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Wyoming The Nucleus of North America

Presented by Ron Frost University of Wyoming One can strongly argue that the North American continent grew around the Wyoming craton by accretion of rocks onto the margins of the continent. This makes the Wyoming craton the true nucleus of North America. The Wyoming Craton, along with the Slave Craton in northern Canada, contains the oldest rocks in North America. The oldest rocks in Wyoming Craton date from around 3.5 Ga (billion years old), but they contain exotic zircon grains that date from around 3.8 Ga. Some of these zircon grains have isotopic compositions that are consistent with the derivation of some chemical components from the very original crust of the Earth. Gray gneisses, (i.e. Na-rich and K-poor granitic gneiss) were emplaced between 3.5 and 3.3 Ga and are exposed across the Wyoming craton from the Granite Mountains to southwestern Montana. A sequence of metasedimentary rocks dating from 2.86 Ga likely records the rifting of the Slave Province from the Wyoming province. Granitic magmatism ranging from 2.8 to 2.6 Ga occurred across the Wyoming province, and along with the 2.7 Ga old belt of high-pressure rocks in the northern Teton Range, record the earliest evidence of plate tectonics in North America – as well as some of the earliest plate tectonics anywhere on earth. Between 1.7 and 1.8 Ga the Wyoming Craton, the Slave Craton, and other Archean cratons with a younger history were stitched together by deformation zones to produce Laurentia, the Precambrian core of North America. North America grew by extensive magmatism at 1.4 to 1.5 Ga, which was followed by the Grenville orogeny a major mountain building event that occurred around 1.1 Ga and extended from Labrador to Texas. The Grenville orogeny records the collision between Laurentia and protocontinents of Europe and Africa. It is very similar to the Appalachian orogeny, which occurred on the eastern margin of North America in the Permian (a mere 300 million years ago). The assembly of North America was completed during Cordilleran orogeny, which occurred along the western margin of the continent in the late Cretaceous to the Eocene (between 65 and 50 million years ago), and which formed nearly all of present-day Wyoming’s mountains and basins. This involved accretion of island arcs from the Pacific onto the western margin of North America.