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Influence of the Permo-Triassic Cordilleran arc on Early to Middle Triassic deposystems, Mojave and Sonoran Deserts

Kwiatkowski, Chad (2021) Influence of the Permo-Triassic Cordilleran arc on Early to Middle Triassic deposystems, Mojave and Sonoran Deserts. Masters thesis, Northern Arizona University.

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Subduction initiated beneath southwestern Laurentia along a major transform fault in Permian time, based on previous dating of ca. 275 Ma plutons in the northern Mojave Desert and northwestern Sonora. The volcanic record of the early Cordilleran arc is rare, so the sedimentary record provides the best evidence of early arc magmatism. The Lower to Middle Triassic Moenkopi Formation and its equivalents are the oldest stratigraphic units that contain evidence of the Cordilleran arc from its retro-arc region. In this study, U-Pb geochronology is used to analyze seven samples of Moenkopi Formation and metasedimentary equivalents from five ranges along a ~450 km north-south transect from southern Nevada to northwestern Sonora. Permo-Triassic grains were present in all Lower Triassic samples except the Timpoweap Member near Las Vegas, and usually were 1–6% of grains. A metaconglomerate from Ejido Serdan in northwestern Sonora, however, is arc-dominated with 83% Permo-Triassic grains. The grains yield ages that span continuously from 268 to 249 Ma, suggesting continuous magmatism throughout this time and a maximum depositional age of 249 ± 2 Ma. The trace element signature of Permo-Triassic grains from the samples suggests that they crystallized in continental crust. Th/U ratios and ages match best with northern Mojave Desert plutons, although plutons in Sonora were a closer potential source for the Ejido Serdan samples. Except for the arc-derived metaconglomerate, samples are dominated by grains sourced from the Ouachita-Marathon-Sonora orogen, peri-Gondwanan sources, the Grenville orogen, southwestern Laurentian basement, and/or recycled from upper Paleozoic strata. This indicates northwest-directed sediment dispersal throughout Early Triassic time with additional contributions from the Cordilleran arc to the southwest. The samples contain progressively fewer arc-derived grains from south to north. In southern Nevada, the arc did not appear in the detrital zircon record until deposition of the lower red member, although Timpoweap Member-equivalents in southeastern California do contain sparse arc detritus. The arc did not have a major effect on regional thickness trends during Moenkopi deposition, but data are sparse off the Colorado Plateau. The arc may have been mostly subaqueous at this time and, compared to the elevated remnants of the Ouachita-Marathon-Sonora orogenic belt, not a significant source region for the Moenkopi deposystem in the arc-distal region.

Item Type: Thesis (Masters)
Publisher’s Statement: © Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords: Cordilleran ; geology; Moenkopi; Permo-Triassic; subduction; zircon; Mojave Desert; Sonoran Desert
Subjects: Q Science > QE Geology
NAU Depositing Author Academic Status: Student
Department/Unit: Graduate College > Theses and Dissertations
College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited: 03 Feb 2022 18:36
Last Modified: 28 Dec 2022 08:30
URI: https://openknowledge.nau.edu/id/eprint/5641

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