CAREER: The Fall of Mountains: Reconstructing Extensional Collapse in the North American Cordillera from the Surface Record

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The seemingly endless chain of high mountains and deep valleys of the northern Rocky Mountains has sparked wonder in visitors since Lewis and Clark looked across it from the continental divide. The goal of this research is to understand when this dramatic landscape formed and how mountains fall apart, a process that is still active in the western U.S. today. The principal investigator and her students are using ancient rainwater, preserved for millions of years in volcanic ash, to measure elevations from the past 50 million years across Idaho, Montana, and Wyoming, combined with the ages of sediments that have filled the valleys over time, to reconstruct the history of mountain building and collapse in the northern Rocky Mountains and Basin and Range. Through this project, the researcher aims to increase accessibility to geoscience for students of all levels. Students benefit from direct support for graduate and undergraduate research projects, development of widely accessible, hands-on investigative Google Earth activities for undergraduate courses, and summer internships for high school students from underrepresented groups, recruited via STEM Access and TRIO-Inspire programs. The project supports to broadening of representation of underrepresented groups in STEM fields through direct research support of an early career female geoscientist researcher and her students, and through the Tuff Talk podcast, which provides distance mentorship, coaching, and professional development to a broad audience through interactive interviews with successful women in Earth science. This research also uses a novel combination of analytical techniques in geochemistry and age dating, with the goal of refining these methods.Elevated regions have an outsized influence on regional hydrology, global climate, erosion and deposition of sediment, and many other Earth processes. Orogens can maintain high topography well after the end of crustal shortening or mantle delamination, despite high gravitational potential energy and thermal weakening of the lithosphere. Although estimates of the timing of uplift have recently become common, we lack precise surface records of orogen collapse, even in the well-studied North American Cordillera. Measuring the timing and pace of surface lowering, and its effects on erosion and basin sedimentation, can provide us with insights into the drivers for that extension, and into the genesis and structure of the original orogen. The overall objective of this research is to reconstruct the Cenozoic evolution of topography across the North American Cordillera, including the timing, magnitude, and mechanisms of surface-lowering, conglomerate deposition, and drainage reorganization, thereby generating a holistic model of orogen collapse as expressed in the surface record. The principal investigator and her students quantify changes in surface topography by combining an innovative, widely distributed stable isotope proxy material with precise radiometric ages, and interpreting those data using an isotope-enabled global climate model to capture changes in elevation over time. Paleoelevation measurements are integrated with new provenance, exhumation, and sediment lag time data from the basin record to determine the onset of extension and test if surface deformation generated widespread conglomerate deposition. Results are used to discriminate between various potential mechanisms for orogenic collapse and to develop a model for the interactions between, and surface expressions of, core complex formation, slab rollback, and magmatism. Through the combination of multiple field and analytical datasets and the integration of findings, the project team is gaining a new understanding of the progression of North American Cordilleran surface lowering and the conditions necessary to initiate range-wide extension and lithospheric thinning. The proposal is supported in part by co-funding by the Division of Earth Sciences Education and Human Resources Program.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.