Testing Plume and Plate Models of Ocean Plateau Formation at Shatsky Rise, Northwest Pacific Ocean (IODP Leg 324 - Shatsky Rise)

The Scientific Summary for IODP Leg 324 is provided in the approved IODP Scientific Prospectus (http://publications.iodp.org/scientific_prospectus/324/324sp_7.htm). It is as follows: One of the most fundamental questions of modern geodynamics is the process of mantle convection and its impact on Earth's surface through volcanism. The greatest source of nonocean-ridge volcanism appears to be massive eruptive episodes that formed oceanic plateaus, volcanic passive margins, and continental flood basalts. A widely accepted hypothesis for such volcanism is that it results from the head of a starting plume, which rises from the deep mantle, spreads out beneath the lithosphere, and melts cataclysmically in a massive outpouring of volcanic activity. Despite the wide acceptance of this hypothesis, a convincing case for a plume head origin has not been made for any oceanic plateau; rather, significant complexities have been revealed by recent drilling of the Kerguelen and Ontong Java plateaus. One great difficulty with research of oceanic plateaus is that the original setting, relative to mid-ocean ridges and plate tectonics, is poorly known for most plateaus because they were formed during the mid-Cretaceous when no magnetic reversals formed ridge-parallel anomalies to record spreading ridge locations. Shatsky Rise, located ~1500 km east of Japan, is unique in being the only large oceanic plateau formed during a time of magnetic reversals, permitting its tectonic setting to be resolved. Magnetic lineations show that the plateau formed along the trace of a triple junction, intimately related to ridge tectonics. Existing data demonstrate that several aspects of Shatsky Rise's history (e.g., massive, rapid initial growth; transition from large to small magma flux; capture of ridges) fit the plume head model. On the other hand, the coincidence of volcanism with the triple junction, ridge jumps, and the lack of isotopic evidence for a hotspot-type mantle source can all be taken as favoring a plate-controlled origin. Its unique combination of features makes Shatsky Rise the best location on Earth to test plume versus plate-tectonic hypotheses of ocean plateau formation. During Integrated Ocean Drilling Program Expedition 324 we propose to core ~800 m of igneous basement at five sites on Shatsky Rise to examine the history, source(s), and evolution of this plateau. From the results of this expedition, we hope to be able to put to rest the question of whether oceanic plateaus like Shatsky Rise were formed from deep-sourced mantle plumes or interaction of plate boundaries and the lithosphere with the shallow mantle.