INTERPRETATION OF REFRACTION SEISMIC AND GRAVITY-DATA ACROSS THE EASTERN MARGIN OF THE TAUPO VOLCANIC ZONE, NEW-ZEALAND

Gravity and seismic refraction data have been interpreted in an attempt to resolve the complex structural relationships between the Kaingaroa Fault (eastern edge of TVZ), Reporoa Caldera, and the Reporoa geothermal System. Seismic data interpretation, using constraints provided by drillhole data, shows that greywacke lies at about 650 metres depth below the Kaingaroa Plateau, 9 km east of the Kaingaroa Scarp. Both gravity and seismic modelling suggests that a previously undetected fault lies beneath the plateau, 11 Ion east of the scarp. This fault extends close to the surface but has no obvious surface trace. Gravity data indicate the eastern edge of the Reporoa Caldera lies about 1 km west of the Kaingaroa Scarp, and that the Kaingaroa Fault lies further eastwards under the plateau, and consists of a number of discrete faults. Within the Reporoa Caldera, basement is modelled at a depth of about 2.5 ion and is vertically displaced by about 1 ion at the caldera boundary. The Reporoa Geothermal System lies entirely within the Reporoa Caldera. There are no significant gravity highs observed along the survey line that may reflect densification brought about by hydrothermal mineral deposition, or a lateral variation in the distribution of near-surface rhyolites.