New (Zealand) Perspectives on Continental Arc Geothermal Systems – Overview and Future Prospects
Isabelle CHAMBEFORT, Colin J.N. WILSON, Julie V. ROWLAND
[GNS Science, New Zealand]
An enduring question in geothermal exploration and development is: what controls the longevities and positions of geothermal systems in areas of active volcanism and rifting? Can deep-seated crustal discontinuities focus the upwards transport of subduction-related volatiles and influence the compositional variability in magmatic and geothermal fluids? Rifting arc models of the Taupō Volcanic Zone (TVZ, North Island, New Zealand) are still challenged to link disparate observations, including deep seismicity, magnetotelluric (MT) models, the location and evolution of geothermal systems, magmatic and aqueous fluid compositions, caldera locations and the North Island tectonic environment. We here synthesise published geophysical, geological and geochemical studies to propose an integrated model and suggest future research avenues. The TVZ represents the on-land continuation of the Tonga-Kermadecs arc/back-arc system, marked in its central part by intense silicic volcanism and associated magmatism expressed as 23 high temperature ( more than 250 °C) geothermal systems. To accommodate the slightly oblique extension in the TVZ, the brittle crust is segmented, with accommodation zones orientated oblique to the arc. These accommodation zones may also be the expression of cross-arc magmatic migration as seen in the northern, offshore continuation of the arc structure in the Havre Trough. Published 3D magnetotelluric inversion models of several TVZ geothermal fields propose deep feeder zones ( more than 3-5 km depth) with a NW-SE cross-arc orientation. These magnetotelluric anomalies are interpreted as resulting from fluids (magma or aqueous) in the crust. We hypothesise that deep, long-lasting, NW-SE crustal discontinuities favour permeability in the proposed ductile crust. These discontinuities enable vertical mass transport from the mantle wedge, enhancing crustal melting and creating ridges on the sub-surface plane of the isotherm that is widely interpreted as the brittle–ductile transition. These ridges create loci for groundwater convective cells and explain the spatial persistence of many of the geothermal systems (despite interruption in some cases by caldera collapse) and the variability of geothermal fluid chemistry.
|        Topic: Geology||Paper Number: 12108|