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1. Decoupled Oligocene mylonitic shearing and Miocene detachment faulting in the East Humboldt Range metamorphic core complex, northeast Nevada, USA

   https://doi.org/10.1130/GES02619.1  

   Zuza, Andrew V. ; Dee, Seth ( July 2023 , Geosphere)

   The relationships between brittle detachment faulting and ductile shear zones in metamorphic core complexes are often ambiguous. Although it is commonly assumed that these two structures are kinematically linked and genetically related, direct observations of this coupling are rare. Here, we conducted a detailed field investigation to probe the connection between a detachment fault and mylonitic shear zone in the Ruby Mountain–East Humboldt Range metamorphic core complex, northeast Nevada. Field observations, along with new and published geochronology, demonstrate that Oligocene top-to-the-west mylonitic shear zones are crosscut by ca. 17 Ma subvertical basalt dikes, and these dikes are in turn truncated by middle Miocene detachment faults. The detachment faults appear to focus in preexisting weak zones in shaley strata and Mesozoic thrust faults. We interpret that the Oligocene mylonitic shear zones were generated in response to domal upwelling during voluminous plutonism and partial melting, which significantly predated the middle Miocene onset of regional extension and detachment slip. Our model simplifies mechanical issues with low-angle detachment faulting because there was an initial dip to the weak zones exploited by the future detachment-fault zone. This mechanism may be important for many apparent low-angle normal faults in the eastern Great Basin. We suggest that the temporal decoupling of mylonitic shearing and detachment faulting may be significant and underappreciated for many of the metamorphic core complexes in the North American Cordillera. In this case, earlier Eocene–Oligocene buoyant doming may have preconditioned the crust to be reactivated by Miocene extension thus explaining the spatial relationship between structures. 

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2. Jurassic–Cenozoic tectonics of the Pequop Mountains, NE Nevada, in the North American Cordillera hinterland

   https://doi.org/10.1130/GES02307.1  

   Zuza, Andrew V. ; Henry, Christopher D. ; Dee, Seth ; Thorman, Charles H. ; Heizler, Matthew T. ( October 2021 , Geosphere)

   Abstract The Ruby Mountains–East Humboldt Range–Wood Hills–Pequop Mountains (REWP) metamorphic core complex, northeast Nevada, exposes a record of Mesozoic contraction and Cenozoic extension in the hinterland of the North American Cordillera. The timing, magnitude, and style of crustal thickening and succeeding crustal thinning have long been debated. The Pequop Mountains, comprising Neoproterozoic through Triassic strata, are the least deformed part of this composite metamorphic core complex, compared to the migmatitic and mylonitized ranges to the west, and provide the clearest field relationships for the Mesozoic–Cenozoic tectonic evolution. New field, structural, geochronologic, and thermochronological observations based on 1:24,000-scale geologic mapping of the northern Pequop Mountains provide insights into the multi-stage tectonic history of the REWP. Polyphase cooling and reheating of the middle-upper crust was tracked over the range of <100 °C to 450 °C via novel 40Ar/39Ar multi-diffusion domain modeling of muscovite and K-feldspar and apatite fission-track dating. Important new observations and interpretations include: (1) crosscutting field relationships show that most of the contractional deformation in this region occurred just prior to, or during, the Middle-Late Jurassic Elko orogeny (ca. 170–157 Ma), with negligible Cretaceous shortening; (2) temperature-depth data rule out deep burial of Paleozoic stratigraphy, thus refuting models that incorporate large cryptic overthrust sheets; (3) Jurassic, Cretaceous, and Eocene intrusions and associated thermal pulses metamorphosed the lower Paleozoic–Proterozoic rocks, and various thermochronometers record conductive cooling near original stratigraphic depths; (4) east-draining paleovalleys with ∼1–1.5 km relief incised the region before ca. 41 Ma and were filled by 41–39.5 Ma volcanic rocks; and (5) low-angle normal faulting initiated after the Eocene, possibly as early as the late Oligocene, although basin-generating extension from high-angle normal faulting began in the middle Miocene. Observed Jurassic shortening is coeval with structures in the Luning-Fencemaker thrust belt to the west, and other strain documented across central-east Nevada and Utah, suggesting ∼100 km Middle-Late Jurassic shortening across the Sierra Nevada retroarc. This phase of deformation correlates with terrane accretion in the Sierran forearc, increased North American–Farallon convergence rates, and enhanced Jurassic Sierran arc magmatism. Although spatially variable, the Cordilleran hinterland and the high plateau that developed across it (i.e., the hypothesized Nevadaplano) involved a dynamic pulsed evolution with significant phases of both Middle-Late Jurassic and Late Cretaceous contractional deformation. Collapse long postdated all of this contraction. This complex geologic history set the stage for the Carlin-type gold deposit at Long Canyon, located along the eastern flank of the Pequop Mountains, and may provide important clues for future exploration. 

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3. How a strong low-angle normal fault formed: The Whipple detachment, southeastern California

   https://doi.org/10.1130/B35386.1  

   Axen, Gary J. ( December 2019 , GSA Bulletin)

   Abstract Many low-angle normal faults (dip ≤30°) accommodate tens of kilometers of crustal extension, but their mechanics remain contentious. Most models for low-angle normal fault slip assume vertical maximum principal stress σ1, leading many authors to conclude that low-angle normal faults are poorly oriented in the stress field (≥60° from σ1) and weak (low friction). In contrast, models for low-angle normal fault formation in isotropic rocks typically assume Coulomb failure and require inclined σ1 (no misorientation). Here, a data-based, mechanical-tectonic model is presented for formation of the Whipple detachment fault, southeastern California. The model honors local and regional geologic and tectonic history and laboratory friction measurements. The Whipple detachment fault formed progressively in the brittle-plastic transition by linking of “minidetachments,” which are small-scale analogs (meters to kilometers in length) in the upper footwall. Minidetachments followed mylonitic anisotropy along planes of maximum shear stress (45° from the maximum principal stress), not Coulomb fractures. They evolved from mylonitic flow to cataclasis and frictional slip at 300–400 °C and ∼9.5 km depth, while fluid pressure fell from lithostatic to hydrostatic levels. Minidetachment friction was presumably high (0.6–0.85), based upon formation of quartzofeldspathic cataclasite and pseudotachylyte. Similar mechanics are inferred for both the minidetachments and the Whipple detachment fault, driven by high differential stress (∼150–160 MPa). A Mohr construction is presented with the fault dip as the main free parameter. Using “Byerlee friction” (0.6–0.85) on the minidetachments and the Whipple detachment fault, and internal friction (1.0–1.7) on newly formed Reidel shears, the initial fault dips are calculated at 16°–26°, with σ1 plunging ∼61°–71° northeast. Linked minidetachments probably were not well aligned, and slip on the evolving Whipple detachment fault probably contributed to fault smoothing, by off-fault fracturing and cataclasis, and to formation of the fault core and fractured damage zone. Stress rotation may have occurred only within the mylonitic shear zone, but asymmetric tectonic forces applied to the brittle crust probably caused gradual rotation of σ1 above it as a result of: (1) the upward force applied to the base of marginal North America by buoyant asthenosphere upwelling into an opening slab-free window and/or (2) basal, top-to-the-NE shear traction due to midcrustal mylonitic flow during tectonic exhumation of the Orocopia Schist. The mechanical-tectonic model probably applies directly to low-angle normal faults of the lower Colorado River extensional corridor, and aspects of the model (e.g., significance of anisotropy, stress rotation) likely apply to formation of other strong low-angle normal faults. 

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4. Reconstruction of Mid‐Cenozoic Extension in the Rincon Mountains Area, Southeastern Arizona, USA, and Geodynamic Implications

   https://doi.org/10.1029/2019TC005565  

   Spencer, Jon E. ; Richard, Stephen M. ; Lingrey, Steven H. ; Johnson, Bradford J. ; Johnson, Roy A. ; Gehrels, George E. ( July 2019 , Tectonics)

   The Rincon Mountains metamorphic core complex, located east of Tucson, Arizona, consists of an arched footwall of foliated crystalline rocks bounded above by the generally outward dipping, Oligocene‐Miocene San Pedro extensional detachment fault. The southwest trending axes of corrugations in the detachment fault, and in footwall foliation and lithologic layering, parallel mylonitic lineation, and inferred top‐southwest displacement on the fault. An upper plate fault block within a synformal fault groove on the west side of the Rincon Mountains contains a thrust fault that is interpreted as displaced 34‐38 km westward from an original position adjacent to a similar thrust in the footwall of the San Pedro detachment fault. Much of the footwall of the detachment fault in the eastern Rincon Mountains consists of metasedimentary tectonites derived largely from Paleozoic carbonates that were buried beneath Proterozoic crystalline rocks forming the hanging wall of the Laramide Wildhorse Mountain thrust. These tectonites were later exhumed by displacement on the San Pedro detachment fault. Structural reconstruction supports the interpretation that the carbonate tectonites localized extensional faulting along the San Pedro detachment fault at crustal depths where carbonates would be weak and deform by crystal plasticity while quartzo‐feldspathic rocks would be strong and brittle. This weak zone is located adjacent to the greatest width of exposed extension‐parallel mylonitic fabrics in southeastern Arizona and may have been associated with the earliest initiation of extension in the region. Domains of low‐strength carbonates may be an underappreciated influence on extensional tectonics in cratonic southwestern North America.

    

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5. Variations in the P-T-t of deformation in a crustal-scale shear zone in metagranite

   https://doi.org/https://doi.org/10.1029/2020GC009384  

   Cawood, T.K. ; Platt, J.P. ( November 2020 , Geochemistry geophysics geosystems)

   null (Ed.)

   Deformation in crustal-scale shear zones occurs over a range of pressure-temperature-time (P-T-t) conditions, both because they may be vertically extensive structures that simultaneously affect material from the lower crust to the surface, and because the conditions at which any specific volume of rock is deformed evolve over time, as that material is advected by fault activity. Extracting such P-T-t records is challenging, because structures may be overprinted by progressive deformation. In addition, granitic rocks, in particular, may lack syn-kinematic mineral assemblages amenable to traditional metamorphic petrology and petrochronology. We overcome these challenges by studying the normal-sense Simplon Shear Zone (SSZ) in the central Alps, where strain localization in the exhuming footwall caused progressive narrowing of the shear zone, resulting in a zonation from high-T shearing preserved far into the footwall, to low-T shearing adjacent to the hanging wall. The Ti-in-quartz and Si-in-phengite thermobarometers yield deformation P-T conditions, as both were reset syn-kinematically, and although the sheared metagranites lack typical petrochronometers, we estimate the timing of deformation by comparing our calculated deformation temperatures to published thermochronological ages. The exposed SSZ footwall preserves evidence for retrograde deformation during exhumation, from just below amphibolite-facies conditions (∼490°C, 6.7 kbar) at ∼24.5 Ma, to lower greenschist-facies conditions (∼305°C, 1.5 kbar) at ∼11.5 Ma, with subsequent slip taken up by brittle faulting. Our estimates fall within the P-T-t brackets provided by independent constraints on the maximum and minimum conditions of retrograde ductile deformation, and compare reasonably well to alternative approaches for estimating P-T. 

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