Burial, Uplift and Exhumation History of the Colorado Plateau

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The Earth's crust is in dynamic motion, driven by plate tectonics and less understood processes, like addition or removal of dense magmatic rocks in the deep crust. One manifestation of these motions is the sinking of large depressions that are continuously filled with sediment as they sink, creating great basins of sedimentary rock, often hundreds of kilometers across and reaching depths of 10 kilometers or more. Rocks in these basins experience heating and reaction with percolating water, before uplifting and eroding due to later crustal motions. These great cycles of burial and exhumation are responsible for creating some of the most important records of earth history, and are the crucibles where key geological resources are forged -- including petroleum, natural gas and ore deposits. Perhaps the most important factor controlling the geological processes in basins is the change in temperature with time, which both reflects the underlying forces driving subsidence and uplift, and controls the chemical processes associated with petroleum and ore formation. This project will bring to bear a new tool for the study of the temperature histories of sedimentary rocks exposed in exhumed basins, focusing on the limestones of the Colorado Plateau, which are exposed in and around the Grand Canyon. We will approach this problem using 'clumped isotope thermometry' -- a technique that examines the extent to which rare, naturally occurring isotopes of carbon and oxygen form bonds with one another in the atomic lattices of mineral structures. These isotopic 'clumps' form in greater abundance at lower temperatures, leading to a kind of thermometry. And, these isotopic clumps can only form or disperse by atomic diffusion above a certain temperature, introducing a second way in which differences in thermal history can be recorded in differences in isotopic composition. This technique was invented about a decade ago, but has only recently become well enough understood to be used as a tool for unraveling the cryptic, often complex thermal histories of exhumed basins. This research focuses on the Colorado Plateau because it is one of the major tectonic and topographic domains of North America and its thermal history has been studied extensively, yet its carbonate-dominated strata have not been amenable to established techniques. The application of clumped isotope thermometry to limestones of this region will complement and build on what has been learned by earlier studies. Second, the exhumation of the Colorado Plateau was associated with incision of the Grand Canyon, and our work will constrain the timing and dynamics of the formation of this important landscape. This project will also be a proving ground for a method that could be used to reconstruct thermal histories in many resource-rich sedimentary basins, expanding our understanding of petroleum and ore formation and of deep-seated crustal waters. In addition to the scientific objectives of the research, the project is contributing to other important societal goals, including the training of postdoctoral scholar in a new and emerging isotopic technique, as well as providing research opportunities for undergraduate and high school students in a STEM (science, technology, engineering, and mathematics) discipline. This proposal examines burial, uplift and exhumation history of the Colorado Plateau, using carbonate clumped-isotopes thermometry. In burial and exhumation settings, the clumped-isotope compositions of carbonate minerals constrain the thermal history of carbonate minerals, possibly including both temperatures of discrete events (cementation, recrystallization, vein formation) and temperature-time thresholds recorded by isotopic mobility, which occurs at 90-180 degrees celsius in calcites and ~250-300 degrees celsius in dolomites. These temperature ranges are important in the study of basin evolution and hydrocarbon exploration, but are only partially covered by conventional, low-temperature thermochronometry. The principal investigators research will bring new constraints that complement better known tools such as fission track and Uranium-Thorium/Helium thermochronometry. In particular, they will characterize the distribution of clumped-isotope apparent temperatures in strata of the Colorado Plateau, including both vertical transects through exposed and cored sections and regional scale horizontal gradients. These data will be combined with existing experimental constraints on the rates of clumped-isotope re-ordering to constrain peak-burial temperatures and peak-burial thermal gradients (based on vertical transects); these derived quantities will be used to infer the distribution of maximum burial depth and total-exhumation pattern across the Colorado Plateau (where appropriate, in combination with existing Uranium-Thorium/Helium and fission track data). More generally, this will be the first detailed study of burial and exhumation of a large-scale tectonic feature using clumped-isotope constraints. The proposed work will advance understanding of the Colorado Plateau burial, uplift and exhumation history, and more generally, of the interactions between tectonic activity and surface processes.