A MAGNETIC MODEL FOR DEEP PLUTONIC BODIES BENEATH THE CENTRAL TAUPO VOLCANIC ZONE, NORTH-ISLAND, NEW-ZEALAND

The central Taupo Volcanic Zone (TVZ) is a region of intense Quaternary rhyolitic volcanism and geothermal activity in the North Island of New Zealand from which about 14,000 km(3) of pyroclastics and lavas have been erupted during the last 1.6 Ma. Analysis of aeromagnetic surveys over the TVZ showed the presence of long-wavelength (10 to 25 km) magnetic anomalies which roughly follow the trend of the currently active eastern TVZ, from the north of Lake Taupo to the east of Lake Rotorua. An interpretation of the long-wavelength magnetic anomalies using 3-D magnetic modelling suggests that these anomalies are caused by the magnetic effects of <3 km thick sequence of volcanic rocks and deeper magnetised bodies within the nonmagnetic upper crust (4-7 km depth) beneath the young (age <0.7 Ma), currently active eastern TVZ. The deep magnetised bodies are interpreted as solidified rhyolitic sub-volcanic plutons that have cooled down to below their Curie temperature. Although the existence of plutonic bodies beneath the TVZ has been postulated prior to this study, this magnetic interpretation result appears to be the first geophysical model of such bodies.