Norfolk Ridge Research Articles

Geochemistry and tectonic significance of basalts in the Poya Terrane, New Caledonia

The Norfolk-New Caledonia Ridge represents a continental slice which drifted away from Australia during the Late Cretaceous breakup of the eastern Gondwana margin. The presence of widespread basaltic rocks beneath the main ophiolite nappe of New Caledonia has been long known but the origin and the age of the Poya Terrane basalts (PTB herein) remained controversial. Recent palaeontologically determined ages date the PTB as Late Cretaceous (Campanian). New geochemical data show that two main discrete groups constitute the PTB: a MORB-like tholeiitic suite, and a more alkaline intra-plate basaltic suite distinguished mainly on immobile HFSE and REE elements. Furthermore, low ϵNd and high ThNb relative to MORB, and weak negative Nb anomalies, reflect limited assimilation of continental crust by these otherwise MORB-like tholeiites. Inter-PTB sedimentary rocks all have a pelagic or hemi-pelagic origin; detrital material originated from the nearby Norfolk-New Caledonia ridge basement. The PTB form a parautochthonous sheet below the main harzburgitic nappe constituting the New Caledonian ophiolite. They are genetically unrelated to the ophiolite, and are interpreted to be 70–85-Ma-old rift tholeiites formed during of the easternmost continental part of Mesozoic Gondwana, and opening the East New Caledonia Basin. The Norfolk-New Caledonia Ridge formed the western passive margin of this new oceanic basin, but the rifted-off eastern block is less easily identified. It may form part of the basement of the Western Belt of the New Hebrides island arc (Vanuatu). The cessation of rifting of the eastern Australian margin around 56 Ma was followed by an eastward-directed subduction which produced boninitic melts and its associated refractory harzburgitic mantle, in the forearc of the primitive Loyalty-d'Entrecasteaux arc. Following the major Pacific plate motion reorganization around 42 Ma, collision of the Norfolk-New Caledonia Ridge with the forearc region of the intra-oceanic Loyalty-d'Entrecasteaux arc around 40 Ma led first to westward thrusting of the PTB as a slice picked up from the upper crustal section of the colliding Norfolk Ridge. Subsequent collisional tectonism led to detachment of the main New Caledonian harzburgitic nappe from its forearc location in the Loyalty arc, and westward emplacement of this nappe over the PTB nearby allochthon. The presence of parautochthonous sheets of basalts unrelated to immediately overlying forearc-derived, boninite-bearing harzburgitic ophiolites is briefly discussed in the light of two other examples in arc-continent collision settings.

Seismic stratigraphy and structural history of the Reinga Basin and its margins, southern Norfolk Ridge system

The Reinga Basin northwest of the North Island of New Zealand was initially formed by crustal extension in Cretaceous time. Gravity models suggest up to 35—40% crustal thinning. The seismic stratigraphy of the basin is continuous with that of the offshore western North Island, where reflectors are well constrained by oil exploration data. In the Reinga Basin, there are two Cretaceous sequences above an older Mesozoic basement. The lower sequence is apparently terrestrial and may include both pre‐rift and syn‐rift subsequences; the upper is a rift‐filling marine sequence. These are overlain by Paleocene and Eocene blanket sequences that were laid down during a period of relative tectonic quiescence consistent with cooling subsidence, continued submergence, a northeast‐facing continental shelf, and absence of a significant active plate boundary. A strong regional reflector, caused by a combined unconformity and Oligocene condensed sequence, separates the Paleogene and Neogene sequences. The Neogene sequences record sedimentary infill from several source directions, not only from the New Zealand landmass, but from the north and west as well. Near the Northland coast, sediment accumulated in clastic wedges and ponded sub‐basins from the Miocene to the present day. Along the flanking ridges to the northwest, similar deposition occurred in the Early and Middle Miocene but changed in the Late Miocene to sedimentation in drifts flanked by scours. This change reflects the end of tectonism, a diminishing clastic sediment supply, and the establishment of a throughgoing oceanic current regime as the marginal ridges submerged. This pattern of sedimentation persists today. Post‐Cretaceous volcanism occurred in two parts of the basin. In the central southeastern part, volcanic bodies in the ?Oligocene to Early Miocene sequences could be a northwestern extension of the Northland volcanic arc. In the western part, small intrusive and extrusive bodies appear to be of Pliocene intraplate origin. Compression (or transpression) had an important role in developing the basin's present form. Miocene compressional structures—asymmetric anticlines, reverse faults, everted basins, and pop‐ups—are present everywhere but at the southeastern end. The present marginal ridges have structurally complex origins. The Reinga Ridge which forms the northeastern margin is a transform boundary with the Norfolk backarc basin. Deformation thought to be caused by the action of the transform is recorded in folded and faulted Cretaceous‐Paleogene sequences and syntectonic Early and Middle Miocene sequences along its length. The southwestern margin of the basin is a double ridge comprising the Wanganella Ridge, an early Middle to early Late Miocene, compressional uplift, and the older, eroded West Norfolk Ridge, which contains Cretaceous half‐grabens. The northern half of the Wanganella Ridge is an everted ?Oligocene to Early Miocene aulacogen in which slivers of basement rock were thrust up along with the sedimentary fill, whereas the southern half is an uplifted block of folded sedimentary rocks of probable Cretaceous or older age. Paleogeographic reconstructions show that Oligocene uplift of the Norfolk Ridge and Miocene uplift of the Reinga Ridge could have provided a means for terrestrial biota of New Caledonian affinities to spread into New Zealand. The total sediment thickness in the Reinga Basin is estimated to be 3.5–5.5 km. Potential source, seal and reservoir rocks are present, and there is an abundance of suitable structures. The potential for petroleum occurrences in the basin is good.

Revision of Southwest Pacific Polymastiidae (Porifera: Demospongiae: Hadromerida) with descriptions of new species of Polymastia Bowerbank, Tylexocladus Topsent, and Acanthopolymastia gen. nov. from New Zealand and the Norfolk Ridge, New Caledonia

Specific diagnostic characters are reviewed and redefined for the genus Polymastia in northern New Zealand waters, to provide a framework for the recognition of new species, and for the revision and redefinition of other Polymastiidae. Eight new species of Polymastia are described, and Polymastia fusca Bergquist, P. hirsuta Berquist, and P. cf. massilis Carter are redescribed in the light of new material. Species of Polymastia which contain exotyles and centrotylote oxea in addition to the usual megasclere complement, provided the opportunity to redefine the polymastid genera Proteleia, and Tylexocladus, which would typically have been reserved respectively, for sponges with these spicules. Anew species of Tylexocladus is described from the Chatham Rise in southern New Zealand, providing a first record of the genus from this location. A new genus Acanthopolymastia is established for southern ocean Polymastiidae with acanthose centrotylote oxea, with Atergia acanthoxa Koltun from Antarctica as the type species. Two additional species from New Zealand and the Norfolk Ridge, New Caledonia are described, and all are compared to Atlantic species of the same genera. Suggestions for partitioning of shallow‐water Polymastia species are put forward.

Mesoscale variability in volume transport in the Tasman and Coral Seas

Mesoscale variability of volume transports for the upper 450 m of the south-west Pacific is examined with repeat expendable bathythermograph temperature cross-sections, using a level of no motion of 1300 m. Salinity and dynamic heights are synthesized. The 136 sections are clustered about seven particular routes between Australia, Fiji and New Zealand. Peak values of net transports across one- degree latitude or longitude bins along routes are found to be an effective method of defining spatial variability, reflecting current features quite strongly. Higher local transports are indicated along the flanks of Lord Howe Rise, West Norfolk Ridge and parts of Challenger Plateau, emphasizing the topographically controlled nature of the currents. Little seasonality is evident. A very broad mean transport pattern is consistent with results in the literature derived by other methods, as are transports for the East Australian Current, indicating that results are representative. The reliability of the mean pattern is examined in terms of directional persistence, e.g. net outflows to the east are strongly directional north and north-west of New Zealand. Error calculations and an overall regularity of results indicate a valid analysis.

Selenoscopus turbisquamatus, a new genus and species of Uranoscopid fish from Japan and the Norfolk Ridge

A new stargazer,Selenoscopus turbisquamatus, is described from 30 specimens from the Kyushu-Palau Ridge, the coasts of Kii Peninsula, Japan, and the Norfolk Ridge, northern Tasman Sea. The species appears to be most closely related to the genusUranoscopus in having two dorsal fins, a spinous dorsal fin consisting of feeble spines, a stout cleithral spine, an externally visible pelvic spur, two supracleithral spines, and no postcleithrum, but differs from it in having a smooth posterior margin of the gill flap, uniserial dentary teeth, random arrangement of body scales, no subopercular spine, an intervention of the pterosphenoid (=alisphenoid) between the frontal and the parasphenoid, and the first and second haemal spines depressed on the centra. A new genus,Selenoscopus, is therefore proposed, based on these characters.

Intraplate earthquakes in the southwest Pacific Ocean Basin and the seismotectonics of the southern Tasman Sea

An examination of 311 intraplate earthquakes in the Australian plate portion of the Pacific Ocean basin reported from 1918 to 1990 reveals that only 113 events are reliably intraplate, with most of the rest relocating to active trenches and transforms. The non‐random distribution of the reliably intraplate events gives insight into the tectonic stresses present. The central Tasman Sea is mostly aseismic except for a swarm of activity at the predicted site of the Tasmantid hot spot. To the north, the broad regions of the Coral Sea, South Fiji Basin and Lord Howe Rise show very little intraplate seismicity, yet the narrow Norfolk Ridge and Three Kings Rise, caught between the double convergence of the New Hebrides and Tonga subduction zones, support many more earthquakes.High levels of intraplate seismicity in the southern Tasman Sea adjacent to the Macquarie Ridge Complex (MRC) indicate that this region may be undergoing internal deformation due to the unusual nature of the Australia‐Pacific plate boundary. Additional support exists in the form of intraplate focal mechanisms similar to those at the plate boundary and a set of parallel gravity rolls which are observed in recent geoid maps. Some aftershocks of the Mw = 8.2 Macquarie Ridge earthquake of 1989 occurred in a fracture zone west of the Macquarie Ridge Complex [Das, 1992], but we have found several earthquakes from as early as 1924 which relocate to this feature, suggesting that its reactivation may be more significant than previously thought. This reactivation of a fossil fracture zone may be the result of the increasing amount of oblique convergence between the Australia and Pacific plates at the Macquarie Ridge Complex, formerly a spreading center, and the stresses associated with subducting recently formed Australian ocean crust beneath the older Pacific plate.

Records of echinoderms (excluding holothurians) from the Norfolk Ridge and Three Kings Rise north of New Zealand

Abstract Fifty-eight species of echinoderms (excluding holothurians) are newly recorded from the southern Norfolk Ridge and the Three Kings Rise to the north of New Zealand. Two species of Asteroidea, in the genera Ceramaster and Pseudoceramaster, ceramaster are described as new. A total of 115 species of echinoderms, not counting the holothurians, are now known from this area. They include 14 crinoids, 35 asteroids, 41 ophiuroids, and 25 echinoids.

Selenoscopus turbisquamatus, a New Genus and Species of Uranoscopid Fish from Japan and the Norfolk Ridge

<I>Selenoscopus turbisquamatus</I>, a New Genus and Species of Uranoscopid Fish from Japan and the Norfolk Ridge

A new look at the New Caledonia Basin, an extension of the Taranaki Basin, offshore North Island, New Zealand

The New Caledonia Basin is a major bathymetric trough extending north-westwards from Taranaki and continuing past New Caledonia. This paper is concerned with the 250 km of the feature closest to New Zealand and adjacent to the Taranaki Shelf. The New Caledonia Basin appears to have formed as the surface expression of a failed rift system. Gravity models, constrained by seismic reflection and refraction data, indicate thinned crust below the basin. The date of inception of rifting is not known, but it must be pre-Cenozoic and may be pre-Cretaceous. New seismic reflection data reveal tensional block faulting within the basin and on the flanks of the Challenger Plateau and the West Norfolk Ridge. Offset gravity anomalies and apparent compressional faulting observed on seismic data provide evidence for transverse, possibly transform, faulting accompanying and facilitating oblique extension. More than 1.5 s (TWT) of sediments were deposited during rifting. This is equivalent to a thickness of up to 3 km. They are likely to consist of Cretaceous clastics and volcaniclastics deposited in fluvial and marginal marine environments. Terrigenous clays and mudstones probably cap the rift sequence. Post-rift sedimentation commenced with thick authigenic limestone, reflecting regional thermal subsidence and remoteness from a clastic source. There is little record of tectonic activity until at least the Late Eocene, and probably not until the Early Miocene when volcanic activity began. Seismic sections show domal uplift at the base of the Challenger Plateau and the formation of several large submarine volcanoes. This activity may have been related to renewed rifting in the Bellona Trough. An average thickness of approximately 0.5 km of Plio-Pleistocene sediments onlap the margins of the basin. Channels and fans are evident on the seismic data and account for the distribution of a significant portion of the sediment eroded from the rising Southern Alps. The fragmentation of this part of the Gondwana margin resembles the evolution of the North-east Atlantic. In particular, the structure of the Rockall Trough closely resembles that of the New Caledonia Basin. Hydrocarbon source rocks may be present in the rift sequence, which has been buried to a depth great enough for thermal maturation. Structural and stratigraphic traps may exist at a number of levels.

Structure and stratigraphy of the New Caledonia Basin

The New Caledonia Basin is a major bathymetric feature extending northwest from Taranaki, New Zealand, between the Challenger Plateau/Lord Howe Rise and the Norfolk Ridge. Interpretations of gravity data and seismic refraction velocities indicate that crustal thinning did not proceed to formation of ocean crust. Rifting was pre-Cenozoic, and may be pre-Cretaceous. Major transverse or transform faults may have accommodated oblique rifting. Cenozoic volcanism was influenced by these earlier structures. Channels and turbidites within the Plio-Pleistocene section reveal the continuation of a submarine distributary system from the Taranaki continental shelf. Seismic stratigraphy, the volume and thicknesses of sediment observed, and the likely maturation levels suggest potential for hydrocarbon accumulations.

New and little-known genera of cheilostome Bryozoa from the New Zealand region

Six new or little-known genera of cheilostome Bryozoa are recorded from the New Zealand region. Villicharixa n. gen. (Electridae), established for a species from South Island and from southern Chile, closely resembles forms from the lower Cretaceous of Europe. Daisyella n. gen. (Calloporidae) includes three uniserial species (two New Zealand and one Antarctic). Rhabdozoum Hincks (Rhabdozoidae), a little-known southern Australian genus, is newly recorded from the Otago Shelf, South Island. Plagiopora MacGillivray (Catenicellidae), a hitherto monotypic? Miocene Australian genus of the little-known subfamily Ditaxiporinae, is represented in Recent seas by a new species from the Norfolk Ridge. Yrbozoon n. gen. (Celleporidae) and Harpago n. gen. (Lekythoporidae), both monotypic, are based on new species from submarine plateaux southeast of New Zealand.

Lateral variation of upper mantle structure beneath New Caledonia determined from P-wave receiver function: evidence for a fossil subduction zone

SUMMARY Short-period teleseismic P-waves recorded at Noumea, New Caledonia, from intermediate and deep focus earthquakes in SE Asia and the SW Pacific are used to investigate crustal and upper mantle structure of the Norfolk ridge, which is currently described as a zone of convergence in its caledonian part. Radial components recorded from various source backazimuths show a succession of large P-to-S converted waves whose timing and polarity suggest the presence of a north-dipping low-velocity zone at approximately 60km depth below the station. The inferred strong lateral variations of structure in the upper mantle are also supported by observations of complex horizontal particle motions and large tangential components which exhibit clear P-to-S phases. The high velocity contrast observed across these dipping interfaces is interpreted as a structural remnant of the New Caledonia basin oceanic crust which would have been underthrust beneath the New Caledonia crust. Using geometrical ray theory in dipping structure, a model valid for the Noumea station has been derived. The crust is apparently quite homogeneous. It is 26 ±1 km thick beneath the station and seems to thicken eastward as deduced from the orientation of the Moho. The low average crustal velocities suggest a continental nature for New Caledonia. Toward the thrust zone, the Moho is a thick transition zone whereas it can be modelled by a sharp boundary elsewhere. The inferred structure can be interpreted in terms of either a dead subduction zone or an underthrusting zone. According to the history of compressive deformations observed in land, the underthrusting would likely have been syntectonic of the New Caledonia late Eocene obduction.

Records of fishes from the West Norfolk Ridge, north-west of New Zealand

Abstract A total of 89 species of elasmobranch and teleost fish was recorded from the West Norfolk Ridge in 1986. Between 50 and 60 of these have not previously been reported from the area. Fish species were caught in bottom trawls at depths between 100 m and 1100 m, principally on the Wanganella Bank. Depth distribution is discussed. Species composition is very similar to that of northern New Zealand.

Structure of the upper mantle beneath the Coral and Tasman Seas, as obtained from group and phase velocities of Rayleigh waves

Abstract Group and phase velocities for Rayleigh waves, in the period range 15–100 s, were analysed for paths crossing the Coral and Tasman Seas. We used data from 14 earthquakes recorded at the 6 World Wide Standard Stations: CTA, HNR, PMG, RIV, TAU, and WEL. By fixing the S-wave velocities at 4.65 km/s from 220 to 400 km and at 5.0 km/s for greater depths, we modelled the upper 220 km. As previous workers have found for oceanic regions, it was necessary to incorporate a significant low-velocity layer (LVL), usually centred at about 150 km, in all models. Typically the lid above the LVL has an S-wave velocity of 4.50 km/s, and the LVL a velocity of 4.25 km/s. Although the resolution of the models obtained precludes the determination of detailed Earth structure, the observations suggest that the thickness of the lid varies significantly between the “oceanic” and “plateaux” regions of Coral and Tasman Seas. It was found to be thicker beneath the Norfolk Ridge (90 km) and the Lord Howe Rise (85 km) than it ...

Tectonic model for the Devonian to middle Permian of the New England Orogen

Abstract Eight sets of stratigraphic layers and igneous rocks are the basis for the recognition of eight tectonic periods, TP1‐TP8, in the history of the New England and Yarrol Orogens from the Devonian to the opening of the Tasman Sea in the Late Cretaceous. The opening of the Tasman Sea caused the removal of an eastern section of the New England Orogen to form parts of the Lord Howe Rise and Norfolk Ridge. The Gwydir‐Calliope and Kuttung volcanic arc systems of TP1 and TP2 in the Devonian and Carboniferous were possibly W‐facing, and probably formed far to the NE of their present positions relative to the Lachlan Orogen. They moved SW as they developed, and in the latest Carboniferous or earliest Permian were cut obliquely by the Mooki Fault on which there was a dextral strike‐slip of about 500 km before the Kuttung volcanic arc became extinct. In the Late Carboniferous a narrow region on the E side of the Peel Fault was elevated to form the Campbell High which was intruded by the Bundarra Plutonic Suit...

Marginal basins of the SW Pacific and the preservation and recognition of their ancient analogues: a review

Summary Marginal basins of the SW Pacific floored by oceanic lithosphere comprise those formed by sea-floor spreading immediately behind active magmatic arcs (back-arc basins) and those created by the rifting of continental crust without obvious connection to an arc system (small ocean basins). The basins opened rapidly, range greatly in shape and size, and show diverse relations to sediment sources. Thick sediment piles mainly accumulated adjacent to emergent continental margins or active arcs, with thin sequences of pelagic sediments, ash, and fine grained turbidites on basin floors. Facies are commonly asymmetrically distributed. Basaltic magmatic activity, mild deformation and submarine erosion affect both types of basins. Ancient back-arc basins should be identifiable on the basis of their temporal relations to magmatic arcs and the volcanic influence in their sedimentary sequence, but distinguishing between small and major ocean basins is often difficult. Narrow marginal basins may collapse following accumulation of a thick sediment pile, but most basins close by subduction. Similarities are recognized between the present-day SW Pacific and the formative stage of the North American Cordillera. The Late Mesozoic palaeogeography of the Western Alps resembles the complex of small ocean basins and continental fragments west of the Norfolk Ridge, and the Ordovician palaeogeography of central Newfoundland bears close resemblance to the assemblage of back-arc basins, remnant arcs and active arc east from the Norfolk Basin.

Eastwards oceanic flow past northern New Zealand

The geostrophic circulation on the north‐east coast of New Zealand in August 1976, as evaluated from hydrographic station data and confirmed by parachute‐drogue trajectories, is very similar to that found previously in individual segments of the region. Near‐surface current velocities, as given by the drogue trajectories in the south‐east‐directed East Auckland Current, were 0.18–0.3 m.s‐1. Re‐examination of the N.Z. Oceanographic Institute block survey hydrographic station data shows that the waters of the East Auckland Current are derived from the eastwards flow between latitudes 30°S and 40°S west of New Zealand; the fastest flow is located over the Norfolk Rise. This latter flow, and the associated Tasman Front, are enhanced by the strong sea floor relief and, therefore, do not provide confirmation of a zonal jet across the Tasman Sea. Short‐term fluctuations in the position of the Tasman Front over the Norfolk Ridge are consistent with probable variations in volume transports produced by local meteor...

Plate tectonics background to biogeographic development in the southwest Pacific over the last 100 million years

About 170 million years ago Greater India, an unknown bit of continent, Greater Australia, Greater New Zealand (includes submarine New Zealand Plateau) an Antarctica were all of a piece and part of Gondwana. The lines of pending dismemberment were present as long-established rift zones. The northern margin of this part of Gondwana (New Guinea), the eastern one (New Caledonia—Norfolk Ridge) and southeastern one (New Zealand—New Zealand Plateau) were active margins with considerable volcanism and both shallow- and deep-water sedimentation. At about 165 m.y. the unknown piece of continent was carried off with the creation of the Wharton Basin. In the Early Cretaceous (140-100 m.y.). Greater India departed to the northwest, a brief marine invasion occurred along the line of the present south margin of Australia and there was a great spread of epeiric seas over the remainder of Greater Australia and possibly also Greater New Zealand. The northern, New Guinea edge, was a volcanic island arc setting; a bordering arc system along the New Caledonia—New Zealand eastern edge made up the Inner Melanesian Arcs. The Rangitata Orogeny culminated in the Early Cretaceous, and was followed by a time of uplift, metamorphism, and regression, over much of Greater New Zealand. Regression of the seas was also a feature of Australia in the Late Cretaceous (100-65 m.y.). The major event of the Late Cretaceous was the creation of the Tasman Sea (78-56 m.y.). The the separation of Greater New Zealand from Antarctica. A large continental block, Tasmantis (Lord Howe and New Caledonia Ridges, New Zealand), was thus detached from Austro-Antarctica. This was itself split by the formation of the New Caledonia Basin at about 65 m.y. The Coral Sea was opened by spreading and sinistral strike-slip of a part of New Guinea north of the Papuan Mobile Belt at roughly the same time. The Inner Melanesian Arc System was disrupted. An Outer Melanesian Arc system now developed from New Guinea to Tonga and a young marginal sea began to form on the east side of the New Caledonia—Norfolk Ridge. The major event, however, was the separation of Australia from Antarctica. Obduction of seafloor and crust took place at the New Guinea and New Caledonia margins. Greater New Zealand was split by a dextral fracture system. The New Zealand Plateau remained on the Pacific Plate. Greatly increased convergence between Australia and Pacific Plates began at about 40 m.y. By mid-Oligocene (30 m.y.) the circum-Antarctic Current had begun its action. At the same time the New Hebrides—South Fiji marginal sea grew very large. By 6 m.y. (end of the Miocene) the northern boundary between Indo-Australian and Pacific Plates (the Melanesian Borderlands) became increasingly a zone of intense sinistral strike-slip and many island blocks moved WNW. Strong vertical movements have been a feature of the Borderlands. The Fiji platform rotated and the New Hebrides arc back-tracked to its present position. The old marginal sea was one-half consumed, the North Fiji Basin created by complex local spreading. The Lau Ridge became a remnant arc with the creation of the Lau—Havre interarc basin. New Zealand was joined to Tonga by way of the Kermadec Islands and subduction zone. Movement along the Alpine Fault has continued, conveying a strike-slip component to Kermadec—Tonga subduction.

Geophysical anomalies and igneous bodies off the West Coast, North Island

Abstract Gravity and magnetic anomalies off the west coast of the North Island are caused by igneous bodies much larger than any observed on land in New Zealand. North of Auckland the bodies resemble seamounts in shape and size; they appear to be volcanic piles about 5-6 km thick with steep sides and flat tops which form the sea floor, and are surrounded by sediments. South of Auckland the anomalies can only be interpreted as due to composite bodies. The greater parts of the gravity anomalies are probably due to igneous material intruding Tertiary sediments; the greater parts of the magnetic anomalies are probably due to igneous rocks intruding late Paleozoic basement. Unlike the northern anomalies which occur as isolated features, the southern anomalies lie on an extensive zone of magnetic activity which may run into the Norfolk Ridge. The relationship of the offshore bodies to the much smaller onshore volcanic features is by no means clear, but it would appear that the coastline separates regions of quite different tectonic styles.

Interpretation of magnetic anomalies across the Campbell Plateau, south of New Zealand

Magnetic anomalies over the Campbell Plateau show up a linear belt of positive anomalies extending for 900 km with an approximate east?west trend. This belt is identified as part of the Stokes Magnetic Anomaly System which extends throughout New Zealand and is associated with rocks of the New Zealand Geosyncline. The system defines a trans-current fault with a dextral displacement of 330 km which can be traced from the SW tip of the Campbell Plateau to the Bounty Trough in the north. Restoration of this Campbell Fault requires the Bounty Trough to be a rift feature, probably associated with Indian-Pacific plates' movements during the Miocene or Oligocene. The extension of this boundary may have been a subduction zone running from the NE tip of the Campbell Plateau past the North Cape of New Zealand to the Norfolk Ridge, along what is now the southern margin of the South Fiji Basin.