Secular Variation in the Cordilleran Continental Arc: Implications for Arc Geodynamics

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In this research project, the investigator and his students will undergo whole rock and zircon geochemical analysis of Permo-Triassic to Cretaceous arc plutons and large-volume ignimbrites in southern California to identify patterns of magma source variations over time. These observed patterns will be used to place limits on regional and temporal variations in mantle and crustal contributions to magmas which ultimately form arc plutons and feed large-volume volcanic eruptions. Preliminary data show large regional variations in the age of magma source rocks, and also show that younger plutons were derived from distinct crustal sources compared to adjacent older plutons. These regional and temporal differences in magma sources will therefore allow a better understanding of the processes occurring at depth, especially changes in thermal structure, melt hybridization, or lithospheric thickening by retro-arc shortening, that may have contributed to high volume magmatism in this long-lived arc setting.Volcanoes commonly form at the boundary between colliding tectonic plates, and these volcanoes are the surface expression of a collision process that is central to the origin of Earth''s continents. However, because the processes that generate this volcanic activity occur out of sight deep within the Earth, we still have much to understand about the volcanic process and its link to colliding plates. This project will examine a series of volcanic and intrusive igneous rocks formed over about 180 million years during a very long-lived, yet now extinct plate collision in the southwestern United States. The purpose of examining this very long record of activity in an extinct setting, where not only the volcanoes but their underlying rocks are exposed to view, is to develop an integrated understanding of the plate collision and magma-generating processes. Thus, this project has broader significance in that it is designed to unify aspects of both igneous petrology and structural geology, as the ideas of these disciplines are applied to plate collisions. The project will also support the training of undergraduate and graduate students in the earth sciences, by supporting supervised independent field and laboratory experiences. These independent studies assist students by helping them to see the application and integration of knowledge they have acquired in the classroom and by encouraging them to persist in their education. Finally, because much of the field research will take place in and around Joshua Tree National Park in California, the knowledge gained during the course of this study will be translated into continuing education of thousands of park visitors each year.