Aspects of the geology of Waimangu, Waiotapu, Waikite and Reporoa geothermal systems, Taupo Volcanic Zone, New Zealand

Abstract

The Waimangu-Rotomahana, Waiotapu, Waikite and Reporoa surface thermal areas are located close to the eastern side of the Taupo Volcanic Zone, within an elongate zone of low apparent resistivity that trends subparallel to the SW-NE structural grain. Salient volcanic features are the Reporoa Caldera in the south, and the SW part of Haroharo Caldera in the north. Between the calderas is the SE-tilted Paeroa Block bounded on the NW by the Paeroa Fault Zone and the Taupo-Rotorua Depression. A variety of extrusive rocks have erupted in the area. Rhyolite lava domes are common in and around Haroharo Caldera but are sparse and largely buried in Reporoa Caldera. Two prominent dacite cones, Maungaongaonga and Maungakakaramea erupted on the Paeroa Block midway between the two calderas, but elsewhere the geology is dominated by extensive ignimbrite sheets and interbedded tuffs. Remnants of a subsurface andesite volcano (>0.75 Ma) destroyed during the formation of Reporoa Caldera are found in Waiotapu drillholes. The Waimangu-Rotomahana geothermal system occurs near the intersection of the Tarawera Linear Vent Zone and the SW boundary of Haroharo Caldera. The conjunction of these two major fracture systems may provide access to a heat source beneath Tarawera Volcano. The Reporoa and southern part of Waiotapu geothermal systems did not exist prior to the eruption of the 0.24 Ma Kaingaroa Ignimbrites and the formation of Reporoa Caldera. The hottest Waiotapu drillholes are adjacent to the north margin of Reporoa Caldera, suggesting that the heat source extends beneath the northern part of the caldera and southern part of Waiotapu Geothermal Field. Reporoa Geothermal Field is considered to be largely the result of southward outflow from Waiotapu through Waiora Formation tuffs and intracaldera rhyolite, confined beneath a siltstone cap. Waikite may be a consequence of westerly lateral flow of hot water from Waiotapu channelling up the Paeroa Fault Zone. Waiotapu Ignimbrite and older tuffs may provide permeable pathways for this flow across the structural grain.