Collaborative research: Evaluating the drivers of exhumation during accretionary orogenesis: A field-based investigation in the Salmon River Suture Zone, Idaho

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Mountain building events typically result from collisions between tectonic plates. During mountain building, rocks that were once deeply buried often become exhumed and transported to the Earth’s surface. This project will investigate a region of central Idaho that experienced an ancient collision between an ocean island system and the North American continent, which resulted in exhumation of rocks that were buried as deeply as 40 kilometers. This study will test whether a portion of the lower crust detached from the overlying tectonic plate and subsequently sank into the mantle below, causing the overlying rocks to be uplifted towards the surface, or alternatively whether movement along steep faults resulted in their exhumation. To evaluate these hypotheses, rock samples will be collected and evaluated to give information on when, and at what rates, these deep rocks were translated towards the surface. The results of this project will improve our understanding of the tectonic processes that are active during plate collisions and mountain building events, which can impact the distribution and quality of geologic resources. This project will improve STEM education by generating field-based learning and research opportunities for undergraduates and graduate students at Washington State and Missouri State universities. The results of this project will introduce STEM research to undergraduates each year through geology lectures. Grade-appropriate presentations will be developed for K-12 students to introduce them to their local geology and allow for the discussion of STEM careers.Quantifying the contributions of processes that exhume deep-seated metamorphic rocks during collisional tectonics is fundamental for understanding how mountain belts evolve. In the North American Cordilleran orogen in central Idaho, the Salmon River suture zone is a ~30 kilometer-wide region that experienced Jurassic-Early Cretaceous contractional deformation associated with collision of a volcanic island arc terrane with the western edge of the North American continent. Rocks in the suture zone record structural burial to and subsequent exhumation from depths as great as 40 kilometers, and therefore are ideal for investigating the processes that drive high-magnitude exhumation during accretionary events. This project will test two primary hypotheses: 1) Rocks in the Salmon River suture zone were exhumed via hinged unroofing following lithospheric delamination, which predicts contemporaneous exhumation-related cooling across the width of the suture zone and eastward-increasing exhumation rates and magnitudes; or 2) Rocks in the Salmon River suture zone were exhumed via forward-breaking thrust propagation, which predicts a systematic westward younging of the timing of peak burial, initial exhumation, and motion on thrust faults. To test these hypotheses, this project will integrate structural analysis and geochronology of field relationships, the timing and pressure-temperature conditions of peak metamorphism, and multi-part temperature-time paths as a proxy for exhumation-related cooling. The results of this project will have implications for our understanding of the geodynamic processes that operate during accretionary events. In addition, project results will address the existence, scale, and effects of lithospheric removal, and the temporal and spatial relationships between accretion-related burial and associated shortening and exhumation in the overlying middle and upper crust.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.