DEPTH EXTENT OF THE LAU BACK-ARC SPREADING CENTER AND ITS RELATION TOSUBDUCTION PROCESSES

Seismic tomography and wave form inversion revealed that very slow vel ocity anomalies (5 to 7 percent) beneath the active Lau spreading cent er 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-a re spreading are related to deep processes, such as the convective cir culation in the mantle wedge and deep dehydration reactions in the sub ducting slab, The slow regions associated with the Tonga are and the L au back are are separated at shallow levels but merge at depths greate r than 100 kilometers, suggesting that slab components of back-are mag mas occur through mixing at these depths.