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1. Exhumation of the Coyote Mountains metamorphic core complex (Arizona): implications for orogenic collapse of the southern North American Cordillera.

   Gottardi, R. (October 2020, Tectonics)

   A microstructural and thermochronometric analysis of the Coyote Mountains detachment shear zone provides new insight into the collapse of the southern North American Cordillera. The Coyote Mountains is a metamorphic core complex that makes up the northern end of the Baboquivari Mountains in southern Arizona. The Baboquivari Mountains records several episodes of crustal shortening and thickening, and regional metamorphism, including the Late Cretaceous-early Paleogene Laramide orogeny which is locally expressed by the Baboquivari thrust fault. Thrusting and shortening were accompanied by magmatic activity recorded by intrusion of Paleocene muscovite-biotite-garnet peraluminous granites such as the ~58 Ma Pan Tak Granite, interpreted as anatectic melts representing the culmination of the Laramide orogeny. Following Laramide crustal shortening, the northern end of the Baboquivari Mountains was exhumed along a top-to-the-north detachment shear zone, which resulted in the formation of the Coyote Mountains metamorphic core complex. Structural and microstructural analysis show that the detachment shear zone evolved under a strong component of non-coaxial (simple shear) deformation, at deformation conditions of ~450 ± 50°C, under a differential stress of ~60 MPa, and a strain rate of 1.5 ×10-11 s-1 to 5.0 × 10-13 s-1 at depth of ~11–14 km. Detailed 40Ar/39Ar geochronology of biotite and muscovite, in the context of the deformation conditions determined by quartz microstructures, suggests that the mylonitization associated with the formation of the Coyote Mountains metamorphic core complex started at ~29 Ma (early Oligocene). Apatite fission track ages indicate that the footwall of the Coyote Mountains metamorphic core complex experienced rapid exhumation to the upper crust by ~24 Ma. The fact that mylonitization and rapid extensional exhumation post-dates Laramide thickening by ~30 Myr indicates that crustal thickness alone was insufficient to initiate extensional tectonic and required an additional driving force. The timing of mylonitization and rapid exhumation documented here and in other MCCs are consistent with the hypothesis that slab rollback and the effect of a slab window trailing the Mendocino Triple Junction have been critical in driving the development of the MCCs of the southwest. 

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2. 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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3. 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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4. Regional exhumation of the Laramide Province

   https://doi.org/10.1130/B37625.1  

   Jepson, Gilby; Carrapa, Barbara; Reeher, Lauren J; DeCelles, Peter G; Afonso, Walter D; Howlett, Caden J; Caylor, Emilia A; Sherpa, Tshering ZL; Wang, Jordan W; Constenius, Kurt N (February 2025, Geological Society of America Bulletin)

   Western North America is the archetypical Cordilleran orogenic system that preserves a Mesozoic to Cenozoic record of oceanic Farallon plate subduction-related processes. After prolonged Late Jurassic through mid-Cretaceous normal-angle Farallon plate subduction that produced the western North American batholith belt and retroarc fold-thrust belt, a period of low-angle, flat-slab subduction during Late Cretaceous−Paleogene time caused upper plate deformation to migrate eastward in the form of the Laramide basement-involved uplifts, which partitioned the original regional foreland basin. Major questions persist about the mechanism and timing of flat-slab subduction, the trajectory of the flat-slab, inter-plate coupling mechanism(s), and the upper-plate deformational response to such processes. Critical for testing various flat-slab hypotheses are the timing, rate, and distribution of exhumation experienced by the Laramide uplifts as recorded by low-temperature thermochronology. In this contribution, we address the timing of regional exhumation of the Laramide uplifts by combining apatite fission-track (AFT) and (U-Th-Sm)/He (AHe) data from 29 new samples with 564 previously published AFT, AHe, and zircon (U-Th)/He ages from Laramide structures in Arizona, Utah, Wyoming, Colorado, Montana, and South Dakota, USA. We integrate our results with existing geological constraints and with new regional cross sections to reconstruct the spatial and temporal history of exhumation driven by Laramide deformation from the mid-Cretaceous to Paleogene. Our analysis suggests a two-stage exhumation of the Laramide province, with an early phase of localized exhumation occurring at ca. 100−80 Ma in Wyoming and Montana, followed by a more regional period of exhumation at ca. 70−50 Ma. Generally, the onset of enhanced exhumation occurs earlier in the northern Laramide province (ca. 90 Ma) and later in the southern Laramide province (ca. 80 Ma). Thermal history models of selected samples along regional cross sections through Utah−Arizona−New Mexico and Wyoming−South Dakota show that exhumation occurred contemporaneously with deformation, implying that Laramide basement block exhumation is coupled with regional deformation. These results have implications for testing proposed migration pathway models of Farallon flat-slab and for how upper-plate deformation is expressed in flat-slab subduction zones in general. 

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5. Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA

   https://doi.org/10.1029/2022TC007541  

   Ronemus, Chance B.; Orme, Devon A.; Guenthner, William R.; Cox, Stephen E.; Kussmaul, Christopher A. L. (January 2023, Tectonics)

   Abstract Archean rocks exposed in the Beartooth Mountains, Montana and Wyoming, have experienced a complex >2.5 Gyr thermal history related to the long‐term geodynamic evolution of Laurentia. We constrain this history using “deep‐time” thermochronology, reporting zircon U‐Pb, biotite⁴⁰Ar/³⁹Ar, and zircon and apatite [U‐Th(‐Sm)]/He results from three transects across the basement‐core of the range. Our central transect yielded a zircon U‐Pb concordia age of 2,805.6 ± 6.4 Ma. Biotite⁴⁰Ar/³⁹Ar plateau ages from western samples are ≤1,775 ± 27 Ma, while those from samples further east are ≥2,263 ± 76 Ma. Zircon (U‐Th)/He dates span 686.4 ± 11.9 to 13.5 ± 0.3 Ma and show a negative relationship with effective uranium—a proxy for radiation damage. Apatite (U‐Th)/He dates are 109.2 ± 23.9 to 43.6 ± 1.9 Ma and correlate with sample elevation. Multi‐chronometer Bayesian time‐temperature inversions suggest: (a) Cooling between ∼1.90 and ∼1.80 Ga, likely related to Big Sky orogeny thermal effects; (b) Reheating between ∼1.80 Ga and ∼1.35 Ga consistent with Mesoproterozoic burial; (c) Cooling to ≤100°C between Mesoproterozoic and early Paleozoic time, likely reflecting continental erosion; (d) Variable Paleozoic–Jurassic cooling, possibly related to Paleozoic tectonism and/or low eustatic sea level; (e) Rapid Cretaceous–Paleocene cooling, preceding accepted proxies for flat‐slab subduction; (f) Eocene–Miocene reheating consistent with reburial by Cenozoic volcanics and/or sediments; (g) Post‐20 Ma cooling consistent with Neogene development of topographic relief. Our results emphasize the utility of multi‐chronometer thermochronology in recovering complex, non‐monotonic multi‐billion‐year thermal histories. 

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