An article in the December issue of GSA's Geology offers evidence for a much older, more complex, and exotic geologic origin for the southwest US [right, Vishnu Schist. Credit, NPS] . Physorg.com summarizes the article:
Researchers Owen Shufeldt and Karl Karlstrom of the University of New Mexico, George Gehrels of the University of Arizona, and Katherine Howard of the University of Adelaide present results from a detailed geochronologic investigation of one of the oldest rock units of the Grand Canyon, the Vishnu Schist. Their data reveal a large population of minerals that range from 1 to 2 billion years older than the rock itself. The ages of these minerals suggest an exotic source for the sedimentary material that became the Vishnu Schist. Possible provenances include Australia, North China, Antarctica, or older blocks within southwestern North America that are not presently exposed. The new data provide evidence for a more complex tectonic history of the southwest United States than presently accepted, including input from nearby cratons and the possibility of undiscovered ancient blocks mixed in with the Proterozoic terranes of southwestern Laurentia.
Ref: Archean detrital zircons in the Proterozoic Vishnu Schist of the Grand Canyon, Arizona: Implications for crustal architecture and Nuna supercontinent reconstructions, Geology, December 2010, v. 38, p. 1099-1102, first published on November 12, 2010, doi:10.1130/G31335.1
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