Tectonic Settings of Low Enthalpy Geothermal Systems in New Zealand: A Review

Abstract

Low-enthalpy geothermal resources are widely distributed in New Zealand and include (1) hot spring systems with <90?C discharge temperatures in the North and South Islands and offshore islands in the north, (2) periphery of high-enthalpy geothermal systems within the Taupo Volcanic Zone, (3) 120-160?C waters at >3.5 km depth in abandoned oil wells and (4) natural heat flow 15-20 m below the surface. Hot spring systems in New Zealand are found in four major tectonic settings characterised by: (1) subduction-related volcanism and rifting in the Taupo Volcanic Zone, (2) intraplate volcanism and associated hot mantle upwelling, (3) rapid rise of hot waters introduced along fractures in the East Cape accretionary prism of North Island, (4) rapid uplift and thrusting in the Alpine Fault and parts of the East Cape. Most hot spring waters, outside the main high-enthalpy geothermal areas, Taupo Volcanic Zone and Ngawha, are derived from deeply circulating groundwaters that may bear some imprints of mantlederived volatiles, saline formation water or metamorphic water. Within the accretionary prism in the East Cape of North Island, reservoir waters originate from seawater and dehydration of marine clays. Although New Zealand has an installed capacity of 308 MWt, its vast geothermal resources remain largely untapped for direct utilisation of heat.