Investigating influences on hydrothermal fluid flow in the Taupo volcanic zone with numerical models constrained by magnetotellurics

Abstract

There are 23 high-temperature geothermal fields in the central Taup? Volcanic Zone (TVZ), discharging ~4200 GW of heat. Magnetotelluric (MT) surveys over large areas in the central TVZ have detected eight low-resistivity anomalies (i.e. plumes) in the brittle part of the crust.
These plumes are interpreted as upwelling hot geothermal fluids with high conductivity compared to the surrounding cold-watersaturated meta-sedimentary basement rocks.
The upper extents of these conductive plumes correspond well with the surface expressions of most geothermal fields, but not all.
We have used TOUGH2 numerical modelling software to explore regional influences on subsurface geothermal fluid circulation. We created simplified models with uniform geology and:
1. localised heat sources at 5 km depth as modelled from MT data.
2. surface elevation variations based on the water table (a muted reflection of topography).
3. a combination of influences 1 and 2.
We then compared resulting modelled zones of high temperature at 500 m depth with geothermal field boundaries as delineated by DC resistivity measurements.
Modelled upflow zones correspond extremely well with some of the known geothermal fields, such as Tikitere and Ngatamariki. In other areas, e.g. Waimangu and Rotokawa geothermal fields, modelled upflow zones are in the right geographic regions but have different lateral extents.
Beneath the Haroharo Volcanic Complex in the Okataina Volcanic Centre, a conductive plume and modelled upflow do not underlie any areas of hydrothermal activity at the surface. Here, we hypothesise that mapped or previously unidentified geological structures such as faults or volcanic domes may be influencing regional geothermal circulation. Some fields are not explained by MT anomalies or topographic effects, and these will be the subjects for future studies.