Structure and evolution of the Wairakei–Tauhara geothermal system (Taupo Volcanic Zone, New Zealand) revisited with a new zircon geochronology

Abstract

The central part of the Taupo Volcanic Zone (TVZ), New Zealand, is notable for the number (23) and size of its high-temperature geothermal systems. Of these, the Wairakei-Tauhara geothermal system collectively represents the largest commercially developed example. Natural thermal output prior to utilisation, i.e. before 1950 was ∼530MW. Since 1950, drilling of over 300 closely spaced wells for power generation has provided geological data for a comprehensive stratigraphic framework. Within this framework, the strata are dominantly rhyolitic pyroclastic rocks and lava bodies, with interbedded fluvial and lacustrine deposits. Until now, no subsurface units had been directly dated and their ages were constrained only by their positions relative to the surficial 25.4 ka Oruanui ignimbrite, or to buried ignimbrite that is correlated with the 349 ka Whakamaru Group. This paper presents 33 suites of U-Pb age data and a limited number of reconnaissance U-Th disequilibrium age data from magmatic zircon crystals (zircons) to date the rocks of the Wairakei-Tauhara system. The oldest units dated are intermediate lavas from ca. 1.7 Ma, while the youngest locally vented eruptives of Tauhara dacite are tentatively dated to ca. 60 ka. Placing the age data within the established drillhole record allows for the recognition of major volcano-tectonic events and spatially clustered local volcanism that represents magmatic heat sources for geothermal activity within the Wairakei-Tauhara area. In contrast to the Rotokawa and Ngatamariki geothermal systems 10–20km to the northeast, the Wairakei-Tauhara area was affected by two episodes of caldera collapse associated with deposits dated at 349 ka and 310 ka, leading to major faulting and abrupt variations in thicknesses of units. The faulting and deposit thickness variations established in these two events remain major controls on present-day permeability of the geothermal system. The new age data and stratigraphy demonstrate development of distinct magma systems through time which may have stimulated previous periods of hydrothermal activity. However, from zircon age data from the rhyolite-andesite mixed Tauhara dacite, we propose that the current geothermal system was initiated at ca. 60 ka and is thermally maintained by continuing contributions from the same magmatic system that has sporadically vented distinctive rhyolite since.