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1. Episodic Late Eocene to Recent Extension in the vicinity of the Ruby Mountains and East Humboldt Range, Elko County, Nevada

   https://doi.org/doi: 10.1130/abs/2021CD-363293  

   McGrew, Allen J.; Metcalf, James M. (May 2021, Abstracts with programs Geological Society of America)

   null (Ed.)

   The Ruby Mountains, East Humboldt Range and Wood Hills (REHW) of Elko County Nevada, one of the classic metamorphic core complexes of the Cordillera, preserves a protracted and episodic record of both ancient and modern crustal extension that has only recently been unraveled based on its thermochronometrically constrained cooling history. Extension began during the Late Eocene synchronously with a major pulse of intermediate to felsic magmatism preserved locally by plutonic rocks intruded into the REHW and regionally by widespread Late Eocene to early Oligocene volcanism (“the ignimbrite flare-up”). The Eocene-Oligocene event accommodated at least 15 km of extension concentrated in the northern half of the complex and associated with deposition in the Elko Basin to the west, a relatively thin (~1 km), broad sequence of Late Eocene lacustrine and related strata that contrasts with the younger sedimentation patterns represented by the narrower, thicker (up to 4+km), coarse clastics of the Miocene Humboldt Basin. Though locally significant, the Eocene-Oligocene extensional phase appears not to have been associated with broadly distributed regional extension, again contrasting with Miocene and younger events. The initial phase of extension slowed or halted by the mid-Oligocene, after which extension re-accelerated in the latest Oligocene to early Miocene (~25 – 21 Ma), correlative with deposition of a coarse clastic and lacustrine sequence known as the Clover Formation. This extensional phase propagated farther south than the earlier phase along the full length of the REHW. Extension likely slowed again between ~21 Ma and ~17.5 Ma, after which it abruptly re-accelerated through the Middle Miocene to ~10 Ma, synchronous with deposition of the thick, coarse clastics of the Humboldt Formation. Middle Miocene extension likely initiated with crustal-scale heating marking the impingement of the Yellowstone hot spot in NW Nevada. Sometime after 10 Ma, the interior of the core complex was transected by east-dipping normal faults that today define the steep eastern face of the Ruby Mountains and East Humboldt Range; these face west-dipping normal faults along the west flank of the Pequop Mountains and Spruce Mountains. Extension continues today at a rate of ~1 mm/yr as represented by the 2008 MW 6.0 Wells Earthquake. 

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2. Cenozoic slip along the southern Sierra Nevada normal fault, California (USA): A long-lived stable western boundary of the Basin and Range

   https://doi.org/10.1130/GES02574.1  

   Lee, Jeffrey; Stockli, Daniel F.; Blythe, Ann E. (April 2023, Geosphere)

   The uplift history of the Sierra Nevada, California, is a topic of long-standing disagreement with much of it centered on the timing and nature of slip along the range-bounding normal fault along the east flank of the southern Sierra Nevada. The history of normal fault slip is important for characterizing the uplift history of the Sierra Nevada, as well as for characterizing the geologic and geodynamic factors that drove, and continue to drive, normal faulting. To address these issues, we completed new structural studies and extensive apatite (U-Th)/He (AHe) thermochronometry on samples collected from three vertical transects in the footwall to the east-dipping southern Sierra Nevada normal fault (SNNF). Our structural studies on bedrock fault planes show that the SNNF is a steeply (~70°) east-dipping normal fault. The new AHe data reveal two elevation-invariant AHe age arrays, indicative of two distinct periods of cooling and exhumation, which we interpret as initiation of normal faulting along the SNNF at ca. 28–27 Ma with a second phase of normal faulting at ca. 17–13 Ma. We argue that beginning in the late Oligocene, the SNNF marked the now long-standing stable western limit, or break-away zone, of the Basin and Range. Slip along SNNF, and the associated unloading of the footwall, likely resulted in two periods of uplift of Sierra Nevada during the late Cenozoic. Trench retreat, driven by westward motion of the North American plate, along the Farallon–North American subduction zone boundary, as well as the gravitationally unstable northern and southern Basin and Range pushing on the cold Sierra Nevada, likely drove the late Oligocene- aged normal slip along the SNNF and the similar-aged but generally local and minor extension within the Basin and Range. We posit that the thick proto–Basin and Range lithosphere was primed for late Oligocene extension by replacement of the steepening Farallon slab with hot and buoyant asthenosphere. While steepening of the Farallon slab had not yet reached the southern Sierra Nevada by late Oligocene time, we speculate that late Oligocene slip along the SNNF reactivated a late Cretaceous dextral shear zone as the Sierra Nevada block was pulled and pushed westward in response to trench retreat and gravitational potential energy. The dominant middle Miocene normal fault-slip history along the SNNF is contemporaneous with high-magnitude slip recorded along range-bounding normal faults across the Basin and Range, including the east-adjacent Inyo and White mountains, indicating that this period of extension was a major regional tectonic event. We infer that a combination of slab-driven trench retreat along the Juan de Fuca–North America subduction zone boundary and clockwise rotation of the southern ancestral Cascade Range superimposed on continental lithosphere pre-conditioned for extension drove this episode of middle Miocene normal slip along the SNNF and extension to the east across the Basin and Range. Transtensional plate motion along the Pacific–North America plate boundary, and likely a growing slab window, continued to drive extension along the SNNF and the western Basin and Range, but not until ca. 11 Ma when the Mendocino triple junction reached the latitude of our northernmost (U-Th)/He transect. 

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3. 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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4. A Revised Cooling and Extensional Exhumation History for the Harrison Pass Pluton, Southern Ruby Mountains Metamorphic Core Complex, Elko County, Nevada

   McGrew, A.J.; Metcalf, J.M. (April 2020, Vision for Discovery: Geology and Ore Deposits of the Great Basin, Geological Society of Nevada 2020 Symposium Proceedings)

   Koutz, F.R.; Pennell, W.M. (Ed.)

   A key question in the tectonic evolution of the Sevier orogenic belt of the western U.S. Cordillera is when and why the overthickened crust of the hinterland plateau began to collapse giving rise to the modern extensional tectonic regime. Delineating the exhumation history of the Ruby Mountains, East Humboldt Range and Wood Hills metamorphic core complex (REHW) of Elko County, Nevada offers important evidence bearing on this question. Recent work from the northern REHW records a three-phase extensional history: (1) ~15–20 km of Late Eocene extension, (2) a second pulse of extension of similar rate and magnitude beginning in the late Oligocene or early Miocene (by 21 Ma) and continuing to approximately 11 Ma, and (3) the Basin-and-Range extensional regime continuing at reduced rate to today. In contrast, previous work from the Harrison Pass area in the southern REHW does not recognize an imprint from the Late Eocene phase of extension, and places the onset of the second extensional phase after ~17 Ma. New intermediate closure temperature thermochronology from the Harrison Pass pluton indicates that it remained at significant depth until at least ~25 Ma, severely limiting any possible Late Eocene to early Oligocene extension, consistent with previous interpretations. However, the new results challenge the previously proposed post-17 Ma onset for extension at Harrison Pass. New, intermediate closure temperature (U-Th)/He titanite and zircon ages from the eastern half of the pluton almost entirely predate 17 Ma and instead support an extensional onset bracketed between the Early Miocene (21 Ma) and the late Oligocene (25 Ma). Integrating potassium feldspar 40Ar/39Ar multi-diffusion domain modeling with the lower closure temperature thermochronometric systems reveals an inflection to faster cooling rates after ~25 Ma and further supports this inference. Nevertheless, all but the farthest east and structurally shallowest of the samples also show a second inflection point at ~17 Ma. We argue that previously reported apatite fission track and apatite (U-Th)/He data captured this post-17.5 Ma reacceleration event but missed the earlier, late Oligocene-early Miocene extension recorded by the higher temperature thermochronometers. The latest Oligocene to early Miocene extensional phase correlates with extensional events reported from southern Nevada and Arizona that may relate to the relaxation of contractional boundary conditions during the early evolution of the San Andreas margin. However, the post-17.5 Ma resurgence in extension probably correlates with large-scale crustal weakening across the northern Basin and Range province attending the arrival of the Yellowstone thermal plume. 

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5. A Revised Cooling and Extensional Exhumation History for the Harrison Pass Pluton, Southern Ruby Mountains Metamorphic Core Complex, Elko County, Nevada

   A.J. McGrew J.R.Metcalf (April 2000, Vision for Discovery: Geology and Ore Deposits of the Great Basin, Geological Society of Nevada 2020 Symposium Proceedings)

   F.R. Koutz W.M. Pennell (Ed.)

   A key question in the tectonic evolution of the Sevier orogenic belt of the western U.S. Cordillera is when and why the overthickened crust of the hinterland plateau began to collapse giving rise to the modern extensional tectonic regime. Delineating the exhumation history of the Ruby Mountains, East Humboldt Range and Wood Hills metamorphic core complex (REHW) of Elko County, Nevada offers important evidence bearing on this question. Recent work from the northern REHW records a three-phase extensional history: (1) ~15–20 km of Late Eocene extension, (2) a second pulse of extension of similar rate and magnitude beginning in the late Oligocene or early Miocene (by 21 Ma) and continuing to approximately 11 Ma, and (3) the Basin-and-Range extensional regime continuing at reduced rate to today. In contrast, previous work from the Harrison Pass area in the southern REHW does not recognize an imprint from the Late Eocene phase of extension, and places the onset of the second extensional phase after ~17 Ma. New intermediate closure temperature thermochronology from the Harrison Pass pluton indicates that it remained at significant depth until at least ~25 Ma, severely limiting any possible Late Eocene to early Oligocene extension, consistent with previous interpretations. However, the new results challenge the previously proposed post-17 Ma onset for extension at Harrison Pass. New, intermediate closure temperature (U-Th)/He titanite and zircon ages from the eastern half of the pluton almost entirely predate 17 Ma and instead support an extensional onset bracketed between the Early Miocene (21 Ma) and the late Oligocene (25 Ma). Integrating potassium feldspar 40Ar/39Ar multi-diffusion domain modeling with the lower closure temperature thermochronometric systems reveals an inflection to faster cooling rates after ~25 Ma and further supports this inference. Nevertheless, all but the farthest east and structurally shallowest of the samples also show a second inflection point at ~17 Ma. We argue that previously reported apatite fission track and apatite (U-Th)/He data captured this post-17.5 Ma reacceleration event but missed the earlier, late Oligocene-early Miocene extension recorded by the higher temperature thermochronometers. The latest Oligocene to early Miocene extensional phase correlates with extensional events reported from southern Nevada and Arizona that may relate to the relaxation of contractional boundary conditions during the early evolution of the San Andreas margin. However, the post-17.5 Ma resurgence in extension probably correlates with large-scale crustal weakening across the northern Basin and Range province attending the arrival of the Yellowstone thermal plume. 

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