ABSTRACT

The gneisses in western Wyoming Province provide a 2.68 - 2.67 Ga. petrologic, structural, and metamorphic record that is best explained as being a product of modern style plate tectonics. Important features include the presence of a possible magmatic arc in the Wind River and Teton ranges; the presence of a possible back-arc basin marked by coeval tholeiitic dike swarms in the Owl Creek Mountains and the eastern Wind River Range at 2678 Ma; a major crustal-scale SW- to W-directed thrust in the Wind River Range; a possible accretionary prism or fore-arc basin in the Teton Range; and a record of possible continent-continent collision in the Teton Range marked by the oldest high-pressure metamorphic rocks in North America. This research is a detailed petrologic, structural, geochemical and geochronologic study in the Northern Teton Range to evaluate whether the basement rocks record modern style plate tectonics. The research involves a combination of thermobarometry to determine the Pressure-Temperature path of the high-pressure gneisses, geologic mapping to determine the structural relations between the high-pressure gneisses and other rocks in the northern Teton Range. Isotopic analyses are being used to determine the age of key plutonic units in the gneisses of the Teton Range, constrain the timing of the decompression of the HP rocks, and more precisely determine the age of thrusting in the Wind River Range. In addition, Neodymium and Strontium isotopic analyses will be used to determine the provenance of metasedimentary rocks in the Teton gneisses and the possible sources of the granitoids that intrude them. The significance of the project lies in addressing the question of when modern-style plate tectonics began, which is a major problem in Earth history. The Earth was much hotter in Archean time (before 2.5 billion years ago). Since the movement of tectonic plates is one way that heat is dissipated from the Earth, many geologists maintain that a hotter Earth should have had a different the tectonic style from what we see today. The 2.67 billion year old rocks in northwestern Wyoming may preserve one of the best records of modern-style plate tectonics in the Archean. Therefore, this study may be a key to understanding the geologic construction of North America and the evolution of tectonics in general. The project is supporting the education of a Ph.D. student and contributing to the training of undergraduate students. Results of the research will contribute to the understanding of the geology of the Grand Teton National Park.

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