Seismic tomography and wave form inversion revealed that very slow velocity anomalies (5 to 7 percent) beneath the active Lau spreading center extend to 100-kilometer depth and are connected to moderately slow anomalies (2 to 4 percent) in the mantle wedge to 400-kilometer depth. These results indicate that geodynamic systems associated with back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. The slow regions associated with the Tonga arc and the Lau back arc are separated at shallow levels but merge at depths greater than 100 kilometers, suggesting that slab components of back-arc magmas occur through mixing at these depths.