A long-standing question in crustal seismology is the nature of rocks with compressional P-wave speeds of 7–7.6 kilometers per second that are found beneath continental rift zones [e.g., Cook, 1962]. Two interpretations are possible: They are either lower crustal rocks, or upper mantle material with unusually low P-wave velocities because of the presence of a small percent of partial melt.
An April 2005 explosion seismology experiment set out to explore the issue of what is crust and what is mantle in the Taupo Volcanic Zone (TVZ), central North Island, New Zealand. The TVZ is the apparent continuation of oceanic back-arc spreading within the Havre Trough into the continental lithosphere of New Zealand (Figure 1). Determining whether crust or mantle exists in such settings is fundamental to understanding back-arc basins and subduction zones, both of which are vital elements in recycling oceanic lithosphere and creating new continental crust