Manfred P. Hochstein

Crustal heat transfer in the Taupo Volcanic Zone (New Zealand): comparison with other volcanic arcs and explanatory heat source models

Abstract The Taupo Volcanic Zone (TVZ) is a 200-km-long volcanic arc segment which developed ≤2 Ma ago within the continental crust of the North Island of New Zealand and lies at the southern end of the much larger Tonga-Kermadec arc system. The total crustal heat transfer of the TVZ is at present c. 2600 MW/100 km, most of the heat being transferred by convective geothermal systems. The rate of transfer is anomalously high in comparison to that of other active arcs, and arguably the highest world wide for such a setting. Heat transfer of other active arcs appear to vary almost linearly with subduction speed (about 150 MW/100 km for 10 mm/yr). The mass rate of common type arc extrusions (basalts, andesites, dacites) also increases almost linearly with subduction speed. This allows separation of the TVZ heat transfer into a “normal” component, associated with extrusions and intrusions of andesites and dacites (about 600 MW/100 km), and an “anomalous” component of about 2000 MW/100 km, related to extrusions and intrusions of rhyolitic melts whose generation is not directly controlled by subduction processes. Rhyolitic melts in the TVZ are partial melts of dominantly crustal origin. Comparison with other arcs indicates that the long-term extrusion rate of TVZ rhyolites (about 400 kg/s per 100 km) is also the highest world wide for this setting. The occurrence of voluminous Quaternary rhyolitic pyroclastics is a rare phenomenon and appears to be associated with a few arc segments (TVZ, Sumatra, Kyushu) that undergo significant crustal deformation. Various models have been proposed to explain the phenomenon of the anomalously high heat transfer within the TVZ. Models which require only heat transfer from plumes and subcrustal melts, either ponded at the crust/mantle boundary or intruding a spreading crust, are not suitable because the associated heat transfer at the contact is too low by a factor 2 to explain the required transfer rate of about 0.8 W/m2 representing the “anomalous” crustal heat component of the TVZ. Heat generation by focussed plastic deformation within the ductile lithosphere is an alternative mechanism to explain “endogenous crustal heating” which yields heating rates that are also too low by a factor of two, although important parameters (average yield strength of lithosphere and opening rate of the TVZ) are not well known. A further search for a suitable combination of heat source models is required.

Geophysical study of the Hauraki Depression, North Island, New Zealand

Abstract Geophysical data show that the southern part of the Hauraki Depression is a rift structure. In W—E cross section it is made up of a fault angle depression, a median horst, and a graben. A maximum thickness of about 3 km of Quaternary and Tertiary sediments fill the two depressions. The fault angle depression and the graben are bounded at the eastern side by major normal faults which dip about 70 ± 10° to the west, namely, the recently discovered Kerepehi Fault in the centre and the Hauraki Fault which forms the eastern boundary of the depression. A minor hinge fault (Firth of Thames Fault) probably runs along the western boundary. Transverse faults cross the depression causing horizontal offsets of the major normal faults of up to 3 km. Recent rifting of the southernmost part is indicated by active faulting and by shallow earthquake activity beneath the depression (focal depth ≤ 12 km). The chemistry of numerous hot springs implies that high temperatures (250–300°c) occur at depths of less than 5...

Geochemistry and heat transfer processes in Quaternary rhyolitic systems of The Taupo Volcanic Zone, New Zealand

Abstract The Taupo Volcanic Zone (TVZ) is a Quaternary volcanic arc associated with subduction at the southern end of the Taupo-Kermadec arc-trench system. At least 10,000 km 3 (dense equivalent) of volcanic rocks have been extruded during the last 2 Ma along the 200 km long segment of the exposed arc. These consist dominantly of rhyolite lavas and pyroclastics (80% by volume) with subordinate andesite, dacite and basalt. The TVZs crustal heat transfer of about 5650 MW is anomalously high and is a result of production of large volumes of dominantly rhyolitic magma by crustal anatexis. About 20% of this magma reaches the surface. The erupted rhyolites have high-Si metaluminous compositions showing only small variations between and within eruption centres. Magma batches of essentially homogeneous composition have been identified which feed single eruption sequences as well as multiple sequences with timespans of up to 20 Ka. The chemical composition of the TVZ rhyolites indicates that they are partial melts derived from an apparently homogeneous lower crust. The TVZ has been classified in the past as a back-arc basin heated by ponded mafic melts associated with a concealed, spreading asthenosphere plume (exogenous crustal heating). However, geochemical and geophysical arguments indicate that there is little evidence for any asthenosphere plume beneath the TVZ. An alternative heat source is proposed in which crustal heating is produced by plastic deformation of the ductile crust concentrated within a narrow tectonic hinge line (endogenous crustal heating). Such deformation promotes crustal fusion and leads to fractionation of the lower crust into an upper “felsic layer” and a lower mafic to ultramafic “restite layer”.

Downwasting of the Tasman Glacier, South Island, New Zealand: Changes in the terminus region between 1971 and 1993

Abstract Downwasting has altered the morphology of the terminus region of the Tasman Glacier between 1971 and 1993. Rapid melting began in the late 1960s in a few isolated melt ponds in the centre and in a small elongated lakelet at the eastern lateral moraine. These ponds and lakes grew rapidly in size during the 1970s and coalesced to form a large melt lake by about 1990. This melting has led to a disintegration of the entire terminus region, and now occurs as far as 3 km upstream from the old terminus. The main front of the glacier has retreated c. 1.5 km since 1982. The breaking up of the glacier has been accelerated by the onset of iceberg calving—a process which probably started in 1991. The icebergs can have volumes of several millions of cubic metres before they break up into smaller ice masses that melt slowly during the summer. A temperature survey has shown that the melt lake is almost isothermal (0.3–0.5°C). A poorly understood convection mechanism prevents suspended silt from settling and cau...

Assessment and modelling of geothermal reservoirs (small utilization schemes)

Abstract Geothermal reservoirs are characterized by their fluid chemistry, natural heat loss and reservoir structure. A revised classification of low ( >225°C ) temperature reservoirs has been given. Almost all types of geothermal reservoirs can be used for small plant development. Three different approaches have been used for assessment of geothermal reservoirs, namely: (a) assessment of gross power potential (volume approach), (2) assessment of productivity of wells and, (3) assessment of productivity potential by computer modelling. Assessment of reservoirs to be developed for small plants is restricted in that it can only be based on data of a few exploration wells; such assessment has to occur at the end of the exploration drilling phase. Power potential estimates are of limited use for adequate reservoir assessment since the permeability structure of the reservoir is neglected. Productivity assessments of exploratory wells can explain the behaviour of wells but do not allow an assessment of any major part of the reservoir. A gross permeability structure of convective reservoirs, however, can be obtained by modelling the reservoir in its natural state. The model can be used to predict the reservoir response for various utilization schemes with different fluid production characteristics. The predicted productivity potential depends therefore both on the permeability structure of the reservoir and fluid production parameters. Numerous examples (mainly from developing countries) have been cited which show that assessment of productivity potential can be extended to most types of geothermal reservoirs. Although reservoir assessment by modelling appears to be a generally suitable method, it requires access to large computers and is time consuming. Its predictive power is limited by equivalence problems and poor input models, but the same problems also affect other assessments. Poor input models are usually the result of poor exploration models which, in turn, are often caused by misidentification of reservoir type and of secondary reservoirs (concealed outflow structures).

Magnetic anomalies associated with high temperature reservoirs in the taupo volcanic zone (New Zealand)

Abstract Distinct magnetic total force anomalies are associated with many, but not all, liquid-dominated geothermal reservoirs in the Taupo Volcanic Zone, which stand in thick, young (

Geothermal resources of Sumatra

Abstract There are at least 30 high temperature systems (with inferred reservoir temperatures >200°C) along the active Sumatra Arc that transfer heat from crustal intrusions to the surface. These systems, together with eleven active volcanoes, five degassing volcanoes and one caldera volcano (Lake Toba), are controlled by the Sumatra Fault Zone, an active mega shear zone that follows the median axis of the arc. At least half of the active and degassing volcanoes are associated with “volcanic geothermal reservoirs” containing magmatic gases and acid fluids. Large, low temperature resources exist in the Tertiary sedimentary basins of cast Sumatra (back-arc region), where anomalously high thermal gradients (up to 8°C/100 m) have been measured. Volcanic activity was not continuous during the Cenozoic; subduction and arc volcanism probably decreased after the Eocene as a result of a clockwise rotation of Sumatra. In the Late Miocene, subduction started again, and andesitic volcanism reached a new peak of intensity in the Pliocene and has been continuous ever since. Rhyolitic volcanism, which has produced voluminous ignimbrite flows, began later (Pliocene/Pleistocene). All known rhyolitic centres associated with ignimbrite flows appear to lie along the Sumatra Fault Zone.

Geophysical evidence for widespread reversely magnetised pyroclastics in the western Taupo Volcanic Zone (New Zealand)

Low-altitude aeromagnetic data show that negative residual anomalies are widespread over the western Taupo Volcanic Zone, New Zealand. Paleomagnetic study of eight rhyolitic ignimbrite units and two lava flows which are exposed in this area, together with new K-Ar dates of four of the ignimbrite units, indicate that the two lava units and seven of the ignimbrite units were erupted during the Matuyama geomagnetic epoch (>0.73 Ma B.P.) and suggest that rhyolitic volcanism in the western Taupo Volcanic Zone began as early as 1.6 Ma B.P. These results provide the basis for an interpretation of our aeromagnetic data which confirms the hypothesis that the magnetic anomalies observed in the western Taupo Volcanic Zone are caused by widespread, thick, reversely magnetised pyroclastic and lava flows. Magnetic modelling also allows thickness estimates of the younger, normally magnetised cover rocks which reach a maximum thickness of the order of 0.5 km in the Mangakino area. The magnetic structure of these volcanic rocks defines approximately the lateral extent of the Mangakino Volcanic Centre.

Steam cloud characteristics and heat output of fumaroles

Abstract The heat transferred to the atmosphere by fumarole clouds is linearly proportional to the cloud area that is visible in vertical air-photos. Using measured outputs of a few accessible fumaroles, the total heat output of all fumaroles in a fumarole field can be assessed from the total area of the steam clouds. Normalised heat outputs, Q j , and normalised cloud areas, A j , were used as variables. Two surveys of the large Karapiti fumarole field (Wairakei, New Zealand) show that the total heat discharged by fumaroles with cloud areas >10 m 2 was 99 MW and 88 MW in 1999 and 2000, respectively. The output of vents with smaller cloud areas has to be obtained from ground surveys. The method also gives a representative (order of magnitude) estimate of the total heat discharged by fumarole clouds if measurements from a single, large fumarole can be used in conjunction with a recent air-photo.

Rapid melting of the terminal section of the Hooker Glacier (Mt Cook National Park, New Zealand)

Abstract A 3 km long section of the Hooker Glacier near its terminus was studied in 1996 using GPS, tacheometric, and bathymetric surveys, as well as ground penetrating radar and gravity surveys. With reference to sparse surface levels and oblique photos dating back to 1889, the studies indicate that between c. 1915 and 1964 downwasting of an axial strip along the terminal section occurred at a rate of c. 0.7 m/yr. Between 1964 and 1986 the rate increased to 1.0 m/yr. Marginal segments of the glacier near the terminus experienced positive buoyancy from 1982 onwards, which promoted rapid melting. Apparent subaqueous melting rates of c. 9 m/yr occurred between 1986 and 1996 over large stretches of the downmelting terminal area. By 1996, a 1 4 km long sector of the glacier had melted down forming a melt lake (Hooker Lake) with a volume of c. 40 × 106 m3 covering an area of 0.78 × 106 m2. A maximum water depth of 135 m was measured near the retreating glacier front where the ice wall descends as a vertical cl...