We are utilizing multiple disciplines in the Earth Sciences to better understand processes associated with the subduction of oceanic ridges at convergent plate boundaries. In particular, we are studying a well-preserved example of ancient ridge subduction in the Pacific Northwest, where we have an unprecedented opportunity to document a 20 million year history of ridge subduction phenomena from development of sedimentary basins to magmatism and metamorphism in the middle crust. This ancient example thus offers a critical compliment to studies of modern ridge subduction systems where deeper processes must be evaluated indirectly. We are testing a model of the changing position of the ridge from approximately 60 to 40 million years ago, a period that marks the transition from the Mesozoic North Cascades arc to modern Cascadia. We are also addressing several central questions for ridge subduction. 1) How do fore-arc sedimentary basins respond to changes in stresses and strains that accompany ridge subduction and jumps in the positions of oceanic ridges? 2) How is deformation, such as faulting, partitioned in space and time across the continental margin during ridge subduction? 3) What are the diagnostic geochemical features of magmas within a convergent margin after ridge subduction. As part of our research, we are using a geographic information system to reconstruct the distribution of geological features during the interval of 60-40 million years. We are making the digital paleotectonic and paleogeographic maps and data derived from this study available to other researchers and educators in order to allow others to build upon and supplement our efforts. In summary, this multidisciplinary collaborative research should provide significant contributions to understanding of processes associated with ridge subduction, a fundamental plate tectonic process, and the effects of ridge subduction on the Eocene tectonic evolution of the Pacific Northwest. Our project involves a collaboration between several universities of diverse character, including San Jose State University, University of Puget Sound, Northern Arizona University, and the Massachusetts Institute of Technology. Involvement of graduate and undergraduate students is a critical component of this project. Several undergraduate students are participating in the research. Their involvement includes participating in the geologic fieldwork, conducting individual research projects, traveling to other universities to use analytical facilities not available at their home institutions, and the preparation of undergraduate theses based on their research. The results are of the research are also being integrated in geologic classroom curricula as part of an advanced undergraduate course at the University of Puget Sound where students are studying the rocks collected during this study as part of a semester-long class project. Results of the research are being disseminated at professional society meetings (including student presentations on their research) and the peer-reviewed scientific literature.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Stephen S. Harlan
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University of Puget Sound
United States
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