calculated mineral equilibria for eclogites in cao na2o feo mgo al2o3 sio2 h2o application to the pouebo terrane pam peninsula new caledonia the high p medium t pouebo terrane of the pam peninsula northern new caledonia includes barroisite and glaucophane bearing eclogite and variably rehydrated equivalents the metamorphic evolution of the pouebo terrane is inferred from calculated p t and p t xh2o pseudosections for bulk compositions appropriate to these rocks in the model system cao na2o feo mgo al2o3 sio2 h2o the eclogites experienced a clockwise p t path that reached p 19 kbar and t 600 c the eclogitic mineral assemblages are preserved because reaction consequent upon decompression consumed the rocks fluid extensive reaction occurred only in rocks with fluid influx during decompression of the pouebo terrane

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

development of p t prograde and p retrograde t prograde isogradic surfaces during blueschist to eclogite regional deformation metamorphism in new caledonia as indicated by progressively developed porphyroblast microstructures the mid tertiary blueschists eclogites and eclogitic gneisses of northern new caledonia are the products of four phases of regional meta morphism and deformation d d4 omphacite lawsonite and mn rich garnet isogradic surfaces were developed during the second deformation d2 under prograde pressure and temperature conditions subsequent deformations d3 d folded these d isogradic surfaces however within the p retrograde t prograde metamor phic environment of the d phase omphacite altered to albite and chlorite as a result a late stage sub horizontal isogradic surface developed for omphacite out where this mineral is preserved as relics within syn d albite porphyroblasts other minerals that crystallized for the first time epidote or had rim additions almandine phengite during d also form nearly horizontal isogradic surfaces porphyroblastic garnet and albite contain inclusion trails which allow their microstructural development and crystallization of the matrix to be traced from d to d late syn d the temperature increased mark edly in association with an extensive exothermic decarbonation even though the rocks were in a state of pressure retrogression this caused considerable neocrystallization recrystallization and growth of matrix and porphyroblasts such that although s foliation crenulated by d and d is readily observable almost all signs of stored strain due to d and d have been removed and the deeper schists and eclogitic gneisses superficially appear to have undergone a drastic annealing recrystallization post dating deformation

the liuyuan complex in the beishan nw china a carboniferous permian ophiolitic fore arc sliver in the southern altaids the tectonic history and time of closure of the palaeo asian ocean of the altaids are issues of lively current debate to address these issues this paper presents detailed geological petrological and geochemical data of the liuyuan complex lc in the beishan region in nw china located in the southernmost altaids in order to constrain its age origin and tectonic setting the lc mainly comprises massive basalts pillow basalts basaltic breccias gabbros and ultramafic rocks together with cherts and tuffs most prominent are gabbros and large volumes of basaltic lavas these mafic rocks have high tio 2 contents flat rare earth element ree patterns and show high field strength elements hfses similar to those of mid ocean ridge basalts morb the mafic rocks exhibit positive e nd t 6 6 9 0 values representing magmas derived from the mantle but these basic rocks are also enriched in th relative to rees and are systematically depleted in nb ta ti relative to rees there is also a large range in initial 87 sr 86 sr 0 7037 0 7093 all these variables indicate that mantle derived magma was contaminated by fluids and or melts from a subducting lithospheric slab and formed in a supra subduction zone ssz setting a gabbro intruded in the complex was dated by la icp ms on 20 zircons that yielded a 206 pb 238 u weighted average age of 286 2 ma considering the fact that all these basalts are imbricated against permian tuffaceous sediments and limestone we propose that the lc formed as an ophiolite in a fore arc in carboniferous permian time this indicates that the palaeo asian ocean still existed at 286 2 ma in early permian time and thus the time of closure of the palaeo asian ocean was in or after the late permian

a drastic lower miocene regolith evolution triggered by post obduction slab break off and uplift in new caledonia a lower miocene coarse conglomerate that crops out in the nepoui peninsula does not represent the base of the marine transgression that followed obduction in new caledonia instead the conglomeratic alluvial fan that contains peridotite cobbles and reworked weathering products records a short lived episode of terrestrial erosion intercalated between two intervals of subsidence marked by marine carbonate deposition considering the miocene sea level evolution reported in the literature it is concluded that neither lower miocene transgression nor erosion were driven by sea level variation in contrast a southeastward propagating slab tear that initiated at the latitude of the high pressure low temperature metamorphic complex of northern new caledonia likely generated east to west tilting of new caledonia subsidence along the west coast and hence fringing reef development together with moderate erosion of older regolith coincidence between conglomerate deposition and hence prominent erosion that closely followed emplacement of postobduction granitoids influenced by a slab window suggests a genetic link therefore it is concluded that short lived lower miocene erosion was due to slab breakoff and subsequent uplift that occurred at 22 ma lower miocene erosion profoundly dissected the peridotite nappe and in the northern half of new caledonia only left isolated klippes along the west coast

the north fiji basin basalts and their magma sources part ii sr nd isotopic and trace element constraints abstract sr nd isotope and trace element data are reported for basalts from the north fiji basin nfb nfb basalts are characterized by extreme variations in isotopic ratios and trace element abundances which are related to mantle heterogeneities values of 87 sr 86 sr for basalts from the northern segments n160 and triple junction range from 0 7029 to 0 7041 whereas 143 nd 144 nd values vary from 0 51281 to 0 51313 most of the basalts from these segments are characterized by strong relative enrichments in rb ba sr k nb ta la ce and ti that are comparable to oib components the central segments n15 and n s have 143 nd 144 nd ratios between 0 51298 and 0 51363 and 87 sr 86 sr ratios between 0 7029 and 0 7033 and are depleted in large ion lithophile light rare earth and high field strength elements similar to n morb covariation between trace element and isotopic ratios among nfb basalts supports a model in which melts from the nfb rift system are derived by mixing of oib type and depleted n morb mantle components the sr nd isotopic and trace element variability indicates that the nfb basalt source is heterogeneous on the scale of individual melt batches

geochemistry of an ophiolitic complex from new caledonia the ophiolites of new caledonia are composed of ultramafics overlain by mafic rocks all of which were affected by low p metamorphism the mafic rocks studied gabbroic cumulates and basaltic flows and dikes from montagne des sources are similar to recent mid ocean ridge rocks they are olivine normative with mg mg fe2 ratios ranging from 0 69 in lavas to 0 90 in gabbroic cumulates and show tholeiitic fractionation trends such as a negative correlation of ti and v with the mg fe ratio the lavas have a flat ree pattern with a slight depletion of light ree and a la yb ratio 2 the dikes have three different types of ree patterns the first type is nearly parallel to that of lavas the second one is enriched in lree la yb 4 and the third type with the lowest ree contents and a distinct lree depletion is similar to that of cumulitic pyroxene gabbro the variations in chemical compositions of the mafic rocks can be accounted for by the dynamic partial melting process of langmuir et al 1977 in agreement with structural and tectonic observations the geochemical data suggests that the ophiolites were formed during the spreading of a mid ocean ridge with a spreading half rate of about 1 cm year

material transfer in accretionary wedges from analysis of a systematic series of analog experiments the structural evolution of accretionary wedges was modeled in a series of analog experiments with systematically varying initial boundary conditions the two principle parameters investigated were basal friction and thickness of sediment input relative to subducted sediment output material transfer in terms of degree and location of accretion and erosion is quantified as a function of these two parameters low basal friction produces long thin wedges by continuous frontal accretion of imbricate thrusts simultaneously substantial basal erosion may occur with little influence on the frontal dynamics the formation of an outer arc high with an accompanying forearc basin depends upon the relative amount of basal erosion the opening of the subduction window spacing of thrust faults is proportional to the layer thickness confirming the principle of self similar wedge growth from critical wedge theory high basal friction typically generates steep wedges marked by an erosional mass wasting frontal slope in the cases i o this is due to underthrusting and or subduction of the entire sedimentary section the case i o also features the late development of normal faulting and an extensional forearc basin for both low and high basal friction the material balance affects the wedge slope erosive conditions i o for low basal friction the primary modes of material transfer are frontal accretion and basal erosion for high basal friction the primary modes of material transfer are underthrusting underplating and frontal erosion applying the experimental results to active convergent margins parallels may be drawn thus the barbados ridge complex is interpreted to be a good example of a low basal friction wedge with an accretionary material balance while the japan and mariana margins provide examples of an erosive material balance

biostratigraphy of the supra ophiolite sediments of the troodos massif cyprus the cretaceous perapedhi kannaviou moni and kathikas formations the troodos massif of cyprus exposes a classic and much studied ophiolite sequence representing oceanic crust of late cretaceous age k ar dating of the sheeted dykes and of the overlying pillow lavas gives a range of 83 3 ma earliest campanian and 75 5 ma late campanian respectively for the formation of the upper levels of the ophiolite an autochthonous sequence of late cretaceous to recent age sediments is exposed resting on the ophiolite the oldest part of which reflects sedimentation in an apparently deep marine oceanic setting little biostratigraphical information is available to constrain the chronostratigraphy of these sediments in relation to the complex geological history of the island including the uplift and unroofing of the ophiolite despite their richness in microfauna and flora this paper provides an integrated biostratigraphical study based on radiolaria planktonic foraminifera and calcareous nannofossils of the oldest part of the supra ophiolite succession of the cretaceous perapedhi kannaviou moni and kathikas formations for the first time well defined micropalaeontological evidence establishes the relative ages of these formations the umbers of the perapedhi formation are no younger than campanian in age while the volcaniclastic sediments of the kannaviou formation were also deposited during campanian times the matrix of the moni formation contains a microfauna consistent with the hypothesis that it is derived from the kannaviou in contrast the kathikas formation is composed of sediment derived mainly from the allochthonous mamonia complex but autochthonous pelagic interbed horizons demonstrate that it was deposited probably within a very short time interval during the late maastrichtian key index species are figured including the first published micrographs of cretaceous planktonic foraminifera from cyprus

overthrust emplacement of new caledonia ophiolite geophysical evidence geophysical studies support inferences from outcrop geology that during the late eocene an ophiolite sheet exposed on new caledonia was thrust southward over a rock complex consisting of sedimentary volcanic and metamorphic rocks that range in age from pre permian to eocene the outcropping ultramafic complex consists of a layered sequence approximately 3000 m thick including harzburgite dunite wehrlite serpentinite and gabbro the absence of pillow basalts and sheeted dikes on land suggests that during or after overthrusting these units were removed either by thrust faulting or by erosion seismic refraction profiles collected over the adjacent loyalty basin show that the lower crust and upper mantle are characterized by velocities of 4 7 5 8 6 8 and 8 1 8 4 km s suggesting the presence of a complete oceanic section that includes sediment basalt gabbro and peridotite the lower crust is probably 6 5 km thick beneath the loyalty basin a seismic reflection profile that extends across the loyalty basin is interpreted to show the oceanic crust rising toward new caledonia we suggest that the ophiolite on new caledonia is continuous with this rising oceanic crust studies of gravity anomalies observed both on land and offshore indicate the presence of a short wavelength high amplitude 180 mgal asymmetrical free air gravity anomaly along the northeastern coast of new caledonia a gravity anomaly profile calculated for a geological model characterized by a 10 km thick slab of oceanic crust and mantle material extending continuously from the ophiolite on new caledonia to the oceanic crust of the loyalty basin successfully matches the observed gravity data we interpret the extension of the high gravity anomaly over the whole length of the eastern lagoon as evidence for the lateral extension of the root zone of the ultramafic complex a tentative geodynamic reconstruction suggests that the proto loyalty basin crust formed a marginal basin along the eastern margin of gondwanaland prior to upper cretaceous time after the opening of the new caledonia basin during upper cretaceous to early paleocene time a subduction zone developed along the western margin of new caledonia while this zone was active the loyalty basin lithosphere overthrust new caledonia in a southeasterly direction and locally rotated clockwise this compressional tectonic event was terminated by upper eocene time

tectonic stresses seaward of an aseismic ridge trench collision zone a remote sensing approach on the loyalty islands sw pacific abstract the loyalty islands are a series of limestone karstified islands on the australian plate that are presently approaching the vanuatu subduction zone sw pacific they are deformed due to the combined effects of the bulging of the subducting lithosphere and the beginning collision between the loyalty ridge and the vanuatu subduction zone therefore they constitute a series of markers for early phases of ridge arc collision lineaments deduced from remote sensing images aerial photos spot3 spot4 and envisat data termed here as fractures are analyzed by comparison with planar structures measured during field studies and termed as geological data fracture data indicate a stable main n110 direction with a large variance of nearly 15 in rms and two minor directions 45 apart which may constitute shear directions associated to the major one a scale dependent analysis shows that fractures longer than 2000 m are close to the n110 direction and that their orientation shifts progressively to reach the n125 direction for l

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

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

geochemical evolution and tectonic significance of boninites and tholeiites from the koh ophiolite new caledonia the central chain ophiolites in new caledonia are fragments of a supra subduction zone ssz ophiolite now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick middle triassic to upper jurassic volcaniclastic sequence most of the fragments were formed by a single tholeiitic magmatic episode but one of these the koh ophiolite was formed by two tholeiitic magmatic episodes separated by boninites the first event in the koh ophiolite formed cumulate gabbros dolerites plagiogranites and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements lree and abnormally low tio2 0 5 at mg 60 shortly after their eruption these tholeiitic lavas were overlain by a high ca boninitic unit with a basal section of boninite pillows flows and breccias and an upper section of boninitic dacites and tuffs the last magmatic phase involved eruption of evolved tholeiitic basalts as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite this second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts babb erupted during the early rifting history of modern back arc basins the boninitic volcanics belong to a high ca series with slightly lower sio2 al2o3 and tio2 compared to those from modern island arc systems and they lack the positive zr spike relative to adjacent rare earth elements ree in normalised element variation patterns these boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40 60 extension of the initial basin crust and eruption of the upper tholeiites the dominance of babb like tholeiites throughout the central chain ophiolites in new caledonia the restricted occurrence of boninites and the stratigraphy and chemistry of the koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation we suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre the occurrence of a thick blanket of calc alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the koh boninites were not associated with the initiation of subduction a close modern analogy for the koh ophiolite exists on the hunter ridge protoisland arc between southernmost vanuatu new hebrides island arc and the fijian islands there high ca boninites lacking positive zr spikes occur together with low ti tholeiites and more typical babb tholeiites where the southern spreading centre of the north fiji basin is propagating into the protoarc crust of the hunter ridge

geodynamic models of continental subduction and obduction of overriding plate forearc oceanic lithosphere on top of continental crust continental subduction takes place in the final stage of subduction when all oceanic lithosphere is consumed and continental passive margin is pulled into the mantle when the overriding plate is oceanic dense forearc oceanic lithosphere might be obducted onto light continental crust forming an ophiolite tethyan style ophiolite obduction four dimensional dynamic analog subduction models have been constructed to evaluate the mechanical feasibility of continental subduction and forearc oceanic lithosphere obduction on top of continental crust the roles of continental crust thickness passive margin length subducting lithosphere thickness and overriding plate thickness were investigated to determine the maximum continental subduction depth maximum forearc obduction distance and forearc deformation during continental subduction our buoyancy driven experiments indicate that deep continental subduction occurs in most circumstances down to 560 km and that obduction of dense oceanic forearc lithosphere on top of light continental crust is mechanically feasible maximum obduction distances are relatively small 26 37 km but are sufficient to explain obduction of short ophiolite sheets such as observed in new caledonia when including the thin 5 10 km thick accretionary wedge of off scraped deep sea sediments oceanic crust and mantle then maximum obduction distances are much larger 60 160 km sufficient to account for the obducted northland allochthon in new zealand results indicate that increasing continental crust thickness decreases continental subduction depth whereas increasing passive margin length and subducting lithosphere thickness increases continental subduction depth notably during continental subduction backarc extension continues while forearc deformation shortening increases moderately compared to the preceding phase of normal oceanic subduction

hydration of eclogite pam peninsula new caledonia garnet glaucophanite and greenschist facies assemblages were formed by the recrystallization of barroisite bearing eclogite facies metabasites in northern new caledonia the mineralogical evolution can be modelled by calculated p t and p xh2o diagrams for appropriate bulk rock compositions in the model system cao na2o feo mgo al2o3 sio2 h2o the eclogites having developed in a clockwise p t path that reached p 19 kbar and t 590 c underwent decompression with the consumption of free h2o as the volume of hydrous minerals increased eclogite is preserved in domains that experienced no fluid influx following the loss of this fluid garnet glaucophanite formed at p 16 kbar during semi pervasive fluid influx fluid influx after further isothermal decompression was focused in shear zones and resulted in chlorite albite bearing greenschist facies mineral assemblages that reflect p 9 kbar

seagrass related carbonate ramp development at the front of a fan delta burdigalian new caledonia insights into mixed carbonate siliciclastic environments abstract past shallow water carbonate environments of the main island of new caledonia nc have been subject to high terrigenous influx derived from the erosion of ultramafic obducted nappes and are therefore a relevant case study for characterizing neritic carbonate production in mixed carbonate siliciclastic systems under a tropical climate more particularly we focused on burdigalian carbonate sedimentary records cropping out on the western coast of nc in the nepoui area based on a comprehensive sedimentological study of cores and outcrops we established a new depositional model of an alluvial fan to carbonate ramp transition shallow water euphotic carbonate production was dominated by seagrass related biota and corals derived from small sized bioconstructions extensional tectonics and associated normal faulting driven by post obduction isostatic rebound favored carbonate ramp aggradation and preservation the carbonate ramp was incised by conglomerate filled terminal distributary channels which indicate that terrigenous inputs remained significant during marine transgression and did not inhibit the development of seagrass and scleractinian carbonate factories phytal substrates induced by seagrass and or macro algae seafloor colonization strongly controlled the nature of carbonate production and promoted the accumulation of foraminiferal coralline algal sediments seagrass development at the front of the fan delta is interpreted to have controlled the preservation of a diverse and significant carbonate production by reducing water turbidity and by limiting the risk of suffocation for filter feeding biota in such a high terrigenous influx setting

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

morphotectonic evolution of the new caledonia ridge pacific southwest from post obduction tectonosedimentary record abstract the tectonostratigraphic and geomorphic study of two post obduction fluvial sedimentary systems on mainland new caledonia and imaged offshore on seismic reflection lines provides a new perspective on the post orogenic evolution of the new caledonia ridge relations between sedimentary sequence boundaries erosion surfaces and faults both on land and on offshore seismic reflection profiles indicate that an episode of extensional tectonics initiated in the early neogene led to the disruption and collapse of the island landsurface previously shaped during a latest oligocene phase of planation microtectonic analysis further suggests early slip on the normal faults was associated with ridge normal extension a later set of faults accompanied ridge parallel to ridge oblique extension that is interpreted to result from a shift toward a transtensional regime driven by the initiation of east verging subduction of the australian plate beneath the pacific plate starting at least in the late mid miocene

latest cretaceous to late paleocene radiolarian biostratigraphy a new zonation from the new zealand region the scarcity of records of early paleocene radiolarians has meant that while radiolarian biostratigraphy is firmly established as an important tool for correlation there has been a long standing gap between established zonations for the cretaceous and from latest paleocene to recent it has also led to considerable speculation over the level of faunal change across the cretaceous tertiary k t boundary consequently the discovery of rich and diverse radiolarian assemblages in well delineated k t boundary sections within siliceous limestones of the amuri limestone group in eastern marlborough new zealand is of great significance for biostratigraphy and k t boundary research this initial report is restricted to introducing a new latest cretaceous to mid late paleocene zonation based on the radiolarian succession at four of these sections and a re examination of faunas from coeval sediments at dsdp site 208 lord howe rise three new paleocene species are described amphisphaera aotea amphisphaera kina andstichomitra wero six new interval zones are defined by the first appearances of the nominate species in ascending order these are lithomelissa hoplites foreman zone rk9 cretaceous amphisphaera aotea n sp zone rp1 paleocene amphisphaera kina n sp rp2 stichomitra granulata petrushevskaya rp3 buryella foremanae petrushevskaya rp4 andburyella tetradica rp5 good age control from foraminifera and calcareous nannofossils permits close correlation with established microfossil zonations where age control is less reliable radiolarian events are used to substantially improve correlation between the sections no evidence is found for mass extinction of radiolarians at the end of the cretaceous however the k t boundary does mark a change from nassellarian to spumellarian dominance due to a sudden influx of actinommids which effectively reduces the relative abundance of many cretaceous survivors an accompanying influx of diatoms in the basal paleocene of marlborough together with evidence for an increase of total radiolarian abundance suggests siliceous plankton productivity increased across the k t boundary possible causes for this apparently localised phenomenon are briefly discussed

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

chloritoid bearing rocks associated with blueschists and eclogites northern new caledonia chloritoid bearing metasedimentary rocks occur in close proximity to blueschists and eclogites in the tertiary high pressure metamorphic belt of northern new caledonia the typical assemblage of chloritoid bearing rocks in the epidote zone is quartzchlorite muscovite garnet chloritoid in the omphacite zone epidote is an additional member of the chloritoid bearing assemblage paragonite is rare plagioclase was not detected and rutile and ilmenite are the fe ti oxide phases chloritoid glaucophane is not a common assemblage chloritoid bearing rocks have relatively low ca k na al ratios and the chloritoids are relatively mg rich with mg mg fe up to about 0 4 a comparison of the mineral assemblages and mineral chemistry with experimental and computed phase equilibria suggest an upper temperature limit near 560 c in the omphacite zone and a minimum temperature limit near 450 c at 10 kbar an empirical garnet chlorite fe mg exchange thermometer does not yield consistent results for the higher grade rocks suggesting ts ranging from 390 to 535 c in the omphacite zone and 420 465 c in the epidote zone the distribution coefficient kd fe mg ctd fe mg chl for chloritoid and chlorite ranges from 3 9 to 6 4 values which are lower than those 10 from lower greenschist facies rocks but are near those of upper greenschist facies and albite epidote amphibolite facies

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

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

regional implications of u pb shrimp age constraints on the tectonic evolution of new caledonia abstract u pb shrimp ages for zircons from plagiogranites indicate a late carboniferous age of formation for ophiolitic basement of the koh terrane in the central chain mountains of new caledonia samples from ophiolites at koh and koua yielded ages of 302 7 ma and 290 5 ma respectively the similarity of these ages to those of plagiogranites in the dun mountain ophiolite belt maitai terrane of new zealand and the comparable structural positions of the two terranes potentially constrain reconstructions of the cretaceous sw pacific margin detrital zircons from late mesozoic sediments that overlap the koh terrane also provide potential constraints on the location of new caledonia prior to its break up and dispersal from eastern australia

inferring the heterogeneity transmissivity and hydraulic conductivity of crystalline aquifers from a detailed water table map abstract estimating the transmissivity or hydraulic conductivity field to characterize the heterogeneity of a crystalline aquifer is particularly difficult because of the wide variations of the parameters we developed a new approach based on the analysis of a dense network of water table data it is based on the concept that large scale variations in hydraulic head may give information on large scale aquifer parameters the method assumes that flux into the aquifer is mainly sub horizontal and that the water table is mostly controlled by topography rather than recharge it is based on an empirical statistical relationship between field data on transmissivity and the inverse slope values of a topography reduced water table map this relationship is used to compute a transmissivity map that must be validated with field measurements the proposed approach can provide a general pattern of transmissivity or hydraulic conductivity but cannot correctly reproduce strong variations at very local scale less than10 m and will face of some uncertainties where vertical flows cannot be neglected the method was tested on a peridotite ultramafic rock aquifer of 3 5 km 2 in area located in new caledonia the resulting map shows transmissivity variations over about 5 orders of magnitude average logt 5 2 0 7 comparison with a map based on measured water level data n 475 shows that the comparison between logt computed values and logt data deduced from 28 hydraulic tests is estimated with an error less than 20 in 71 of cases logt 0 4 and with an error less than 10 logt 0 2 on average in 39 of cases from this map a hydraulic conductivity map has been computed showing values ranging over 8 orders of magnitude the repeatability of the approach was tested on a second data set of hydraulic head measurements n 543 the mean deviation between both logt maps is about 11 these encouraging results show that the method can give valuable parameter estimates and can characterize aquifer heterogeneity the computed logt and logk maps highlight the spatial distribution of parameters that show a pattern clearly controlled by the fault network of this ultramafic massif however the faults are mainly characterized by low permeability zones this differs from results on other crystalline aquifers and may be due to the fact that weathering products of peridotite are clay like materials the resulting transmissivity map can be used as a starting point for modelling or to direct additional fieldwork

first palaeomagnetic dating of ferricrete in new caledonia new insight on the morphogenesis and palaeoweathering of grande terre terra nova 24 77 85 2012 abstract comparing palaeolatitudes estimated from remanent magnetization data of ferricrete developed over laterite at two sites goro and tiebaghi against the apparent polar wandering path of australia provides new time constraints for the peridotite weathering in new caledonia the tiebaghi massif of the west coast klippes displays a single episode of ferricrete development during latest oligocene times by contrast the goro site in the southern part of the massif du sud reveals a more complex history with several episodes of ferricrete development from latest oligocene to pliocene quaternary times these data have made it possible to constrain the post obduction geodynamics and associated morphogenesis of new caledonia since related uplift probably prevented regolith development until latest oligocene times moreover the later uplift and deep erosion of the west coast klippes during the early miocene probably prevented further regolith preservation

strain localization and fluid infiltration in the mantle wedge during subduction initiation evidence from the base of the new caledonia ophiolite despite decades of petrological and geochemical studies the nature and setting of obducted ophiolites remain controversial the influence of supra subduction zone environments on pre existing oceanic lithosphere is yet to assess and the processes leading to subduction obduction initiation are still poorly constrained our study documents successive influx of slab derived fluids and progressive strain localization within the upper mantle in a supra subduction environment during the first few my of the subduction history we focus on strongly sheared mafic amphibolites intruding peridotites near the mantle crust transition of the new caledonia obducted ophiolite and 50 to 100 m above the basal thrust contact of the ophiolite these m to hm long and several m thick shear bands are interpreted as inherited small scale intrusions of mafic melts probably dikes or sills which were derived from a moderately refractory mantle source refertilized by supra subduction zone fluids 40ar 39ar age constraints on pargasite at ca 90 ma suggest that they could be inherited from the former pacific west dipping subduction secondary deformation of these mafic intrusions is intimately associated to three major stages of fluid infiltration 1 the first stage of deformation and metasomatism is marked by syn kinematic growth of ca amphibole at 700 800 c and 3 5 kbar with a distinctive supra subduction zone signature and controlled later channelization of aqueous fluids 40ar 39ar dating on magnesio hornblende indicates that this deformation episode occurred at ca 55 ma coincident with east dipping subduction initiation 2 the main metasomatic stage characterized by the development of a phlogopite rich matrix wrapping peridotites and amphibolite boudins points to the percolation of alkali rich aqueous fluids at still high temperature 650 750 c 3 the last low temperature 600 c metasomatic stage results in the formation of deformed veinlets containing talc chlorite and serpentine

thermo mechanical modeling of the obduction process based on the oman ophiolite case obduction emplaces regional scale fragments of oceanic lithosphere ophiolites over continental lithosphere margins of much lower density for this reason the mechanisms responsible for obduction remain enigmatic in the framework of plate tectonics we present two dimensional 2d thermo mechanical models of obduction and investigate possible dynamics and physical controls of this process model geometry and boundary conditions are based on available geological and geochronological data and numerical modeling results are validated against petrological and structural observations of the oman semail ophiolite our model reproduces the stages of oceanic subduction initiation away from the arabian margin the emplacement of the oman ophiolite on top of it and the domal exhumation of the metamorphosed margin through the ophiolitic nappe a systematic study indicates that 350 400 km of bulk shortening provides the best fit for both the maximum pressure temperature conditions of the metamorphosed margin 1 5 2 5 gpa 450 600 c and the dimension of the ophiolitic nappe 170 km width our results confirm that a thermal anomaly located close to the arabian margin 100 km is needed to initiate obduction we further suggest that a strong continental basement rheology is a prerequisite for ophiolite emplacement

slab derived origin of tremolite antigorite veins in a supra subduction ophiolite the peridotite nappe new caledonia as a case study hydration of mantle peridotites provides information on the exhumation history and on the fluid regime accompanying exhumation of these rocks reservoirs involved fluid rock ratios temperature of interaction highly depleted harzburgites and dunites of the peridotite nappe of new caledonia are crosscut by fractures which have been pervasively serpentinized producing lizardite brucite magnetite and minor chrysotile in a near static environment probably by sea water circulating in cooling joints this event is generally referred to as primary serpentinization in a next step already serpentinized joints were re opened to produce tension and shear cracks sealed by higher temperature synkinematic fibrous minerals locally tremolite bearing veins and pockets which do not display evidence for void infill were generated by metasomatic replacement of the wall rock peridotite most veins only contain fibrous antigorite but some display tremolite antigorite intergrowths or even pure tremolite the latter rocks yield high contents of ca and incompatible elements which contrast with the overall depletion of the peridotite host rock and suggest contribution of an external source whole rock geochemical and isotopic features 87sr 86sr 18o and h suggest that antigorite veins which bear highly radiogenic sr isotope signatures were strongly influenced by fluids emitted by the subducted slab and associated sediments in contrast the geochemical and isotopic signatures of tremolite bearing rocks suggest a genetic link with early eocene supra subduction dykes and fluids that leached them calcium strontium and ree bearing oxidized fluids reacted with already serpentinized fracture wall rock as shown by fibre nucleation chromite alteration and high cr contents in tremolite the bulk of syn tectonic fluid rock interaction associated with shear and tensile fracture development probably occurred at the onset of eocene intra oceanic subduction when the buoyant lower plate the south loyalty basin obliquely forced its way beneath the nascent loyalty fore arc mobile elements extracted from supra subduction dykes temporarily enriched the circulating fluids and generated tremolite antigorite mg chlorite and magnetite instead of antigorite and magnetite solely tremolite crystallization probably ceased due to the exhaustion of ca rich fluids in a globally cooling fore arc environment

lateral variation of upper mantle structure beneath new caledonia determined from p wave receiver function evidence for a fossil subduction zone summary short period teleseismic p waves recorded at noumea new caledonia from intermediate and deep focus earthquakes in se asia and the sw pacific are used to investigate crustal and upper mantle structure of the norfolk ridge which is currently described as a zone of convergence in its caledonian part radial components recorded from various source backazimuths show a succession of large p to s converted waves whose timing and polarity suggest the presence of a north dipping low velocity zone at approximately 60km depth below the station the inferred strong lateral variations of structure in the upper mantle are also supported by observations of complex horizontal particle motions and large tangential components which exhibit clear p to s phases the high velocity contrast observed across these dipping interfaces is interpreted as a structural remnant of the new caledonia basin oceanic crust which would have been underthrust beneath the new caledonia crust using geometrical ray theory in dipping structure a model valid for the noumea station has been derived the crust is apparently quite homogeneous it is 26 1 km thick beneath the station and seems to thicken eastward as deduced from the orientation of the moho the low average crustal velocities suggest a continental nature for new caledonia toward the thrust zone the moho is a thick transition zone whereas it can be modelled by a sharp boundary elsewhere the inferred structure can be interpreted in terms of either a dead subduction zone or an underthrusting zone according to the history of compressive deformations observed in land the underthrusting would likely have been syntectonic of the new caledonia late eocene obduction

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

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

eclogites and blueschists of the pam peninsula ne new caledonia a reappraisal high p rocks of the pam peninsula ne new caledonia are divided into three zones 1 an uppermost ferroglaucophane lawsonite zone of cretaceous to eocene metasediments and metavolcanics of the diahot terrane that experienced peak conditions involving p 7 9 kbar and t 400 58 c 2 albite epidote omphacite zone diahot terrane rocks that experienced blueschist facies conditions of p 12 6 1 2 kbar and t 570 36 c 3 lowermost metabasic eclogites of uncertain age that form the pouebo terrane which experienced high p conditions of p 23 9 3 0 kbar and t 600 c eclogite occurs as metre to kilometre scale pods in coarse grained hydrous mineral rich glaucophanite formed during hydration and decompression of the pouebo terrane metamorphism and deformation were consequent to 44 51 ma eocene convergence when sedimentary and ophiolitic nappes were thrust over the eclogites in a sw direction white mica ages constrain metamorphism to have ended by 37 1 ma large steps in metamorphic grade are coincident with sw dipping and ne dipping faults that separate the three zones and were formed during two stages 1 comparatively slow uplift and hydration of the pouebo terrane before it was juxtaposed with the albite epidote omphacite zone at p 14 kbar 2 comparatively rapid uplift of both the pouebo terrane and the albite epidote omphacite zone to form a domal core of eclogite flanked by significantly lower grade rocks to the sw and ne

the emplacement of the peridotite nappe of new caledonia and its bearing on the tectonics of obduction the peridotite nappe of new caledonia is one of the few ophiolites worldwide that escaped collisional orogeny after obduction here we describe the deformation associated with serpentinization in two klippes of the nappe in northwestern new caledonia the klippes are flat lying and involve s sw vergent reverse slip shear zones which are true compressional structures in origin further northeast the nappe is folded in association with the development of a steep schistosity in low grade metasediments this difference in structural style indicates that the peridotite nappe experienced compression at greater depths toward its root zone suggesting a push from the rear mechanism of emplacement this supports the view that the nappe has been emplaced through horizontal contraction sustained by plate convergence we establish a crustal scale cross section at the end of the obduction event before neogene extension this involves a large fold nappe of high pressure rocks bounded from below by a major thrust furthermore we show that obduction in new caledonia occurred through dextral oblique convergence oblique convergence probably resulted from the initial obliquity between the subduction trench and the continental ribbon that became incorporated in it this obliquity can solve the paradox of the peridotite nappe seemingly being emplaced at the same time the high pressure rocks were exhumed oblique convergence together with focused erosional denudation on the northeastern flank of the island led to exhumation of the metamorphic rocks in a steep fold nappe rising through the rear part of the orogen

40ar 39ar dating of white micas from the epidote to the omphacite zones northern new caledonia tectonic implications an 40ar 39ar study of white micas from high pressure metamorphic rocks of northern new caledonia yielded cooling ages of 37 1 ma for both epidote and omphacite zone samples whole rock samples from the lawsonite zone yielded ages in the range 44 51 ma with complicated age spectra probably reflecting both detrital and newly grown micas the areal extent of the mica samples over 300 km2 suggests that the epidote and omphacite zone rocks cooled through the muscovite closure temperature about 350 c as a coherent cooling unit simple thermal modeling suggests that these rocks could have closed at similar times if the unroofing rate were greater than 2 10 mm a 1 lawsonite zone rocks occur structurally within about 0 5 km of garnet omphacite rocks suggesting the possibility of major postmetamorphic tectonic displacement

eocene 46 44 ma onset of australia pacific plate motion in the southwest pacific inferred from stratigraphy in new caledonia and new zealand

southwest pacific absolute plate kinematic reconstruction reveals major cenozoic tonga kermadec slab dragging tectonic plates subducting at trenches having strikes oblique to the absolute subducting plate motion undergo trench parallel slab motion through the mantle recently defined as a form of slab dragging we investigate here long term slab dragging components of the tonga kermadec subduction system driven by absolute pacific plate motion to this end we develop a kinematic restoration of tonga kermadec trench motion placed in a mantle reference frame and compare it to tomographically imaged slabs in the mantle estimating tonga kermadec subduction initiation is challenging because another new caledonia subduction zone existed during the paleogene between the australia and pacific plates we test partitioning of plate convergence across the paleogene new caledonia and tonga kermadec subduction zones against resulting mantle structure and show that most if not all tonga kermadec subduction occurred after ca 30 ma since then tonga kermadec subduction has accommodated 1 700 to 3 500 km of subduction along the southern and northern ends of the trench respectively when placed in a mantle reference frame the predominantly westward directed subduction evolved while the tonga kermadec trench underwent 1 200 km of northward absolute motion we infer that the entire tonga kermadec slab was laterally transported through the mantle over 1 200 km such slab dragging by the pacific plate may explain observed deep slab deformation and may also have significant effects on surface tectonics both resulting from the resistance to slab dragging by the viscous mantle

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

prograde sulfide metamorphism in blueschist and eclogite new caledonia in new caledonia blueschist and eclogite preserve as inclusions in porphyroblastic minerals a record of sulfide present during prograde subduction processes sulfide inclusions in prograde garnet and lawsonite became chemically isolated from the matrix whereas sulfide minerals in the matrix continued to equilibrate with matrix fluids or grew later during retrogression cu fe sulfide mineral inclusions have been found across metamorphic grade within silicate defined metamorphic mineral zones spanning a crustal profile of 30 km bulk area scans of sulfide inclusions provide compositions that represent mixtures of the solid sulfide that were included as the host silicate minerals grew in general single sulfide inclusion compositions and aggregate sulfide assemblages are distinct from those of matrix phases high cu contents in sulfide inclusions are interpreted to be a consequence of fe lost from sulfide to growing garnet rather than the result of intrinsically high cu in the bulk rock the distribution of sulfide inclusion compositions across metamorphic grade considered together with the available thermodynamic data suggests that covellite nukundamite bearing inclusions in lawsonite high in both cu and s disappear at higher grades as these sulfide minerals are no longer stable similarly clustering near the ratio fe cu 1 1 may cease with increasing grade owing to the replacement of chalcopyrite intermediate solid solution iss by the denser assemblage pyrite pyrrhotite bornite digenite

assessing seawater basalt exchange of strontium isotopes in hydrothermal processes on the flanks of mid ocean ridges we have measured the87sr 86sr of discharging geothermal waters from three ridge flank hydrothermal systems 1 the galapagos mounds hydrothermal field on 0 7 myr old crust 2 a region with crustal age of 4 6 myr on the west flank of the east pacific rise at 19 s and 3 an area having a crustal age of 3 myr in the mariana trough a back arc spreading center half spreading rates at our study sites range from 1 to 8 cm yr and our sites include both normal spreading centers and a back arc basin our sites thus span the range of ridge flank tectonic environments at each of our study sites the hydrothermal nature of our study samples is confirmed by supporting heat flow and chemical evidence our results show that the87sr 86sr of these ridge flank hydrothermal solutions is virtually identical to that of modern seawater indicating that sr isotope exchange on the ridge flanks occurs too slowly to be quantitatively important in the oceanic mass balance of the sr isotopes

tectonic and metamorphic architecture of the hp belt of new caledonia abstract new caledonia includes a well exposed example of high pressure orogenic belt formed as a result of cenozoic plate tectonic reorganizations at the australia pacific plate boundary the metamorphic belt is relatively well accessible and has deserved extensive attention mainly for its metamorphic and petrological features the architecture of this belt however still lacks general consensus the aim of this paper is i to review the nature and origin of the main tectonic units of the belts and ii to revisit the available structural models describing its architecture and geodynamic evolution for that we integrate the available field petrological geochemical and geochronological data together with new results from large scale field survey and focused ar ar geochronological data all together these data allow proposing a new model for the tectonic and metamorphic architecture of the belt also comprising a newly established tectonostratigraphic unit this architecture resembles that characterizing alpine type metamorphic belts formed in non pacific settings and clearly differs from cordilleran type belts found in eastern pacific regions moreover the collected data also allow refining our understanding of the oceanic and transitional oceanic lithosphere involved in the eocene subduction and plausible analogies with hyper extended margins known in present day ocean continent transition zones

comment on geochemistry of the early miocene volcanic succession of northland new zealand and implications for the evolution of subduction in the southwest pacific by m a booden i e m smith p m black and j l mauk in a recent paper booden et al 2011 present new geochemical and petrological data of early miocene volcanics from the northland region northland volcanic belt in new zealand and interpret these data to support a particular regional tectonic model this tectonic model involves early miocene westward subduction of cretaceous pacific oceanic lithosphere below the northland volcanic belt and the authors interpret the volcanic belt as a continental magmatic arc although the new data are not in disagreement with such a tectonic model they provide more support for an alternative interpretation that involves a northeast dipping subduction zone furthermore geometric and plate kinematic data show that the west dipping subduction model is unviable geological and geophysical data contradict the model while geodynamic arguments indicate that the model is implausible here it will be shown that a subduction model involving a northeast dipping southwestward retreating slab made of the late cretaceous paleocene south loyalty backarc basin lithosphere that subsequently detaches is in agreement with the local geology geophysics and geochemistry is geometrically kinematically and geodynamically viable and fits within the regional southwest pacific tectonic framework

geologic structure of the northern new caledonia ridge as inferred from magnetic and gravity anomalies bathymetric gravity and magnetic data collected in the southwest pacific ocean over the northern new caledonia ridge show that the main geological units known from the island of new caledonia extend northward from this island beneath the grand lagon nord the grand passage and the d entrecasteaux reefs these data support the model of tectonic evolution of the new caledonia region proposed by kroenke 1984 we interpret a linear axial gravity low that extends from southern new caledonia to the grand passage as evidence for the continuity of the thick pre permian to jurassic core of the island the belep magnetic pattern which covers the southwestern half of the grand lagon nord shows linear high relief 1000 2000 nt anomalies that are locally associated with a 120 mgal gravity high these magnetic and gravity anomalies resemble anomalies measured over the west coast of new caledonia suggesting that the cretaceous to eocene basaltic complex of the coastal area is overlain by ophiolite remnants as far north as the western d entrecasteaux reefs the similarity between the belep magnetic pattern and a highly magnetic province evident 200 km southwest across the new caledonia basin along the fairway ridge indicates that volcanic rocks lie symmetrically on both sides of the new caledonia basin we suggest that part of these volcanic rocks were emplaced during the middle cretaceous when the new caledonia ridge rifted from the australian margin simple gravity models of an elongated gravity high having peak values in the range of 130 to 150 mgal suggest that the ophiolite which was thrust over new caledonia during the upper eocene extends along the east side of the grand lagon nord and as far north as the d entrecasteaux reefs gravity and magnetic models suggest that in the area of the d entrecasteaux reefs the ophiolite belt includes two subunits the first subunit underlies the huon la surprise platform and may include highly serpentinized ultramafic rocks without basaltic oceanic crust the second subunit which lies beneath the guilbert ridge may include ultramafic rocks as well as thin imbricate slices of oceanic crust at the northern termination of the new caledonia ridge seismic and geopotential data evidence a major east west trending tectonic zone that separates the basement of the new caledonia ridge from the d entrecasteaux zone an arcuate oceanic feature extending northward from the ridge differences in structure geophysical signatures and morphology evident between areas north and those south of the grand passage together with the nearness of the le noroit massif west of the grand passage suggest that contemporaneously with eocene to early oligocene subduction along the western new caledonia margin an arc ridge collision may have occurred near the northern termination of this subduction zone

origin of the new caledonian ophiolites based on a french australian seismic transect abstract the origin age and nature of the new caledonian ophiolites are still debated recently as part of the joint french australian seismic transect faust program the australian research vessel rig seismic recorded a series of multichannel deep seismic profiles between the new hebrides arc and the australian margin these profiles which ran south of new caledonia image the southern prolongation of the overthrusted ophiolites and allow a new interpretation of the eocene compressive tectonism along the new caledonia norfolk ridge a model involving abduction of the whole oceanic lithosphere of the loyalty basin is consistent with the age and origin of the ophiolite variations in tectonic style along strike in the north south trending part of the norfolk ridge have produced ophiolite exposures related to uplifted and partially overthrust upper mantle slivers within the southern part of the loyalty basin

mg fe carpholites from aluminous schists in the diahot region and implications for preservation of high pressure low temperature schists northern new caledonia abstract mg fe carpholite is widespread in the diahot region of new caledonia in highly aluminous schists and as veins in what was originally a clay rich hydrothermal alteration envelope about massive suphide deposits these carpholites have fe fe mg ratios of 0 03 0 65 and no significant mn component mg carpholite quartz occur in assemblages with chlorite or pyrophyllite pyrophyllite kaolinite and pyrophyllite diaspore temperatures of 230 320 c and minimum pressures of 7 kbar are indicated for the mg fe carpholite bearing rocks the regional distribution of aragonite and mg fe carpholite parallel to a major zone of dominantly transcurrent movement and oblique to the trend of the subduction complex indicates the high p low t schists owe their rapid uplift and preservation to the vertical component of the transcurrent faulting

observation of sections of oceanic crust and mantle cropping out on the southern wall of kane fz n atlantic the objective of the 20 nautile dives of the recent kanaut cruise was to study the southern wall of the kane fracture zone from its eastern intersection with the mid atlantic ridge mar to 5 myr in age the geological mapping shows four successive massifs wrench faulted and slightly tilted the transform facing walls of these massifs exhibit outcrops of fresh and serpentinized peridotites gabbros and basalts the entire crustal exposure is cataclased and metamorphosed to greenschist facies

10be concentrations and the long term fate of particle reactive nuclides in five soil profiles from california concentration depth profiles of cosmic ray produced 10 be t 1 2 1 5m y have been measured by accelerator mass spectrometry in five soil profiles these measurements were made in an effort 1 to understand the retentivity of soil surfaces for particle reactive tracers depositing from the atmosphere on time scales of 10 4 10 6 years and 2 to explore the application of 10 be as a chronometer of geomorphic surface age the profiles sampled are from two wave cut terraces located near mendocino california a table mountain top and an alluvial fan both located near friant california the ages of the mendocino terraces are inferred to be 1 5 10 5 years based on amino stratigraphic correlations and models of terrace evolution those of the table mountain top and alluvial fan are 9 5 10 6 years and 6 0 10 5 years respectively based on k ar analyses all the surfaces sampled are nearly flat and exhibit few erosional features in addition to 10 be we measured 210 pb 239 240 pu and 7 be to ascertain the retentivity of the soils for particle reactive nuclides and to assess the present day delivery rate of nuclides from the atmosphere the 7 be inventory is 4 0 dpm cm 2 similar to those observed at nearby locations the inventories of 210 pb and pu isotopes conform to those predicted from model calculations and suggest that the soil surfaces sampled retain the entire burden of particle reactive nuclides delivered to them over short time scales 100 years the 10 be concentrations in the sample range between 0 2 and 7 10 8 atoms g soil and show strong correlations with leachable fe and or al the inventory of 10 be in the soil domain sampled is 1 2 orders of magnitude lower than that expected from the geological age of the surface and an average delivery rate of 10 be from the atmosphere 5 2 10 5 atoms cm 2 yr the low inventory of 10 be is attributed to its loss from the soil domain sampled by solution transport based on a simple model type calculation with a first order removal process for 10 be the residence time of 10 be in the soil domains sampled is determined to be of the order of 10 4 years the low residence time of 10 be in the soil domains sampled requires that it be found either deeper in the regolith or in ground waters in either case the application of 10 be as a chronometer of geomorphic surface age is severely constrained however the study of 10 be in soils provides the only entry into the long term 10 4 10 6 years behavior of particle reactive nuclides in soils and hence could be important for understanding the behavior of analogous nuclides introduced into soils by natural and anthropogenic processes

obduction why how and where clues from analog models obduction is an odd geodynamic process characterized by the emplacement of dense oceanic ophiolites atop light continental plates in convergent settings we herein present analog models specifically designed to explore the conditions i e sharp increase of plate velocities herein coined as acceleration slab interaction with the 660 km discontinuity ridge subduction under which obduction may develop as a result of subduction initiation the experimental setup comprises an upper mantle modeled as a low viscosity transparent newtonian glucose syrup filling a rigid plexiglas tank and high viscosity silicone plates convergence is simulated by pushing a piston with plate tectonics like velocities 1 10 cm yr onto a model comprising a continental margin a weakness zone with variable resistance and dip w an oceanic plate with or without a spreading ridge a preexisting subduction zone s dipping away from the piston and an upper active continental margin below which the oceanic plate is being subducted at the start of the model as for the neotethyan natural example several configurations were tested over thirty five parametric models with special emphasis on comparing different types of weakness zone and the degree of mechanical coupling across them measurements of displacements and internal deformation allow for a precise and reproducible tracking of deformation models consistently demonstrate that once conditions to initiate subduction are reached obduction may develop further depending on the effective strength of w results 1 constrainthe range of physical conditions required for obduction to develop nucleate and 2 underline the key role of such perturbations for triggering obduction particularly plate acceleration they provide an explanation to the short lived peri arabic obduction which took place along thousands of km almost synchronously within 50 10 myr from turkey to oman while the subduction zone beneath eurasia became temporarily jammed they also demonstrate that the emplacement of dense oceanic material on continental lithosphere is not a mysterious process requiring extraordinary boundary conditions but results from large scale normal oceanic then continental subduction processes

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

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

deepwater fold and thrust belt along new caledonia s western margin relation to post obduction vertical motions classically deepwater fold and thrust belts are classified in two main types depending if they result from near or far field stresses and the understanding of their driving and triggering mechanism is poorly known we present a geophysical dataset off the western margin of new caledonia sw pacific that reveals deformed structures of a deepwater fold and thrust belt that we interpret as a near field gravity driven system which is not located at a rifted passive margin the main factor triggering deformation is inferred to be oversteepening of the margin slope by post obduction isostatic rebound onshore erosion of abnormally dense obducted material combined with sediment loading in the adjacent basin has induced vertical motions that have caused oversteepening of the margin detailed morpho bathymetric seismic stratigraphic and structural analysis reveals that the fold and thrust belt extends 200 km along the margin and 50 km into the new caledonia trough deformation is rooted at depths greater than 5 km beneath the seafloor affects an area of 3500 km2 and involves a sediment volume of approximately 13 000 km3 this deformed belt is organized into an imbricate fan system of faults and one out of sequence thrust fault affects the seabed the thrust faults are deeply rooted in the basin along a low angle floor thrust and connected to new caledonia island along a major detachment this study not only provides a better knowledge of the new caledonia margin but also provides new insight into the mechanisms that trigger deepwater fold and thrust belts

subduction of the bougainville seamount vanuatu mechanical and geodynamic implications new bathymetric data gathered during a seabeam survey seapso cruise leg 1 enable us to re examine the flexural response of the oceanic lithosphere subducting under the new hebrides vanuatu island arc the bougainville seamount and sabine bank are interpreted as immerged fossil atolls the recent subsidence of which is related to the subduction of the oceanic lithosphere the position and altitude of the different fossil atolls which belong to the d entrecasteaux or loyalty ridges are in good agreement with predictions of elastic flexure of the lithosphere we deduce an average effective elastic thickness of the lithosphere of about 22 km for the area under study this value is slightly smaller than the one corresponding to the lithospheric age as given by the magnetic anomalies but is in good agreement with the age after a correction for thermal rejuvenation this assumption of a thermal rejuvenation of the north loyalty basin is also supported by previously reported high heat flow values and attenuation of sn seismic waves however the location and depth of the trench in front of the north loyalty basin do not agree with the model which fits the other data this discrepancy is interpreted as the result of variations of the value of pb vertical force per unit length of trench applied at the edge of the plate along the subduction zone such variations may be related to the length of the subducted slab which is shorter in front of the d entrecasteaux and loyalty ridges than in front of the north loyalty basin according to the hypocentral distribution of earthquakes and a tomographic image of the slab

successive rifting events in marginal basins the example of the coral sea region papua new guinea reactivation of extensional structures is commonly inferred during rift evolution in that context we present original seismic interpretation to explore the geometry and interactions of three successive rifting events in the coral sea region papua new guinea the first event r 1 poorly documented occurred during the triassic along an older n s permian structural fabric during the jurassic extensional faults were reactivated through a second extensional episode r 2 which formed small 10 20 km basins bounded by n s ne sw and e w listric faults extension prolonged during the lower cretaceous with seafloor spreading in the owen stanley oceanic basin now incorporated in the papuan fold and thrust belts a third late cretaceous extensional phase r 3 gently reactivated some of the faults with very limited landward tilt in most basins and deformation located along the present continent ocean transition seafloor spreading in the coral sea followed from danian to ypresian this extensional system is sealed by unequally preserved eocene strata that mark the onset of postrift thermal subsidence prior to the margin inversion from oligocene onward this overall evolution suggests various extensional systems that are geographically and temporarily defined the one another the early rifting of the crust is controlled by preexisting continental features resulting in the local pangaea breakup in contrast the coral sea propagator cuts through the rifted margin and is controlled by a subduction complex in accordance with the tasman sea opening this evolution underlines the interactions existing between two extension modes in agreement with variations of the regional geodynamical setting around australia

arc continent collisional orogenesis in the sw pacific and the nature source and correlation of emplaced ophiolitic nappe components abstract southwest pacific ophiolitic nappes emplaced upon the former margin of eastern australia provide a record of cyclical episodes of arc continent collisional orogenesis that occurred throughout the paleogene cenozoic nappes in papua new guinea new caledonia and northland new zealand consist of two key components 1 a basal and apparently older late cretaceous eocene metamorphosed normal to weakly enriched morb like basaltic ocean floor terrane and 2 an uppermost supra subduction zone ssz ophiolite that represents basement of the associated magmatic forearc a detailed comparison of geochemical characteristics of unequivocal ssz basalts with those considered as representative of the basaltic ocean floor terranes illustrates that formation of the majority of the latter is consistent with melting of a source transitional to normal n and enriched e morb these slight ree enriched or transitional t morb have generally been considered as the result of seafloor spreading in a back arc basin bab or basins to the east of australia in the late cretaceous paleocene 85 55 ma the enrichment recorded by the bab basalts is likely a result of upwelling plumes associated with initial fragmentation of eastern gondwana that left their signature on the mantle source consequently tapped by the bab basalts a subset of slightly more enriched basalts may owe their enrichment to source contamination minor assimilation of thinned gondwana derived continental crust associated lree depleted n mor basalts previously considered as part of the basaltic ocean floor terranes on the other hand are similar to ssz basalts of the northland and papuan ultramafic belt ophiolites refinement of existing tectonic models is hindered by ambiguities in the stratigraphic relationships of the underlying basaltic ocean floor terranes with the overlying ssz ophiolites a paucity of age data and uncertainties in the tectonomagmatic affinities of the underlying basalts nonetheless the geochemical characteristics displayed by lree depleted basalts of the basaltic ocean floor terranes suggest that these may instead represent younger magmatic forearc basement components subsequently overthrust by lower sections of a genetically related ophiolite whether these and for that matter the majority of basalts of the basaltic ocean floor terranes indeed represent complementary forearc crust to the ssz ophiolite as opposed to a slice of late cretaceous eocene bab floor backthrust beneath the ssz ophiolite upon emplacement will have to await the acquisition of corresponding radiometric age data