TIMING OF COOLING AND EXHUMATION OF LARAMIDE UPLIFTS INFORMS MODELS OF FLAT-SLAB SUBDUCTION

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This project intends to determine the ages of uplift of the major mountain ranges that together constitute the Rocky Mountains of the western interior United States. These ranges are referred to in the geological literature as "Laramide uplifts", after the city and county of Laramie, Wyoming, which is situated in the region where these uplifts are notably developed. The timing of uplift of the Laramide ranges is poorly understood, but critical for assessing the shape and movement direction of tectonic plates in the eastern Pacific basin and western regions of the North American plate. This work is important to the public for several reasons: (a) The Rocky Mountains are the definitive landscape of the mountain west, a landscape that has inspired writers, artists, entrepreneurs, emigrants, and scientists for more than 200 years; yet, no consensus exists among geologists to explain the origins of these mountains. (b) By virtue of its high elevation and north-south orientation, this spectacular landscape impacts the ecology and climate of the entire U.S. (c) The Laramide region is of enormous economic importance to the U.S., containing strategically important groundwater, mineral, and hydrocarbon resources. (d) Although the Laramide uplifts are no longer "active", results of this project will useful for understanding seismic hazards in regions on Earth today where similar tectonic processes are still active (for example, western South America and southern Alaska). Finally, a group of undergraduate and graduate students will be trained in the context of this project.The primary objective of this project is to determine the timing of exhumation of Laramide uplifts in Montana, Wyoming and Utah. This is essential to inform tectonic and geodynamic models of the Laramide orogeny, which is generally attributed to flat-slab subduction. Such models rely heavily on spatio-temporal patterns of Laramide uplift. Flat-slab subduction of oceanic plateaus and aseismic ridges are important geodynamic phenomena in convergent margins, with respect to regional plateau uplift, termination of arc magmatism, and active seismicity in the overriding plate. This project will apply a multi-component dating approach, including apatite fission track and (U-Th-Sm)/He thermochronology, which, when combined with thermo-kinematic modeling and the record of sedimentation, will constrain the cooling and erosion histories of the Laramide region. Regional patterns in timing and distribution of uplift events will provide constraints for plate tectonic models of the western U.S. Knowledge gained by this research will be exportable to other active and ancient flat-slab tectonic systems such as southern Alaska, Central America and western Argentina.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.