Long offset tensor apparent resistivity surveys of the Taupo Volcanic Zone, New Zealand

Abstract

The Taupo Volcanic Zone (TVZ) is the centre of rhyolite volcanism in New Zealand and is characterised by numerous calderas and active geothermal systems. Three long-offset multiple-source (tensor) bipole–dipole surveys, which together span the width of the TVZ, have been used to determine the resistivity structure down to depths of 5–8 km. These data have been interpreted using the constraints provided by extensive shallow (<1 km) resistivity mapping made using the Schlumberger electrode array. The deep resistivity structure defines three distinct zones within the TVZ, each with boundaries approximately parallel to the eastern margin. The eastern zone is nearly 20 km wide and contains highly conductive rocks (<10 Ωm) that extend from about 2 km to at least 3.5 km depth. The conductive nature of these rocks is believed to reflect clay-rich volcaniclastics which have infilled a series of overlapping and coalescing calderas. About 70% of the geothermal discharge of the TVZ (>4000 MW) occurs through this eastern zone. Along the centre of the TVZ lies the Taupo Fault Belt in which resistivities are an order of magnitude higher (>200 Ωm to depths of at least 7 km) than in the eastern zone. The Taupo Fault Belt also has low heat-flux, high seismicity and is undergoing rapid tectonic extension. A third zone along the western side of the TVZ contains conductive rocks below 500 m depth. Drilling shows that the high conductivities occur in thick ignimbrite sheets with ages greater than 1 Ma. These low resistivities are caused by clay minerals produced by low-temperature alteration.