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Episodic Sierran arc magmatism expressed in Mesozoic rocks of the Mount Morrison pendant, east-central Sierra Nevada, California

Field, Derek M. (2018) Episodic Sierran arc magmatism expressed in Mesozoic rocks of the Mount Morrison pendant, east-central Sierra Nevada, California. Masters thesis, Northern Arizona University.

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The Mount Morrison pendant contains approximately 40 km2 of exposed Mesozoic volcanic rocks. Triassic strata include a 1.2-km-thick intra-caldera ignimbrite; 1.5 km of dacitic rocks including a large hypabyssal sill, tuff, breccia, and sedimentary rocks; and two local deposits of subaqueous volcanic breccia. Jurassic rocks include 1.3 km of intercalated dacitic to rhyolitic tuff and breccia that are highly deformed. The Cretaceous section contains subaqueous, finely laminated sedimentary strata interlayered with rhyolite tuffs and flows, and a >400-m-thick mass of andesite porphyry. U-Pb geochronology shows that these rocks were emplaced during periods of high magmatic flux beneath the Sierran arc prior to juxtaposition by faulting. The ages of Triassic rocks range from 217 to 213 ± 2 Ma, which corresponds to the first pulse of Sierran volcanism. Rhyolite tuff in the Jurassic block was dated at 170 ± 2 Ma and thus corresponds to the second high-flux episode. Volcanic rocks of the Cretaceous block were dated at 101 ± 2 Ma, and so were sourced from the final magmatic pulse. All these volcanic rocks have geochemical signatures of arc magmas sourced from evolved crust. Structural patterns show that Triassic and Jurassic rocks underwent at least one period of deformation prior to emplacement of Cretaceous rocks, providing evidence for crustal contraction between the Middle Jurassic and latest Early Cretaceous. All rocks were deformed by the Sierra Crest shear zone during or soon after the final pulse of arc magmatism. Dextral shear indicators are consistent with Sierra Crest shear zone tectonics. The Mesozoic Mount Morrison pendant records the major stages of development of the Sierran arc as it was built on the continental crust of southwest Laurentia, offering insight into eruption style, paleogeography, and arc tempo.

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: Earth sciences; Arc; Geochronology; Morrison; Paleogeography; Pendant; Sierra
Subjects: Q Science > QE Geology
NAU Depositing Author Academic Status: Student
Department/Unit: Graduate College > Theses and Dissertations
College of Engineering, Forestry, and Natural Science > School of Earth Sciences and Environmental Sustainability
Date Deposited: 10 Oct 2018 20:09
URI: http://openknowledge.nau.edu/id/eprint/5278

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