Postdoctoral Fellowship: EAR-PF: Field tests of Mesozoic terrane accretion in South America

The Andes of South America are a classic example of how mountains are built along continental margins. For decades, scientists have believed that this range formed through a relatively steady, long-term process of oceanic tectonic plates sliding beneath the western edge of South America. However, recent imagery of Earth’s deep interior challenges this view, suggesting instead that a chain of volcanic islands—perhaps resembling modern Japan—may have crashed into and merged with South America about 100 million years ago. By studying the chemistry, age, and characteristics of ancient volcanic and sedimentary rocks in central Chile, this research will test these two models. Answering this fundamental question will improve our understanding of how continents grow, how Earth’s crust is recycled back into the mantle, and how and where valuable mineral resources are formed. This fellowship will advance science education by providing hands-on research opportunities for undergraduate students and developing geology field trips for rural Idaho high schoolers. Additionally, the project will strengthen U.S. science by contributing to growth of a new cutting-edge laboratory facility at Idaho State University, strengthening international collaborations, and preparing the fellow for a professorship at a research-intensive U.S. institution.

This project will test competing tectonic models for the Mesozoic evolution of the South American margin. Whereas traditional interpretations favor uninterrupted, east-dipping subduction since Carboniferous time, recent geophysical studies propose Mesozoic accretion of an intra-oceanic island arc following closure of a remnant ocean by west-dipping or divergent double subduction. This research will evaluate these hypotheses by investigating two Mesozoic outcrop belts in the Coastal and Principal Cordilleras of central Chile (~31–36 °S). Igneous zircon U-Pb geochronology will be combined with trace element and Lu-Hf geochemistry to determine the number and character of Mesozoic magmatic arc(s) preserved in the region. The project will also investigate the sedimentary record to determine if Mesozoic deposition occurred within a single back-arc basin, or two oppositely-facing forearc basins later joined along a cryptic suture. Detrital zircon petrochronology will be integrated with sandstone petrography to test whether these deposits received input from distinct source terranes or if they shared a common South American provenance. Results showing a single magmatic trend and similar sediment sources between basin segments will support a history of continuous east-dipping subduction, whereas evidence for two coeval, geochemically distinct arcs feeding separate forearc basins may indicate a major terrane collision.

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.

Award Number: 2518506
Principal Investigator: Chance Ronemus
Funds Obligated: $291,976
State: ID