S. L. Bennie

Fluid and deformation regime of an advancing subduction system at Marlborough, New Zealand

Newly forming subduction zones on Earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes. The transpressional subduction regime of New Zealand, which is advancing laterally to the southwest below the Marlborough strike–slip fault system of the northern South Island, is an ideal setting in which to investigate these processes. Here we acquired a dense, high-quality transect of magnetotelluric soundings across the system, yielding an electrical resistivity cross-section to depths beyond 100 km. Our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops. Massive fluid release just inland of the trench induces fault-fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates. Narrow strike–slip faults in the shallow brittle regime of interior Marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones. Deep subduction-generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high-angle thrusts that are poorly oriented for failure under dry conditions. The fluid-deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models.

Wide‐band magnetotelluric measurements across the Taupo Volcanic Zone, New Zealand‐Preliminary results

The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by intensive geothermal activity and frequent rhyolitic volcanism. Sixteen wide-band (0.01-1,800 s) magnetotelluric soundings were measured along a 110 km-long profile approximately perpendicular to the strike of the TVZ. A model obtained from 2D inversion of the soundings shows two near-surface regions of high conductance which correspond to low density volcaniclastic sediments, up to 3 km thick, which infill a sequence of collapse calderas. At deeper levels (approximately 5-10 km) a resistive layer underlies the entire TVZ. Modelling shows other conductive zones occur beneath the TVZ, with the shallowest lying below the central part at a depth of 10 -15 km. Given the high heat flux and volcanic history of the TVZ, the high conductivity at depth may indicate the presence of connected melt. At greater depth (20-30 km) the upper mantle beneath the TVZ appears to be anomalously conductive, consistent with observed high seismic attenuation.

Resistivity structure of the Waimangu, Waiotapu, Waikite and Reporoa geothermal areas, New Zealand

Abstract Electrical resistivity measurements have been used to investigate a 650 km2 region containing four separate areas of geothermal surface activity: Waimangu, Waiotapu, Waikite and Reporoa. Measurements were made using the Schlumberger array with fixed spacings (AB/2) of 500 m (1167 measurements) and 1000 m (720 measurements). The data outline an area of over 100 km2 where near surface electrical resistivity is less than 30 Ωm; the most extensive low resistivity region known in the Taupo Volcanic Zone (TVZ). Within this large area, three smaller zones of resistivity less than 10 Ωm can be distinguished which contain the geothermal activity of Waimangu, Waiotapu and Reporoa. A fourth area of surface geothermal activity, Waikite, lies about 5 km to the west of the Waiotapu Springs, and is separated from them by the high elevation Paeroa Range. Interpretation of the resistivity measurements suggests a single deep source for the geothermal fluid discharged at both Waiotapu and Waikite, that lies beneath the high elevation ground. This Waiotapu-Waikite system straddles a northwest-southeast drainage divide, and thus subsurface movement of geothermal waters occurs to both the north and south, masking the northern and southern extent of the deeper parts of the system. The Waimangu Geothermal Field, as outlined by the resistivity data, is much more extensive than indicated by the distribution of surface features. The field is characterised by abrupt resistivity discontinuities to the north, west and east, with the northern boundary crossing Lake Rotomahana. It is not possible to identify a southern boundary to Waimangu because of the masking effects of the near surface movement of geothermal fluids from Waiotapu. The resistivity signature of the Reporoa thermal area is not consistent with the commonly accepted model in which the Reporoa thermal waters originate as groundwater drainage from Waiotapu. The sharp boundaries to the resistivity anomaly, and the resistivity variation with depth at Reporoa, are quite different from the region between the Reporoa and Waiotapu. These characteristics, observed in many geothermal systems in the TVZ, suggest that the resistivity anomaly marks an independent geothermal system at Reporoa.

Electrical resistivity and magnetic investigations of the geothermal systems in the Rotorua area, New Zealand

Electrical and magnetic data are used in an investigation of a 450 km2 region in order to delineate the Rotorua City Geothermal system and determine its relationship with other geothermal systems in the region. Electrical resistivity mapping of the region has been carried out using Schlumberger array measurements with nominal current array (half) spacings of 500 m (586 measurements) and 1000 m (410 measurements). In addition continuous profiling using an equilateral dipole array provides close spaced data along 6.5 km of profile in Lake Rotorua. Modern draped aeromagnetic data (200 m line spacing, 60 m ground clearance) has been made over about 100 km2 covering two areas. Where aeromagnetic data could not be obtained, these data have been augmented with previously measured ground magnetic (vertical force) and waterborne (total field) data. Three distinct regions of low (< 30 Ωm) apparent resistivity are delineated. The southern of these outlines the Rotorua City Geothermal System which has an area of about 18 km2, with the northern third covered by Lake Rotorua. The boundary of the system is characterised by a rapid lateral change in apparent resistivity which can be modelled as a single, near vertical zone in which the distance between hot and cold water is very narrow. Magnetic properties also change in the vicinity of the discontinuity in some areas, consistent with hydrothermal alteration having destroyed the magnetite in the rocks of the geothermal system. Hot water is believed to be rising, driven by buoyancy forces across the whole of the low resistivity region. There is some indication, particularly in the south, that the boundary between hot and cold fluids dips away from the field. A second low resistivity zone (the East Lake Rotorua anomaly) with an area of about 8 km2, is believed to outline a second independent geothermal system, with surface manifestations on Mokoia Island, and on the eastern shore of the lake. High heat flow in lake bottom sediments, and a reduction in magnetic signature over this region supports this conclusion. A third resistivity low under the west of Lake Rotorua has no associated thermal features and is believed to be a fossil hydrothermal system. There is no apparent relationship between the location of the geothermal systems and the Rotorua caldera. The aeromagnetic measurements have delineated several highly magnetic bodies which cannot be linked with surface geology. These are believed to be caused by buried rhyolite dome complexes at shallow depth.

Appraisal of the Tokaanu-Waihi geothermal field and its relationship with the Tongariro geothermal field, New Zealand

Abstract Tokaanu–Waihi geothermal field is situated near the southern end of the Taupo Volcanic Zone, New Zealand. Neutral chloride thermal waters discharge at Tokaanu and Waihi in the north of the field on flat land between the andesite volcanoes Tihia and Kakaramea and the shore of Lake Taupo, while steam-heated thermal features occur at Hipaua on the northern flanks of Kakaramea. Electrical resistivity surveys have been made over the field using several different measurement techniques. In the north of the field where roads and tracks allow vehicle access, resistivity profiling using Schlumberger arrays with electrode spacings (AB/2) of 500 m and 1000 m show that Tokaanu, Waihi and Hipaua all lie within a continuous region of low apparent resistivity (5–20 Ωm) and are thus part of the same geothermal system. Along the eastern edge of the system there is a sharp transition to apparent resistivities greater than 100 Ωm in the cold surrounding region. Surveys on Lake Taupo using an equatorial bipole-bipole electrode array towed behind boats (spacing equivalent to AB/2=500 m) found that the low resistivity zone extends offshore by about 1 km. The steep, bush-clad, southern part of the field was surveyed with magnetotelluric (MT) resistivity measurements using both naturally occurring signals and the 50 Hz radiation from the power wires as sources. These measurements found low resistivities over the north-eastern slopes and around the summits of Tihia and Kakaramea, indicating thermal activity. However, the measurements were too widely spaced to allow the field boundary to be clearly delineated. Interpretation of the resistivity and other data suggests that the Tokaanu–Waihi thermal waters rise nearly vertically from a source deep beneath the elevated southwestern part of the field to the water table. These waters then flow north to discharge at the surface near Lake Taupo. Neighbouring geothermal systems, which occur at Tongariro about 18 km south of Tokaanu–Waihi, and at Motuoapa about 10 km to the northeast, are separated from the Tokaanu–Waihi field by high resistivity ground. This suggests that the thermal fluids discharging at the three fields do not have a common source, as has been suggested previously.

Mapping subduction interface coupling using magnetotellurics; Hikurangi Margin, New Zealand

The observation of slow-slip, seismic tremor, and low-frequency earthquakes at subduction margins has provided new insight into the mechanisms by which stress accumulates between large subduction (megathrust) earthquakes. However, the relationship between the physical properties of the subduction interface and the nature of the controls on interplate seismic coupling is not fully understood. Using magnetotelluric data, we show in situ that an electrically resistive patch on the Hikurangi subduction interface corresponds with an area of increased coupling inferred from geodetic data. This resistive patch must reflect a decrease in the fluid or sediment content of the interface shear zone. Together, the magnetotelluric and geodetic data suggest that the frictional coupling of this part on the Hikurangi margin may be controlled by the interface fluid and sediment content: the resistive patch marking a fluid- and sediment-starved area with an increased density of small, seismogenic-asperities, and therefore a greater likelihood of subduction earthquake nucleation.

Geophysical techniques for low enthalpy geothermal exploration in New Zealand

Shallow warm water resources associated with low enthalpy geothermal systems are often difficult to explore usinggeophysicaltechniques,mainlybecausethewarmwatercreatesaninsufficientphysicalchangefromthehostrockstobe easily detectable. In addition, often the system also has a limited or narrow size. However, appropriate use of geophysical techniques can still help the exploration and further investigation of low enthalpy geothermal resources. We present case studies on the use of geophysical techniques for shallow warm water explorations over a variety of settings in New Zealand (mostly in the North Island) with variable degrees of success. Asimpleanddirectmethodfortheexplorationofwarmwatersystemsisshallowtemperaturemeasurements.InsomeNew Zealand examples, measurements of near surface temperatures helped to trace the extent of deeper thermal water. ThegravitymethodwasutilisedasastructuraltechniquefortheexplorationofsomewarmwatersystemsinNewZealand. Ourcasestudiesshowthetechniquecanbeusefulinidentifyingbasementdepthsandtracingfaultsystemsassociatedwiththe occurrence of hot springs. Direct current (DC) ground resistivity measurements using a variety of electrode arrays have been the most common method for the exploration of low enthalpy geothermal resources in New Zealand. The technique can be used to detect the extentofshallowwarmwatersthataremoreelectricallyconductivethanthesurroundingcoldgroundwater.Groundresistivity investigationsusingtheelectromagnetic(EM)techniquesofaudiomagnetotellurics(AMTorshallowMT),controlledsource audio magnetotellurics (CSAMT) and transient electromagnetic (TEM) methods have also been used. Highly conductive clays of thermal or sedimentary origin often limit the penetration depth of the resistivity techniques and can create some interpretation difficulties. Interpretation of resistivity anomalies needs to be treated in a site specific manner.