Integrating Geophysical and Geochemical Data to Understand the Hydration and Thermal State of the Colorado Plateau Lithosphere

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The Colorado Plateau, located in the states of Utah, Arizona, New Mexico, and Colorado, is home to some of the most spectacular scenery and geologic exposures within the US. However, the region is enigmatic geologically for its high elevation, the occurrence of modern volcanism along its margins, and rocks that have not experienced the folding and faulting normally associated with mountain building. It is also unusual because mountain building and volcanism typically occur at the edges of tectonic plates, whereas the Colorado Plateau is located far from the margins of the North American plate. Previous work has suggested that the mantle beneath the margins of the Colorado Plateau is undergoing small-scale convection, which brings hot material at depth closer to the surface. This hot mantle material then heats the lower crust, which is believed to contain an unusual amount of water-rich minerals due to the absorption of water that was released from a subducting oceanic plate passing beneath the area earlier in its history. Water lowers the melting temperature of rocks, and heating may contribute to uplift and melting within the region. This project will use geophysical and geochemical data along with thermodynamic and temperature modeling to better understand the subsurface structure, the amount of water, and the temperatures beneath the margins of the Colorado Plateau. The goal of this work is to improve our understanding of the tectonic evolution of the Colorado Plateau and better understand the causes of volcanism along its margins in places such as the western Grand Canyon and the San Francisco Volcanic field. This project will train students in modern geochemical and geophysical techniques and will support a workshop to bring together researchers who address these and similar problems. Ultimately, this work will provide insight into the global tectonic processes that build and support high elevations and produce volcanic activity far from plate margins.The tectonics of western North America have long been influenced by plate boundary processes occurring along the continent's western margin. These processes have a complex history in which variations in plate convergence direction, slab dip, and the transition from a convergent to a transform margin have significantly altered the state of the lithosphere and asthenosphere and led to multiple episodes of deformation across the region. The Colorado Plateau is located within this intraplate deformation zone and is notable for its resistance to deformation and high surface elevations with no obvious driving mechanism for uplift. Despite its long-term stability, as evidenced by exposures of flat-lying Paleozoic rocks, the Colorado Plateau has recently been identified as a region influenced by small-scale mantle convection along its margins and it is argued that the plateau is succumbing to volcanism and extension encroaching into its interior. For these reasons, the Colorado Plateau provides an ideal location to study how stable continental lithosphere evolves with time and how "cratonic" lithosphere may destabilize. To develop a better understanding of these processes, we will conduct an integrative study focused on the Colorado Plateau's upper mantle that will compile existing seismic and geochemical data, analyze a small targeted set of geochemical samples to address knowledge gaps, inverse model seismic and geochemical data to constrain melt conditions, and utilize thermodynamic modeling to determine stable mineral assemblages, melt percentages, density, and potential hydration states. The synthesized results will be used to test the hypotheses that lithospheric hydration is an important control on the uplift of the Colorado Plateau and that small-scale convection is driving both melting and volcanism, and contributing to uplift and deformation on the edges of the plateau. The specific questions to be addressed with our geophysical, geochemical, and modelling work are: What is the modern density structure of the Colorado Plateau lithosphere, how did Farallon flat-slab subduction hydrate the overriding lithosphere, how does continental lithospheric density evolve through time, and have lithospheric downwellings occurred beneath the Colorado Plateau? The findings from this work will have global implications for the tectonic evolution of the region and for the density evolution of continental lithosphere in both active deformational belts and stable regions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.