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Department of Geosciences

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https://hdl.handle.net/10217/100434
These digital collections include theses, dissertations, faculty publications, and datasets from the Department of Geosciences. Due to departmental name changes, materials from the following historical departments are also included here: Earth Resources, Geology.

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Browsing Department of Geosciences by Subject "Andes"

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    Magmatism, deformation, and mineralization along the intra-arc Atacama fault system, northern Chile
    (Colorado State University. Libraries, 2020) Seymour, Nikki M., author; Singleton, John S., advisor; Kuiper, Yvette D., committee member; Hannah, Judith L., committee member; Ridley, John R., committee member; Wilson, Robert J., committee member
    Oblique plate motion is nearly universal across subduction margins, and the lateral component of motion produced by oblique subduction may be accommodated through distributed strain and/or along crustal-scale strike-slip faults in the overriding (upper) plate. Magmatic arcs, another fundamental features of the upper plate, have been suggested to play a key role in the initiation and development of localized intra-arc crustal-scale faults. Significant hydrothermal fluid flow derived from arc plutons has also produced world-class metal deposits along intra-arc faults, and may suggest the processes responsible for the initiation, continued deformation, and eventual abandonment of strike-slip faults may also play an important role in focusing economic mineralization. Here I study the sinistral Atacama fault system (AFS), a fossil intra-arc strike-slip fault that occurs within the Mesozoic Coastal Cordillera arc to better understand how oblique plate motion is accommodated in the upper plate and related to arc magmatism. Mapping along the northern ~70 km of the El Salado segment of the AFS documents the distribution of arc plutons and style of deformation. Petrology, geochemistry, and geo/thermochronology were used to characterize and correlate plutons, and structural data were analyzed to understand progressive changes in the style of deformation. New zircon U-Pb dates document a major pulse of magmatism from 150–120 Ma, with the 135–119 Ma plutons most directly tied to AFS ductile deformation. Mylonitic fabrics along the AFS are uniquely associated with the margins of Early Cretaceous plutons, and are cut by late kinematic intrusions at 120–110 Ma. Steeply dipping N- to NE-striking mylonitic fabrics with sinistral shear sense indicators strike ~8–12° clockwise of the steeply dipping, N- to NW-striking AFS strands, indicating deformation occurred during progressive ductile to brittle sinistral strain. The distinctive synkinematic Cerro del Pingo tonalite was mapped on both sides of the eastern branch of the El Salado segment. Petrography, geochemistry, and geochronology all overlap within error, and therefore I interpret two sinistrally separated exposures of the Cerro del Pingo Complex as an offset marker along the AFS. In addition, I correlate a chain of offset leucocratic granites and hypabyssal intrusions across the western branch of the El Salado segment. The sinistral slip magnitude across the entire the El Salado segment is ~50 ± 6 km and occurred almost entirely between ~133 and ~110 Ma at an average slip rate of ~2.0–2.5 km/Myr. I postulate that thermal softening as a result of Early Cretaceous pluton intrusion into the shallow crust locally elevated geothermal gradients, allowing for ductile deformation at ~5–7 km depths. Spatially variable Early Cretaceous pluton emplacement set up a heterogeneous rheology that produced a segmented system that never evolved into a single regional-scale fault. Zircon (U-Th)/He dates record cooling through ~180°C by 116–99 Ma and relaxation of elevated gradients coeval with the end of slip along the El Salado segment. Along the central El Salado segment ~150–200 km south of the offset Cerro del Pingo Complex tonalites, clear fault branches no longer define the AFS. The main branch of AFS in this region is defined by a ~200–500-m-thick steeply NW-dipping shear zone that does not show evidence for brittle overprint. Zircon U-Pb dates document synkinematic emplacement of a tonalite in the shear zone at ~122 Ma. Kinematic indicators record oblique sinistral-reverse shear, but locally coaxial fabrics dominate, indicating an overall transpressional regime. Shear zone activity overlaps in age with other sections of the AFS. The tonalite records a synkinematic sodic-calcic assemblage of actinolite+epidote+titanite+plagioclase, but mylonitic microstructures are completely annealed. The shear zone is cut by an unstrained ~115 Ma diorite body that contains pervasive actinolite+epidote+andradite+plagioclase sodic-calcic mineralization. Similar hydrothermal alteration assemblages are also present ~20 km east of the AFS in the economic Punta del Cobre copper district near Copiapó. The absence of brittle faulting is likely related to continued magmatism associated with the Copiapó batholith complex, which is younger than most arc plutons in the Coastal Cordillera. Postkinematic mineralization along the AFS is unique to the Copiapó area, and magmatic fluids responsible for alteration were most likely derived from the Copiapó batholith. Together, these data document the development of the AFS as a highly segmented fault system that localized mineralization and slipped at a relatively slow rate over ~20 Myr, and was abandoned as plate motion vectors shifted in the middle Cretaceous and arc magmatism migrated eastward.