Auckland, New Zealand
Auckland, New Zealand

Mighty River Power Limited is a New Zealand electricity generation and electricity retailing company. It was formed from the breakup of the Electricity Corporation of New Zealand in 1999 as a result one of the reforms of the New Zealand Electricity Market and corporatised to become a state-owned enterprise with its own board of directors and Ministerial shareholders. A state-owned enterprise from its founding, it was partially privatised by the Fifth National Government in May 2013, with the government retaining a 51.78 percent shareholding and the remaining 48.22 percent listed on the stock market.The company owns and operates the hydroelectric generating stations on the Waikato River as well as geothermal plants in the Taupo area, the gas fired Southdown plant in south Auckland and the largely unused plant at Marsden Point near Whangarei.In 2013, Mighty River Power generated 15% of the country's electricity and had a market share of 19%. According to its own website, the company supplies 22% of New Zealand peak energy demand, with about 80% of this coming from hydro-power. Wikipedia.

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Chambefort I.,Institute of Geological & Nuclear Sciences | Lewis B.,Institute of Geological & Nuclear Sciences | Simpson M.P.,Institute of Geological & Nuclear Sciences | Bignall G.,Institute of Geological & Nuclear Sciences | And 2 more authors.
Economic Geology | Year: 2017

The Ngatamariki geothermal system is one of more than 20 high enthalpy (>225°C) geothermal systems in the Taupo Volcanic Zone, North Island, New Zealand. At shallow levels (<2 km), they are analogous to lowintermediate sulfidation state epithermal ore-forming environments. Unique to Ngatamariki is the occurrence of an intrusive complex with an associated magmatic-hydrothermal alteration halo that was intersected by deep geothermal drilling (3 km depth) and that resembles hydrothermal alteration associated with high-intermediate sulfidation state epithermal mineralization. This paper describes the results of a study involving alteration petrography, X-ray diffraction, shortwave infrared (SWIR) reflectance spectroscopy, backscattered electron scanning electron microscopy/energy dispersive spectroscopy (BSE-SEM/EDS), plus whole-rock and trace element geochemistry to document and characterize distinct hydrothermal alteration found nowhere else in the Taupo Volcanic Zone. Two separate phases of hydrothermal activity are distinguished, old and modern, as defined by a paleosurface that is dated at 0.68 Ma, which occurs 500 m below sea level. A composite plutonic body comprising intrusions of diorite to tonalite was encountered in three adjacent drill holes (NM4, NM8, and NM9) between 2,000 and 3,200 m below sea level (2,300-3,500 m depth below the surface) in the northern part of the Ngatamariki system. The associated hydrothermal alteration is zoned and made up of potassic, advanced argillic, phyllic, and propylitic mineral assemblages that occur between 500 and 2,500 m below sea level. Subtle potassic alteration consisting of biotite + magnetite ± K-feldspar mantles the intrusive complex. It is crosscut by a hypogene advanced argillic alteration containing pyrophyllite ± minor andalusite ± topaz ± anhydrite ± rare aluminophosphates (AP) and fluorine-bearing minerals, but lacks alunite. The deep-formed advanced argillic alteration appears in some samples to be overprinted by phyllic alteration, made up of quartz + muscovite + pyrite. Between 500 and 1,000 m below sea level, the intense phyllic alteration is less pervasive, and the hydrothermal alteration is dominated by kaolinite, rare dickite, and localized occurrences of highly silicified rocks that resemble vuggy quartz, which is bounded at the top by the paleosurface (defined as the unconformity at the base of the overlying Whakamaru group ignimbrite). In the central and southern part of the system below the paleosurface, propylitic hydrothermal alteration consisting of chlorite + calcite + epidote ± wairakite ± actinolite (along with ± albite and ± illite) is widespread, and could equally have formed during old hydrothermal activity associated with emplacement of the intrusive complex or in the deep hot parts of the modern hydrothermal system. The mineralogy and geochemistry of advanced argillic altered rock indicates that acidic magmatic-hydrothermal fluids leached base cations, resulting in the loss of elements typically considered immobile (Al, Ti, Y, Zr, Nb, as well as rare earth elements) to form cation-depleted minerals such as pyrophyllite, andalusite, and topaz. The strongest enrichments in Au (0.6 g/t), Ag (4.6 g/t), Te, As, Sb, and Bi coincide with intense acid alteration at <250-m depth beneath the paleosurface. The results of this study reveal a complex history of intrusion and hydrothermal activity that provides a modern example of successive development (600,000 years apart) of acid and neutral pH hydrothermal alteration assemblages, which are associated with the two end member types of epithermal mineralization. © 2017 Society of Economic Geologists, Inc.

Milicich S.D.,Victoria University of Wellington | Milicich S.D.,Institute of Geological & Nuclear Sciences | Wilson C.J.N.,Victoria University of Wellington | Bignall G.,Institute of Geological & Nuclear Sciences | And 4 more authors.
Journal of Volcanology and Geothermal Research | Year: 2013

Crystallisation-age spectra have been obtained by SIMS techniques (SHRIMP-RG) on zircons from altered volcanic units penetrated by drillholes at Kawerau Geothermal Field in the central Taupo Volcanic Zone (TVZ), New Zealand. Drillholes penetrate 700-1300. m of volcanic rocks and sediments before reaching the basement Mesozoic greywacke. Twenty-seven samples of altered volcanic lithologies and two surficial, fresh rock units have been studied in order to constrain ages of the major stratigraphic units. Within the volcanic/sedimentary pile the oldest in-situ ignimbrites that can be widely correlated have ages of ~. 1.45. Ma. Between them and the basement greywacke is a variable thickness of sediments, mostly greywacke gravels and minor volcaniclastic units, reflecting localised basinal deposition associated with strike-slip faulting. Two ignimbrites within this sequence yield age estimates of c. 2.4 and 2.1. Ma, consistent with these being distal southern Coromandel Volcanic Zone deposits, pre-dating TVZ activity. Below the regional marker plane of the 0.32. Ma Matahina ignimbrite, three main ignimbrite groups occur, with ages around 1.45. Ma, 1.0. Ma and 0.6-0.5. Ma, which are separated by sediment-dominated intervals and andesite volcanics. All of these ignimbrites represent marker horizons from other volcanic centres and do not reflect the presence of local magmatic heat sources.Numerous bodies of coherent rhyolite, previous labelled as Caxton and Onepu rhyolites, have been intersected at all pre-Matahina ignimbrite levels (including within the basement greywacke) and reflect earlier local magmatic heat sources. Our geochronological data resolve these rock bodies into three packages. The youngest is represented by the surficial rhyodacite Onepu domes, 40Ar/39Ar dated at 0.138±0.007Ma. U-Pb ages on zircons from dome material yield a spectrum that can be matched (consistent with petrography) with two dikes intersected at 880m and 2.67km depth, and with an estimated age of 0.15±0.01Ma (Onepu Formation). The older two packages consist of older crystal rich (~15%) and younger crystal-poor (~5%) rhyolite, here grouped as Caxton Formation and with eruption/intrusion age of 0.36±0.03Ma. The shallowest Caxton rhyolite bodies are interpreted to be domes, whilst deeper intersections are inferred to be sills based on the lateral extents relative to thicknesses.Net subsidence rates inferred from depths to key units do not reflect the present-day situation. Modern rates of subsidence (2 ± 1. mm/yr) associated with TVZ rifting processes can have been active for no more than ~. 50,000. years, based on elevation differences of the Matahina ignimbrite top surface. An inferred change in intrusion geometry from sill (Caxton) to dike (Onepu) indicates a change in principal stress orientations reflecting onset of the modern Whakatane Graben. This change is dated at ~. 0.37. Ma in coastal sedimentary sequences 23. km to the north of Kawerau, consistent with our age data. Although previously interpreted to be a long-lived system, the modern Kawerau Geothermal Field is a Holocene entity reflecting the rejuvenation of magmatic heat flux associated with Putauaki volcano superimposed on an area of multiple reactivated fault structures, sporadic magmatism and variable rates of subsidence. © 2012 Elsevier B.V.

Cole J.W.,University of Canterbury | Spinks K.D.,Mighty River Power | Deering C.D.,University of Canterbury | Nairn I.A.,45 Summit Road | Leonard G.S.,Institute of Geological & Nuclear Sciences
Journal of Volcanology and Geothermal Research | Year: 2010

The Okataina Volcanic Centre (OVC) contains the northeasternmost caldera complex in onshore Taupo Volcanic Zone (TVZ), New Zealand, sited largely within the Taupo Rift. Pyroclastic fall deposits and ignimbrites deposited in the Bay of Plenty coast area between 420 and 625 ka were probably erupted from OVC, and these provide a maximum age for the centre. The earliest ignimbrite for which there is good evidence for eruption from OVC is the c.550 ka Quartz-biotite Ignimbrite, exposed to the west of Lake Okataina and to SE of OVC. This ignimbrite probably correlates with one of the early ignimbrites found in the Kawerau geothermal wells, and is large enough to have been accompanied by caldera collapse. It was followed by extensive rhyolitic explosive eruptions of the Murupara Subgroup, culminating with eruption of the Matahina Ignimbrite at c.325 ka. This c.160 km3 (magma volume) eruption was accompanied by caldera collapse to form the southern part of the present day Okataina caldera complex. A long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of OVC, before eruption of the > 100 km3 Rotoiti Pyroclastics at c.61 ka was accompanied by caldera collapse on the northern side of the centre. The Rotoiti episode was followed by an intensive period of intra-caldera volcanic activity which is still going on today. The Mangaone Subgroup pyroclastics were erupted between 40 and 31 ka, and include the c.33 ka Kawerau Ignimbrite (∼ 20 km3), large enough to have caused further minor caldera collapse. In the last 26 ka, nine rhyolite eruption episodes have built the Haroharo and Tarawera lava and pyroclastic massifs (> 85 km3 magma volume) within the caldera complex. The structural boundaries of the OVC calderas are buried by the products of later eruptions, but are probably controlled by regional tectonic features. Both the Matahina and Rotoiti calderas appear to have embayments which represent downsags where magma has migrated along regional structures associated with the Taupo Rift. OVC is sited at a major offset within the young Taupo Rift and represents a structurally complex transfer zone. Some early rhyolite domes are aligned north-northwest suggesting control by structures in the subvolcanic basement, while more recent domes are aligned northeastwards, reflecting the orientation of the Taupo Rift. Southwestward propagation of the axial rift of the Whakatane segment and northeastward propagation of the Kapenga segment have created two linear vent zones through OVC (Haroharo and Tarawera). At Tarawera, fissures and near surface dikes formed during the 10 June 1886 basalt eruption are oblique to the vent lineation suggesting some near surface strike-slip component consistent with OVC being in a zone of transtension. © 2009 Elsevier B.V. All rights reserved.

News Article | March 22, 2016

The Australian CleanTech Index rose from 42.97 to 43.89 over the month of February recording a 2.2% gain. This compared to the S&P ASX200 loss of 2.5% and the S&P ASX Small Ordinaries Index gain of 0.7%. The Australian CleanTech 20 rose 2.3% for the month. The CleanTech Index continues to outperform the wider market over each of the longer periods reported in the table below. The 12-month performance now leads the ASX200 by 11.1%. The best performing sub-indices for the month were the Australian Renewable Energy Index with a 3.7% gain and the Australian Waste Index with a 3.6% gain. The weakest sub-index through February was the Australian Environment Index recording a loss of 4.0%. The market capitalisation of the 62 stocks in the Australian CleanTech Index is A$16.7 billion down from the March 2015 record of $18.9 billion but a long way up from its low of A$6.2 billion in July 2012. The month’s performance included 9 companies with gains of more than 10%. The greatest percentage gains were recorded by HRL Holdings (HRL), Neometals (NMT) and RedFlow (RFX). The greatest market capitalisation gain was recorded by Meridian Energy (MEZ). These gains were partially offset by 10 companies recording losses of more than 10% led by GO Energy Group (GOE), Kalina Power (KPO), Traffic Technologies (TTI) and Enerji (ERJ). The greatest market capitalisation loss was recorded by Mighty River Power (MYT).   Get CleanTechnica’s 1st (completely free) electric car report → “Electric Cars: What Early Adopters & First Followers Want.”   Come attend CleanTechnica’s 1st “Cleantech Revolution Tour” event → in Berlin, Germany, April 9–10.   Keep up to date with all the hottest cleantech news by subscribing to our (free) cleantech newsletter, or keep an eye on sector-specific news by getting our (also free) solar energy newsletter, electric vehicle newsletter, or wind energy newsletter.  

Amend B.,DNV GL | Yee J.,Mighty River Power
NACE - International Corrosion Conference Series | Year: 2013

The objective of this paper is to describe a structured process for response to unexpected metal loss in geothermal resource production. Application of the process is illustrated by describing a case history. The Kawerau geothermal power generating station in New Zealand uses pH modification to mitigate scaling in flashed brine piping and injection wells. After nine months of operation, significant metal loss was observed in various areas of the steel piping and pressure vessels. The metal loss was generally localized and most extreme in areas of high velocity, turbulence, two-phase flow and low pH. Mitigation of the metal loss included studies of the conditions responsible for the damage, the related process controls, evaluation of alternative alloys, and tests of various methods of optimizing the cost effectiveness of corrosion resistant alloys. In addition, inspection and monitoring methods were reviewed and revised. Ultimately, the evaluation program demonstrated that a combination of formats for using corrosion resistant alloys could be the most cost effective. Those formats included: • Stainless steel or NiCrMo weld metal overlays • Application of stainless steel plates and platelets welded to steel substrates in corrosion susceptible areas • NiCrMo alloys applied by thermal spraying (high velocity arc spraying) • Solid corrosion resistant alloys in selected areas © 2013 by NACE International.

Cole J.W.,University of Canterbury | Deering C.D.,University of Wisconsin - Oshkosh | Burt R.M.,University of Canterbury | Sewell S.,Mighty River Power | And 3 more authors.
Earth-Science Reviews | Year: 2014

The Okataina Volcanic Centre (OVC) is one of eight caldera systems, which form the central part of the Taupo Volcanic Zone, New Zealand. During its ~. 625. kyr volcanic history, which perhaps equates to ~. 750. kyr of magmatic history, the OVC has experienced two definite periods of caldera collapse (Matahina, ~. 322. ka, and Rotoiti, for which dates of 61 and 45. ka have recently been published), one probable collapse (Utu, ~. 557. ka) and one possible collapse (Kawerau, ~. 33. ka). Each collapse accompanied voluminous ignimbrite eruptions. Rhyolite dome extrusion and explosive tephra eruptions have occurred throughout the history of OVC.This paper reviews volcanological observations, and geochemical and geophysical data that provides evidence for the nature and evolution of the mid- to upper crustal magma system below OVC. The chemistry of the largely rhyolitic juvenile pyroclastic deposits and lavas (most with 73-78wt.% SiO2) is reviewed, together with evidence provided by plutonic and mafic lithic blocks found within some pyroclastic deposits to reconstruct reservoir development. Detailed studies of zircon crystals provide age control for the longevity of the supersolidus state of the magmatic system of the OVC, while geophysical measurements, in particular resistivity and magnetotelluric (MT) data, suggest the present day existence of partial melts at depths of between 8 and 15km.A comparison with older exposed high-level plutonic systems helps explain some of the features found in the erupted plutonic lithic blocks at OVC, and provides an indication of the potential longevity of the system. An integration of these disparate datasets allows a model to be developed in which an extensive, intermediate composition 'mush' zone occurs at 8-15. km depth, from which more silicic melt fractions periodically rise to higher level sill or laccolith-like 'pods' in the crust. Sometimes one of these pods may erupt to produce lava or pumice of a single composition, while at other times a number of pods are tapped to form large-scale, caldera-forming eruptions. Periodically, the magmatic system reaches its solidus or near-solidus, which allows ascending basalt to reach the shallow magmatic system. In the last 50. kyrs, some of these basalts have reached the surface, for example during the 1886. AD fissure eruption from Tarawera volcano.A comparison with other active caldera complex systems in TVZ and overseas suggests that while the general model may apply, there are variations because of different tectonic setting, crustal thickness and age of the system. However, the general model has implications for geothermal reservoir evaluation and studies of epithermal ore deposition. The high crustal level magma system beneath OVC is probably part way through its evolution, so further intrusions and eruptions can be expected in the future, with clear implications for hazard evaluation. © 2013 Elsevier B.V.

Wyering L.D.,University of Canterbury | Villeneuve M.C.,University of Canterbury | Wallis I.C.,Mighty River Power | Siratovich P.A.,University of Canterbury | And 3 more authors.
Journal of Volcanology and Geothermal Research | Year: 2014

Mechanical characterization of hydrothermally altered rocks from geothermal reservoirs will lead to an improved understanding of rock mechanics in a geothermal environment. To characterize rock properties of the selected formations, we prepared samples from intact core for non-destructive (porosity, density and ultrasonic wave velocities) and destructive laboratory testing (uniaxial compressive strength). We characterised the hydrothermal alteration assemblage using optical mineralogy and existing petrography reports and showed that lithologies had a spread of secondary mineralisation that occurred across the smectite, argillic and propylitic alteration zones. The results from the three geothermal fields show a wide variety of physical rock properties. The testing results for the non-destructive testing shows that samples that originated from the shallow and low temperature section of the geothermal field had higher porosity (15 - 56%), lower density (1222 - 2114kg/m3) and slower ultrasonic waves (1925 - 3512m/s (vp) and 818 - 1980m/s (vs)), than the samples from a deeper and higher temperature section of the field (1.5 - 20%, 2072 - 2837kg/m3, 2639 - 4593m/s (vp) and 1476 - 2752m/s (vs), respectively). The shallow lithologies had uniaxial compressive strengths of 2 - 75MPa, and the deep lithologies had strengths of 16 - 211MPa. Typically samples of the same lithologies that originate from multiple wells across a field have variable rock properties because of the different alteration zones from which each sample originates. However, in addition to the alteration zones, the primary rock properties and burial depth of the samples also have an impact on the physical and mechanical properties of the rock. Where this data spread exists, we have been able to derive trends for this specific dataset and subsequently have gained an improved understanding of how hydrothermal alteration affects physical and mechanical properties. © 2014 Elsevier B.V.

Zarrouk S.J.,University of Auckland | Moon H.,Mighty River Power
Geothermics | Year: 2014

The conversion efficiency of geothermal power developments is generally lower than that of all conventional thermal power plants. Confusion can be found in literature concerning the estimation of this conversion efficiency. Geothermal power plants conversion efficiency estimates that is based on the enthalpy of the produced geothermal fluid can be the most desirable for use during the first estimates of power potential of new wells and for resource estimation studies.The overall conversion efficiency is affected by many parameters including the power plant design (single or double flash, triple flash, dry steam, binary, or hybrid system), size, gas content, dissolved minerals content, parasitic load, ambient conditions and other parameters.This work is a worldwide review using published data from 94 geothermal plants (6 dry-steam, 34 single flash, 18 double flash, 31 binary, 2 hybrid steam-binary and 1 triple flash plant) to find conversion efficiencies based on the reservoir enthalpy.The highest reported conversion efficiency is approximately 21% at the Darajat vapour-dominated system, with a worldwide efficiency average of around 12%. The use of binary plants in low-enthalpy resources has allowed the use of energy from fluid with enthalpy as low as 306. kJ/kg, resulting in a net conversion efficiency of about 1%.A generic geothermal power conversion relation was developed based on the total produced enthalpy. Three more specific correlations are presented for single flash/dry steam plants, double flash plants and binary plants. The conversion efficiency of binary plants has the lowest confidence, mainly because of the common use of air cooling which is highly affected by local and seasonal changes in ambient temperatures. © 2013 Elsevier Ltd.

Chambefort I.,Institute of Geological & Nuclear Sciences | Lewis B.,Institute of Geological & Nuclear Sciences | Wilson C.J.N.,Victoria University of Wellington | Rae A.J.,Institute of Geological & Nuclear Sciences | And 3 more authors.
Journal of Volcanology and Geothermal Research | Year: 2014

Recent drilling at the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand has provided new constraints on the stratigraphy and volcanic evolution of the region. Over 2800m thickness of volcanic products are present at Ngatamariki, mainly comprised of rhyolitic ignimbrites linked to large caldera-forming events at sources outside the field area, but locally sourced andesite and rhyolite lavas and domes are also encountered. Most of the rocks are allocated to the pre-0.35Ma Tahorakuri Formation. Crystallisation age spectra (and consequent best estimates of eruption age) have been obtained by U-Pb dating on zircons from otherwise severely hydrothermally altered magmatic rocks by Secondary Ion Mass Spectrometry techniques using a SHRIMP-RG instrument. The oldest rock dated is an ignimbrite, which yields an eruption age estimate of 1.85±0.06Ma. This ignimbrite, plus comparable-aged units dated at the adjacent Rotokawa and Ohaaki geothermal fields, are interpreted to represent the oldest silicic volcanic deposits in the area, and onlap the basal andesite lava pile that is best developed at Rotokawa to the south. Other pyroclastic units and associated volcaniclastic sediments (with another intercalated andesite lava unit) return age estimates between 1.85±0.06 and 0.701±0.039Ma. Between ~0.7 and 0.35Ma, contemporaneous surface lithologies in the Ngatamariki area are dominated by sediments, with subordinate lava domes. Between 0.716±0.017 and 0.655±0.016Ma at least three shallow (to<2km depth) intrusions were emplaced under the northern part of the field: a diorite, microdiorite and a large tonalite body totalling >5km3. The intrusions generated a large alteration halo (~25km3 minimum) and intense silicification of the wall rocks. At 0.35 and 0.34Ma the area was buried by two ignimbrite packages of the Whakamaru Group, erupted from sources just west and well north, respectively, of the field. Ignimbrites of the Waiora Formation and several rhyolite lava domes were then emplaced over a period bracketed by domes dated at 0.282±0.011 and 0.257±0.011Ma, coeval with more extensive volcanic activity in the Maroa dome complex west of the field. Sediments of the Huka Falls Formation and deposits of the 25.4±0.2ka Oruanui eruption then cap and seal the system. The new U-Pb data coupled with detailed petrographical studies allow us to build the history of the area encompassed by the Ngatamariki Geothermal Field. The field, despite >2.8km of subsidence, does not lie in a caldera and is the only one known to date to have a plutonic intrusive complex of Quaternary age. Two chemically and temporally distinct hydrothermal events are located at the Ngatamariki field, with no evidence of continuity between the two. © 2014 Elsevier B.V.

Milicich S.D.,Victoria University of Wellington | Milicich S.D.,Institute of Geological & Nuclear Sciences | Wilson C.J.N.,Victoria University of Wellington | Bignall G.,Institute of Geological & Nuclear Sciences | And 2 more authors.
Journal of Volcanology and Geothermal Research | Year: 2013

The utilisation of geothermal systems benefits from an understanding of the host-rock geology, locations and controls of permeability pathways, and the nature and timing of magmatic sources providing thermal energy. Kawerau Geothermal Field in the central Taupo Volcanic Zone (TVZ) of New Zealand is currently developed for electricity generation and direct uses of high-temperature steam to ~200MW electrical output. The Kawerau geothermal system is hosted in a sequence of volcanic lithologies (tuffs, lavas and intrusive bodies) and sediments that overlie faulted Mesozoic metasedimentary (greywacke) basement. Identification of lithologies in the volcanic/sedimentary sequence is challenging due to the levels of hydrothermal alteration and lithological similarities. A combination of detailed petrological investigations, consideration of the emplacement processes and greater certainty of crystallisation or eruption ages through U-Pb age determinations on zircons is used to reconstruct the depositional and faulting evolution of the rocks hosting the currently active hydrothermal system. The oldest event inferred is faulting of the greywacke along northwest-southeast orientated, dominantly strike-slip structures to generate half-grabens that were filled with sediments, incorporating two dated ignimbrites (2.38±0.05 and 2.17±0.05Ma). A 1.46±0.01Ma ignimbrite was deposited relatively evenly across the field, implying that any topographic relief was subdued at that time. Subsequent deposition of ignimbrites occurred in episodes around 1.0, 0.55-0.6, and 0.32Ma, interspersed with thin sedimentary sequences that accumulated at average rates of 0.06mmyr-1. Andesite lavas from a buried composite cone occur as a conformable package between units dated at 1.0 and 0.6Ma. Bodies of coherent rhyolite occur at multiple stratigraphic levels: two magma types with associated tuffs were emplaced as domes and sills at 0.36±0.03Ma, and a third type at 0.138±0.007Ma as dikes, and domes that are exposed at surface. The andesitic Putauaki composite cone southwest of the field first erupted around 8ka, but earlier hydrothermal eruption breccias imply that magma was intruded to shallow depths as early as ~16ka.Age data and associated correlations show that post-1.5. Ma normal faulting has accompanied episodic subsidence of the Kawerau area, with fault movement focused between northeast-southwest structures (associated with the geometry of the modern TVZ) and the reactivated northwest-southeast structures associated with most displacement in the area prior to 1.5. Ma. Contrasts between emplacement of coherent rhyolite as sills at 0.36. Ma and a dike at 0.138. Ma reflect a shift in orientation of the principal stress axes in response to initiation of the modern TVZ rifting regime. Most volcanic rocks at Kawerau are distally sourced from elsewhere in the TVZ but form local marker horizons that delineate topographic relief within the field, and additionally constrain past subsidence rates. Current rates of subsidence and thermal output at Kawerau are geologically recent features associated with latest Quaternary rifting processes (<. ~. 50. ka) and emplacement of the magmatic system for Putauaki volcano (~. 16. ka) respectively. © 2013 Elsevier B.V.

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