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1. Isolating non-subduction-driven tectonic processes in Cascadia

   https://doi.org/10.1186/s40562-021-00181-z  

   McKenzie, K. A. ; Furlong, K. P. ( December 2021 , Geoscience Letters)

   null (Ed.)

   Abstract Several tectonic processes combine to produce the crustal deformation observed across the Cascadia margin: (1) Cascadia subduction, (2) the northward propagation of the Mendocino Triple Junction (MTJ), (3) the translation of the Sierra Nevada–Great Valley (SNGV) block along the Eastern California Shear Zone–Walker Lane and, (3) extension in the northwestern Basin and Range, east of the Cascade Arc. The superposition of deformation associated with these processes produces the present-day GPS velocity field. North of ~ 45° N observed crustal displacements are consistent with inter-seismic subduction coupling. South of ~ 45° N, NNW-directed crustal shortening produced by the Mendocino crustal conveyor (MCC) and deformation associated with SNGV-block motion overprint the NE-directed Cascadia subduction coupling signal. Embedded in this overall pattern of crustal deformation is the rigid translation of the Klamath terrane, bounded on its north and west by localized zones of deformation. Since the MCC and SNGV processes migrate northward, their impact on the crustal deformation in southern Cascadia is a relatively recent phenomenon, since ~ 2 –3 Ma. 

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2. Mesozoic Tectonic Evolution in the Kurgovat‐Vanch Complex, NW Pamir

   https://doi.org/10.1029/2021TC007180  

   Li, Yipeng ; Robinson, Alexander C. ; Zucali, Michele ; Gadoev, Mustafo ; Oimuhammadzoda, Ilhomjon ; Lapen, Thomas J. ; Carrapa, Barbara ( October 2022 , Tectonics)

   Different crustal deformation histories between Tibet and the Pamir reflect along‐strike variations in geodynamics of the Tethys orogen. To investigate the less well‐documented deformation history of the Pamir, which has been a barrier in understanding the nature of these differences, we conducted an integrated study in the Kurgovat‐Vanch region, NW Pamir. The lithologies are primarily Ediacaran‐to‐Carboniferous metasedimentary rocks intruded by Carboniferous plutons, which then experienced Late Triassic to Early Jurassic regional metamorphism. Structural mapping and analyses document a low‐angle NW‐directed thrust fault, the Poshkharv thrust, separating the overlying upper‐greenschist facies Poshkharv complex from the underlying amphibolite facies Kurgovat complex. Regional geologic maps indicate the Poshkharv thrust continues for ∼300 km across the NW Pamir. Our study also documents another regional thrust fault, the top‐to‐the‐SE Vanch thrust that juxtaposes the Southern Kurgovat complex above the lower‐grade Vanch complex in the south. Biotite⁴⁰Ar/³⁹Ar thermochronology indicates Early Cretaceous movement on all structures with ∼135–125 Ma exhumation along the NW‐directed Poshkharv thrust and ∼125–115 Ma exhumation along the SE‐directed Vanch thrust. Regional crustal deformation in the Northern Pamir was formed in a Cretaceous retro‐arc setting, unrelated to the Cenozoic India‐Asia collision. Cretaceous deformation in the NW Pamir was broadly coeval with the NE Pamir, but preceded Cretaceous shortening and coeval arc magmatism in the Southern Pamir. We interpret Early Cretaceous thrusting and crustal thickening followed by southward migration of shortening and magmatic flare‐up in the Pamir to have resulted from a transition of Neotethys subduction from northward flat‐slab advancing to southward retreating.

    

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3. 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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4. 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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5. SCENTAR: A High-Density Nodal Array to Study the Structure and Seismogenic Behavior of the Southern Cascadia Forearc

   https://doi.org/10.1785/0220220251  

   Delph, Jonathan R. ; Thomas, Amanda M. ; Stanciu, A. Christian ; Aslam, Khurram ; Chatterjee, Avigyan ; Sassard, Vincent ( November 2022 , Seismological Research Letters)

   Abstract Tectonic and seismogenic variations in subduction forearcs can be linked through various processes associated with subduction. Along the Cascadia forearc, significant variations between different geologic expressions of subduction appear to correlate, such as episodic tremor-and-slip (ETS) recurrence interval, intraslab seismicity, slab dip, uplift and exhumation rates, and topography, which allows for the systematic study of the plausible controlling mechanisms behind these variations. Even though the southern Cascadia forearc has the broadest topographic expression and shortest ETS recurrence intervals along the margin, it has been relatively underinstrumented with modern seismic equipment. Therefore, better seismic images are needed before robust comparisons with other portions of the forearc can be made. In March 2020, we deployed the Southern Cascadia Earthquake and Tectonics Array throughout the southern Cascadia forearc. This array consisted of 60 continuously recording three-component nodal seismometers with an average station spacing of ∼15 km, and stations recorded ∼38 days of data on average. We will analyze this newly collected nodal dataset to better image the structural characteristics and constrain the seismogenic behavior of the southern Cascadia forearc. The main goals of this project are to (1) constrain the precise location of the plate interface through seismic imaging and the analysis of seismicity, (2) characterize the lower crustal architecture of the overriding forearc crust to understand the role that this plays in enabling the high nonvolcanic tremor density and short episodic slow-slip recurrence intervals in the region, and (3) attempt to decouple the contributions of subduction versus San Andreas–related deformation to uplift along this particularly elevated portion of the Cascadia forearc. The results of this project will shed light on the controlling mechanisms behind heterogeneous ETS behavior and variable forearc surficial responses to subduction in Cascadia, with implications for other analogous subduction margins. 

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