3-D CONFIGURATION OF SUBDUCTED LITHOSPHERE IN THE WESTERN PACIFIC

Computer graphics techniques are used to portray the shape of the subducted lithosphere as seen from different views. The configuration of the subducted lithosphere is represented by a surface that approximately fits the upper envelope of the slab-like spatial distribution of earthquake hypocentres in the Benioff-Wadati zone. Regions considered include Tonga-Kermadec, Izu-Bonin-Mariana, Japan-Kuril-Kamchatka, and Sunda-Banda in the western Pacific. These regions include all known zones of deep focus earthquakes except the tectonically complex regions of New Hebrides, Solomon-New Britain, Molucca Sea (Phillipines), the Mediterranean, and South America. Shown at the same scale and with the effect of Earth's sphericity included, the views are very useful for illustrating variations in the configuration of the subducted lithosphere between different regions and within the same region. The results illustrate the segmentation of the upper several hundred kilometres of subducted lithosphere into major units of relatively uniform configuration. These segments are also clearly defined by major features of surface morphology of the trench and upper plate, the distribution of subduction-related volcanism, and by the intersection with the subduction zone of major bathymetric features of the subducting plate. In contrast, the deeper portions of the slabs have a more complex and variable configuration. Discordances between the deep and shallow structures are often important on a regional scale and suggest that there are relative horizontal movements between the surface trace of the subduction zone and the material at depth into which the subducted lithosphere is sinking. These relative movements appear to be intimately associated with the tectonic evolution of the region and indicate a regional aspect of subduction that is often overlooked in an attempt to explain the global variability of the shapes of subducted lithosphere. Although earthquake activity ceases at 680 km, possibly because of a phase transition, the geometry of the deep portion of subducted lithosphere suggests that subducted lithosphere does penetrate to greater depth.