Earth and Planetary Sciences ETDs

Publication Date

8-1-2001

Abstract

The lithosphere of the southwestern United States is a mosaic of Proterozoic arc terranes assembled to the southern margin of the Archean Wyoming craton between 1.8 and 1.6 Ga. This mosaic was profoundly transformed by ca. 1.4 Ga intracontinental plutonism, metamorphism, and tectonism. These processes of assembly and tectonic reactivation during the early stages of continental evolution laid the foundation for the development of the continental architecture of the southwestern United States. This dissertation comprises three independent studies focused on different aspects of the Proterozoic evolution of tectonic boundaries exposed in the Southern Rocky Mountains. The first study is a literature review aimed at clarifying the nature of the Yavapai-Mazatzal province boundary in southern Colorado and northern New Mexico. Synthesis of available evidence suggests that this boundary may represent a complex overlap zone between crustal domains with different ages, geochemical signatures, and tectonic histories. The second study explores the timing and kinematics of Proterozoic deformation in the Homestake shear zone, a major Proterozoic structure within the Colorado mineral belt. Mapping, microstructural analysis, and U-Th-Pb monazite geochronology indicate that the Homestake is a composite of Paleoproterozoic high-temperature tectonites and Mesoproterozoic greenschist-grade mylonitic rocks. The high-temperature fabrics record northwest-southeast shortening related to a polyphase orogenic event ~1720-1630 Ma. Mesoproterozoic mylonite-ultramylonite zones overprint steep domains of the folded Paleoproterozoic fabrics and record first southeast-side-down, then southeast-side-up displacement around 1380 Ma. The third study uses over seventy new 40Ar/39Ar dates from Colorado and northern New Mexico to elucidate the ca. 1.4 Ga regional temperature field gradient of the southern Rocky Mountains and nearly 700 compiled dates to extend this analysis throughout the southwestern United States. The data indicates that Archean rocks of the Wyoming craton were largely unaffected by ca. 1.4 Ga metamorphism, rocks of northern and central Colorado reached regional greenschist-grade conditions with higher temperatures (>500°C) within pluton aureoles, and rocks of northern New Mexico and southern Colorado reached amphibolite-grade. We suggest that the high-temperature zone straddling the New Mexico-Colorado border may represent a somewhat deeper mid-crustal exposure level of the ca. 1.4 Ga continental crust. Thus the southern Rocky Mountains may provide an oblique section through part of the ancient middle crust representing conditions during the ca. 1.4 Ga thermo-tectonic episode.

Degree Name

Earth and Planetary Sciences

Level of Degree

Doctoral

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Selverstone, Jane

Second Committee Member

Geissman, John

Third Committee Member

Pazzaglia, Frank

Fourth Committee Member

Mousumi, Roy

Project Sponsors

Caswell Silver Foundation, the National Science Foundation, the Geological Society of America, and the Colorado Scientific Society.

Language

English

Keywords

Proterozoic, Rocky Mountains, Crustal Boundaries, Crustal Boundaries Evolution

Document Type

Dissertation

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