A seismic investigation of the crustal and upper mantle structure within the Central Volcanic Region of New Zealand

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Abstract
The structure of the crust and upper mantle of the Central Volcanic Region, New Zealand, are deduced from long-range seismic refraction and wide-angle seismic reflection data. Three shots were detonated in the Bay of Plenty and one in Lake Taupo; seismographs were located at approximately 10 km intervals between these shots. The geometry of the experiment was such that there was a partially reversed coverage for the crustal phases, but correlatable upper mantle phases were only recorded from the Bay of Plenty shots. A nondipping, plane-layer model to account for the refraction observations consists of a 15 ± 2 km thick crust of seismic velocity 5.5–6.15 km/s, underlain by an upper mantle with an anomalously low velocity of 7.5 ± 0.2 km/s. Wide-angle upper mantle reflections were also recorded from the Lake Taupo shot; a simple T2 versus X2 analysis of these reflections gives a depth of about 16±1 km to the upper mantle with a root-mean-square (r.m.s.) velocity for the crust of 5.94 ± 0.2 km/s. The anomalously thin crust and low crustal and upper mantle seismic velocities are consistent with the Central Volcanic Region being an area of high heat flow, past and present volcanism, and active back-arc spreading.