Tectonic Controls on Hydrothermal Fluid Flow in a Rifting Arc, Taupo Volcanic Zone, New Zealand

Abstract

Tectonic controls on the localization of high-temperature (>250°C) fluid flow are evaluated for geothermal systems of the Taupo Volcanic Zone (TVZ), New Zealand. Most geothermal systems occur within an actively rifting arc (~150 km-long) dominated by silicic volcanism and they occur in association with major faults near caldera bounding structures or within accommodation zones that transfer extension between rift segments. Geothermal systems are hosted in a thick sequence (1 to >3 km) of Quaternary volcanic deposits that rest unconformably on weakly metamorphosed Mesozoic argillite and sandstone (termed greywacke). Permeability controls in selected geothermal systems, including Kawerau, Rotokawa, and Wairakei-Tauhara, can be summarized as follows: 1) intergranular host-rock porosity and permeability; 2) fault-fracture network permeability produced by tectonism, volcanism, and/or dike injection (magmatism); 3) pipe-like vertical conduits produced by volcanic and hydrothermal eruptions; and 4) hydrothermal alteration, mineral deposition and dissolution that may cause heterogeneity in the porosity and permeability of a fluid reservoir. Such controls influence fluid flow within three distinctive depth zones: 1) a feed zone (>2000 m depth), 2) a reservoir zone (