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1. New interpretations on the Reindeer Hills mélange using structural and geochronological data

   Clark, G; Waldien, T (May 2024, Geological Society of America Abstracts with Programs)

   The North American Cordillera formed by protracted subduction that led to the accretion of multiple exotic terranes during the Mesozoic. Subduction and terrane accretion are recorded throughout the Cordillera by fault-bounded mélange belts exposed between disparate terranes. South of the Denali fault in central Alaska, the Reindeer Hills Mélange (RHM) consists of pervasively sheared carbonate, ultramafic, and sandstone blocks in a shale and chert-breccia matrix. The presence of these oceanic rock types and correlation with nearby Cretaceous flysch has led to the interpretation that the RHM formed by subduction of oceanic lithosphere during the Cretaceous. However, the age of the RHM and its genetic relationship to surrounding terranes remain unclear. New structural and kinematic analysis along a ~5 km across-strike transect through the RHM reveals a steeply N-dipping penetrative cleavage, and asymmetric sandstone blocks in the shale matrix record distributed top-to-the-south shear. Detrital zircon U-Pb geochronology of grains taken from a sandstone block at the southern end of the transect present a dominant population of Silurian-Devonian grains that yield a youngest statistical population maximum depositional age of 416 +/- 6 Ma. Abundant Proterozoic grains ranging from 900-2000 Ma permit sediment input from peri-Laurentian sources, yet a distinctive population of 1450-1500 Ma grains may suggest input from Baltica basement or other Baltica-derived terranes recognized in the Cordillera (e.g., Alexander, Farewell). The new age data, along with Silurian-Devonian fossils from limestone blocks in the mélange and our recognition of Triassic diabase dikes that crosscut the mélange fabric, suggest that deposition and imbrication of Reindeer Hills clastic sediments took place in the Paleozoic. The new U-Pb data, Triassic mafic dikes, and published displacement estimates for the Denali fault suggest that the RHM correlates with the Mirror Creek Formation northeast of the Denali Fault in western Yukon, Canada, and may also have a link to Silurian-Devonian igneous rocks in the Alexander terrane of southeast Alaska. Altogether, the preliminary data presented here suggest that the RHM provides a record of early Devonian(?) subduction spatially associated with other Baltica-derived Cordilleran terranes. 

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2. A crucial geologic test of Late Jurassic exotic collision versus endemic re-accretion in the Klamath Mountains Province, western United States, with implications for the assembly of western North America

   https://doi.org/10.1130/B35981.1  

   LaMaskin, Todd A.; Rivas, Jonathan A.; Barbeau, David L.; Schwartz, Joshua J.; Russell, John A.; Chapman, Alan D. (July 2021, GSA Bulletin)

   Abstract Differing interpretations of geophysical and geologic data have led to debate regarding continent-scale plate configuration, subduction polarity, and timing of collisional events on the western North American plate margin in pre–mid-Cretaceous time. One set of models involves collision and accretion of far-traveled “exotic” terranes against the continental margin along a west-dipping subduction zone, whereas a second set of models involves long-lived, east-dipping subduction under the continental margin and a fringing or “endemic” origin for many Mesozoic terranes on the western North American plate margin. Here, we present new detrital zircon U-Pb ages from clastic rocks of the Rattlesnake Creek and Western Klamath terranes in the Klamath Mountains of northern California and southern Oregon that provide a test of these contrasting models. Our data show that portions of the Rattlesnake Creek terrane cover sequence (Salt Creek assemblage) are no older than ca. 170–161 Ma (Middle–early Late Jurassic) and contain 62–83% Precambrian detrital zircon grains. Turbidite sandstone samples of the Galice Formation are no older than ca. 158–153 Ma (middle Late Jurassic) and contain 15–55% Precambrian detrital zircon grains. Based on a comparison of our data to published magmatic and detrital ages representing provenance scenarios predicted by the exotic and endemic models (a crucial geologic test), we show that our samples were likely sourced from the previously accreted, older terranes of the Klamath Mountains and Sierra Nevada, as well as active-arc sources, with some degree of contribution from recycled sources in the continental interior. Our observations are inconsistent with paleogeographic reconstructions that are based on exotic, intra-oceanic arcs formed far offshore of North America. In contrast, the incorporation of recycled detritus from older terranes of the Klamath Mountains and Sierra Nevada, as well as North America, into the Rattlesnake Creek and Western Klamath terranes prior to Late Jurassic deformation adds substantial support to endemic models. Our results suggest that during long-lived, east-dipping subduction, the opening and subsequent closing of the marginal Galice/Josephine basin occurred as a result of in situ extension and subsequent contraction. Our results show that tectonic models invoking exotic, intra-oceanic archipelagos composed of Cordilleran arc terranes fail a crucial geologic test of the terranes’ proposed exotic origin and support the occurrence of east-dipping, pre–mid-Cretaceous subduction beneath the North American continental margin. 

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3. A detrital zircon test of large-scale terrane displacement along the Arctic margin of North America

   https://doi.org/10.1130/G48336.1  

   Gibson, Timothy M.; Faehnrich, Karol; Busch, James F.; McClelland, William C.; Schmitz, Mark D.; Strauss, Justin V. (January 2021, Geology)

   null (Ed.)

   Abstract Detrital zircon U-Pb geochronology is one of the most common methods used to constrain the provenance of ancient sedimentary systems. Yet, its efficacy for precisely constraining paleogeographic reconstructions is often complicated by geological, analytical, and statistical uncertainties. To test the utility of this technique for reconstructing complex, margin-parallel terrane displacements, we compiled new and previously published U-Pb detrital zircon data (n = 7924; 70 samples) from Neoproterozoic–Cambrian marine sandstone-bearing units across the Porcupine shear zone of northern Yukon and Alaska, which separates the North Slope subterrane of Arctic Alaska from northwestern Laurentia (Yukon block). Contrasting tectonic models for the North Slope subterrane indicate it originated either near its current position as an autochthonous continuation of the Yukon block or from a position adjacent to the northeastern Laurentian margin prior to >1000 km of Paleozoic–Mesozoic translation. Our statistical results demonstrate that zircon U-Pb age distributions from the North Slope subterrane are consistently distinct from the Yukon block, thereby supporting a model of continent-scale strike-slip displacement along the Arctic margin of North America. Further examination of this dataset highlights important pitfalls associated with common methodological approaches using small sample sizes and reveals challenges in relying solely on detrital zircon age spectra for testing models of terranes displaced along the same continental margin from which they originated. Nevertheless, large-n detrital zircon datasets interpreted within a robust geologic framework can be effective for evaluating translation across complex tectonic boundaries. 

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4. Effect of cyclic loading at elevated temperatures on the magnetic susceptibility of a magnetite-bearing ore

   https://doi.org/10.1093/gji/ggab400  

   Dudzisz, Katarzyna; Walter, Mario; Krumholz, Ralf; Reznik, Boris; Kontny, Agnes (November 2021, Geophysical Journal International)

   SUMMARY Cyclic loading at elevated temperatures occurs either naturally during tectonic or volcanic-induced earthquakes or can be human-induced due to various geological engineering activities. The aim of this study is to test if mechanical fatigue in rocks can be monitored by magnetic methods. For this purpose, the effect of cyclic-mechanical loading (150 ± 30 MPa) on the magnetic susceptibility and its anisotropy of a magnetite-bearing ore with varying temperatures (400 and 500 °C) and environment (air and vacuum) was investigated. Our study shows that magnetic susceptibility decreases significantly (up to 23 per cent) under air conditions and in vacuum (up to 4 per cent) within the first ca. 1000 cycles. Further loading does not significantly affect the magnetic susceptibility which then remains more or less constant. The decrease of susceptibility parameters is stronger at 500 °C compared to 400 °C under both experimental conditions. Magnetic susceptibility was always measured after decompression of the loaded sample at room temperature so that magnetostriction can be excluded as a reason for these changes. The higher the temperature at which samples were loaded the more pronounced is the oxidation of magnetite to haematite. The transformation of magnetite into haematite under ambient conditions is the most important mechanism influencing bulk magnetic properties. The weak changes in magnetic susceptibility after vacuum loadings are probably caused by intragranular microcracks formed on the surface of magnetite grains. These surface deformation structures are accompanied by the refinement of magnetic domains, which is observed by magnetic force microscopy. Bulk magnetic grain size modifications are also confirmed by hysteresis parameters as well as by the increasing Hopkinson peak ratios determined from magnetic susceptibility measurements over Curie point. The degree of magnetic anisotropy and shape factor only change for the air-treated samples and are therefore related to the haematite formation and not to irreversible ductile deformation in magnetite. Our experimental study shows that cyclic loading can change significantly the magnetic properties of a rock due to mineral transformation below < 1000 cycles and that the first stages of mechanical fatigue, which are a precursor of the failure of rock, are closely associated with these transformations. 

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5. Restoration and Transformation: The Response of Shocked and Oxidized Magnetite to Temperature

   https://doi.org/10.1029/2023JB027244  

   Mendes, Bruno Daniel; Kontny, Agnes (January 2024, Journal of Geophysical Research: Solid Earth)

   Abstract Large impact craters on Earth are associated with prominent magnetic anomalies, residing in magnetite of the shocked target rocks and impactites. Shock experiments on magnetite suggest that up to 90% of magnetic susceptibility is lost at pressures >5 GPa, but can be partially restored by post‐shock thermal annealing. The magnetic property changes are caused by shock induced grain size reduction and fragmentation, as well as domain wall‐pinning at crystal lattice defects. A recent study of granitoids from the peak‐ring of the Chicxulub crater found that annealing may occur naturally, but can also be overprinted by high‐temperature hematite‐to‐magnetite transformation in non‐oxidizing environments. In this study, we isolate the effect of defect annealing and hematite‐to‐magnetite transformation using the evolution of hysteresis, isothermal remanent magnetization components and first order reversal curve (FORC) diagrams at different high‐temperature steps. We used a laboratory‐shocked magnetite‐quartz ore, a non‐shocked naturally oxidized granite, and a naturally shocked and oxidized granite. Our findings suggest that annealing of shock‐induced lattice defects partially restores some pre‐shock magnetic behavior and causes an apparent average bulk‐sample domain state increase. Hematite‐to‐magnetite transformation creates new fine‐grained magnetite that strongly overprints the original signal, and decreases the average bulk‐sample domain state. Where annealing and hematite‐to‐magnetite transformation both occur, the new magnetite masks the annealing‐induced property restoration and apparent domain state modification in the shocked magnetite. As magnetite oxidation is a ubiquitous process in surface rocks, these findings are fundamental to understand hematite‐to‐magnetite transformation as a potential overprint mechanism, and could have broad implications for paleomagnetic interpretations. 

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