Collaborative Research: Volcanic Material in Triassic Sedimentary Units of Southwestern North America: Understanding the Initiation of Arc Magmatism

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An outstanding problem in understanding the tectonic evolution of southwestern North America is the extent and nature of the retroarc record of Permo-Triassic magmatism. Triassic sedimentary sequences hold a record of the earliest stages of arc volcanism along the Cordilleran continental margin, a record poorly preserved in the remnant plutonic arc dismembered by voluminous Middle to Late Mesozoic plutonism. Drawing on the rich base of previously published information about the stratigraphy of the Chinle and Moenkopi formations on the Colorado Plateau, this study is expanding the understanding of volcanic material found in the Triassic rocks of the Plateau, and characterizing proposed or likely equivalents to the Colorado Plateau section in southern and western Arizona and in eastern California. A variety of methods used to address these issues includes U-Pb dating of detrital zircons from ash beds and tuffaceous sandstones to provide a complete age spectrum for the Chinle Formation; analysis of volcanic clasts in the Chinle Formation for age and complete chemistry; study of detrital zircons in known or suspected Triassic clastic rocks in southern and western Arizona for provenance data; and complete geochemical characterization of igneous clasts in these units and in the El Antimonio Group of Sonora, Mexico. These data will mesh with the growing body of information being obtained from similar-age plutons in the Mojave Desert of southeastern California. The comparison will allow testing of the hypothesis that the remnant plutonic arc and its volcanic cover were the source of airborne ash and stream-borne clastic material that mixed with locally derived detritus to deposit the Buckskin Formation and related units in the proximal retroarc area of western Arizona, the Gardner Canyon and other, unnamed arc-proximal units in southern Arizona, and the Moenkopi and Chinle formations in the relatively distal retroarc region.The research will provide a new view of plate movement along the west coast of North America during Mesozoic time, including an improved understanding of the hypothesis that the western margin of the continent was translated southeastward into northern Mexico. The study will also give clues as to the ancient topography of western North America, including where major volcanoes stood, and where major rivers flowed. As part of this project, collaborations with Flagstaff Unified School District high-school Earth Sciences classes to bring young students into the world of active research, in hopes of strongly encouraging them to follow into scientific studies and careers.