Collaborative Research: RUI: Continental-Scale Study of Jura-Cretaceous Basins and Melanges along the Backbone of the North American Cordillera-A Test of Mesozoic Subduction Models

The geologic processes that formed the North American Cordillera, mountains that extend from western Mexico to Alaska, are disputed. These mountain ranges and neighboring basins (such as the Sierra Nevada and Great Valley basin within California) are conventionally interpreted to have formed due to eastward subduction of an oceanic plate beneath the western margin of the North American continent. Recent studies challenge this model and propose that much of the western Cordillera formed as an archipelago above westward subduction of an oceanic plate far from the margin of North America. In this alternative model, the archipelago became part of the Cordillera when the North American continent collided with the archipelago. This continental-scale study will test these models along four east-west transects across the western Cordillera in central Alaska, SE Alaska, British Columbia, and California. The project will provide support for four PIs, two of whom are female, the PhD work of three graduate students, and each PI’s commitment to provide undergraduate students with authentic field and laboratory experiences. The research methods are ideal for training undergraduate students to think across a range of spatial and temporal scales in the context of field observations and quantitative analyses, and undergraduate students will benefit from near-peer mentoring by graduate students. The researchers will also lead a multi-day field trip for Native American students centered on the connections between geologic processes that formed western North America and the homelands of Indigenous communities. The research addresses the fundamental problem of determining subduction polarity along ancient convergent margins. This project will couple the study of mélange belts with basin strata, and integrate the results along the hypothesized continental-scale suture within the North American Cordillera. This collaborative project will evaluate 1) the age and provenance of mélange belts, 2) the basins’ structural and sedimentological configuration (including inboard and outboard contacts), 3) the timing of basin formation and subsequent collapse, and 4) basin and mélange provenance comparison in the context of east-dipping versus west-dipping subduction. Analysis of the basin strata and adjacent mélange belts using detrital zircon analysis (U-Pb, Lu-Hf, and trace element geochemistry), sandstone petrography, and whole-rock Sm-Nd isotopic analysis and trace-element geochemistry will provide a continental-scale test of proposed models to better understand the Mesozoic development of the North American Cordillera. This project will also refine methods for establishing the geometry and timing of convergent margin systems in other orogens, particularly pre-Mesozoic systems for which tomographic information is not available. 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.