twin active spreading ridges in the north fiji basin southwest pacific recent work on the north fiji basin axial spreading ridge and on the active propagating ridge located west of the fiji islands has established the existence of two active spreading ridges that have been functioning synchronously in the basin since at least 1 or 1 5 ma this hypothesis is tested and the kinematics of the twin active ridges are refined further on the basis of analysis of bathymetric and magnetic data

a late cretaceous and cenozoic reconstruction of the southwest pacific region tectonics controlled by subduction and slab rollback processes abstract a cenozoic tectonic reconstruction is presented for the southwest pacific region located east of australia the reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for pacific australia and lord howe rise pacific relative plate motion the reconstruction is based on a conceptual tectonic model in which the large scale structures of the region are manifestations of slab rollback and backarc extension processes the current paradigm proclaims that the southwestern pacific plate boundary was a west dipping subduction boundary only since the middle eocene the new reconstruction provides kinematic evidence that this configuration was already established in the late cretaceous and early paleogene from 82 to 52 ma subduction was primarily accomplished by east and northeast directed rollback of the pacific slab accommodating opening of the new caledonia south loyalty coral sea and pocklington backarc basins and partly accommodating spreading in the tasman sea the total amount of east directed rollback of the pacific slab that took place from 82 ma to 52 ma is estimated to be at least 1200 km a large percentage of this rollback accommodated opening of the south loyalty basin a north south trending backarc basin it is estimated from kinematic and geological constraints that the east west width of the basin was at least 750 km the south loyalty and pocklington backarc basins were subducted in the eocene to earliest miocene along the newly formed new caledonia and pocklington subduction zones this culminated in southwestward and southward obduction of ophiolites in new caledonia northland and new guinea in the latest eocene to earliest miocene it is suggested that the formation of these new subduction zones was triggered by a change in pacific australia relative motion at 50 ma two additional phases of eastward rollback of the pacific slab followed one during opening of the south fiji basin and norfolk basin in the oligocene to early miocene up to 650 km of rollback and one during opening of the lau basin in the latest miocene to present up to 400 km of rollback two new subduction zones formed in the miocene the south dipping trobriand subduction zone along which the solomon sea backarc basin subducted and the north dipping new britain san cristobal new hebrides subduction zone along which the solomon sea backarc basin subducted in the west and the north loyalty south fiji backarc basin and remnants of the south loyalty santa cruz backarc basin subducted in the east clockwise rollback of the new hebrides section resulted in formation of the north fiji basin the reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere a change in plate kinematics or slab mantle interaction

a vertically averaged circulation model using boundary fitted coordinates abstract a two dimensional vertically averaged circulation model using boundary fitted coordinates has been developed for predicting sea level and currants in estuarine and shelf waters the basic idea of the approach is to use a set of coupled quasi linear elliptic transformation equations to map the physical domain to a corresponding transformed plane such that all boundaries are coincident with coordinate lines and the transformed mesh is rectangular the hydrodynamic equations are then solved by a multi operation finite difference technique in the rectangular mesh transformed grid comparisons of the circulation model predictions for tidally forced flows in a wedge section with both flat and quadratic bottom topography and in a flat channel with exponential variation in width were in excellent agreement with corresponding analytic solutions simulation of steady state wind induced setup in a closed basin formed using elliptic cylindrical coordinates also was in excellent agreement with the analytic

backarc rifting constructional volcanism and nascent disorganised spreading in the southern havre trough backarc rifts sw pacific high resolution multibeam em300 and seabeam data of the southern havre trough sht combined with observations and sample collections from the submersible shinkai6500 and deep tow camera are used to develop a model for the evolution and magmatism of this backarc system the havre trough and the associated kermadec arc are the product of westward subduction at the pacific australian plate boundary detailed studies focus on newly discovered features including a seamount saito seamount and a deep graben ngatoroirangi rift 4000 m water depth floored with a constructional axial volcanic ridge 5 km in length and in excess of 200 m high both of which are characterised by pillow and lobate flows estimated at elongate volcanic ridges at 35 15 s and 34 30 s and backarc seamounts 35 30 s 178 30 e occur at the eastern margin of the sht similar seafloor morphology is observed in the central and western portions of the basin suggesting that recent volcanism may be broadly distributed across the backarc mass balance modelling indicates a maximum crustal thickness of 11 km to 4000 m spreading systems these discontinuous spreading systems are characterised by failed rifts rift segmentation and propagation successive episodes of magmatic intrusion into thinned faulted arc basement results in defocused asymmetrical accretion cross arc volcanic chains isolated volcanoes and underlying basement plateaus are interpreted to represent a cap of recent extrusives however they may also be composed entirely of newly accreted crust and the spatially extensive basement fabric of elongated volcanic ridges may be the surface expression of pervasive dike intrusion that has thoroughly penetrated and essentially replaced the original arc crust with newly accreted intrusives

isotopic evidence b c o of deep fluid processes in fault rocks from the active woodlark basin detachment zone we report results from boron carbon and oxygen stable isotope analyses of faulted and veined rocks recovered by scientific ocean drilling during odp leg 180 in the western woodlark basin off papua new guinea in this area of active continental extension crustal break up and incipient seafloor spreading a shallow dipping seismically active detachment fault accommodates strain defining a zone of mylonites and cataclasites vein formation and fluid infiltration syntectonic microstructures and vein fill mineralogy suggest frictional heating during slip during extension and exhumation of moresby seamount low carbon and oxygen isotope ratios of calcite veins indicate precipitation from hydrothermal fluids 13cpdb down to 17 18opdb down to 22 formed by both dehydration and decarbonation boron contents are low 7 ppm indicating high grade metamorphic source rock for the fluids some of the 11b signatures 17 35 parent solutions to calcite vein fills are low when compared to deep seated waters in other tectonic environments likely reflecting preferential loss of 11b during low grade metamorphism at depth pervasive devolatilization and flux of co2 rich fluids are evident from similar vein cement geochemistry in the detachment fault zone and splays further updip multiple rupture and healing history of the veins suggests that precipitation may be an important player in fluid pressure evolution and hence seismogenic fault movement

calculating tidal current profiles from vertically integrated models near the critical latitude in the barents sea the method of extracting current profiles from vertically integrated hydrodynamic models is extended to incorporate more general boundary conditions and the consistency of the method is investigated using this method the tidal current profiles for semidiurnal and diurnal constituents are studied in high latitudes near the critical latitude where the coriolis frequency equals the tidal frequency for the semidiurnal tide the current profiles are computed at stations in the barents sea where current measurements exist at up to six different depths the observation stations are located close to the critical latitude for the largest semidiurnal constituents and the current profiles are therefore sensitive to variations of magnitude and vertical distribution of the eddy viscosity at this latitude the current is strongly depth dependent and in deep water the tidal ellipse changes direction of rotation near the seabed at four of the stations there are under ice measurements and the effects of an ice covered ocean upon the tidal current profiles are also investigated

from rifting to active spreading in the lau basin havre trough backarc system sw pacific locking unlocking induced by seamount chain subduction associated with pacific australia plate convergence the lau basin havre trough is an active back arc basin that has been opened since 5 5 ma by rifting and southward propagating oceanic spreading current back arc opening rates decrease from 159 mm yr 1 in the northern lau basin to 15 mm y 1 in the southern havre trough major tectonic changes occur at the transition between havre trough rifting and full oceanic spreading of the eastern lau spreading center elsc where the oblique to trench westward subducting louisville seamount chain lsc sweeps southwards along the tonga trench new swath bathymetry seismic reflection data and limited rock sampling in this area constrain a tectonic and kinematic back arc model that incorporates the effects of lsc subduction the elsc which extends southward to 24 55 s forms a deep rift valley propagating southward through older rifted arc basement present day seismicity and fresh and fractured pillow lavas at 23 42 s are consistent with rift valley neovolcanism conversely the northern havre trough has low seismicity and rifted volcanic basement ridges trending 25 45 oblique to the basin axis consistent with low levels of extensional tectonism and volcanism this latter structural fabric is interpreted as an early stage of rifting that is now locked due to compression on the arc exerted by lsc subduction while in the lau basin such effects have passed as the lsc swept along the tonga trench it is proposed that the lau havre back arc opening is controlled by tectonic constraints exerted at the limits of the system by the lsc subduction which determines the southward migration of the tonga arc pole of rotation and associated lau basin opening a discrete three stage back arc opening evolution is proposed comprising 1 an initial phase of back arc rifting along the whole length of the plate boundary beginning at 6 5 ma 2 a subsequent phase mostly present in the southern part of the back arc domain and still active in the havre trough of transpression and transtension starting at 4 ma in the north as the lsc starts to subduct and sweeps southward along the tonga trench and 3 a renewed opening phase in the northern segment of the back arc domain with rifting and spreading starting at 3 5 ma as subduction of the lsc along the northern tonga trench is progressively completed

perspectives on cretaceous gondwana break up from detrital zircon provenance of southern zealandia sandstones detrital zircon u pb ages in 37 sandstones from late early late cretaceous marine and non marine successions across southern zealandia indicate a provenance from local basement within present day zealandia samples from taranaki basin were derived from median and karamea batholith granitoids with transport directions from west to east samples from west coast western southland and great south basins contain components derived more locally and more variably from median batholith and rahu suite granitoids and or the palaeozoic buller terrane west coast basin samples have more plutonic contributions and great south basin localities have more albian aged c 110 100 ma zircons samples from canterbury basin were sourced from torlesse composite terrane basement the provenance variations are present in both marine and non marine sandstones and suggest localized watersheds this fits an interpretation of late cretaceous deposition in rift controlled basins across southern zealandia during pre gondwana break up regional extension more speculatively some additional source areas may have been created at the rifted margins of zealandia during this break up

on the importance of time varying eddy viscosity in generating higher tidal harmonics by using a spectral modal expansion in the vertical a method is developed for determining tidal current profiles subject to either a slip or no slip bottom boundary condition an arbitrary vertical profile of eddy viscosity can be used in the calculation the magnitude of which can be related to the flow field the nature of the modal solution is such that the high shear bottom boundary layer can be accurately resolved using a few modes of order five compared with the order of 45 grid boxes on a logarithmic transformation which would be required in a finite difference approach calculations show the importance of a time varying eddy viscosity in generating higher harmonics when a no slip condition is applied at the seabed by relating the current one meter above the bed to the bed stress a value of friction coefficient c100 in reasonable agreement with observations was obtained from these no slip calculations however the value of friction coefficient was strongly dependent upon the height above the bed at which the current was calculated with friction coefficient increasing rapidly in the near bed region current profiles on the continental shelf determined using the modal method developed in the paper were in good agreement with observations with computed profiles in the celtic sea giving the observed midwater maximum

a simple three dimensional model of the circulation off newfoundland a simple numerical model is developed to study the three dimensional structure of the steady circulation off newfoundland the model uses linear dynamics a vertical eddy viscosity to parameterize the vertical mixing and a linear parameterization of bottom friction in which the bottom stress is related to the bottom velocity realistic geometry and bottom topography are included the results presented here are for an ocean of uniform density the model is driven by specifying the sea level along the northern boundary the inflow corresponding to the inshore and offshore branches of the labrador current the model can reproduce the circulation pattern found by greenberg and petrie 1988 who used a vertically integrated nonlinear primitive equation model it also enables dynamical interpretation to be given in terms of flow along f h contours and frictional diffusion across these contours in this connection we investigate the dependence on the friction parameters of the transport through several sections we also examine the onshore offshore mass exchange over the southeastern edge of the grand banks we find that the model labrador current flowing southward along the shelf break leads to an onshelf flux in the upper part of the water column and an offshelf flux in the bottom ekman layer this is caused by the frictional torque and leads to an along slope decay of the current amplitude finally the limitations of the model and plans for its further development are discussed

on formulating two dimensional vertically integrated hydrodynamic numerical models with an enhanced representation of bed stress a convolution method is developed for computing the bottom stress in a vertically integrated hydrodynamic numerical model by this means the bottom stress is no longer related to the depth mean current as in a conventional vertically integrated model and an enhanced representation of the bottom stress is possible an enhanced vertically integrated model consequently the enhanced model can reproduce phase lags and differences in direction between bed stress and depth mean current which cannot be taken into account in conventional vertically integrated models a linear or quadratic frictional formulation can be used with the method idealized calculations of wind induced flow in a rectangular basin show that the bed stress computed by the convolution method is in excellent agreement with that derived using a full three dimensional model however significant differences in the bed stress between the three dimensional and conventional two dimensional model are evident also in this paper a method is developed for extracting current profiles from vertically integrated models current profiles extracted from the enhanced two dimensional model for a range of frictional effects are found to be in good agreement with those computed using a full three dimensional model

new sw pacific tectonic model cyclical intraoceanic magmatic arc construction and near coeval emplacement along the australia pacific margin in the cenozoic various reconstructions of the sw pacific for the late cretaceous and cenozoic suggest that northeast dipping subduction began in the south loyalty basin slb at 55 50 ma and that subsequent closure of the slb resulted in the diachronous emplacement of cretaceous paleocene ophiolitic nappes onto the norfolk ridge in new caledonia at 40 34 ma and in northland new zealand around 24 21 ma a fundamental problem with these models is that they do not account for the fact that ne dipping subduction had already been established offshore papua new guinea by at least 65 60 ma which resulted in the emplacement of the papuan ultramafic belt pub ophiolite at 59 58 ma a second issue is that the reconstructions are based largely upon unfounded assumptions as to the age and nature of the basement beneath the loyalty arc and three kings ridge finally reconstructions of the northland region are based upon the erroneous assumption that the age of the majority of the igneous component comprising the northland allochthon is late cretaceous paleocene when in fact it is oligocene a new model is presented whereby the pub new caledonia and northland ophiolites formed and were emplaced in a cyclical fashion above an extensive ne dipping cenozoic intraoceanic arc system which diachronously propagated n s along the entire eastern margin of the australian plate these infant arc ophiolites represent fragments of suprasubduction zone lithosphere sszl generated in the earliest stages of magmatic arc formation that were emplaced shortly after 20 m y as a result of forearc australian plate collision subduction inception was the result of subsidence of older morb like lithosphere generated within an extensive back arc basin to the east of the norfolk ridge during the earliest stages of slb formation above a southwest dipping pacific plate during emplacement of each ophiolite a crustal fragment of the older lithosphere was scraped off the ne dipping slab and subsequently back thrust beneath each ophiolite during its emplacement

seismic stratigraphy of a buried volcanic arc northland new zealand and implications for neogene subduction abstract for over 6 million years in the early miocene the northland peninsula of new zealand s north island was the site of a volcanic arc comprising north westerly trending twin belts of marine and subaerial andesitic volcanoes approximately 50 volcanic edifices from major massifs to small cones have been newly identified on marine seismic profiles of the western belt which is largely buried the volcanoes were important point sources of sediment and the volcaniclastic deposits of each of the major and intermediate sized volcanoes form distinct seismic sequences the three main elements of the volcanic sequence are i a main body ii a sloping apron and iii a wide almost flat ring plain the downlap of the oldest discernible internal clinoform reflector relative to a known regional marker reflector underlying it indicates the approximate time of onset of activity whereas downlap of the top bounding reflector relative to those of other volcanic sequences provides a relative order of extinction sequence stratigraphic principles and super position determine relative stratigraphic hierarchy the volcanoes studied here were active only in the early miocene with few exceptions most were active by 22 ma and extinct by 16 ma with many extinct by 19 ma there was no pattern in their order of inception or extinction although a few became dominant burying or surrounding their neighbours at the south eastern end of the chain there is a rapid change over a short distance in both age and orientation from the northwest trending early miocene volcanism in the northland basin to north east trending middle and late miocene volcanism in the contiguous taranaki basin the northland volcanoes are interpreted to have been an arc facing the south westwards subducting south fiji basin whereas the taranaki volcanoes are interpreted to have been part of a separate pacific facing volcanic arc the lau colville ridge

hurricane storm surge simulations comparing three dimensional with two dimensional formulations based on an ivan like storm over the tampa bay florida region 1 we provide a dynamics based comparison on the results from three dimensional and two dimensional simulations of hurricane storm surge we begin with the question what may have occurred in the tampa bay florida vicinity had hurricane ivan made landfall there instead of at the border between alabama and florida this question is explored using a three dimensional primitive equation finite volume coastal ocean model the results show that storm surges are potentially disastrous for the tampa bay area especially for landfalls located to the north of the bay mouth the worst case among the simulations considered is for landfall at tarpon springs such that the maximum wind is positioned at the bay mouth along with such regional aspects of storm surge we then consider the dynamical balances to assess the importance of using a three dimensional model instead of the usual vertically integrated two dimensional approach to hurricane storm surge simulation with hurricane storm surge deriving from the vertically integrated pressure gradient force tending to balance the difference between the surface and bottom stresses we show that three dimensional structure is intrinsically important two dimensional models may overestimate or underestimate bottom stress necessitating physically unrealistic parameterizations of surface stress or other techniques for model calibration our examination of the dynamical balances inherent to storm surges over complex coastal topography suggests that three dimensional models are preferable over two dimensional models for simulating storm surges

deepwater sedimentation and cenozoic deformation in the southern new caledonia trough northern zealandia sw pacific abstract the new caledonia trough nct is a 2000 3000 m deep bathymetric feature that extends 2500 km from taranaki new zealand to the western margin of new caledonia northern zealandia sw pacific the underlying sedimentary basin originates from cretaceous extension but underwent a significant eocene tectonic event that shaped its present physiography we present an analysis of the basin based on multibeam data seismic profiles and rock samples collected on the tan1312 and tan1409 expeditions onboard r v tangaroa combined with legacy data we focus on the southern part of the basin where new data reveal a link between compressive deformation of paleocene strata and their potential reworking into the basin on the western basin side upper cretaceous to paleocene strata were deformed by local reverse faults and folds that created a sub circular bathymetric ridge and seabed exposure this folded unit seismic unit 2 is sharply overlain by a restricted interval imaged as chaotic high amplitude reflections that are interpreted as syntectonic mass transport deposits due to slope oversteepening seismic unit 1b this unit is stratigraphically below the main basin onlap surface and seismic mapping revealed that it rapidly thins away from the mouth of a present day submarine canyon imaged on the western slope of the basin and that is diverted by exposures of unit 2 deformed strata deformed and syntectonic intervals are in turn overlain by a flat lying unit seismic unit 1a we interpret as reflecting a passive basin fill our new data provide insight into an extensive deep water basin that is remote from terrigenous sediment sources and new constraints on its cenozoic tectonic history stratigraphic ages are constrained by seismic ties to taranaki basin petroleum wells and biostratigraphic dating of dredged samples this specific site has particular significance for understanding tectonic events in the southwest pacific indeed our observations show that deformation is younger than the paleocene and is envisaged to be a local expression of a widespread cenozoic compressive event called tecta tectonic event of the cenozoic in the tasman area which affected the region after the mesozoic rifting on a regional perspective this study provides new insights on the evolution of the submerged zealandia continent and associated geodynamic processes such as gondwana break up and initiation of the tonga kermadec subduction

the structure of the south fiji basin abstract new magnetic seismic and bathymetric data show that the south fiji basin is a structurally complex marginal basin a gap in the identifiable magnetic anomaly lineations exist over the central part of the basin and prevents the unequivocal linking of the anomaly lineations anomalies 7a to 12 associated with the ridgc ridge ridge triple junction in the north with an apparent single spreading centre of the same age in the south this gap which makes a detailed synthesis of the historical development of the basin difficult may arise from post spreading intraplate tectonics if symmetric spreading is assumed part of the oceanic lithosphere formed during the oligocene episode of seafloor spreading has subsequently been consumed presumably by subduction westward under three kings rise

location and migration of miocene quaternary volcanic arcs in the sw pacific region abstract we report new ar ar and u pb ages from ten rocks in the sw pacific region our results 1 establish a northward late miocene australian plate movement rate of 57 mm a for the lord howe hotspot chain 2 reinforce the previously established widespread nature of early miocene subduction related volcanism in onland and nearshore northern new zealand 3 indicate that leucotonalite xenoliths from raoul island are the products of quaternary kermadec arc magmatism rather than being cretaceous pliocene basement a synthesis of available sw pacific data emphasises that while there is a reasonably complete record of subduction related volcanism from at least 23 ma to the present day the process of back arc basin formation is highly episodic and asymmetric subduction related arcs stabilised along the taupo kermadec tonga arc from 2 ma to the present along the taranaki colville lau trend from 17 6 ma and along the three kings ridge northland plateau northland trend from 23 18 ma south of latitude 25 s back arc basin opening occurred during the early miocene and quaternary arc volcanic episodes but does not appear to have accompanied the 17 5 ma volcanism

oligocene miocene spreading history of the northern south fiji basin and implications for the evolution of the new zealand plate boundary a tectonic model of the evolution of the northern half of the south fiji basin including the minerva triple junction and cook fracture zone is developed from regional gravity multibeam bathymetry and a new interpretation of magnetic anomalies pinned to radiometric dates of oceanic crust in the basin the geometry and age of a portion of the minerva triple junction and the cook minerva spreading center the connection from the triple junction to the cook fracture zone which accommodated coeval opening of the norfolk basin are resolved with multibeam bathymetry and magnetics the south fiji basin opened from about 34 to 15 ma in an anticlockwise sweep about an euler pole located at the northern end of the present lau ridge this rotation and a rigidly straight southeastward motion of the three kings ridge were accommodated by the configuration of the triple junction changing from ridge fault fault to ridge ridge fault to ridge ridge ridge during this evolution the southeastern arm of the system the julia fracture zone underwent several transformations and the cook minerva spreading center experienced repeated ridge jumps the kinematics of the northern south fiji basin dictate to a large extent the evolution of the southern south fiji basin and the norfolk basin this in turn leads to the interpretation of a complex trench trench double transform fault framework at the northern new zealand margin which explains most aspects of the geology structure and arc volcanic history of the margin and provides a radical new setting for the origin of the northland allochthon

modeling ice dynamics of coastal seas a coupled sea ice barotropic ocean model with a 1 km resolution and a seaward domain of 200 km quantifies three coastal processes coupling of ice motion to wind driven coastal currents ice thickness redistribution under compaction at the coast and formation of coastal shear zones the model consists of an ice momentum balance mass concentration and two parameter ice thickness distribution and equations for horizontal water motion and continuity using vertical structure functions an appropriate constitutive law appears to be a hardening plastic based on qualitative observations from alaskan continental shelves for first year sea ice strength is taken to be proportional to the square of ice thickness a north wind example of 10 m s with the coastline to the west shows the depth dependence of rotational shear in the sea ice ocean boundary layer and sea surface tilt which contributes an alongshore slope current there is slight convergence of sea ice over the shelf a coastal shear zone of 4 km and an alongshore ice speed seaward of the shear zone of 6 of the wind speed caused by the combination of an under ice shear layer and an alongshore slope current for an onshore wind ice is near free drift at 3 of the wind seaward of a ridging front which propagates seaward a square dependence of ice strength on thickness is required for the rubble field to approach a limiting thickness consistent with observations the hardening plastic interpretation of the rubble field has the stress state at the yield limit in contrast with a rigid plastic of high constant strength that yields only at the coast we conclude that 1 ice thickness motion feedback is important on scales less than 10 km 2 the observational base to discriminate between mesoscale constitutive laws is not yet available and 3 the relation of ice velocity to wind stress is variable because the ocean slope current responds only to the alongshore component of the wind

tidal spectroscopy of the english channel with a numerical model abstract the possibility of reproducing the complexity of tides in shallow water areas with a classical finite difference numerical model is examined this hydrodynamic model is two dimensional but incorporates topography nonlinear advection and quadratic bottom friction particular care is taken to prescribe sea surface elevations at the open boundaries a one month simulation of real tides is run with a simplified spectrum restricted to only 24 constituents corresponding to the nine main astronomical tides and their nonlinear significant interactions the results are analyzed by spectral decomposition elevations and vertically integrated currents and compared with observational data from the tide gages and current meters and with other solutions produced in the literature it is found that the dominant m2 constituent greatly influences the damping of the other constituents so that it is necessary to run them together for any correct simulation however the quadratic friction law introduced in

cretaceous fore arc basalts from the tonga arc geochemistry and implications for the tectonic history of the sw pacific the tonga fore arc preserves a complex history of subduction initiation back arc basin formation and arc volcanism which has extended from the cretaceous to the present in this paper we discuss the geochemistry of a cretaceous basalt dolerite gabbro suite recovered in two dredges from the tonga fore arc at 19 s the geochemistry of the tonga fore arc suite is unlike that of the uniformly depleted morb basalts of the subducting pacific plate and therefore is unlikely to be accreted from pacific cretaceous crust the 102 ma age obtained for one tongan fore arc dolerite is contemporaneous with a major phase of cretaceous subduction related volcanism recorded both in detrital zircon age populations and associated volcanics from new caledonia and new zealand we believe that the tonga fore arc basalts are a remnant of a hypothesized once extensive cretaceous back arc basin called the east new caledonia basin which we propose to have existed from 102 to 50 ma the allochthonous poya terrane of new caledonia is geochemically very similar to the tonga fore arc basalts and represents a younger 84 55 ma remnant of the same basin subduction related cretaceous volcanics from the sw pacific representing both arc and back arc settings all appear to have similar zr nb values suggesting a common mantle component in their petrogenesis the tonga fore arc basalts are also similar to fore arc basalts recovered from the izu bonin mariana fore arc but unlike these basalts they are not associated with subduction initiation

unroofing of fore arc ranges along the hikurangi margin new zealand constraints from low temperature thermochronology abstract the axial ranges of north island new zealand parallel the hikurangi subduction margin they consist of uplifted and exhumed mesozoic meta sedimentary basement rocks of the overriding australian plate beneath which the pacific plate has been subducting since at least the late oligocene we investigate the unroofing histories of these fore arc mountains during the evolution of the hikurangi margin based on new and previous zircon and apatite fission track and apatite u th sm he data which we interpret based on inverse modelling the results suggest that the exhumation of rocks in the axial ranges initiated in the west and migrated trench wards towards the east onsets of accelerated exhumation in different parts of the ranges indicate significant eastwards thrusting on the margin parallel ngamatea fault before 20 17 ma and on the wellington mohaka fault before 10 7 ma the exhumation rate has varied significantly along strike since the late miocene lower in the central part of the axial ranges and significantly higher to the south since the late miocene the increasing exhumation rate from the central to southern axial ranges is consistent with the clockwise vertical axis rotation of eastern north island relative to the australian plate in the hikurangi margin although underplating of subducted material at the basal upper plate may have contributed to localised rock uplift e g in the raukumara range we suggest that the shortening of the fore arc upper plate was the chief driver of unroofing of the proto axial ranges

on extracting tidal current profiles from vertically integrated two dimensional hydrodynamic models a numerically accurate and computationally efficient method is developed for extracting tidal current profiles from vertically integrated two dimensional models the method can incorporate an arbitrary profile of eddy viscosity and determines tidal current profiles satisfying a no slip condition at the sea bed comparisons with observations and current profiles computed with full three dimensional models confirm the accuracy of the method bottom boundary layer thicknesses determined from these current profiles clearly show regions of the european continental shelf where the bottom boundary layer extends to the sea surface the boundary between these regions and other areas where the bottom layer is less than the water depth corresponds quite closely to the known position of tidal fronts

vertical flow structure during sandy duck observations and modeling abstract observations of the vertical distribution of time averaged cross shore and alongshore flows during the sandy duck field experiment are compared with model predictions to assess the parameters governing the flow behaviour the measurements were obtained with a vertical stack of eight two component current meters with the lowest and highest sensor at respectively o 0 1 and o 2 7 m above the bed observations under breaking wave conditions within the surfzone show that the maximum return flow velocities occur in the lower part of the water column consistent with laboratory observations under non breaking conditions outside the surfzone the maximum return flow velocities are observed closer to the water surface again in line with laboratory results analogous to previous observations the measured longshore current velocity profiles are logarithmic under non breaking conditions and become more depth uniform under breaking conditions the model description of the vertical structure of the flow includes the presence of wind stresses wave stresses pressure gradients turbulent eddy viscosity and a wave boundary layer the model utilizes parabolic shape functions to describe the vertical distribution of the turbulent eddy viscosity in the middle layer and within the bottom boundary layer eddy viscosity is enhanced in regions where turbulence is produced i e near the surface in the case of breaking waves and within the bottom boundary layer estimates of the wave breaking induced turbulent eddy viscosity and bottom friction are obtained by minimizing the model measurement discrepancies predictions utilizing calibrated expressions for both the turbulent eddy viscosity and bottom friction are in general agreement with the observations provided the wave transformation and associated mass flux are modeled correctly and a parabolic eddy viscosity distribution is used using a piecewise constant eddy viscosity distribution generally results in a degrading of the agreement between measurements and model results

petrogenesis of dacite in an oceanic subduction environment raoul island kermadec arc abstract raoul volcano in the northern kermadec arc is typical of volcanoes in oceanic subduction systems in that it is composed mainly of low k high al basalts and basaltic andesite however during the last 4 ka raoul volcano has produced mainly dacite magma in pyroclastic eruptions associated with caldera formation the rocks produced in these episodes are almost aphyric containing only sparse crystals of plagioclase clinopyroxene orthopyroxene and magnetite these apparent phenocrysts have chemical compositions that suggest that they did not crystallise from melts with the chemical composition of their host rocks rather they are xenocrysts and only their rims show evidence for crystallisation from their host melt chemical compositions of samples of the dacites show that each eruption has tapped a distinct magma batch compositional variations through the analysed suite cannot be accommodated in any reasonable model of fractional crystallisation from likely parental magma compositions the hypothesis that best fits the petrology of raoul island dacites is one of crustal anatexis this model requires heating of the lower crust by a magma flux to the point where dehydration melting associated with amphibole breakdown produces magma from a preconditioned source it is suggested that raoul is passing through an adolescent stage of development in which siliceous melts are part of an open system in which felsic and mafic magmas coexist

pre spread rifting and heterogeneous volcanism in the southern havre trough back arc basin abstract the havre trough is an active back arc basin associated with pacific australian plate convergence two segments within the southern havre trough sht surveyed with gloria sea beam and sys09 swath data now provide the first detailed data to reliably document the structure and morphology of this complex back arc region at the southern segment 35 40 37 s along a strike length of some 120 km the sht structure comprises a series of segmented en echelon generally flat floored sediment filled axial rift grabens flanked by ridge and basin rift block topography prominent fault escarpments with relief of 500 1000 m and a marked zig zag arrangement mark the outer margins of the rift system within the rift grabens extensional tectonism is pervasive extensive shallow seismicity within the sht can only partly be associated with the rift graben system at present youthful constructional volcanic terrains within the rift grabens are not common and invariably have a restricted areal extent and a relief an identical rift morphology has been identified at the northern 33 34 s site data from both sites show individual rifts are 15 35 km in length 6 10 km wide and have axial depths of 3100 4000 m apart from at least one 15 20 km wide cross trough ridge available data indicate the rift graben system is contiguous between 33 s and 37 s this rift graben morphology by analogy with other known rifting back arc basins is considered here not to be consistent with true oceanic back arc spreading rather it is argued that the sht possibly indeed all of the havre trough is undergoing back arc rifting prior to true spreading further the rifting tectonic fabric within the sht and in the onshore taupo to the south extension is consistently dextrally oblique relative to the bounding colville and kermadec ridges the degree of obliquity increases from south to north if accepted that the sht is rifting rather than spreading magnetic anomaly data within the trough do not record the accretion of new oceanic crust and consequently can no longer be used to establish the spreading history of the sht the sht magnetic anomalies are here interpreted as pseudo linear magnetic anomalies resulting from the generally irregular spatial and temporal emplacement of magnetic sheeted lava and dike intrusives between older low magnetisation arc basement rift blocks flanking the axial rift proxy data from onshore north island new zealand and offshore sht suggest that the rate of extension and age of initiation of back arc rifting are between 35 and 37 s respectively 15 20 mm yr 1 and 5 ma

on improving the bed stress formulation in storm surge models a method of improving the representation of bottom stress in storm surge models an enhanced bed stress model is tested using a fine mesh model of the celtic and irish seas and is compared with results from a conventional two dimensional model the approach uses a convolution method to determine the bed stress rather than relating it directly to the depth mean current as in a conventional storm surge model for the region considered comparisons of surge elevations computed with both models and observations show only a slight improvement of surge elevation in the near coastal region with the enhanced bed stress model however spatial distributions of bed stresses from the two models do exhibit significant differences which are important in considering sediment movement paths during major storms the magnitude and direction of the bed stress vectors computed from the enhanced bed stress model are in excellent agreement with calculations using a full three dimensional model of the area this suggests that the enhanced bed stress model can compute a bed stress that is not significantly different from that in a full three dimensional model at a fraction of the computational cost

stratigraphy of the southern norfolk ridge and the reinga basin a record of initiation of tonga kermadec northland subduction in the southwest pacific abstract seismic stratigraphic interpretations of a large new 2d seismic reflection dataset from the reinga basin region northwest of new zealand constrain the history of cretaceous fragmentation of gondwana and cenozoic initiation of tonga kermadec northland subduction the southern norfolk ridge system lay in a proximal location to plate boundaries that were active during both cretaceous and cenozoic events and persistent marine conditions led to a relatively complete record of sedimentation cretaceous extension was followed by regional subsidence and transgression two cenozoic contractional events are separated by substantial 1 km subsidence late eocene contraction led to reverse faulting and folding between new caledonia and the southern norfolk ridge system and topographic highs created in the north western sector were locally eroded by wave abrasion reinga basin was located at the southern tip of this event which had only limited impact in adjacent northern new zealand regional oligocene and miocene subsidence was contemporaneous with late oligocene and early miocene emplacement of nappes in northern new zealand and the onset of arc volcanism early miocene contraction in the reinga basin led to formation of the wanganella ridge these events can be related to calculated plate motions and are shown to be consistent with models of induced subduction nucleation that require c 150 km of convergence the first contraction is associated with regional inception of tonga kermadec subduction whereas oligocene to miocene events and the onset of hikurangi northland subduction were in response to a local change in plate boundary displacement rate and consequent linkage between subduction north of new zealand with alpine fault formation farther south

a three dimensional model of the northwest european continental shelf with application to the m4 tide abstract the solution of the fully nonlinear three dimensional hydrodynamic equations in polar coordinates for tides is developed using a finite difference grid in the horizontal and the galerkin method in the vertical using this method a three dimensional model of the northwest european continental shelf is used to investigate the spatial distribution of the m4 tidal elevations and currants over the shelf computed m4 currents are compared with observations and found to he in good agreement in the celtic sea although there are significant errors in the north sea the reasons for these discrepancies and the spatial and depth variations in the m4 currants are discussed in detail

application of a three dimensional turbulence energy model to the determination of tidal currents on the northwest european shelf a three dimensional turbulence energy tidal model employing a sigma coordinate system through the vertical with a logarithmic type transformation to enhance near bed resolution is used to examine m2 tidal currents on the european continental shelf a no slip bottom boundary condition is applied in the model and vertical eddy viscosity is computed from a turbulence energy closure scheme comparisons of observed and computed current profiles determined using the present model and an earlier model employing a spectral representation davies 1986 in the vertical and a slip bottom boundary condition are made in this spectral model a simple relationship between eddy viscosity and depth mean current was employed these comparisons of current profile suggest that above the near bed layer the spectral model can reproduce tidal currents with a comparable accuracy to that of the present turbulence energy model however in the near bed region the turbulence energy model is significantly more accurate computed spatial distributions of current magnitude turbulence energy intensity and eddy viscosity show similar distributions over the model region suggesting that a relationship between eddy viscosity and current intensity the form used in the spectral model is a valid alternative to using a turbulence energy model turbulence energy intensity in particular exhibits significant spatial variations both horizontally and vertically with surface contours of turbulence corresponding to known positions of tidal fronts

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

storm surges 1967 1982 summary an earlier review of storm surges covering the forty years up to 1967 is updated again the emphasis is on storm surges around the british isles but as before references are also made to surges which occur in other areas it is shown that the main advance in the work during the period considered has been the widespread development and use of two dimensional numerical storm surge models the application of these models to operational storm surge forecasting is described research evaluating storm surge currents using three dimensional models is considered

review of recent developments in tidal hydrodynamic modeling i spectral models in this paper we present a brief overview with a comprehensive set of references to recently published papers for detail of a range of techniques used in tidal hydrodynamic modeling and recent progress in the field although the paper aims to introduce the topic to someone new to the area the main emphasis is on new developments in near coastal shallow sea tidal modeling and consequently we concentrate on three dimensional modeling which has progressed significantly in recent years the paper gives a brief account of methods used to discretize the hydrodynamic equations in the horizontal namely finite difference finite element and boundary fitted coordinates and through time the representation of tidal current structure in a three dimensional model using a spectral method in the vertical is also described in this method the subgrid scale vertical diffusion of momentum which has a major influence on tidal current profiles is parameterized using a flow dependent eddy viscosity recent work on

four dimensional numerical modeling of crustal growth at active continental margins 1 crustal growth and topography development in subduction related arcs are intimately related to magmatic processes and melt production above subducting slabs lateral and temporal variations in crustal thickness and composition have been observed in nature but until now no integrated approach has been developed to comprehensively understand magmatic activity in subduction related arcs here we investigate the 4 d spatial temporal and compositional character of continental crustal growth at active margins using a new 4 d space time petrological thermomechanical numerical model of a subduction related magmatic arc based on a series of numerical experiments we demonstrate that crustal growth inside the arc is inherently clustered in both space and time the characteristic wavelength of variations in crustal thickness and topography along the arc is on the order of 30 80 km and is comparable to volcano clustering in natural arcs the clusters of new crust are formed mainly by basaltic melt episodically extracted from partially molten peridotite due to lateral variation of water release and transport in the mantle wedge melts derived from subducted oceanic crust and sediments could contribute up to 15 50 vol to the arc crust growth and their relative proportion is maximal at the onset of subduction the total amount of newly formed crust correlates mainly with the amount of convergence since the beginning of subduction and is not strongly influenced by the plate convergence velocity indeed slower subduction and lower melt extraction efficiency helps partially molten sediments and oceanic crust to be transported into the mantle wedge by hydrated partially molten diapiric structures for the modeled regime of stable subduction the maximum crustal additional rate 25 40 km3 km myr occurs when the amount of convergence reaches around 700 km mantle wedge structures developed in our models correlate well with available geophysical seismological observations for the alaskan subduction zone in particular partially molten mantle plumes found in our models could explain low seismic anomalies in the mantle wedge whereas mobile water and water release patterns could reflect paths and sources for magmatic activity evidenced by seismic b value and vp vs ratio analysis

three dimensional modal model of wind induced flow in a sea region abstract the solution of the linear three dimensional hydrodynamic equations describing wind induced flow in a sea region is developed using the galerkin method through the vertical a basis set of b splines is shown to have some computational advantages over a set of eigenfunctions vertical modes however a basis set of modes leads to a system of essentially uncoupled equations and current profiles can be interpreted in terms of vertical modes the influence of wind induced surface turbulence and turbulence at depth due to tidal motion upon current profiles in both deep 260 m and shallow 35 m sea regions is examined the variation in the angle between surface current and surface wind for different viscosity profiles and the effect of bottom friction upon it is considered the magnitude and direction of the surface current is significantly influenced by surface eddy viscosity however viscosity at depth due to tidal motion also has an important effect upon the surface current the time evolution of current structure following the sudden onset of a wind is examined using the modal model calculations show that the rate of damping of the internal modes is inversely proportional to the square of the depth consequently wind induced current structure takes longer to reach a steady state in a deep sea region than a shallow area the influence of sea surface elevation gradients in determining the direction of surface current is also considered

large magnitude oligo miocene extension in southern sonora implications for the tectonic evolution of northwest mexico new structural and 40ar 39ar data from the santa rosa area southeastern sonora shed light on the magnitude and timing of extensional tectonism episodic magmatic activity from the late cretaceous to the miocene is recorded by a 3 4 km thick structural stratigraphic section that includes a composite laramide granodiorite batholith and its andesitic wall rocks early to middle tertiary rhyolite ignimbrites and silicic domes and late oligocene to middle miocene mafic lavas interbedded with tuffaceous sediment and fanglomerate the older units are complexly faulted and steeply tilted by several generations of normal faults palinspastic reconstructions indicate cumulative extension of 90 early gently dipping nw striking normal faults document major extension oriented n50 e angular unconformities and growth fault relations within oligo miocene sequences bracket this extension between 26 and 20 ma younger widely spaced ns to nnw trending high angle normal faults cut the previously faulted and tilted sections and produced modest 10 15 e w extension that is largely bracketed between 20 and 17 ma the magnitude of neogene extension in this region is greater and the timing is older than previously recognized reconnaissance of other areas between the sierra madre occidental and the coast at this latitude suggests that most extension in sonora occurred between 27 and 12 ma while remnants of the farallon plate were still being subducted plate tectonic models that predict substantial extension in sonora during proto gulf 10 5 ma transtensional deformation may need to be reevaluated an alternative model presented here is that baja california began moving with pacific plate motion shortly after the 11 ma termination of subduction and that northwest motion of baja relative to mainland mexico might total 500 km distributed across the previously extended oligo miocene magmatic arc

north eastward subduction followed by slab detachment to explain ophiolite obduction and early miocene volcanism in northland new zealand oligocene miocene models for northern new zealand involving south westward subduction to explain early miocene northland volcanism do not fit within the regional southwest pacific tectonic framework a new model is proposed which comprises a north east dipping south loyalty basin slab that retreated south westward in the eocene earliest miocene and was continuous with the north east dipping subduction zone of new caledonia in the latest oligocene the trench reached the northland passive margin which was pulled it into the mantle by the slab resulting in obduction of the northland allochthon during and after obduction the slab detached from the unsubductable continental lithosphere inducing widespread calc alkaline volcanism in northland the new model further explains contemporaneous arc volcanism along the northland plateau seamount chain and sinking of the northland basement followed by uplift and extension in northland

stratigraphy and structure of the ngatamariki geothermal system from new zircon u pb geochronology implications for taupo volcanic zone evolution abstract 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 2800 m 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 35 ma 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 06 ma 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 0 039 ma between 0 7 and 0 35 ma contemporaneous surface lithologies in the ngatamariki area are dominated by sediments with subordinate lava domes between 0 716 0 017 and 0 655 0 016 ma at least three shallow to 5 km 3 the intrusions generated a large alteration halo 25 km 3 minimum and intense silicification of the wall rocks at 0 35 and 0 34 ma 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 0 011 ma 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 2 ka 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 8 km 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

enigmatic formation of the norfolk basin sw pacific a plume influence on back arc extension 1 the norfolk basin is a small back arc basin in the sw pacific with an unknown age and origin for its formation moreover the process behind its formation as a back arc basin in a region of extensive volcanism has previously never been resolved we have conducted a detailed interpretation of recently acquired swath bathymetry gravity and magnetic data from the northern norfolk basin faust 2 survey together with existing data from the region to address the age and origin of the norfolk basin and propose a new model for basin evolution the norfolk basin is characterized by complex bathymetry which formed during three distinct tectonic phases 1 regions of elevated topography termed plateaus show characteristics of thickened oceanic crust formed during the cretaceous normal superchron in either a back arc basin environment or as a trapped piece of pacific phoenix oceanic crust 2 regions of deep bathymetry termed basins separate the cretaceous plateaus and show basement textured by seafloor spreading fabric of miocene age 3 widespread intraplate plume related volcanic activity post dating the formation of the plateaus but predating the formation of the basins a miocene model for the formation of the basin regions in the norfolk basin requires a 700 km landward jump in the location of back arc spreading even with continued subduction at the tonga kermadec trench and the seaward migration of the subduction hinge the cessation of spreading in the basin regions would have been followed by an eastward jump of back arc spreading by about 1000 km into the current lau basin we propose that the driving force for such an enormous landward jump in back arc spreading is active rifting driven by a mantle plume this scenario is supported by the extensive miocene intraplate volcanism observed in the norfolk basin and the elevated bathymetry of the entire region in this model mantle driven uplift and excess volcanism would have favored back arc rifting landward rather than seaward of the existing south fiji basin in an extensional regime on the over riding plate caused by the continued roll back of the subduction hinge this scenario is supported by the numerous miocene volcanic cones in the norfolk basin which are mainly located on the preexisting plateaus probably predating the formation of the basins as well as their plume melt signature the norfolk basin appears to be the first known example of a major back arc basin whose position relative to the slab was not controlled by subduction driven mantle flow and melting but by plume tectonics this requires a re evaluation of the traditional back arc basin theory in supra subduction zones

basement geology from three kings ridge to west norfolk ridge southwest pacific ocean evidence from petrology geochemistry and isotopic dating of dredge samples abstract we present new petrographie x ray fluorescence electron microprobe ar ar radiometric fission track and sr nd and pb isotopic data for igneous and metamorphic rocks from 18 dredges from a number of basins and ridges in the southwest pacific ocean the dredge samples can be divided into four groups 1 permian to cretaceous gabbroids granitoids and hornfelses from the west norfolk and wanganella ridges 2 mainly cretaceous mafic igneous rocks from the reinga ridge and vening meinesz escarpment 3 subduction related late oligocene and early miocene basalts and shoshonites from the three kings and southern norfolk ridges and 4 back arc basin and intraplate basalts of early miocene age in the south and north norfolk basins respectively and intraplate basalts of pliocene age in the reinga basin our dredge data combined with regional magnetic and gravity data provide much needed tests of earlier tectonic interpretations of the region correlatives of the permian brook street terrane and mesozoic median tectonic zone of onshore new zealand clearly extend along the west norfolk and wanganella ridges the three kings ridge was active at least in the 19 to 20 ma period and was probably generated above a west dipping subduction zone the norfolk basin was a back arc basin to this arc and had opened by 18 20 ma the oldest cenozoic volcanic rocks in the region are 26 ma subduction related volcanic breccias on the southern norfolk ridge they probably predate opening of the norfolk basin and associated eastward migration of the three kings ridge

morphology and history of the kermadec trench arc backarc basin remnant arc system at 30 to 32 s geophysical profile microfossil and k ar data abstract knowledge of the time span of arc activity essential for correct tectonic reconstructions has been lacking for the kermadec arc system but is supplied in this paper through study of microfossils contained in dredge samples and k ar ages on dredged basalt clasts the kermadec system at south latitudes 30 to 32 in the southwest pacific comprises from west to east the colville ridge remnant arc havre trough backarc basin kermadec ridge active arc and kermadec trench site of west dipping subduction of pacific plate lithosphere beneath the australian plate data are presented from two traverses dredge magnetic single channel seismic across the whole system an important transverse tectonic boundary the 32 s boundary lies between the two traverse lines and separates distinct northern 32 25 s and southern 32 36 s sectors the northern sector is shallower and well sedimented with broad ridges and a diffuse backarc basin the southern sector is deeper with narrow ridges and steep escarpments facing inwards to a little sedimented rifted backarc basin the kermadec ridge slopes smoothly trenchward to a mid slope terrace forearc basin with minor sediment fill at 5 6 km water depth a steeper 10 24 and more rugged lower trench slope is mantled with new zealand sourced rhyolitic vitric mud diamictons containing locally derived mafic volcanic clasts one clast is of late miocene age k ar age 7 84 ma the arc kermadec ridge is capped by active volcanoes very young k ar ages