© Hersh Gilbert, University of Colorado

Deep discountinuity structure of the Tonga subduction zone from common depth point stacks. NEIC seismicity superimposed on discontinuity image.

Hersh J. Gilbert, Anne F. Sheehan (Dept. of Geological Sciences and CIRES, University of Colorado, Boulder, CO 80309; (303) 735-4916; hersh@mantle.colorado.edu); Doug Weins (Washington University, St. Louis), Spahr Webb, LeRoy Dorman and John Hildebrand (Scripps Institution of Oceanography)

In this study we examine the mantle structure below the Lau Basin Spreading Center and Tonga Subduction Zone in order to examine the geometry of the Tonga slab at depth and its interaction with the 410- and 660-km discontinuities. We utilize data from stations in both the Lau Basin ocean bottom experiment and island stations of the Southwest Pacific Seismic Experiment. The tectonic complexity of this region, containing both the Tonga subduction zone and the associated back-arc Lau Basin back arc spreading center, make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as CMP stacking of receiver functions. Previous studies indicate that the slab deflects along the 660-km discontinuity in this region. Our study will produce a high resolution image of the interaction between the 660 and the subducting slab to determine the extent to which the slab is deflected and how much the discontinuity is depressed as a result of the slab. Previous work in this area which used data from these same arrays found extremely slow seismic velocities in the upper 100-km of the mantle (Xu and Wiens, 1997; Zhao et al., 1998). With detailed images of upper mantle and transition zone discontinuity structures, we hope to reach a greater understanding of the depths to which mantle structure influences surface tectonics.