We study the mantle structure below the southwest Pacific in order to examine the geometry of the Tonga slab at depth and its interaction with the 410- and 660-km discontinuities (hereafter called the 410 and the 660). We utilize data from stations of both the Lau Basin Ocean Bottom Seismogram 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 Lau back arc spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point stacking of receiver functions. We produce a high-resolution image of the upper mantle near the Tonga subduction zone to show the interaction between the discontinuities and the subducting 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 olivine phase transitions. ¿ 2001 American Geophysical Union