Lithospheric structure of an active backarc basin: the Taupo Volcanic Zone, New Zealand

Abstract

S U M M A R YSeismic data from both explosive and earthquake sources have been used to model the crustal and upper-mantle velocity structure beneath the Taupo Volcanic Zone (TVZ), an active backarc basin in central North Island, New Zealand. Volcanic sediments with P-wave velocities of 2.0-3.5 km s −1 reach a maximum thickness of 3 km beneath the central TVZ. Underlying these sediments to 16 km depth is material with velocities of 5.0-6.5 km s −1 , interpreted as quartzo-feldspathic crust. East and west of the TVZ, crust with similar velocities is found to depths of 30 and 25 km, respectively. Beneath the TVZ, material with P-wave velocities of 6.9-7.3 km s −1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust. The base of the crust at 30 km depth under the TVZ is marked by a strong seismic reflector, interpreted as the Moho. Modelling of arrivals from deep (>40 km) earthquakes near the top of the underlying subducting Pacific Plate reveals a region with low mantle velocities of 7.4-7.8 km s −1 beneath the crust of the TVZ. This region of low mantle velocities is best explained by the presence of partially hydrated upper mantle, resulting from dehydration of hydrous minerals (e.g. serpentinite) carried down by the underlying subducting plate. Within the lower crust beneath the TVZ, a region of high (0.34) Poisson's ratio is observed, indicating the presence of at least 1 per cent partial melt. This melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust, where it provides a source for the voluminous rhyolitic magmas of the TVZ.

The effect of crustal melt on rift dynamics: case study of the Taupo Volcanic Zone

“…Crust is assumed to extend to 30 km depth (e.g. Harrison & White 2006), with a bulk composition controlled by the greywacke basement. A mantle layer is included below this with a uniform linear viscosity of 10 18 Pas, consistent with weakened fluid-rich mantle above the Hikurangi subduction zone (Reyners et al 2006;Eberhart-Phillips et al 2008).…”

Investigations of deep resistivity structures at the Wairakei geothermal field

“…tion in the resistivity (Heise et al, 2007). A further significant drop of resistivity can be seen at about 15 km depth, which coincides with the base of the quartzo-feldspathic crust (Harrison and White, 2006;Stratford and Stern, 2006) beneath which underplating is believed to be occurring, and a higher proportion of melt is present (Charlier et al, 2005).…”

Application of Raman spectroscopy to distinguish adularia and sanidine in drill cuttings from the Ngatamariki Geothermal Field, New Zealand

“…Recognition and correlation of units can be difficult from fine-grained cuttings, particularly when trying to distinguish between units with similar lithologies. Within the Taupo Volcanic Zone (TVZ), New Zealand, it can be difficult to distinguish between different siliciclastic units which make up much of the upper 3 km of the crust (Bibby et al 1995;Harrison & White 2006). An additional complication in the TVZ is the presence of large regional-scale depressions (calderas and graben structures) that cause overthickening of siliciclastic deposits of both primary and secondary origin (e.g., Wilson et al 2008).…”