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Example of active fault scarp (Paeroa Fault). 2.2. 050-065° trend (Figure 1) Active faults north of the Ngakuru Graben strike 050-065°, approximately 20°E of the dominant rift trend, and intersect the margins of the Okataina Volcanic Centre (OVC). The intersection zone between 030-045° and 050-065° trending faults results in a highly complex zone of cross cutting and intersecting fault geometries (Fig. 3). This fault trend is dominant in the northern-most extent of the onshore rift across the Rangitaiki Plains.
Source publication
The bulk permeability of upper crustal rocks in the Taupō Rift, central North Island, New Zealand is a function of the development and intersection of active and inherited faults, caldera collapse structures, and/or lithology. To date, major fault trends within the rift have been described as: 1) 030-045°, the dominant rift trend of north of Lake T...
Citations
... Throughout the TVZ, surface geothermal activity appears to occur between mapped faults as often as on them ( Fig. 1; Rowland and Sibson, 2004). Although active faults intersect the boundaries of the OVC, there is an absence of known faulting in large areas within it (Villamor et al., 2017) making any relationship between faulting and geothermal features difficult to assess. However, the Waimangu-Rotomahana-Tarawera geothermal system (from here on referred to as the Waimangu geothermal system) is located at the intersection of the OVC boundary and a highly faulted region associated with the Tarawera Linear Vent Zone (Fig. 1). ...
The Okataina Volcanic Centre (OVC) in the Taupo Volcanic Zone (TVZ) is New Zealand's most recently active caldera and lava dome complex, erupting 85 km3 of magma equivalent in the past 21 ka. Successive caldera collapses over the past ~550 kyr have created a complex crustal structure through which fluids now migrate, evidenced by hot springs and several geothermal fields located predominantly around the caldera margins. While recent magnetotelluric (MT) modelling suggests that high-temperature upwelling fluids originate from mid-crustal locations at the margins of an inferred magmatic complex, it is still uncertain to what degree various factors influence these fluid pathways from source to surface. Here, we use heat and fluid flow models to explore the roles that simplified geological structures, topography, and deep, localised heat sources play in determining the locations of upflow at the OVC.
To isolate and understand individual processes that modulate fluid flow, we created TOUGH2 heat and fluid flow models that encompass a 35 by 39 km2 area extending beyond the topographic boundaries of the OVC calderas. An average heat input of 700 mW/m2 was set at the base of the models at 5 km depth, either as a uniform hot plate or as discrete zones of higher heat input with locations derived from MT models. The upper boundary of the models followed water table elevation, which is a slightly muted reflection of topography. We constructed a suite of models, ranging from uniform rock properties to more complex permeability variations that correspond to basement/volcanic rocks and large-scale faults inferred from gravity data and surface mapping. Models ran for 50 kyr, the approximate time since the most recent caldera-forming event. Elevated model temperatures were compared with low-resistivity zones from shallow (<500 m) Direct Current (DC) apparent resistivity values and deeper sensing MT resistivity models.
Our models suggest that topography and localised basal heat sources are the largest influences on the locations of geothermal upflows. In the western half of the study area, observed upflows beneath Waimangu and Tikitere can be modelled with localised heat sources and topography alone. Upflow under Rotoma to the north-east is not replicated by any of our suite of models, suggesting that unmapped heat sources are important in this area. In the east of the study area, modelled upflow is dominated by present-day topography which is strongly influenced by <25ky Tarawera and Haroharo Volcanic Complexes and < 1ky Tarawera River valley. Our models suggest that strong permeability contrasts associated with large-scale faults would have effects on regional-scale fluid circulation that are not observed in real life, and that the coincidence of OVC topographic boundaries and geothermal systems is due to the boundaries' also being topographic lows rather than fluid conduits.
... The TVZ geothermal systems are predominantly within the Taupo Reporoa Basin, or are to the northwest of the Taupo Fault Belt (Figure 1). Only a few of the geothermal systems appear to be associated with major faults, for example at Waiotapu, Te Kopia, and Orakei Korako (Villamor et al., 2017). ...
Plain Language Summary
Geothermal systems are a clean, renewable energy source. They are created by water that is heated at several kilometers depth and rises to the surface in distinct “plumes.” We don't know why the plumes rise where they do or why they seem to stay in the same place for hundreds of thousands of years. We used specialized computer code to look at whether topography can keep these plumes in place. With uniform geology and heat flow at depth, our simplified models show that high topography can drive water downwards even at 3 km depth. The geothermal systems form between these highs, in valleys and topographical lows. Faults and localized heat sources will also be playing a part, but topography is known accurately and can explain the locations of 14 of the 19 geothermal systems in the Taupo Volcanic Zone, New Zealand. Before, it was thought that topography mainly affected nearby shallow fluid flow. Future geothermal exploration and modeling of warm water extraction should take these results into account.
... This unique setting and the resulting thick sequences of pyroclastic deposits from the intense volcanic activity favours the convection of descending cold groundwater. The regional active faults, defining the Taupo rift, cutting through the volcanic sequence and underlying metasedimentary greywacke are inferred to favour deep penetration of the meteoric water in the crust close to the partial melt area ( Villamor et al., 2017). The meteoric water is then heated and convects toward the surface, along with some magmatic fluids, underneath focussed (5-25 km 2 ) hydrothermal surface expressions ( Fig. 1). ...
... What controls the location and the deep heat source of the geothermal systems is thus likely to be linked to structures or pathways beneath the brittle crust. This inference is in agreement with recent structural investigations ( Villamor et al., 2017), which show that while many active systems are located at fault intersections, not all intersections are favoured by hot plumes, implying that deep permeable structures are necessary. We hypothesise that deep, longlasting, crustal cross-arc discontinuities (occurring to accommodate the oblique extension and caldera volcano processes), favour permeability in the ductile crust. ...
... Downward recharge of cool meteoric water is enhanced by the abundant faulting of the Taupo Rift as suggested by Villamor et al. (2017). These waters, once heated, react with the volcanic sequence and greywacke basement rocks resulting in an enrichment in sulfur and chlorine (as suggested by Mountain et al., 2017). ...
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.
