THE DEEP CONTROLS ON HIGH ENTHALPY GEOTHERMAL SYSTEMS: A MULTI-DISCIPLINARY OVERVIEW FROM NEW ZEALAND

Abstract

An enduring question is: what controls the longevity and position of geothermal systems in areas of active volcanism and rifting? Can deep-seated crustal discontinuities focus the upwards transport of subduction-related melts and volatiles and influence the compositional variability in magmatic and geothermal fluids? Rifting arc models of the Taupo Volcanic Zone (TVZ, New Zealand) still are challenged to link diverse observations, including deep seismic anisotropy and tomography, 3D magnetotelluric inversions, the location and evolution of geothermal systems, magmatic and aqueous fluid compositions, locations of caldera development and the North Island tectonic environment. By reviewing and combining recent geophysical, geological, geochemical and structural studies we here consider an integrated model to suggest future research avenues.
The TVZ is an extensional arc, representing the on-land continuation of the Tonga-Kermadec arc/back-arc system, marked in its central part by intense magmatism associated with 23 high-enthalpy geothermal systems. To accommodate the slightly oblique extension, the brittle crust is segmented, expressed in the form of accommodation zones. These zones may be the expression of cross-arc magmatic migration as seen just north of the TVZ in the Havre Trough.
Three-D inversion magnetotelluric (MT) models of several TVZ geothermal fields show deep feeder zones perpendicular or oblique to the overall arc alignment below ~5 km depth. These MT anomalies have been interpreted as reflecting concentrations of fluids (magma or aqueous) in the crust. We hypothesise that deep, long-lasting, crustal cross-arc discontinuities (occurring in response to the oblique extension and caldera forming processes), favour permeability in the deeper, ductile crust. These discontinuities enable vertical mass transport, enhancing crustal melting and creating upwarped ridges on the surface representing the brittle–ductile transition. These ridges in turn create pinning loci for groundwater convective cells, and explain the persistence of many of the geothermal systems, despite interruption in some cases by caldera collapse and/or active faulting, and the variability of geothermal fluid chemistry and magma compositions.