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.