northland ophiolite new zealand and implications for plate tectonic evolution of the southwest pacific the northland ophiolite and coeval ocean floor sedimentary rock form the northland allochthon obducted from the northeast onto new zealand in the late oligocene the ophiolite was probably emplaced as a single sheet and separated into individual massifs during subse quent movement of the allochthon chemically the bulk of the igneous rocks are normal mid ocean ridge basalts but the ophiolite also includes a younger suite of hornblende modal within plate alkalic rocks believed to represent seamounts two alternative models one involving a subduction flip the other involving continuous westward subduction are proposed to account for the obduction of the upper part of the northland ophiolite the oceanic crust from which the ophiolite originated was formed simultaneously with tasman sea spreading on the western rim of a once much larger south fiji plate assemblage

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

new caledonia obducted peridotite nappe offshore extent and implications for obduction and postobduction processes one of the largest ophiolitic peridotite masses in the world covers a quarter of the island of grande terre new caledonia the peridotite nappe was obducted during the eocene is weakly deformed and corresponds to the highest of a structurally simple pile of thrust nappes we present new marine seismic data that allows us to track the offshore continuation of the peridotite nappe along strike for a distance of more than 500 km south of new caledonia and to image its pre syn and post obduction sedimentary records offshore the peridotite nappe underlies a c 150 km wide and 2 km deep basin flat topped horsts of peridotite are clearly bounded by major normal faults in contrast faults are obscure onland to the east the peridotite nappe roots along the eastern margin of the felicite ridge new name a c 300 x 25 km dome shaped ridge which we interpret as being the southern extension of the hp lt metamorphic core complex observed in new caledonia two alternative tectonic models address the relative timing and relationships between peridotite nappe emplacement uplift of a metamorphic core complex and extensional tectonics these models provide new ideas for the understanding the formation of the eastern margin of the zealandia continent our results contribute to an understanding of how oceanic mantle is emplaced onto continental margins

morphology and origin of the osbourn trough the osbourn trough is a 900 km long east west trending gravity low visible in satellite altimetry data which intersects the tonga trench at 25 30 s a recent survey collected gravity magnetic echo sounder and swath bathymetry data on three adjacent north south trending ship tracks centered on the trough the linear gravity low is 20 30 mgal less than the regional value and is accompanied by a flat lying 200 500 m deep sediment filled valley swath bathymetry images reveal several parallel east west trending linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers including a prominent inside corner high and possible pseudo fault trace the presence of magnetic anomalies possibly anomalies 33 and 32 suggests that the seafloor here was formed after the end of the cretaceous normal superchron anomaly 34 these data support the conclusion that this trough is a spreading center which stopped spreading in late cretaceous time the existence of this feature has important implications for tectonic reconstructions in this region the osbourn trough could be part of the fossil spreading center between the pacific plate and a fragment of the phoenix plate the bellingshausen plate this would require the seafloor between the osbourn trough and the chatham rise to the south to be a remnant fragment of the bellingshausen plate this remnant may have joined to the pacific plate when the hikurangi plateau entered the gondwana subduction zone at the chatham rise possibly causing the cessation of spreading on the osbourn trough

geologic and kinematic constraints on late cretaceous to mid eocene plate boundaries in the southwest pacific abstract starkly contrasting tectonic reconstructions have been proposed for the late cretaceous to mid eocene 85 45 ma evolution of the southwest pacific reflecting sparse and ambiguous data furthermore uncertainty in the timing of and motion at plate boundaries in the region has led to controversy around how to implement a robust southwest pacific plate circuit it is agreed that the southwest pacific comprised three spreading ridges during this time in the southeast indian ocean tasman sea and amundsen sea however one and possibly two other plate boundaries also accommodated relative plate motions in the west antarctic rift system wars and between the lord howe rise lhr and pacific relevant geologic and kinematic data from the region are reviewed to better constrain its plate motion history during this period and determine the time dependent evolution of the southwest pacific regional plate circuit a model of 1 west dipping subduction and basin opening to the east of the lhr from 85 55 ma and 2 initiation of northeast dipping subduction and basin closure east of new caledonia at 55 ma is supported west dipping subduction and basin opening were not driven by convergence as has previously been proposed our plate circuit analysis suggests that between at least 74 ma and subduction initiation at 55 ma there was little net relative motion between the pacific plate and lhr

u pb zircon dating of post obduction volcanic arc granitoids and a granulite facies xenolith from new caledonia inference on southwest pacific geodynamic models in new caledonia the occurrence of one of the world s largest and best exposed subduction obduction complex is a key point for the understanding of the geodynamic evolution of the whole southwest pacific region within the ophiolite pre and post obduction granitoids intrude the ultramafic allochthon and provide new time constraints for the understanding of obduction processes at 27 4 ma a new east dipping subduction generated the active margin magmatism along the western coast of the island saint louis massif at 24 3 ma the eastward shift of the magma activity and slightly different geochemical features koum borindi massif was either related to the older slab break off or alternatively due to the eastward migration of the mantle wedge following the collision of the eastern margin of the low howe rise finally the occurrence of a granulite facies xenolith in the koum borindi massif with comparable 24 5 ma u pb zircon age and isotopic features initial end 5 1 suggests that these evolved magmas were generated within the lithospheric mantle beneath a continental crust of normal thickness geochronological evidence for continuous convergence during the oligocene infers an east dipping eocene oligocene subduction obduction system to have existed in the southwest pacific from the d entrecasteaux zone to the north island of new zealand

a comparative study of late cretaceous ophiolitic basalts from new zealand and new caledonia implications for the tectonic evolution of the sw pacific abstract our understanding of the tectonic evolution of the sw pacific between the cretaceous and the miocene is incomplete during this period there were a number of ophiolitic complexes formed and emplaced including the tangihua ophiolite complex of new zealand and the poya terrane ophiolites of new caledonia both ophiolitic complexes contain rocks from only the uppermost levels of the oceanic crust have a similar age of formation and were emplaced during the eocene consequently it is thought that the two systems are somehow related however the poya terrane and the tangihua ophiolite complex have significantly different geochemistry the poya terrane basalts are predominantly p and n morb tholeiites generated during the opening of a small marginal basin north east of new caledonia whereas the tangihua basalts range from back arc tholeiites to calc alkaline arc basalts suggesting formation in an arc back arc setting the similarities in age emplacement and stratigraphy between the two systems must be a result of the tectonic regime operating within the sw pacific as the new geochemical information clearly shows that the poya terrane and the tangihua ophiolite complex are not directly related

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

geochemistry and tectonic significance of the tangihua ophiolite complex new zealand abstract the ophiolitic igneous complexes of the tangihua complex of northland new zealand consist of massive and pillowed basaltic lava sequences with intercalated sediments and lesser gabbro microgabbro and basaltic intrusions the complex also includes minor felsic derivatives younger alkalic intrusions which exhibit within plate characteristics and rare ultramafic rocks the lavas are relatively homogeneous and are dominantly tholeiitic basalts with minor calc alkaline and alkaline affinities the complex nature of the chemistry in particular those lavas with transitional arc signatures and the presence of non arc lavas is clear evidence that these volcanics formed in a suprasubduction zone environment the combined geochemical and tectonic constraints suggest that the tangihua complex formed in either a transitional zone between an arc and a back arc setting or in a zone of migration from arc to back arc volcanism as such the field age and geochemical data support the model in which these rocks formed suprasubduction zone crust and are fragments recording some or all stages of arc initiation arc rifting and back arc basin formation the orientation of the arc and back arc volcanic rocks in relation to the over tectonic regime suggests subduction towards the west recent work in the sw pacific shows that late cretaceous ophiolites are present in papua new guinea new caledonia and new zealand the formation of these ophiolitic systems supports the presence of a large scale tectonic regime convergence and consequent westward subduction stretching from papua new guinea to new zealand during the late cretaceous to early palaeocene

significance of palaeomagnetic data from the oceanic poya terrane new caledonia for sw pacific tectonic models results from a palaeomagnetic study of pillow basalts and associated pelagic sediments of the late cretaceous palaeocene poya terrane of new caledonia are presented the poya terrane forms a nappe that was thrust sw over the island i e the pacific side in the middle eocene data from four outcrops nine sites spread over 250 km yield a normal polarity tilt corrected mean inclination of 57 2 where 95 11 2 and k 67 4 which suggests formation at 37 8 12 1 s the between outcrop declinations are unfortunately too scattered to be used in regional tectonic discussions they indicate rotations that are localised and sometimes very large following reference to sw pacific late cretaceous palaeogene plate tectonic reconstructions the inclination data suggest that the poya terrane formed close to the new caledonian portion of the indo australia plate this interpretation is consistent with tectonic models where the poya terrane is generated in a marginal basin immediately to the northeast of new caledonia during the break up of east gondwana the terrane was thrust over new caledonia in the middle eocene possibly in response to a change in relative motion of the australian and pacific plates at about this time a short time later it was overthrust by a supra subduction zone ultramafic nappe and together they have since drifted northwards as passengers on the australia plate

baroclinic developments in jet entrances and exits i linear normal modes this study examines the effect of the ageostrophic circulations associated with jet entrance and exit regions on the structure and growth rates of baroclinic normal modes a linearized gcm is used to find the most unstable normal mode disturbances for a zonally symmetric basic state consisting of a zonal jet and meridional circulation normal mode growth rates are reduced by up to 20 for realistic strengths of meridional overturning and normal mode structures are significantly affected for example the normal mode poleward tilt with height varies affecting the conversion of basic state available energy a mechanism for the change in poleward tilt is proposed ageostrophic meridional circulations may play a significant role in determining the magnitude and length of mid latitude storm tracks

bay of islands and little port complexes revisited age geochemical and isotopic evidence confirm suprasubduction zone origin the bay of islands complex of the humber arm allochthon west newfoundland contains the best exposed ophiolite in the appalachian orogen associated structural slices the little port and skinner cove complexes also contain rocks formed in an oceanic domain although their relationship to the bay of islands complex remains controversial to constrain the origin of the complexes and obtain a better understanding of the geology of the humber arm allochthon we have undertaken an integrated geochronological geochemical and isotopic study a u pb zircon age of ma for the little port complex and a zircon and baddeleyite age of 484 5 ma for the bay of islands complex have been obtained geochemical and isotopic data on trondhjemitic rocks from the two complexes indicate that petrogenetic models for these rocks must account for fundamental differences in source materials and mineralogy during differentiation the little port complex trondhjemites are characterized by initial end of 1 to 1 whereas those

drumlin formation related to inverted melt water erosional marks drumlin forms are described from maps and air photographs of a part of the athabasca plains northern saskatchewan three major forms spindle parabolic and transverse asymmetrical are recognized these forms which may show superimposed minor elements depart from classical descriptions of drumlins but are similar to moulds of erosional marks created by separated fluid flows assemblages of drumlins also show characteristics similar to those of erosional marks the form analogy between drumlins and moulds of erosional marks is carried to a conclusion that drumlins may be formed by the infilling of erosional marks created on the under side of glaciers by separated subglacial melt water flows estimates of specific discharge are obtained by means of an expected range of reynolds number geomorphological evidence is given for large scale erosion by subglacial melt water a discussion of the sedimentology stratigraphy and deformational structure of the interiors of drumlins shows that they may be explained by the erosional mark hypothesis this paper emphasizes the importance of melt water as a geomorphic agent and may have broad implications for ice sheet dynamics and profiles rates of deglaciation and the occurrence of bedrock thrusting by ice

wave zonal flow interaction and ultra low frequency variability in a simplified global circulation model the interactions between baroclinic waves and the zonal mean mid latitude flow are investigated using a multilevel primitive equation model of moderate resolution with idealized surface friction and diabatic heating terms integrations corresponding to 100 years were performed under perpetual solstice conditions empirical orthogonal function eof analyses were carried out on the zonal mean zonal wind u adjustments to the winter hemisphere zonal jet due to the growth and decay of baroclinic waves can be described in terms of the first two eof patterns which exhibit substantial interannual variability for a single lifecycle such adjustments take the form of a circuit of a phase space whose axes are defined by the amplitudes of the two eofs the cumulative effects of multiple lifecycles lead to a low frequency circulation in the phase space with a timescale of about 150 days we suggest that this effect may be intrinsically related to the internal generation of ultra low frequency variability in the model normal mode lifecycle calculations including dissipation reveal similar phase space properties the growth rates of normal mode disturbances are shown to be highly sensitive to the phase of the first eof

eocene arc continent collision in new caledonia and implications for regional southwest pacific tectonic evolution new caledonia preserves evidence that constrains models for the tectonic evolution of the southwest pacific region onland geology reflects four main tectonic phases 1 early mesozoic development of subduction related terranes and their accretion to the gondwana margin 2 cretaceous passive margin development and sea floor spreading during the gondwana breakup 3 foundering of an oceanic basin and the eocene arrival of thinned gondwana margin crust at a southwest facing subduction zone resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast and 4 detachment faulting during extensional collapse resulting in unroofing of metamorphic core complexes the last phase explains supposedly anomalous metamorphic gradients in the northeast of the island

linear and nonlinear baroclinic instability of the northern hemisphere winter zonal flow abstract this study was carried out to investigate the extent to which the results of linear or f plane solutions for baroclinic instability of simple flows can be applied to the northern hemisphere winter zonal mean zonal flow in particular the idea of baroclinic adjustment postulated by stone 1978 in response to a parameterization of poleward heat flux presented by held 1978 and based on charney s plane model is considered this parameterization is very dependent on a simple formulation for the depth scale of baroclinic waves and it is shown here that this is not an appropriate scaling for many of the modes under consideration the baroclinic adjustment hypothesis is examined by considering first the linear stability of a variety of atmospheres then the normal modes are integrated using a spectral primitive equation model and their nonlinear development is found to depend on the process of barotropic decay which is not a feature of the simple theoretical models the time averaged wave struc

formation and emplacement of the northland ophiolite northern new zealand sw pacific tectonic implications petrological geochemical geochronological and palaeomagnetic data for rocks of the northland ophiolite terrane of northern new zealand suggest that it formed in a suprasubduction zone setting between c 29 and 26 ma at c 35 s close to its late oligocene obduction site cretaceous igneous rocks formerly considered to be part of the ophiolite probably represent the basement upon which the ophiolite was emplaced and are probably part of the mount camel arc related terrane the ophiolite is believed to have been generated in the southeastern south fiji basin close to a nw se oriented transform fault located to the sw of the vening meinesz fracture zone and was probably emplaced in response to the collision of the hikurangi plateau with eastern new zealand at the end of the oligocene this collision would have involved a major adjustment on the transform fault thereby allowing a portion of the upper crustal section of the southern south fiji basin to be emplaced southwestward onto northern new zealand as well as the coeval emplacement of the east cape allochthon to the south concomitant subduction of the lower crust mantle section led to the initiation of arc volcanism that resulted in the northland lower miocene volcanic plutonic suite

erosion of bedrock by subglacial meltwater georgian bay ontario a regional view a spectacular series of sculpted erosional forms s forms is mapped and described from a 70 km wide area along the shore of georgian bay ontario which except for a scattered boulder lag has been swept clean of sediment a great variety of sculpted forms is described and illustrated and grouped into three classes transverse longitudinal and nondirectional forms transverse forms comprise transverse troughs muschelbruche sichelwannen and comma forms longitudinal forms comprise spindle flutes cavettos and furrows and nondirectional forms consist of undulating surfaces and potholes transverse forms are preferentially located on stoss slopes and longitudinal forms on lee slopes of rock rises undulating nondirectional forms are found on distal slopes and potholes at major breaks in slope this correlation of form and bed topography suggests that relief exerts considerable control on both form and location form geometry is also inferred to be related to coherent flow structures and their int

a seismic crustal section across the east cape convergent margin new zealand abstract crustal seismic reflection measurements were made along a 250 km profile across the pacific australian plate boundary north of new zealand data were recorded to 16 s two way time and have defined a major sedimentary basin corresponding to the southern end of the raukumara basin a fore arc basin reaching a depth of about 13 km this basin contains a lower generally conformable sequence of sediments and an upper sequence dominated by several major uncomformities the eastern part of this basin is cut by several recently active eastward dipping high angle thrust faults which are interpreted to be caused by the recent convergence at the adjacent pacific australian plate boundary the eastern margin of the basin is marked by a sharp change in character on the seismic data we interpret this feature as a terrane boundary coinciding with a major transcurrent fault which has juxtaposed a detached fragment of an island arc fore arc with a thin 15 km crust to the east against a passive margin sedimentary sequence to the west the southern raukumara basin is floored by cretaceous age oceanic crust fragments of which were obducted onto the north island about 22 ma ago the area has had a complex history a schematic model for the evolution of this margin during the past 22 ma is developed suggesting that the raukaumara basin has developed as a combination of fore arc basin processes and oblique crustal extension

hikurangi plateau a cretaceous large igneous province in the southwest pacific ocean the first dredge haul of basement rocks obtained from the hikurangi plateau in the southwest pacific ocean consists dominantly of volcanic and volcaniclastic rocks of probable pre late cretaceous age all samples have undergone extensive seafloor weathering to phyllosilicate and zeolite bearing assemblages petrography mineral chemistry and whole rock element concentrations and ratios of the least altered lavas e g tio2 1 3 1 6 wt cr 132 224 ppm zr y 2 9 3 6 zr nb 19 29 ti v 22 29 suggest a restricted igneous compositional range broadly comparable with normal to enriched mid ocean ridge basalt however isotopic compositions of leached samples 87sr 86sri 0 70361 0 70374 endi 5 7 6 2 are comparable with oceanic island basalt suites collectively these petrological characteristics are similar to rocks from large igneous provinces of cretaceous age in the western pacific ocean e g manihiki and ontong java plateaus consequently we interpret the hikurangi plateau as another of these early cretaceous basaltic oceanic plateaus speculatively parts of the obducted tangihua and matakaoa igneous complexes of the north island may have been derived from the hikurangi plateau as the plateau collided with the new zealand continent in the early miocene

palaeoecology and palaeoceography near cochrane alberta canada just after the last major high stand of glacial lake calgary abstract this paper describes a fossiliferous section through a mound of lake calgary sediments at 1201 m elevation at grid reference 6671 6701 south of cochrane alberta canada the section lies just west of the zone of interaction of cordilleran and laurentide ice sheets these lake deposits can be traced laterally to sections where they overlie gravels of the bighill creek formation date previously at about 10 200 11 300 radiocarbon years b p the lake maximum is therefore younger the sediments indicate that forest occurred around the glacial lake until shortly after 9650 yr b p and that contemporary forest fires produced charcoal as air temperatures rose grassland replaced the forest and the ice melted the lake had drained by 8145 yr b p

tectonic implications of paleomagnetic data from the northland ophiolite new zealand abstract paleomagnetic sampling of 12 sites from two separate massifs of the northland ophiolite new zealand yields consistent results two characteristic nrm directions are recognised one of normal polarity associated with weakly magnetised rocks particularly intrusive dolerites and another of reversed polarity associated with strongly magnetised rocks particularly basaltic pillow lavas thermal and af demagnetisation shows that the normal directions are due to a secondary viscous component which reflects the present day field direction whilst the reversed directions are stable typically to 500 c reflecting either the primary trm or a hard secondary component the declination and inclination of this stable magnetisation differ substantially from that of the present day field implying significant tectonic movement or deformation of the ophiolite since formation anomalous declinations and inclinations are interpreted here in terms of rotations about vertical axes and tentatively latitudinal translation since no widespread significant tilting has been recognised within the massifs declinations are consistent with clockwise rotations of 50 and 120 for the cape reinga and mangakahia massifs respectively probably associated with obduction at a convergent margin with a right lateral slip component situated to the northeast of northland the anomalously low inclination vectors 30 40 are consistent with an origin at lower than present latitudes which if true implies long lived n s translation of oceanic crust in the southwest pacific region between cretaceous and oligocene times

tectonic accretion and underplating of mafic terranes in the late eocene intraoceanic fore arc of new caledonia southwest pacific geodynamic implications this paper deals with the tectonic events that result in the accretion of mafic terranes in the fore arc region and a close juxtaposition of ultramafic rocks low grade and high grade mafic terranes in many collisional orogens the example is taken from new caledonia where tectonic accretion subduction underplating and obduction of mafic terranes took place during the late eocene in an intra oceanic forearc setting the late eocene tectonic complex comprised three major terranes an overlying ultramafic mainly harzburgitic allochthon named the ophiolitic nappe an intermediate mafic mainly basaltic off scraped melange composed of kilometre scale slices of oceanic upper crust called the poya terrane parts of which have been metamorphosed into an eclogite blueschist facies complex the pouebo terrane and a lower continental basement formed by the norkolk ridge terranes based upon exhaustive sampling of the mafic terranes and field surveys our tectonic micropaleontologic and geochemical data reveal that poya and pouebo terranes rocks originally formed within one single campanian to late paleocene oceanic basin floored by tholeiitic basalt associated with some minor seamount related intraplate alkali basalt the tholeiitic basalt displays a continuous range of compositions spanning between undepleted and depleted end members the former being volumetrically predominant the overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin the south loyalty basin which almost completely disappeared during eocene convergence the opening of this basin originally located to the east of the norfolk ridge was synchronous with that of tasman sea basin as a consequence of oceanward migration of the west dipping pacific subduction zone establishing the origin of the ultramafic ophiolitic nappe is beyond the scope of this paper however it appears to be genetically unrelated to the mafic poya and pouebo terranes although it was located in the late eocene fore arc the ophiolitic nappe and the corresponding oceanic lithosphere originated before the late cretaceous to the east of the south loyalty basin in a back arc setting or alternatively in a much older trapped basin for reasons that remain unclear a new east dipping subduction started in the eocene and consumed most of the south loyalty basin forming the intra oceanic loyalty arc due to a changing subduction regime underplating of the diahot terrane the mafic slices that now form the poya terrane were tectonically accreted in the loyalty fore arc region and remained under low pressure low temperature conditions possibly at the subsurface until the norfolk ridge reached the subduction zone diachronously this resulted in the final obduction of the fore arc area the two step obduction involved first the mafic complex forming the poya terrane and thereafter the lithospheric mantle that now forms the ophiolitic nappe in contrast pieces of the accretionnary complex were dragged down into the subduction zone underplated at depth ca 70 km and metamorphosed into high temperature eclogite to form the pouebo terrane metamorphics that display the same geochemical features as the poya terrane basalt a mid to late eocene syntectonic piggy back sedimentary basin the nepoui flysch basin mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore arc region due to the accretion and underplating of mafic ocean related and other material in contrast a slightly younger foreland basin located upon the norfolk ridge the priabonian bourail flysch basin received a massive input of detrital material derived from the norfolk ridge itself and a time increasing amount of mafic poya derived material that recorded the first step of obduction thereafter the bourail flysch was overthrust by the poya terrane and finally by the ophiolitic nappe at the same time buoyancy driven uplift and exhumation of the high pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone finally for a short time a new subduction started along the west coast of new caledonia and generated small amounts of active margin related magmas these events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre collision analogue of mafic ultramafic belts found in many collisional mountain ranges

link between ssz ophiolite formation emplacement and arc inception northland new zealand u pb shrimp constraints cenozoic sw pacific tectonic implications abstract new u pb age data from zircons separated from a northland ophiolite gabbro yield a mean 206 pb 238 u age of 31 6 0 2 ma providing support for a recently determined 28 3 0 2 ma shrimp age of an associated plagiogranite and 29 26 ma 40 ar 39 ar ages n 9 of basalts of the ophiolite elsewhere miocene arc related calc alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206 pb 238 u ages of 20 1 0 2 and 19 8 0 2 ma the ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122 104 ma the early cretaceous zircons in the arc andesites are interpreted as xenocrysts from the mt camel basement terrane through which magmas of the northland miocene arc lavas erupted the inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation we postulate that the tholeiitic suite of the ophiolite represents the crustal segment of ssz lithosphere sszl generated in the southern south fiji basin sfb at a northeast dipping subduction zone that was initiated at about 35 ma the subduction zone nucleated along a pre existing transform boundary separating circa 45 20 ma oceanic lithosphere to the north and west of the northland peninsula from nascent back arc basin lithosphere of the sfb construction of the sszl propagated southward along the transform boundary as the sfb continued to unzip to the southeast after subduction of a large portion of oceanic lithosphere by about 26 ma and collision of the sszl with new zealand compression between the australian plate and the pacific plate was taken up along a new southwest dipping subduction zone behind the sszl renewed volcanism began in the oceanic forearc at 25 ma producing boninitic like ssz and within plate alkalic and calc alkaline rocks rocks of these types temporally overlap ophiolite emplacement and subsequent miocene continental arc construction

the geochemistry of tholeiitic and alkalic plutonic suites within the northland ophiolite northern new zealand magmatism in a back arc basin abstract the northland ophiolite of new zealand consists mainly of tholeiitic basalts with subordinate alkalic volcanic rocks the alkalic rocks occur together with but are generally younger than the tholeiitis both the tholeiitic and alkalic suites include minor plutonic intrusives which range from gabbro to plagiogranite variations within the tholeiitic plutonic suite can be modelled in terms of low pressure crystal fractionation the evolution of the alkalic suite is more problematic because each massif appears to be derived from a slightly different parental magma new trace element chemistry presented in this study shows that the tholeiitic rocks formed from a mantle source that was compositionally modified by supra subduction zone processes but that the alkalic suite has a different chemical signature nevertheless both suites of rocks are interpreted as having formed in a back arc spreading system in this environment the addition of fertile mantle to the supra subduction wedge facilitated by subduction roll back provided a source for the alkalic rocks with mixed geochemical characteristics

change of tectono stratigraphic regime in the australian plate during the 99 ma mid cretaceous and 43 ma mid eocene swerves of the pacific the clockwise bend at 99 ma mid cretaceous in linear volcanic chains in the tropical pacific coincides with a change from pre 99 ma head on chilean type subduction of the pacific plate beneath eastern gondwana to 99 43 ma sinistral oblique mariana type subduction and strike slip breakup by simple sea floor spreading between australia and antarctica and by backarc spreading in the southwest pacific the 99 ma breakup of australia from antarctica is documented by a mid cretaceous unconformity this tectono stratigraphic change founded modern australia with a mountain chain along an upper plate margin in the east and lowlands on the lower plate margin in the south the counterclockwise bend at 43 ma mid eocene the emperor hawaiian bend coincides with the onset of structure in the challenger rift of new zealand the eromanga cooper basin of central australia and the oil shale grabens of coastal queensland

trace element variations in atlantic ocean basalts and proterozoic dykes from northwest scotland their bearing upon the nature and geochemical evolution of the upper mantle abstract trace element data on ocean floor and ocean island basalts from the atlantic and from early proterozoic dyke swarms are considered in relation to the nature and causes of upper mantle heterogeneity studies of dredged and drilled basalts from the north atlantic have enabled the compositions of erupted ridge basalts to be monitored in space and time and have revealed the following features a that while melting processes may give rise to appreciable ree variation at a particular site ratios of the more incompatible elements remain remarkably consistent in basalts erupted at a particular ridge segment for tens of millions of years and b that there are major differences in incompatible element ratios in basalts erupted at different ridge segments the relationships suggest that the volumes of mantle with these characteristic incompatible element ratios must be large but that the processes associated with basalt generation are not capable of significantly changing incompatible element ratios nevertheless the processes which have produced these incompatible element variations must be incorporated into any model for mantle evolution our preferred model invokes both crustal extraction and incipient mantle melting as the major mechanisms for changing incompatible element mantle source regions both represent very small degrees of mantle melting a depleted upper mantle variably veined with incompatible element enriched liquids or fluids derived through incipient melting at greater depths is most consistent with the trace element and isotopic compositions of north atlantic basalts many of which are not normal mid ocean ridge basalts n type morb ocean island basalts may represent more intensely veined and in some cases at least more refractory sources sampled at lower degrees of partial melting mantle from which incipient melts have been removed probably represents the n type morb source extraction of crustal material having high contents of the large ion lithophile elements but low contents of the high field strength ions ie ta and nb from a veined metasomatized mantle source leaves the upper mantle residue enriched in ta and nb relative to the large ion lithophile elements this might explain the enrichment of ta and nb in most mantle sources of continental and ocean island basic lavas mantle sources enriched by incipient melting processes are continually dissipated by mantle convection but may be preserved for a significant time beneath stable continents trace element data on early proterozoic tholeiitic to picritic dyke swarms from scotland suggest that short term mantle heterogeneities may have existed beneath the region at that time and could have been related to the processes of crustal generation 600 m y earlier

cretaceous termination of subduction at the zealandia margin of gondwana the view from the paleo trench abstract during the permian to cretaceous zealandia occupied a position on the proto pacific facing convergent margin of gondwana subduction on this margin ceased somewhere between 105 ma and perhaps 70 ma but the timing of this tectonic transition remains controversial resolution of this uncertainty is important for tectonic reconstructions of the southwest pacific and for global plate tectonic models here we revisit the problem by reference to new stratigraphic and geochemical data from the east coast basin of new zealand which occupied a position adjacent to and possibly superimposed on the relict cretaceous subduction trench at the time subduction ceased this basin is expected to preserve direct structural and stratigraphic evidence for or against late cretaceous subduction in western parts of the east coast basin the western sub belt a conspicuous unconformity separates undoubted accretionary prism rocks of the torlesse composite terrane from younger cretaceous cover units strata beneath and overlying this unconformity vary in age from place to place but abundant paleontological data and published ages of detrital zircons some reinterpreted herein indicate that exposed torlesse rocks are nowhere younger than 100 ma overlying zealandia megasequence cover strata are mostly younger than 110 ma between 110 and 85 ma the entire length of the western sub belt reveals complex stratigraphic architecture of relatively small scale depocentres subjected to alternating episodes of subsidence and local uplift and erosion there is widespread evidence for compression on the western sub belt over this period although the overall amount of shortening is likely to be relatively modest in contrast the eastern sub belt preserves a record of near continuous and apparently simple deposition over the same interval of time we see little and somewhat equivocal evidence for extension in either sub belt through the late cretaceous superimposed on this general pattern we observe discrete widespread or basin wide tectonic events at 95 ma and within the intervals 86 83 ma and 83 81 ma indicated by the presence of unconformities and sedimentary facies changes importantly all these events can be correlated with coeval events recorded elsewhere in zealandia suggesting that the east coast basin was subject to the same tectonic regime as the rest of zealandia and shared a common history during the mid to late cretaceous integrating these findings with data from elsewhere in zealandia we argue that there is diverse evidence to indicate that subduction finished along the new zealand segment of the gondwana margin by 100 ma that said the causes of on going late cretaceous compression in the east coast basin and in other parts of zealandia are not well constrained although several possible explanations are plausible there are few analogue abandoned subduction margins described in the literature however the situation in the south shetland trench shows strong similarities with patterns and complexities observed in the east coast basin

comments on palaeoecology and palaeogeography near cochrane alberta canada just after the last major high stand of glacial lake calgary

geochemistry and tectonic setting of matakaoa volcanics east coast allochthon new zealand suprasubduction zone affinity regional correlations and origin in north eastern new zealand the late cretaceous to eocene submarine volcanic rocks of the east coast allochthon referred to as matakaoa volcanics formed a dominantly mafic melange with minor abyssal sediments during the subduction of oceanic crust accreted and melanged pillow lavas and dolerites display the geochemical and isotopic features of tholeiites with some diversity referred to as midocean ridge basalts island arc tholeiites and back arc basin basalts the association of supra subduction tholeiitic magmas abyssal sediments and polymetallic volcanogenic sulphide deposits is typical of many back arc basins therefore a correlation with the alkaline and transitional basalts of the hikurangi manihiki ontong java plateau may be ruled out these petrological features and the fossil ages as well are closely similar to those of the tangihua complex northland allochthon and allow matakaoa and tangihua mafic volcanic rocks now separated by the modern havre trough to be correlated within one single late cretaceous to eocene 2 basin the consideration of possible younger ages oligocene is also discussed the occurrence of late cretaceous eocene oceanic crust with supra subduction zone affinities to the east of the norfolk ridge poses the problem of the intervening southwest dipping subduction the missing late cretaceous eocene arc and their relationship with southeast gondwana marginal break up

subglacial meltwater origin and subaerial meltwater modifications of drumlins near morley alberta canada abstract drumlins in the bow valley west of calgary alberta are interpreted as the product of subglacial and subaerial meltwater erosion on the sides of the bow valley highly elongated streamlined first generation drumlins are aligned in an en echelon fashion and are not significantly modified by post glacial processes in the valley bottom scalloped stubby irregularly spaced second generation drumlins are found in close association with valley fill the drumlins are composed of bedrock and diamicton and some are overlain by a thin gravel veneer stratified gravel is found on the leeward flanks of some second generation drumlins meltwater erosional forms s forms found on the limestone bedrock up flow of the drumlins and crescentic furrows around their proximal ends indicate that subglacial meltwater beneath the bow valley ice was probably the main erosive agent responsible for the shaping of the drumlins the second generation drumlins are believed to be the remnants of the first generation drumlins that were dissected and reworked by postglacial subaerial meltwater flow s meltwater drainage from ice dammed glacial lake kananaskis and possibly other glacially dammed tributary valley lakes resulted in the deposition of megaripples 3 5 m high 40 m wavelength 8 km down flow from the drumlins and reverse eddy deposits on the flanks of the drumlins post glacial bow river incision and terrace development has continued to further sculpt these forms

cavalli seamount northland plateau sw pacific ocean a miocene metamorphic core complex cavalli seamount 34 06 s 174 10 e is an irregular shaped flat topped seamount on the northland plateau sw pacific ocean in contrast to miocene lavas recovered from other seamounts in the area the rocks dredged from cavalli seamount consist of biotite schist with minor calc silicate bands mineralogy whole rock chemistry u pb dating of detrital zircons and sr and nd isotopic composition collectively indicate a siliciclastic carbonate protolith of latest cretaceous to paleogene stratigraphic age metamorphic index minerals are sillimanite andesine garnet ilmenite and k feldspar and indicate peak p t conditions of c 650 c and c 0 4 gpa u pb ar ar and fission track dating of minerals and micropalaeontological dating of associated limestones indicate rapid cooling in the interval 23 21 ma the schist contains chloritized and hematized slickenslides parallel to a penetrative lineation s c planes and conjugate microfractures and has limestone geopetal infillings parallel to foliation these features are consistent with the interpretation of cavalli seamount as part of the lower plate of an early miocene metamorphic core complex although other explanations are possible the core complex interpretation fits with regional early miocene events including crustal thickening as a result of allochthon emplacement in northern new zealand crustal thinning associated with rapid extension in the south fiji basin pacific trench rollback and abundant early miocene igneous activity

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