ABSTRACT

Despite remarkable advances over the past few years in our understanding of the dynamics and evolution of Earth's deep mantle, one major aspect has become increasingly contentious. Is hotspot volcanism fed by deep mantle plumes or not? This has been a central tenet of the Standard Model for three decades, but is now contrasted with a "top down" model that derives hot spot volcanism strictly from the upper mantle, via fracture tectonics of the surface plates. While the Hawaiian hotspot chain is the "archetype" for the Plume model, the Samoan chain is cited frequently as evidence for the Top Down model. Recent studies in Samoa show that, while fracture tectonics plays an important role in the volcanism, it is only the "end-game" of a plume-driven hotspot.

This project uses the unique juxtaposition of the Samoa hotspot and adjacent Tonga subduction zone to illuminate the dynamics of this interaction, and to contribute critical understanding to the debate about plumes more generally. As the Tonga slab rolls back rapidly to the east, it forces an eastward propagation of the hinge fault between the slab and the Pacific plate. The central thesis of this project is that this roll-back drives a northward flow of mantle "escaping" from beneath the slab, forcing the Samoa plume to step progressively northward, leaving behind a series of en echelon volcanic lineaments. Concomitantly, the propagation of the Tonga-Pacific hinge, and associated return flow of mantle into the back-arc region, spawns rejuvenated volcanism that is incidentally contiguous with the Samoan hotspot lineament. This study comprises a multi-disciplinary multi-institutional group of dynamicists (M. Billen, UC Davis, and C. Kincaid, URI), seismologists (M. Behn and J. Collins, WHOI), a geochemist and a mineral physicist (S. Hart and G. Hirth, WHOI). 3-D computational modeling and experimental "tank" modeling is being used to study the dynamic interaction of slab rollback and the (putative) plume conduit. Four seismic stations have been deployed along the Samoa chain and, in conjunction with existing permanent stations, will be used to make shear-wave splitting measurements. These regional observations of seismic anisotropy (along with published values from Fiji and the Lau back-arc) are interpreted in the context of the numerical and experimental flow modeling, and constrain the pattern of mantle flow around the Tonga slab. Ps and Sp receiver functions from these same stations provide estimates of lithospheric thickness, and thickness of the 410-660 km transition zone. The nature of (putative) plume-plate interaction is being delineated and, combined with the geochemistry and mapping of tectonic features, allows distinctions between plate-fracture models and plume-driven hotspot models. These studies provide clear and direct tests of the specific set of questions explicit in our concept for the Samoa-Tonga interaction.

Graduate students from each of the three institutions are involved and are active participants in all aspects of the project. A meeting involving all participants has been scheduled for June 8, 2007 at the Woods Hole Oceanographic Institution. The students will also be hosted for short visits to the other institutions in the coming year. Frequent conference calls and video-conferences are used to further enhance the student participation. The students are encouraged to attend the 2008 CIDER Summer Program, as a way of providing context and breadth to their studies. Kinematic models for lattice-preferred orientation are being integrated into the CITCOM 3-Dconvection code, and these will be made available to the entire community through the Computational Infrastructure for Geodynamics. On a broader outreach front, the first trip to Samoa to service the seismic stations has been scheduled for early July 2007; visits to high schools on Savai`i and Ta`u are planned, with a day of science classes that will focus on volcanoes, plumes and plate tectonics. Given the exquisite volcanic terrain surrounding the local schools, informal half-day field trips will be run for both students and teachers.

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