Paleogeography of Western North America: Insights from Detrital Zircon U-Pb Geochronology and Hf Isotope Geochemistry

Abstract

Reconstructing the geologic and tectonic evolution of western North America from Paleozoic through Eocene time is the goal of this dissertation. Three geographical regions were selected for analysis using modern analytical techniques including U-Pb geochronology and Hf isotope geochemistry of detrital zircons, as well as, geologic mapping and paleocurrent analysis. The Neoproterozoic through Pennsylvanian Yukon-Tanana terrane (YTTs) in the Coast Mountains of southeastern Alaska, the Cretaceous through Eocene strata in the San Juan basin of northwestern New Mexico and southwestern Colorado, and the Paleocene Wilcox Group in the Gulf of Mexico are the regions selected for analysis. The project also includes a large compilation of εHft from the eight main elements that comprise the North American Cordilleran arc: Coast Mountains batholith, North Cascades Range, Idaho batholith, Sierra Nevada batholith, Transverse Ranges, Peninsular Ranges, Sierra Madre Occidental, and the Porphyry Copper Province. Identifying changes in sediment provenance from Sevier thin-skinned thrusting through the formation of Laramide basement block uplifts was accomplished by conducting U-Pb geochronologic and Hf isotopic analyses on 32 detrital zircon samples from across the entire San Juan basin. The detrital zircon U-Pb results indicate four stratigraphic intervals with internally consistent age peaks. Based on a combination of U-Pb ages and paleocurrent indicators, three transitions in sediment provenance were identified, resulting in a refined paleogeographic model for Late Cretaceous through Eocene time. This model includes a transition from initial reworking of the Paleozoic and Mesozoic cratonal blanket to unroofing of distant basement cored-uplifts and Laramide plutons, then a final transition to more local Laramide uplifts. Hf isotopes were also collected on zircons from the San Juan basin, and also for a select group of samples of the Paleocene Wilcox Group in the Gulf of Mexico. This was done because Hf isotopes in zircon not only provide crustal evolution information, they also supply complementary isotopic information that is combined with U-Pb age to create a zircon fingerprint. This information refines the understanding of the Late Paleocene sediment pathways across the western United States, including a new provenance connection between the Coast Mountains batholith in British Columbia, Canada and the Gulf of Mexico. This information also allowed for the generation of a Laramide-age (ca. 80-50 Ma) detrital signature map of zircons that can now be used for referencing detrital zircons for researchers working in and around the Cordilleran magmatic arc.