Collaborative Research: Brittle-plastic deformation, fluid-rock interaction, and extensional reactivation along Laramide thrust faults in the Sangre de Cristo Mountains, Colorado

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This collaborative project investigates the deformation history of structures involved in formation of the Rocky Mountains and Rio Grande rift in southern Colorado. Through detailed fieldwork and laboratory analyses, the investigators will evaluate the timing and directions of motion along fault zones and the role of fluids in weakening these zones and facilitating tectonic activity. These results will help geoscientists better understand how rock strength affects fundamental mountain-building processes. This project will directly support thesis work for three Colorado State University graduate students and three Washington and Lee University undergraduate students, who will disseminate research results via publications and conference presentations. The study area in the Sangre de Cristo Mountains borders Great Sand Dunes National Park, which receives over half a million visitors each year, and the investigators will communicate research results with the park geologist to help provide additional geological context for educational outreach. This project will also support three years of an interdisciplinary student-led research seminar on the Sangre de Cristo Mountains within the Colorado State University Department of Geosciences. This seminar will involve at least 15 students (graduate and undergraduate) and 7 faculty members from across the Department, exposing students to a highly interdisciplinary approach to research. The project area in the Sangre de Cristo Mountains exposes some of the deepest Laramide and Rio Grande rift structural levels in the Rocky Mountain region, offering a rare opportunity to investigate deformation near the brittle-plastic transition. The investigators hypothesize that fluid-rock interactions weakened faults in this region, allowing contractional deformation to take place at relatively low stresses during the Laramide orogeny and extensional reactivation to occur on structures not optimally oriented for failure during development of the Rio Grande rift. Widespread shortening across the Laramide orogen may have been facilitated by fluid-related weakening, and extensional reactivation of thrust faults may represent a fundamental component of orogenic collapse. The investigators will test their hypotheses using a multidisciplinary research approach that includes geologic mapping, microstructural analysis, electron backscatter diffraction, Fourier transform infrared spectroscopy, stable isotope analysis, fluid inclusion analysis, geochemistry, and geo/thermochronology. Data generated in this project will provide key constraints on the stresses, temperatures, deformation mechanisms, and fluid involvement associated with intraplate shortening near the strongest part of the crust. This research will also shed light on the relations between Laramide contraction and Rio Grande rift extension and the role of shear zone geometry and rheology on extensional reactivation, providing a framework for understanding extensional reactivation of thrusts in other orogens.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.