Deep electrical structure of the Central Volcanic Region and Taupo Volcanic Zone, New Zealand

Abstract

Magnetotelluric measurements have been made along a transect crossing the Central Volcanic Region (CVR) and Taupo Volcanic Zone (TVZ) of the North Island of New Zealand. Analysis of the observed data using the phase tensor method shows that at the south-eastern end of the transect, where it crosses the TVZ, the data are largely 2-dimensional in character and respond to electrical structure which has a strike orientation of N45°E. There is however, evidence of some degree of 3-dimensionality in data from the central part of the TVZ. 2-dimensional inversion and forward modelling of data indicate the existence of low resistivity beneath about 10 km depth centred below the Taupo Fault Belt. The existence of low resistivity beneath this depth is consistent with a low velocity zone identified from receiver function analysis and with measurements of the transition from brittle to ductile behaviour and the depth penetration of the convective systems associated with the geothermal fields within the TVZ. The low resistivity provides support for the existence of igneous intrusion to shallow depths beneath the TVZ. Data from the north-western end of the transect, where it crosses the western boundary of the CVR, are 2-dimensional and may respond to electrical structure associated with the Hauraki Rift, an active rift containing Tertiary sediments, which cuts the boundary of the CVR in this region.