Test Between Thermal and Tectonic Hypotheses for North Atlantic Seafloor Spreading Geometry Reorganization

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There is a massive ongoing reorganization of North Atlantic seafloor spreading geometry occurring at present south of Iceland. The previous orthogonal ridge/transform staircase geometry typical of slow-spreading ridge systems is being progressively changed to the present non-segmented oblique spreading geometry on the Reykjanes Ridge as transform faults are successively eliminated. This reorganization is presently interpreted as a thermal phenomenon, with a pulse of warmer mantle expanding away from the Iceland plume causing a progressive change in subaxial mantle rheology from brittle to ductile, so that transform faults can no longer be maintained. This explanation is nearly universally assumed to be true & is used to infer important conclusions about deep Earth & lithospheric geodynamics that are also nearly universally assumed to be true.The PI will undertake a month-long marine geophysical expedition to collect the multibeam, magnetics & gravity data that would provide a definitive test between the fundamentally different thermal & tectonic hypotheses for exactly how the Iceland plume (or whatever form of mantle convection or heterogeneity creates Iceland) caused the reorganization of the Mid-Atlantic Ridge south of Iceland. An important result of this test will be that modelers will either be confident that their thermal Reykjanes Ridge reorganization models are providing accurate information about Earth's behavior, or that they can confidently begin to model the correct mechanism instead. An additional benefit of this project will be the first accurate map of the Bight transform fault/fracture zone complex, known to be an important pathway for westward flow of North Atlantic mid-water circulation across the Reykjanes Ridge boundary, an important control on global climate change.Broader Impacts: Education and Human ResourcesResolving the origin of this large-scale plate boundary reorganization has important implications beyond testing a long-held fundamental assumption of Iceland mantle plume geodynamics. Thermal plume pulses have been interpreted as leading to regional uplift/subsidence events that modulate ocean currents and affect climate and sedimentation patterns throughout the North Atlantic. The proposed study will examine alternative tectonic explanations related to the mechanism and timing of the sequential elimination of transform faults in this area and thus has implications for inferred links among these phenomena. The proposed work will involve significant international collaboration with Icelandic scientists, as has our previous work in the area. The project will support a graduate student who will work on the survey data as a primary part of his/her thesis. As in our previous cruise we plan to mentor several undergraduate students at sea through the NSF-funded Marine Advanced Technology Education (MATE) Internship program and thus further NSF education and human resource development goals. Results will be integrated into classroom teaching by the P.I.s, and thus disseminated to a fairly wide audience including many underrepresented minority groups in Hawaii, enhancing scientific infrastructure. We think that improved understanding of this obviously important plate boundary reorganization will form an invaluable tool for further research and teaching, and we will make these data sets available on a project website. We think a movie of Reykjanes Ridge evolution projected onto a sphere could ultimately be a valuable teaching aid and museum display.