overlying fossiliferous limestone records the first instance of basinal ponding, which was coeval with the cessation of delivery of Challis volcanics−derived sediment into the Green River Basin. We attribute initial ponding to regional drainage reorganization and damning of the paleo−Idaho River due to uplift and doming of the southern Absaroka volcanic province, resulting in its diversion away from the Green River Basin and backfilling of the Lemhi Pass paleovalley. Detrital zircon maximum depositional ages and sanidine 40Ar/39Ar ages show alternating fluvial sandstone and lacustrine mudstone deposition from 46 Ma to 40 Ma in the Muddy Creek Basin. Sediment provenance was dominated by regionally sourced, Challis volcanics−aged and Idaho Batholith−aged grains, while detrital zircon (U-Th)/He (ZHe) data are dominated by Eocene cooling ages. Basin deposition became fully lacustrine by ca. 40 Ma, based on an increasing frequency of organic-rich mudstone with rare interbedded sandstone. Coarse-grained lithofacies became prominent again starting ca. 37 Ma, coeval with a major shift in sediment provenance due to extension and local footwall unroofing. Detrital zircon U-Pb and corresponding ZHe ages from the upper part of the section are predominantly Paleozoic in age, sourced from the Paleozoic sedimentary strata exposed in the eastern footwall of the Muddy Creek detachment fault. Paleocurrents shift from south- to west-directed trends, supporting the shift to local sources, consistent with initiation of the Muddy Creek detachment fault. Detrital zircon maximum depositional ages from the youngest strata in the basin suggest deposition continuing until at least 36 Ma. These data show that extension in the Muddy Creek Basin, which we attribute to continued lithospheric thermal weakening, initiated ∼10 m.y. later than in the Anaconda and Bitterroot metamorphic core complexes. This points to potentially different drivers of extension in western Montana and fits previously proposed models of a regional southward sweep of extension related to Farallon slab removal. 

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5. Heterogenous late Miocene extension in the northern Walker Lane (California-Nevada, USA) demonstrates vertically decoupled crustal extension

   https://doi.org/10.1130/GES02409.1  

   Say, Michael C.; Zuza, Andrew V. (September 2021, Geosphere)

   Abstract The spatial distribution and kinematics of intracontinental deformation provide insight into the dominant mode of continental tectonics: rigid-body motion versus continuum flow. The discrete San Andreas fault defines the western North America plate boundary, but transtensional deformation is distributed hundreds of kilometers eastward across the Walker Lane–Basin and Range provinces. In particular, distributed Basin and Range extension has been encroaching westward onto the relatively stable Sierra Nevada block since the Miocene, but the timing and style of distributed deformation overprinting the stable Sierra Nevada crust remains poorly resolved. Here we bracket the timing, magnitude, and kinematics of overprinting Walker Lane and Basin and Range deformation in the Pine Nut Mountains, Nevada (USA), which are the westernmost structural and topographic expression of the Basin and Range, with new geologic mapping and 40Ar/39Ar geochronology. Structural mapping suggests that north-striking normal faults developed during the initiation of Basin and Range extension and were later reactivated as northeast-striking oblique-slip faults following the onset of Walker Lane transtensional deformation. Conformable volcanic and sedimentary rocks, with new ages spanning ca. 14.2 Ma to 6.8 Ma, were tilted 30°–36° northwest by east-dipping normal faults. This relationship demonstrates that dip-slip deformation initiated after ca. 6.8 Ma. A retrodeformed cross section across the range suggests that the range experienced 14% extension. Subsequently, Walker Lane transtension initiated, and clockwise rotation of the Carson domain may have been accommodated by northeast-striking left-slip faults. Our work better defines strain patterns at the western extent of the Basin and Range province across an approximately 150-km-long east-west transect that reveals domains of low strain (∼15%) in the Carson Range–Pine Nut Mountains and Gillis Range surrounding high-magnitude extension (∼150%–180%) in the Singatse and Wassuk Ranges. There is no evidence for irregular crustal thickness variations across this same transect—either in the Mesozoic, prior to extension, or today—which suggests that strain must be accommodated differently at decoupled crustal levels to result in smooth, homogenous crustal thickness values despite the significantly heterogeneous extensional evolution. This example across an ∼150 km transect demonstrates that the use of upper-crust extension estimates to constrain pre-extension crustal thickness, assuming pure shear as commonly done for the Mesozoic Nevadaplano orogenic plateau, may not be reliable. 

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