Subsurface structure across the axis of the Tongariro Volcanic Centre,New Zealand

The relationship between structure and volcanism in the Tongariro Volcanic Centre, New Zealand, is largely masked by a mantle of young volcanic deposits. Here we report the results of an integrated geophysical investigation (using gravity, multi-level aeromagnetic and magnetotelluric methods) of subsurface deposits and structures in the upper 1–2 km across the axis of the Tongariro Volcanic Centre. Modelling of these data across the Tama Lakes saddle shows that the outcropping volcanic deposits are up to 800 m thick, underlain by Tertiary sediments (of a few 10's to a few 100 m in thickness) and in turn lying above a basement of probable Mesozoic greywacke. Basement faulting is shown to be concentrated in the centre of the rift, which is 18 km wide at this location, but no vertical offset is resolved at the rift axis. Vertical displacements on basement faults of 250–300 m are modelled giving a minimum total basement subsidence of 650 m. A 5 km-wide, deep low resistivity zone occurs at the axis of the rift which is interpreted as either resulting from extensive fracturing and/or hydrothermal alteration within the basement. Steep-sided volcanic bodies with a high proportion of lavas/dykes coincide with the Waihi fault and the rift axis. Coincidence with the Waihi Fault suggests that this fault system may have provided magma pathways to the surface and a focus for dyke emplacement, which could have contributed to rift extension. The lack of offset at the rift axis may reflect the juvenile nature of faulting at this location, which is consistent with the notion of a migration of faulting towards the centre of the graben, alternatively, rifting may have been entirely accommodated by dyke emplacement.     

Radiometric ages for some rocks from Snares and Auckland Islands,Campbell Plateau

On the basis of K/Ar muscovite and biotite ages, and of Rb/Sr whole-rock, muscovite, and feldspar ages, the last cooling of granites from Snares and Auckland Islands on the Campbell Plateau took place in early Late Cretaceous times. The original emplacement is unlikely to have been older than Late Jurassic. These results from the basement rocks of the Campbell Plateau conform with data from Fiordland and southern Stewart Island in southern New Zealand and from Ford Range and other localities in West Antarctica. Two gabbros from Auckland Island yield Miocene K/Ar ages.     

Magnetic polarity stratigraphy of a Plio-Pleistocene marine sequence of North Island, New Zealand

Thick sequences of relatively undisturbed Plio-Pleistocene sediments in the Wanganui Basin, North Island, New Zealand consist of well exposed silts, clayey silts, sandstones, rare limestones, and several tephras. Oriented specimens were collected from a section more than 2500 m thick and palaeomagnetic measurements were made using A.C. demagnetisations in fields up to 35 mT. With the aid of tephrochronology the age of the upper sequence is now well established and falls within the Matuyama epoch. The lower two-thirds of the section except for the basal 500 m is predominantly normally magnetised and is interpreted as a very extended sequence of the Gauss epoch. The lowest 500 m then represents the Gilbert epoch. The Plio-Pleistocene boundary, as defined at Vrica, Italy, falls within the upper part of the section studied, in the Upper Nukumaruan stage. For the first time a reliable correlation is made with the international boundary, using as intermediaries the palaeomagnetic and tephrostratigraphy of deep-sea cores from the southwest Pacific.

As a result of the high deposition rate (of the order of 1.2 m/ky) and the apparent lack of unconformities, the temporal resolution is high; short-lived magnetic events are detected, especially in the lower Matuyama and upper Gauss epochs. These generally correlate well with events reported from other extended sections.     

Reconstructions of the Campbell Plateau and the Lord Howe Rise

Reconstructions of the southeastern margin of Gondwanaland require either a separation of East and West Antarctica or movement between the Lord Howe Rise and the Campbell Plateau. Previous plate tectonic reconstructions based on sea-floor spreading data eliminated the Lord Howe Rise-Campbell Plateau separation prior to 36 m.y. ago because of overlap. This conclusion is dependent on the reconstruction of Australia and Antarctica, interpretation of magnetic anomalies between Antarctica and the Campbell Plateau and the nature of the plate boundary in New Zealand. Revised reconstructions of the fit between Australia and Antarctica, and a reinterpretation of the magnetic anomalies between the Campbell Plateau and Antarctica suggest that there is no problem of overlap between the Lord Howe Rise and the Campbell Plateau, and that continued motion between these plates prior to 36 m.y. ago is a more plausible alternative to separation between East and West Antarctica.     

Monitoring and mapping of hydrogen sulphide emissions across an active geothermal field: Rotorua,New Zealand

Hydrogen sulphide (H₂S) is one of a number of gaseous species associated with geothermal activity in the Taupo Volcanic Zone (TVZ), New Zealand. The city of Rotorua is located within Rotorua Caldera in the TVZ and is one of the few urban areas in the world where a large population (>60,000 people) is frequently exposed to geothermal emissions. In order to evaluate the health hazard from long-term exposure to H₂S being emitted from the Rotorua geothermal field, a passive sampler has been developed to measure concentrations of H₂S at many locations across the city simultaneously. In contrast to other passive or pump-based samplers, the sampler is inexpensive, easily mass-manufactured, and involves the reaction of H₂S with silver halide contained in treated photographic paper. H₂S-exposed paper shows a distinct colour change from white to dark brown as H₂S concentrations increase and is sensitive to concentrations between ≪30 and around 1000 ppb. Rotorua city can be divided into three regions—an area of low H₂S concentration in the west, a ‘corridor’ of high concentrations running north–south through the city centre where H₂S is being emitted, and an area of medium concentration to the east which is influenced by the prevailing wind direction, creating a plume from the central corridor. The data give new insight into the subsurface routes of degassing in the Rotorua geothermal field, by showing the surface expression of the main upflow zone and the direction of the conjectured faulting below.     

Entanglement of New Zealand fur seals in man-made debris at Kaikoura, New Zealand

New Zealand fur seals in the Kaikoura region breed near a town with expanding tourist and fishing industries and commonly come ashore entangled in nets and plastic debris. However, the rate at which entanglement occurs was previously unknown. A decade of Department of Conservation seal callout data was analysed to determine the level of entanglement in the region and the most common debris type. Monitoring of adult female fur seals released from entanglement provided information on the potential for serious wounds to heal and survivorship of released individuals. Entanglement rates of pinnipeds in Kaikoura are some of the highest reported world-wide (average range: 0.6-2.8%) with green trawl net (42%), and plastic strapping tape (31%) together contributing the most to debris types. Nearly half of the reported entangled seals are successfully released (43%) and post-release monitoring shows that with appropriate intervention the chance of an individual surviving even with a significant entanglement wound is high. Our study demonstrates that while entanglement in the region is high, a successful intervention protocol may help reduce the potential for entanglement-related mortality in the region.     

Magnetic anomalies — India and Antarctica

Magnetic anomalies over the continental shelf off the east coast of India (Orissa) suggest the presence of a highly magnetic rock type magnetized with an intensity of 900 nT in a direction, azimuth(A) = 150° and inclination(I) = +65°. This suggest the occurrence of igneous volcanic rocks which is confirmed from samples found below Tertiary sediments from a few boreholes in this region. The depth of this rock type as estimated from magnetic anomalies varies from approximately 1–2 km near the coast to 4–4.5 km towards the shelf margin. This direction of magnetization is the reverse of the reported direction of magnetization for the Rajmahal Traps of the Cretaceous period (100–110 m.y). A small strip of the body near the continental shelf margin appears, however, to possess normal magnetization suggesting the occurrence of normal and reversed polarities side by side, a characteristic typical for oceanic magnetic anomalies. The reversed polarity of the rocks on the continental shelf suggests that they correspond probably to the MO reversal (115 m.y.) on world magnetostratigraphic scale and provide a paleolatitude of 47°S for the land mass of India which agrees with the palaeoreconstruction of India and Antarctica. In this reconstruction, the Mahanadi Gondwana graben on the Indian subcontinent falls into line with the Lambert Rift in Antarctica, suggesting a probable common ancestry. The volcanic rocks on the continental shelf off the east coast of India might represent a missing link, that is, rocks formed between India and Antarctica at the time of the break-up of Gondwanaland. Satellite magnetic anomalies (MAGSAT) recorded over the Indian shield and interpreted in terms of variations in the Curie point geotherm provide a direction of magnetization which also places this continent close to Antarctica. As such MAGSAT anomalies recorded over eastern Antarctica are found compatible with those recorded over the Indian shield.     

Magnetic field anomalies associated with geodynamic phenomena

Significant variations of the Earth's magnetic field seem to be associated with geodynamic phenomena, particularly volcanic cruptions and earthquakes. The variations have small amplitudes, generally only a few nT. These small signals are embedded into larger transient variations due to other natural or artificial causes. An original method of recovering interesting signals having amplitudes down to 1 nT, is to describe the whole variation analytically so that each component may be isolated. Several studies have shown that significant signals exist for tectonically active regions: 10–30 nT for large-scale arrays over periods of years; 5–15 nT for volcanic phenomena; 1–5 nT for earthquakes.Only a few test sites for volcanoes (New Zealand, Hawaii, Antilles) and earthquakes (essentially the San Andreas fault system) have been monitored so far.The cause of the type of magnetic variations studied here, which are associated with volcanism and rock failures, has most often been attributed to piezomagnetism. Some authors have suggested that electrokinetic effects associated with porosity-changes may also play a role.The isolated signals may, in the future, be used for hazard forecasting when the time-constants associated with the effects are known more precisely.     

Magnetic interpretation of dyke anomalies using derivatives

The horizontal and vertical derivative profiles of magnetic anomalies of dykes show some interesting properties. The points of zero derivatives and the points where the derivatives are equal are conjugate point pairs. A method of interpretation of dyke anomalies is suggested, which utilizes the distances between these points.Notation F Magnetic anomaly in total intensity - Z Depth to top of the dyke - 2T Width of the dyke - Geological dip of the dyke - I Effective intensity of magnetisation in the plane of profile - Dip of effective magnetisation vector in the plane of profile - Strike angle of the dyke - i Magnetic dip - Q – - Q _f –+arctan (sin coti) - I _f      

Linking interannual river flow river variability across New Zealand to the Southern Annular Mode, 1979–2011

River flow constitutes an important element of the terrestrial branch of the hydrological cycle, yet knowledge regarding the extent to which its variability, at a range of timescales, is linked to a number of modes of atmospheric circulation is meagre. This is especially so in the Southern Hemisphere where strong candidates, such as El Niño Southern Oscillation and the Southern Annular Mode (SAM), for influencing climate and thus river flow variability can be found. This paper presents the results of an analysis of the impact of the SAM on winter and summer river flow variability across New Zealand, purposefully controlling for the influence of El Niño Southern Oscillation and the tendency for the SAM to adopt a positive phase over the last 10–20 years. Study results, based on identifying hydrological regions and applying circulation‐to‐environment and environment‐to‐circulation approaches commonly used in synoptic climatology, reveal a seasonal asymmetry of the response of river flow variability to the SAM; winter flows demonstrate a higher degree of statistical association with the SAM compared to summer flows. Further, because of the complex orography of New Zealand and its general disposition normal to zonal flows of moisture bearing winds, there are intraseasonal spatial variations in river flow SAM associations with clear rain shadow effects playing out in resultant river flow volumes. The complexity of SAM river flow associations found in this study warns against using indices of large scale modes of atmospheric circulation as blunt tools for hydroclimatological prediction at scales beyond hydroclimatological regions or areas with internal hydrological consistency.     

Phytoplankton biomass and primary production responses to physico-chemical forcing across the northeastern New Zealand continental shelf

Phytoplankton biomass and primary production were monitored in the Hauraki Gulf and on the northeastern continental shelf, New Zealand - using ship surveys, moored instruments and satellite observations (1998-2001) - capturing variability across a range of space and time scales. A depth-integrated primary production model (DIM) was used to predict integrated productivity from surface parameters, enabling regional-specific estimates from satellite data. The shelf site was dominated by pico-phytoplankton, with low chlorophyll-a (<1 mg m⁻³) and annual production (136 g C m⁻² yr⁻¹). In contrast, the gulf contained a micro/nano-phytoplankton-dominated community, with relatively high chlorophyll-a (>1 mg m⁻³) and annual production (178 g C m⁻² yr⁻¹). Biomass and productivity responded to physico-chemical factors; a combination of light, critical mixing depths and/or nutrient limitation—particularly new nitrate-N. Relatively low biomass and production was observed during 1999. This coincided with inter-annual variability in the timing and extent of upwelling- and downwelling-favourable along-shelf wind-stress, influencing the fluxes of new nitrate-N to the shelf and gulf. Relationships with the Southern Oscillation Index are also discussed. Our multi-scaled sampling highlighted details associated with stratification and de-stratification events, and deep sub-surface chlorophyll-a not visible to satellite sensors. This study demonstrates the importance of multi-scaled sampling in gaining estimates of regional production and its responses to physico-chemical forcing.     

Observations on white island volcano,New Zealand

White Island is a complex of two overlapping cones constructed of lava flows, agglomerates and unconsolidated and unsorted ash and tuff beds. Remnants of a welded-tuff flow have been found on the north-east flank of the volcano. Since the extrusion of the youngest lava flow the young cone has been breached to the south-east and deeply eroded. White Island lavas are porphyritic augite-hypersthene-labradorite andesites. One young lava flow is unusually rich in Na₂O and contains groundmass sodian ferroaugite instead of the normal augite and hypersthene. The unusual groundmass features of this andesite are believed to be the result of contamination. Volcanic, plutonic and gneissic xenoliths have been found in the White Island lavas. Three new analyses of White Island andesites are given together with an electron microprobe analysis of a groundmass glass from one of the andesites. The White Island andesites are believed to have formed from the hybridisation of a primary mantle-derived andesitic magma with crustal material below the base of the Mesozoic New Zealand Geosyncline.     

伽利略卫星观测磁异常及其解释

伽利略飞船在木星四颗最大的卫星附近探测到了显著的磁异常.尽管磁异常不同,但是都来自于卫星内部和周围环境的磁流体动力学过程,例如冰下海洋的电磁感应,卫星的轨道运动与外部等离子体的相互作用等等.本文总结了每颗卫星的观测磁异常以及现有解释.我们也提出了磁异常来源的新的可能解释机制:磁异常可能部分来自卫星冰下导电海水或者流体核的磁对流.对该机制的研究有助于进一步理解所观测到的磁异常,也可能提供对卫星内部物理的磁约束.     

Seismology in New Zealand

New Zealand is a country of moderate seismicity. Its earthquakes are well-located and their magnitudes found with closely-spaced modern seismographs. Historical catalogues have been prepared. Deep and shallow shocks are associated with two active continental margins having the usual geophysical associations. The northern one is orientated towards the Pacific and the southern one towards the Tasman Sea. The structure is complex, and below the continental-type crust there are large lateral inhomogeneities in the upper mantle. There is a marked lack of correlation between all except the most superficial earthquake foci and the geological faults and this persists to the microearthquake level. New Zealand seismologists have made theoretical studies of earthquake mechanism and examined the statistical properties of earthquake occurrence. They have also studied the fine structure of the Earth's core, and made microzoning and other studies with engineering implications.     

Processing and preliminary interpretation of noisy high-resolution seismic reflection/refraction data across the active Ostler Fault zone,South Island,New Zealand

In an attempt to understand the structure of active faults as they emerge from bedrock into shallow semi-consolidated and unconsolidated sediments, we have recorded a comprehensive high-resolution seismic reflection/refraction data set across the Ostler Fault zone on the central South Island of New Zealand. This fault zone, which absorbs 1–2 mm/yr of compression associated with oblique convergence of the Pacific and Australian tectonic plates, consists of a series of surface-rupturing N–S trending, west-dipping reverse faults that offset a thick sequence of Quaternary glacial outwash and late Neogene fluvio-lacustrine sediments of the Mackenzie Basin. Our study focuses on a region of the basin where two non-overlapping fault segments are separated by a transfer zone. Deformation in this area is accommodated by offsets on multiple small faults and by folding in their hanging walls. The seismic data with source and receiver spacing of 6 and 3 m and nominal CMP fold of 60 was acquired along twelve 1.2 km long lines orthogonal to fault strike and an additional 1.6 km long tie-line parallel to fault strike. The combination of active deformation and shallow glacial outwash sediments results in particularly complicated seismic data, such that application of relatively standard processing schemes yields only poor quality images. We have designed a pre- and post-stack reflection/refraction processing scheme that focuses on minimising random and source-generated noise, determining appropriate static corrections and resolving contrasting reflection dips. Application of this processing scheme to the Ostler Fault data provides critical information on fault geometry and offset and on sedimentary structures from the surface to ～ 800 m depth. Our preliminary interpretation of one of the lines includes complex deformation structures with folding and multiple subsidiary fault splays on either side of a ～ 50° west-dipping primary fault plane.     

Thermal modeling of the Southern Alps,New Zealand

Finite-element modeling of the thermal regime across the Southern Alps of New Zealand has been carried out along two profiles situated near the Franz Josef and Haast valleys. The modeling involves viscous deformation beneath the Southern Alps, including both uplift and erosion, and crustal/lithospheric thickening, as a result of crustal shortening extending to 20 mm/y of a 25-km thick crust. Published uplift rates and crustal thickness variations along the two profiles are used to constrain the modeled advection of crustal material, and results are compared with the recent heat flow determinations, 190±50 mW/m² in the Franz Josef valley and 90±25 mW/m² in the Haast valley. Comparisons of the model with published K–Ar and fission track ages, show that the observed heat flow in the Franz Josef valley is consistent with observed zircon fission track ages of around 1 Ma, if the present-day uplift rate is close to 10 mm/y. Major thermal differences between the Franz Josef and Haast profiles appear to be due to different uplift and erosion rates. There is weak evidence that frictional heating close to the Alpine fault zone is not significant. The modeling provides explanations for the distribution of seismicity beneath the Southern Alps, and predicts a low surface heat flow over the eastern foothills due to the dominant thermal effect of crustal thickening beneath this region. Predicted temperatures at mid-crustal depth beneath the zone of maximum uplift rate are 50–100°C cooler than those indicated in previously published models, which implies that thermal weakening of the crust may not be the main factor causing the aseismicity of the central Southern Alps. The results of the modeling demonstrate that the different types of reset age data in the region within 25 km of the Alpine fault are critical for constraining models of the deformation and the thermal regime beneath the Southern Alps.     

In-stream water quality,invertebrate and fish community health across a gradient of dairy farming prevalence in a New Zealand river catchment

Dairying is an intensive form of agriculture influencing stream ecosystems worldwide via increased levels of nutrients, deposited fine sediment and other contaminants. However, it is not fully understood how dairy farming affects food supply for stream fish. We investigated relationships between dairy farming prevalence in the catchments of nine tributaries of a New Zealand river (0% to 79% of the catchment area) and fish and invertebrate communities. Streams were sampled four times at monthly intervals for brown trout density, fitness/growth-related trout response variables, native fish density, invertebrate community metrics as well as physical and chemical water quality variables. Densities of both brown trout and native fish declined as dairying increased, with no trout found in streams where dairy farms covered more than 50% of the catchment area. Increasing dairy farming prevalence was also associated with higher in-stream levels of dissolved nutrients and deposited fine sediment. These findings suggest that increasing the extent of dairy farming in New Zealand based on practices at the time of sampling results in less abundant and diverse fish communities.