The volcanic, magmatic and tectonic setting of the Taupo Volcanic Zone, New Zealand, reviewed from a geothermal perspective

The Taupo Volcanic Zone (TVZ) in the North Island of New Zealand is one of the world's most spectacular and productive areas of Quaternary silicic volcanism and geothermal manifestations. The TVZ is only the latest manifestation of NNE-SSW-orientated arcs that have migrated in step-wise fashion to the SSE over the past ca. 16 Ma. The TVZ began erupting around 2 Ma, with early andesitic volcanism being joined and rapidly swamped by voluminous rhyolitic volcanism. The zone shows a pronounced segmentation into northern and southern extremities with andesite composite cones, no calderas and only limited vent-hosted geothermal systems, and a 125 km long rhyolite-dominated central segment. About four times as much magma is trapped at depth below the central TVZ than is erupted, feeding heat, volatiles and chemicals into 23 geothermal systems with a total of ca. 4.2 GW thermal energy release. The modern (post-61 ka) TVZ is an actively rifting arc, widening at 7 mm/year at the south end to 15 mm/year at the Bay of Plenty coastline, with an associated zone of young to active faulting (Taupo Fault Belt: TFB, or the Taupo Rift), but the axes of the modern TFB and TVZ are offset by 15-20 km through much of the central TVZ. Although there is a dominant NNE-SSW tectonic grain within the central TVZ, there are also influences of deeper basement structures that sometimes extend outside the limits of the zone, such as NW-SE, arc-perpendicular accommodation zones linking local domains of extension as well as N-S orientated structures related to the Hauraki Rift that may control fluid flow into the roots of the geothermal systems. Models for the geothermal systems favour either a source in a relatively shallow localised magmatic intrusion (e.g., Kawerau, Ngatamariki) or treat the systems as reflecting large-scale fluid dynamical instabilities from an evenly heated 'hot plate' at ∼7 km depth. Where controls from dating of host lithologies are available, systems at Kawerau and Ngatamariki are seen to represent renewed activity superimposed on a fossil system fed by past intrusions, and it is unclear what is meant by the lifetime of any single geothermal system. TVZ geothermal systems appear in turn to react too sluggishly to respond to disruptive episodes of volcanism, and recover within geologically short periods of time, as seen at Waimangu and Taupo. In the central TVZ, there are complex inter-relationships between volcanism, magmatism, and tectonism. Magmatism and volcanism are obviously linked, but it is uncertain why intense magmatism at Taupo and Okataina should yield voluminous rhyolite volcanism, whereas more intense magmatism in the Taupo-Reporoa Basin has not yielded significant silicic volcanism but instead feeds multiple large geothermal systems. The central TVZ is unique for an arc segment in the intensity of its magmatic-volcanic-geothermal flux (matching the Yellowstone system), and the cause(s) of this uniqueness are not yet established. Any explanation needs to address the segmented nature of the zone, and why the thermal flux should be so geographically and temporally constrained.