

Track map MGL1003 - Shatsky Rise

Oceanic Volcanic Plateaus are a submarine form of Large Igneous Provinces. They are formed from huge amounts of basaltic lava, and their formation is often linked with mass extinctions and climate change. Two principal causes for their formation are hypothesized: mantle plumes and rifting events. Shatsky Rise, in the NW Pacific, is of unknown origin, but will be extensively studied with marine seismic techniques aboard R/V Marcus G. Langseth during the summer of 2010.

Deciphering the origins of the giant large igneous provinces (LIPs), such as continental flood basalts and oceanic plateaus, is a critical element for understanding mantle dynamics and its relation to terrestrial magmatism. The origins for giant LIPs are, however, still poorly understood; they may represent the eruption of plume heads rising from the core-mantle boundary, but non-plume hypotheses have also been proposed. Studying oceanic plateaus is an appealing approach to the giant LIP problem because oceanic lithosphere offers the least tectonic and magmatic complication. Among a dozen or so large oceanic plateaus in the oceans, Shatsky Rise must be a high-priority target because of its unique tectonic setting. It is the only giant LIP plateau formed at a time of frequent magnetic reversal anomalies that show its relationship to coeval spreading ridges. Moreover, having formed at a ridge-ridge-ridge triple junction and being intimately linked with large-scale plate boundary reorganization, this plateau offers a decisive contest pitting the plume head versus non-plume hypotheses, such as plate-driven models. Finally, the triple junction setting provides the optimal tectonic window: minimal lithospheric contamination makes this the easiest place to probe the dynamics of mantle melting imprinted in the crustal section. Despite its potential importance, Shatsky Rise has been inadequately explored and crucial geophysical data are missing. IODP Expedition 324 is currently scheduled to sample the sediments and upper igneous layers at five sites on Shatsky Rise during September-November 2009, and a better geophysical context is essential to get the most out of this drilling project.

We have been funded to carry out a collaborative geophysical project with two foci: (1) constraining the whole crustal structure by an OBS reflection and refraction experiment, and (2) delineating the tectonic history by extensive MCS profiling and reanalysis of bathymetry and magnetic data. New data are needed because existing seismic refraction data are outdated and inaccurate and other geophysical data are simply too sparse. A new model of crustal seismic structure will constrain the nature of mantle melting on the basis of correlation between thickness and velocity. The detailed upper crustal structure revealed by MCS will help to reconstruct the tectonic history of Shatsky Rise, which appears to be a huge volcano formed at a triple junction and then split apart by seafloor spreading. Underway geophysical data will be assimilated with existing data to improve the bathymetry and magnetic anomaly maps of the rise. Gravity data will be combined with seismic constraints to map out 3-D variations in crustal thickness and also estimate a plausible density structure for crust and shallow mantle. We will synthesize the results of these geophysical studies to build a comprehensive tectonic framework, including better estimations of eruption rate, its temporal variation, its relation to ridge kinematics, and the coeval evolution of the parental mantle