Browsing by Author "Seymour, N. M."

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    Magnitude, timing, and rate of slip along the Atacama fault system, northern Chile: implications for Early Cretaceous slip partitioning and plate convergence
    (GEOLOGICAL SOC PUBL HOUSE, 2021) Seymour, N. M.; Singleton, J. S.; Gomila, R.; Mavor, S. P.; Heuser, G.; Arancibia, G.; Williams, S.; Stockli, D. F.
    Displacement estimates along the Atacama fault system (AFS), a crustal-scale sinistral structure that accommodated oblique convergence in the Mesozoic Coastal Cordillera arc, vary widely due to a lack of piercing points. We map the distribution of plutons and mylonitic deformation along the northern c. 70 km of the El Salado segment and use U-Pb geochronology to establish the slip history of the AFS. Along the eastern branch, mylonitic fabrics associated with the synkinematic c. 134-132 Ma Cerro del Pingo Complex are separated by 34-38 km, and mylonites associated with a synkinematic c. 120-119 Ma tonalite are separated by 20.5-26 km. We interpret leucocratic intrusions to be separated across the western branch by c. 16-20 km, giving a total slip magnitude of c. 54 +/- 6 km across the El Salado segment. Kinematic indicators consistently record sinistral shear, and zircon (U-Th)/He data suggest dip-slip motion was insignificant. Displacement occurred between c. 133-110 Ma at a slip rate of c. 2.1-2.6 km Myr(-1). This slip rate is low compared to modern intra-arc strike-slip faults, suggesting (1) the majority of lateral slip was accommodated along the slab interface or distributed through the forearc or (2) plate convergence rates/obliquity were significantly lower than previously modeled. Supplementary material: Full U-Pb, (U-Th)/He, petrographic, and structural data with locations is available at Thematic collection:This article is part of the Isotopic dating of deformation collection available at:
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    Sodic-calcic alteration and transpressional shear along the Atacama fault system during IOCG mineralization, Copiapo, Chile
    (2024) Seymour, N. M.; Singleton, J. S.; Gomila, R.; Arancibia, G.; Ridley, J.; Gevedon, M. L.; Stockli, D. F.; Seman, S. M.
    The Punta del Cobre district near Copiapo is a center of iron oxide-copper-gold (IOCG) mineralization spatially and temporally associated with regional sodic-calcic hydrothermal alteration, the Atacama fault system (AFS), and two phases of Early Cretaceous magmatism. Here, we investigate the spatiotemporal and geochemical relationships between magmatism, ductile deformation, and hydrothermal alteration along the similar to 200 to 300-m-thick steeply NW-dipping Sierra Chicharra shear zone, interpreted to be the major strand of the AFS. Mylonitic fabrics and oblique sinistral-reverse kinematic indicators together record coaxial flattening in a transpressional regime. Deformation on the AFS took place before, during, and after intrusion of the synkinematic Sierra Chicharra quartz diorite of the Coastal Cordillera arc at similar to 122 Ma and terminated before intrusion of the unstrained similar to 114 Ma Sierra Atacama diorite of the Copiapo batholith. Geochemical data show that the Copiapo batholith was more mafic and more K-rich than the calc-alkaline Coastal Cordillera arc. This time period thus overlaps IOCG mineralization in the Punta del Cobre district (similar to 120 to 110 Ma). Multiple phases of sodic-calcic alteration in and around the AFS shear zone are recognized. Textures of altered rock in the shear zone show both synkinematic assemblages and post-kinematic hydrothermal oligoclase. A similar to 775-m-long andradite vein that cuts the shear zone formed broadly at the end of magmatism in the district (similar to 95 Ma). Oxygen isotope ratios from the vein indicate that hydrothermal fluids were likely magmatically derived. Together, this work shows the AFS-related shear zone and nearby IOCG mineralization developed in a regional transpressional regime produced by SE-directed oblique convergence across a NE-striking shear zone. IOCG-related magmatic-hydrothermal fluids exploited this transcrustal shear zone to produce multiple episodes of regional sodic-calcic alteration formed from fluids exsolved from magmas or driven by the heat of the Coastal Cordillera arc and Copiapo batholith.
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    The Relationship Between Magmatism and Deformation Along the Intra-arc Strike-Slip Atacama Fault System, Northern Chile
    (2020) Seymour, N. M.; Singleton, J. S.; Mavor, S. P.; Gomila, Rodrigo; Stockli, D. F.; Heuser Amaya, Gert Klaus; Arancibia Hernández, Gloria Cecilia
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    Timing, Kinematics, and Displacement of the Taltal Fault System, Northern Chile : Implications for the Cretaceous Tectonic Evolution of the Andean Margin
    (2020) Mavor, S. P.; Singleton, J. S.; Gomila, Rodrigo; Heuser Amaya, Gert Klaus; Seymour, N. M.; Williams, S. A.; Arancibia Hernández, Gloria Cecilia; Johnston, S. M.; Kylander Clark, A. R. C.; Stockli, D. F.