Upper mantle discontinuity structure in the region of the Tonga SubductionZone

We study the mantle structure below the southwest Pacific in order to exami ne the geometry of the Tonga slab at depth and its interaction with the 410 - and 660-km discontinuities thereafter called the 410 and the 660). We uti lize data from stations of both the Lau Basin Ocean Bottom Seismogram exper iment and island stations of the Southwest Pacific Seismic Experiment. The tectonic complexity of this region, containing both the Tonga subduction zo ne and the associated Lau back are spreading center make it an ideal area t o investigate the upper mantle discontinuities using a high resolution tech nique such as common conversion point slacking of receiver functions. We pr oduce a high-resolution image of the upper mantle near the Tonga subduction zone to show the interaction between the discontinuities and the subductin g slab. Our results show the 410 uplifted by 30 km near the Tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivi ne phase transitions.