Zhao, Dapeng, Yingbao Xu, Douglas Wiens, LeRoy Dorman, John Hildebrand, Spahr Webb, Depth Extent of the Lau back-arc spreading center and its relationship to subduction processes, Science, V278, 254-257, 1997.

This image is of compressional seismic velocities under the Tonga island arc and Lau Basin. The size of the grid is about 1000 km horizontally and about 625 km in depth.  The blue (high-velocity) object is the Pacific plate, which is being subducted beneath the island arc.  The reddish material is low-velocity (presumably hot and magma-rich) material which forms the source for the back-arc spreading center and the arc volcanism.  The "new science" was that the volcanoes and the spreading center are fed from sources which are more-or-less connected, and that the spreading center (not indicated on the figure) is somewhat to the east of the main body of the source material.  This seems consistent with the observation that the spreading center is migrating to the west. Adjusting the lower slider will vary the amount of the low-velocity material which is visible.  The chart atop the structure shows the islands of Fiji on the west and Samoa at the northeast.  Red dots indicate land seismic stations which operated for 2 years, while the blue squares show the positions of the OBSs which operated for 4 months.  The green triangles are active volcanoes.  The red lines show the lau spreading centers and associated faults. The scale bars on the chart are 2 degrees (222 km) in length.