Using array MT data to image the crustal resistivity structure of the southeastern Taupo Volcanic Zone, New Zealand
Introduction
The Taupo Volcanic Zone (TVZ), located in the central North Island of New Zealand, is an actively spreading, rifted volcanic arc, and is the on-land extension of the Tonga–Kermadec arc to the northeast (e.g. Seebeck et al., 2014). Since ~ 1.8 Ma ago (Eastwood et al., 2013) extension in the central TVZ has stretched and down-faulted the former surface rocks, comprised of Mesozoic meta-sediments (greywacke) and overlying (andesitic) arc-volcanics. In the past few decades, geothermal production wells drilled along the southeast margin of the TVZ have intersected these Mesozoic greywackes and arc-volcanics at progressively greater depths to the northwest, with a maximum intersection at 3.4 km depth near the Ngatamariki geothermal field (Horton et al., 2012). Extensive layers of rhyolitic ignimbrite and volcaniclastics from numerous caldera-forming eruptions since ~ 1.8 Ma ago now overlie this ‘basement’ of Mesozoic greywacke (Houghton et al., 1995, Wilson et al., 1995, Wilson et al., 2009, Wilson and Rowland, 2011, Chambefort et al., 2014). Very few andesitic volcanics remain exposed at the surface within the central TVZ, where high-silica rhyolite lava domes, thick ignimbrite sequences, volcaniclastics and Quaternary sediments blanket the older arc-volcanoes below.
Since ~ 0.34 Ma ago, the central TVZ has been the most productive silicic volcanic system on Earth (e.g. Houghton et al., 1995), and has an exceptionally high heat flux of 700 mW/m2 (Bibby et al., 1995) that is more than 10-times the global mean value (65 mW/m2) for continental crust (Pollack et al., 1993). This heat flux is discharged at the surface through 23 localised high-temperature geothermal fields. The conceptual model of heat transport in the central part of the TVZ envisages these geothermal fields as the tops of vertical convection plumes of high-temperature fluid at near-hydrostatic pressure in the brittle part of the crust (e.g. Bibby et al., 1995 and references therein). Numerical modelling of convection in homogeneous media (McNabb, 1975, Kissling and Weir, 2005, McLellan et al., 2010) and other geological and geophysical evidence (e.g. Grindley, 1965, Browne, 1979, Simmons et al., 1993, Bibby et al., 1994) suggests that the locations of the geothermal fields, and hence the underlying plumes have been stable for periods of at least 0.2 Myr. Convection is driven by a basal layer of partial melt located below the brittle–ductile transition (Heise et al., 2007), which is associated with the cut-off of shallow seismicity at ~ 6–7 km depth (Bibby et al., 1995, Bryan et al., 1999). Between the geothermal fields, upward conductive heat flow is suppressed by down-flowing, cold meteoric water that recharges the hydrothermal systems. Importantly, this conceptual model does not require a direct relationship between individual geothermal fields and localised sources of heat (e.g. magmatic intrusions).
The lateral extent of the geothermal fields at shallow depth (i.e. ~ 300 m) is well-delineated by zones of low-resistivity, identified from ~ 24,000 direct-current (DC) resistivity measurements (Bibby, 1988). More recently, inversion modelling of magnetotelluric (MT) measurements in the southeast TVZ has imaged ‘plume-like’ low-resistivity zones within the basement rocks that connect to the shallow geothermal fields (Bertrand et al., 2012). These low-resistivity zones have been interpreted as high-temperature saline fluids upwelling in fractures, supplying heat (and fluids) to the geothermal fields above, consistent with the overall model of convective heat flow.
However, heterogeneity in the MT models suggests that other influences (e.g. geological structure and magmatic intrusions) also play an important role. For example, a vertical low-resistivity zone is imaged directly beneath the Rotokawa geothermal field (Bertrand et al., 2012), while at Ohaaki, a deep low-resistivity zone is offset and dips to the northwest (Bertrand et al., 2013), suggesting a pathway of preferential permeability associated with pre-existing rift-faults or a collapse feature. Although problematic from the point-of-view of a (vertical) convective driving force, an elongated low-gravity anomaly centred northwest of Ohaaki (the Mihi Volcanic Depression; Soengkono, 2012) may provide the structural (permeable) pathway that causes the low-resistivity offset.
Furthermore, while the locations of the geothermal fields appear to be stable, geological evidence suggests that the heat output of several TVZ geothermal fields have episodically waxed and waned in response to nearby magmatism (e.g. Arehart et al., 2002, Milicich et al., 2013). For example, a 0.7 Ma old intrusive complex (Chambefort et al., 2014) with an overlying halo of magmatic–hydrothermal alteration was intersected at ~ 2.6 km depth beneath the Ngatamariki geothermal field (Browne et al., 1992). This intrusion is much too old to represent the heat source for the present-day geothermal system, and thus implies that at least two episodes of hydrothermal activity have occurred at this location in the past 0.7 Ma.
Here we use data from 169 MT soundings to construct a 3-D resistivity inversion model of a 700 km2 area in the southeast TVZ (Fig. 1). These MT data were collected as part of a research programme to study processes that transport heat through the brittle part of the crust, targeting depths between 3 and 7 km; below drilled depths and above the base of the seismogenic zone, respectively (Bignall, 2010). These MT measurements encompass the Rotokawa, Ngatamariki, Ohaaki, Orakei-Korako and Te Kopia geothermal fields, and several major geological features in the TVZ: the southeast rift margin, parts of the Whakamaru and Reporoa collapse calderas, the Paeroa Fault, and the Maroa Volcanic Centre (a group of young 280–16 ka rhyolite domes; Leonard et al., 2010). The 3-D inversion model images the crustal resistivity structure beneath the 700 km2 study area to a depth of ~ 10 km, extending earlier DC-resistivity surveys that mapped the near-surface electrical resistivity.
Section snippets
MT data and dimensionality
Five-component broadband (0.01 s < period < 1000 s) MT measurements were made at ~ 2 km spacing to form a regular array in the southeast TVZ (Fig. 1). All measurements were made using Phoenix Geophysics MT recorders and magnetic field sensors, with lead chloride electrodes forming ~ 70 m dipoles. Electromagnetic field time-series data were measured for a minimum of 40 h (2 night's duration) at each site. These time-series data were all processed using a remote reference (Gamble et al., 1979), located at a
3-D inversion modelling
The 3-D inversion algorithm WSINV3DMT (Siripunvaraporn et al., 2005) was used to generate smooth resistivity inversion models from the impedance tensor data (i.e. both diagonal and off-diagonal components) at the 169 MT-sites shown in Fig. 1. Impedance data at 18 periods (between ~ 0.01 s and 1000 s) were inverted, using a model mesh comprised of 3-D blocks with 500 m lateral dimensions (i.e. a quarter of the station spacing) covering the survey area. Block dimensions increase laterally outside of
Shallow resistivity structure
The standard electrical resistivity model of a high-temperature, liquid-dominated geothermal system includes a low-resistivity ‘clay-cap’ located above a deeper, more resistive geothermal reservoir (e.g. Simmons and Browne, 1990, Johnston et al., 1992). The clay-cap is formed by hydrothermal alteration of young volcanics to smectitic clays, and the deeper reservoir reflects the transition to more resistive illitic clays at temperatures above ~ 200 °C (Ussher et al., 2000). However,
Conclusions
We have constructed a comprehensive 3-D inversion model of the electrical resistivity structure in the upper crust of the southeastern TVZ using 169 broadband MT measurements. Localised low-resistivity (< 30 Ωm) zones observed within the basement rocks beneath the volcanic infill of the TVZ require the presence of partial-melt and/or interconnected saline fluid. At Rotokawa, where thermally induced seismicity (marking the transition from hydrostatic to lithostatic pressure) occurs at the top of a
Acknowledgements
Cooperation from landowners in the survey area is greatly appreciated and we thank Weerachai Siripunvaraporn for making his 3-D inversion programme WSINV3DMT available. We also thank Peter McGavin for his assistance implementing the parallel version of WSINV3DMT. This work was supported by GNS Science's core-funded geothermal research programme, with additional funding from the Waikato Regional Council (WRC). In particular, we would like to acknowledge Dr. Jim McLeod (WRC) for his support of
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2022, Journal of Volcanology and Geothermal ResearchCitation Excerpt :However, contemporaneous deformation indicates that the present day TVZ is largely undergoing contraction (Holden et al., 2015, Haines and Wallace, 2020, Dimitrova et al., 2016, Hamling et al., 2015). The source of the contraction has been previously explained by the cooling and contraction of magma at depth (Hamling et al., 2015; Holden et al., 2015) consistent with low degree partial melt, imaged by Magnetotellurics (Heise et al., 2010; Bertrand et al., 2015), and seismic velocity models suggesting a heavily intruded shallow crust beneath the central TVZ (Harrison and White, 2004, 2006; Stratford and Stern, 2004, 2006). Recent activity within the TVZ has been focussed within its northern and southern limits where andesitic-dacite volcanism dominates (Wilson and Rowland, 2016) with eruptions at Ruapehu (1994–96, (Bryan and Sherburn, 1999)), Tongariro (2012, (Jolly et al., 2014)) in the south and from White Island (2016, 2019, (Hamling, 2017, 2021)) in the north.