Using array MT data to image the crustal resistivity structure of the southeastern Taupo Volcanic Zone, New Zealand

Magnetotelluric (MT) data from 169 locations covering a 700km2 region in the southeastern part of the Taupo Volcanic Zone, New Zealand, are used to generate a 3-D resistivity inversion model to a depth of 10km. Isolated zones of low-resistivity (<30Ωm) below 3km are imaged in the basement that appear to be associated with the geothermal fields at Rotokawa, Ngatamariki and Ohaaki, but not with the geothermal fields at Orakei-Korako and Te Kopia. These low-resistivity zones are interpreted to be recent intrusions that feed heat and magmatic fluid to the overlying system of hydrothermal convection supplying the geothermal fields. At Rotokawa, hypocentres of thermally induced seismicity coincide with the top of the deep low-resistivity zone and suggest that temperatures sufficient for melting are reached at ~4.5km depth. In most cases these deep low-resistivity zones are located at the margins of areas of low-gravity, which suggests that the intrusions lie at the margins of old caldera collapse structures. The 3-D resistivity model indicates that geological structure and magmatic intrusions play a more direct role than previously envisaged, and significantly influence the overarching system of convective heat and mass transport in the Taupo Volcanic Zone.