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

landward and seaward directed thrusting accompanying the onset of subduction beneath new zealand a short 5 m y episode of deformation accompanied the onset of the present phase of subduction beneath northern and eastern new zealand in the earliest miocene at many places along the continental margin the structures produced were dominated by subhorizontal thrust sheets with tens of kilometres of displacement thrusting was directed both landward with or without involvement of the upper part of the downgoing plate and seaward the structures are different in style from those typical of active trench slopes which are predominantly seaward directed imbricate thrusts and associated folds the structures show that delamination of the downgoing plate occurred at some places and suggest that a high degree of coupling between upper and lower plates occurred at others

determination and significance of the mn ii zero field splitting zfs interaction in the geochemistry of travertines an analytical approach based on the electron paramagnetic resonance epr spectroscopy of mn ii in travertines has been developed in order to obtain relevant information about the local inhomogeneity of calcite and about the thermodynamic conditions which control the formation of travertine deposits this information is crucial to constrain the precipitation of travertine under different geochemical contexts an empirical correlation between the spectral features and the zero field splitting zfs interaction has been established through numerical simulations of epr spectra the variability of the investigated parameters and the applicability of the method have been tested on several travertines from central italy

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

paleogene seafloor spreading in the southeast tasman sea magnetic anomalies 11 18 have been identified in the southeast tasman sea the area of ocean crust southwest of fiordland new zealand west of the puysegur trench the present plate boundary and southeast of the older 80 58 ma ocean crust of the tasman sea basin structures associated with two changes in spreading direction are preserved in the area spreading between the australian and pacific plates began in the eocene about 40 ma in this area orthogonal to cretaceous paleocene spreading between them in the tasman sea the boundary between the two ages of ocean crust is abrupt and associated with what we have interpreted as marginal uplift blocks and rift basins of the resolution ridge system rifting appears to have propagated northeast along a fracture zone the northern blocks of the resolution ridge system may be isolated fragments of continental crust of the campbell plateau between anomalies 18 and 11 40 30 ma the spreading rate was about 1 5 cm yr swath mapping and satellite altimetry data show that a second change in relative plate motion direction began shortly after anomaly 11 about 30 ma and led to a change from tension to transcurrent motion along the plate boundary the timing of the changes in spreading direction agrees with tectonic events interpreted from onshore data

thermoluminescence dating of the kamil impact crater egypt thermoluminescence tl dating has been used to determine the age of the meteorite impact crater at gebel kamil egyptian sahara previous studies suggested that the 45 m diameter structure was produced by a fall in recent times less than 5000 years ago of an iron meteorite impactor into quartz arenites and siltstones belonging to the lower cretaceous gilf kebir formation the impact caused the complete fragmentation of the impactor and the formation of a variety of impactites e g partially vitrified dark and light materials present as ejecta within the crater and in the surrounding area after a series of tests to evaluate the tl properties of different materials including shocked intra crater target rocks and different types of ejecta we selected a suite of light colored ejecta that showed evidence of strong thermal shock effects e g partial vitrification and the presence of high temperature and pressure silica phases the abundance of quartz in the target rocks including the vitrified impactites allowed tl dating to be undertaken the variability of radioactivity of the intracrateric target rocks and the lack of direct in situ dosimetric evaluations prevented precise dating it was however possible to constrain the impact in the 2000 bc 500 ad range if as we believe the radioactivity measured in the fallback deposits is a reliable estimate of the mean radioactivity of the site the narrower range 1600 400 bc at the 2 confidence level can be realistically proposed

seismic stratigraphic record of transition from mesozoic subduction to continental breakup in the zealandia sector of eastern gondwana abstract the southwest pacific between australia new zealand and new caledonia is a block of continental crust zealandia that moved away from australia and antarctica after a long period of subduction beneath eastern gondwana we use 100 000 line km of seismic reflection profiles to identify intra continental basins related to the gondwana active margin overlain with erosional unconformity by retrogradational strata we interpret this regional scale first order unconformity the eastern gondwana composite surface and seismic stratigraphic megasequence pattern to represent the transition from subduction to continental breakup and separation of eastern gondwana rocks that make up the lower seismic stratigraphic megasequence zealandia 3 permian to early cretaceous have been drilled near new zealand and can be correlated with murihiku supergroup rocks farther north on the lord howe rise we correlate this megasequence with triassic to jurassic rocks of the clarence moreton basin of eastern australia we reinterpret the fairway aotea basin to be underlain by a wedge of zealandia 3 deposits thrust in a retro arc like foreland basin that was active immediately prior to breakup the lower rift and retrogradational megasequence zealandia 2 late cretaceous to eocene we correlate with the pakawau and kapuni groups of taranaki basin new zealand and the formation a charbon and phtanites of new caledonia the boundary with the shallowest megasequence zealandia 1 late eocene to present is the eocene oligocene unconformity which is primarily overlain by pelagic carbonate rocks this regional unconformity likely represents the onset of the modern tonga kermadec subduction system

geological and geophysical investigation of kamil crater egypt we detail the kamil crater egypt structure and refine the impact scenario based on the geological and geophysical data collected during our first expedition in february 2010 kamil crater is a model for terrestrial small scale hypervelocity impact craters it is an exceptionally well preserved simple crater with a diameter of 45 m depth of 10 m and rayed pattern of bright ejecta it occurs in a simple geological context flat rocky desert surface and target rocks comprising subhorizontally layered sandstones the high depth to diameter ratio of the transient crater its concave yet asymmetric bottom and the fact that kamil crater is not part of a crater field confirm that it formed by the impact of a single iron mass or a tight cluster of fragments that fragmented upon hypervelocity impact with the ground the circular crater shape and asymmetries in ejecta and shrapnel distributions coherently indicate a direction of incidence from the nw and an impact angle of approximately 30 to 45 newly identified asymmetries including the off center bottom of the transient crater floor downrange maximum overturning of target rocks along the impact direction and lower crater rim elevation downrange may be diagnostic of oblique impacts in well preserved craters geomagnetic data reveal no buried individual impactor masses 100 kg and suggest that the total mass of the buried shrapnel 100 g is approximately 1050 1700 kg based on this mass value plus that of shrapnel 10 g identified earlier on the surface during systematic search the new estimate of the minimum projectile mass is approximately 5 t

the flux of small near earth objects colliding with the earth asteroids with diameters smaller than approximately 50 100 m that collide with the earth usually do not hit the ground as a single body rather they detonate in the atmosphere these small objects can still cause considerable damage such as occurred near tunguska siberia in 1908 the flux of small bodies is poorly constrained however in part because ground based observational searches pursue strategies that lead them preferentially to find larger objects a tunguska class event the energy of which we take to be equivalent to 10 megatons of tnt was previously estimated to occur every 200 300 years with the largest annual airburst calculated to be approximately 20 kilotons kton tnt equivalent ref 4 here we report satellite records of bolide detonations in the atmosphere over the past 8 5 years we find that the flux of objects in the 1 10 m size range has the same power law distribution as bodies with diameters 50 m from this we estimate that the earth is hit on average annually by an object with approximately 5 kton equivalent energy and that tunguska like events occur about once every 1 000 years

seafloor spreading in the north fiji basin southwest pacific abstract the seapso leg 3 cruise december 1985 was carried out in the north fiji basin aboard the ifremer r v jean charcot the main purpose of the cruise was to carry out a geophysical and geological study of the central part of the basin between 16 s and 22 s to this end bathymetric profiles with the help of the seabeam multichannel echosounder single channel seismic reflection profiles and magnetic and gravimetric profiles were made moreover geological samples were taken by dredging and piston coring as well as water samples with the help of a multiprobe one of the aims of the cruise was to explore the present day spreading axis of the north fiji basin we were able to show that accretion the rate of which is about 7 cm yr in the north fiji basin has been constrained in a n s direction for 3 ma very recently between 0 6 and 0 7 ma ago the accretion process changed resulting in the creation of a ridge ridge trench triple junction centered at 173 30 e and 16 40 s water samples taken in the vicinity of this triple junction show significant anomalies of methane and manganese which are probably linked to hydrothermal events in its morphological characteristics transversal as well as longitudinal the ridge of the north fiji basin shows striking similarities to the east pacific rise it presents the same differences in level the same sizes of structures and the same particular features overlapping spreading centers offsets etc another aim of the seapso cruise was to study the complex deformation area located immediately to the west of the fiji islands which we interpret as a distensive type deformation in a strike slip system

new style of honeycomb structures revealed on 3d seismic data indicate widespread diagenesis offshore great south basin new zealand abstract in the great south basin within the eocene section at time depths around 700 900 ms two way time below the seafloor unusual features are observed on 3d seismic data closely associated with polygonal faults the features referred to as honeycomb structures hs cover an area of 600 km 2 are packed circular oval to polygonal depressions 150 400 m across in plan view and several to 10 m in amplitude polygonal faults rapidly die out at the marshall paraconformity which is overlain by the oligocene penrod formation hence the polygonal faults are inferred to have formed prior to the marshall paraconformity and they cross cut hs features consequently the top of the hs probably formed at burial depths of around 375 500 m which is their decompacted depth below the paraconformity the interval containing hs is about 125 m vertical thick there are several possible origins for the hs the most probable is related to bulk contraction of the sediment volume accompanied by fluid expulsion which suggests a diagenetic origin in particular the opal a ct transition there are actually two polygonal fault systems pfs present in the area the southern tier 1 pfs lies laterally to the hs and overlaps with it the northern pfs tier 2 lies above the hs appears to be independent of the hs and formed in the upper 200 300 m of the sediment column the tier 1 pfs probably formed by shear failure related to the same diagenetic effects that caused the hs

meteorite reconnaissance in saudi arabia meteorite searches in saudi arabia between 2008 and 2014 yielded 46 meteorites from the yabrin area 23 n 49 e 35 meteorites from the rub al khali sand desert 19 20 n 48 51 e and 1 meteorite from al haddar no meteorites were found near hafar al batin 29 n 45 e the 82 new meteorites represent 57 falls comprising 43 ordinary chondrites 4 carbonaceous chondrites 2 enstatite chondrites 3 ureilites 3 eucrites 1 acapulcoite and 1 lunar meteorite the median of 31 14c terrestrial ages is 6 2 ka significantly younger than the oman population 19 5 ka n 128 a further assessment of terrestrial 14c contamination is advised by a 11 15 ka 14c terrestrial age of heavily weathered meteorite khawr al fazra 014 geology indicating a terrestrial age 100 ka find densities of 0 4 2 8 km 2 for yabrin and the western rub al khali are similar to 0 5 km 2 observed in oman higher find densities of 135 km 2 29 km 2 for masses 10 g exist on small pleistocene outcrops in blowouts in the south central rub al khali 21 unpaired meteorites four 10 g were found in 11 blowouts with a combined area of 0 14 km2 the rub al khali meteorites show a relatively high degree of weathering median w 3 6 2 5 for yabrin low median mass 4 3 138 g and a high h l ratio 2 3 1 1 the high density of small meteorites is explained by prolonged sand protection and recent deflation the high meteorite density and relatively high proportion of rare meteorite types render the rub al khali blowouts an interesting target for future exploration

biotic and inorganic control on travertine deposition at bullicame 3 spring viterbo italy a multidisciplinary approach abstract a multidisciplinary characterization of an active thermal spring in central italy has been undertaken with the aim of i ascertaining whether microbiological activity plays a relevant role in hot depositing travertines and ii establishing an experimental protocol able to identify similar effects in fossil travertines water gas and travertine samples were investigated by chemical icp ms sem eds physical dta dtg isotopic 18o d and 13c mineralogical xrpd and spectroscopic epr techniques twenty four samples three for each phase were collected every 5 c temperature drop along a 100 m long artificial channel near viterbo bullicame 3 latium central italy a microbiological characterization was carried out in parallel sampling the channel every 10 c temperature drop the bullicame 3 system is revealed to be composed of two markedly different subsystems a water gas interface where a kinetically fast exchange allows equilibrium of components both in water and in gases a solid water interface where travertine precipitation occurs influenced by microbiological activity a peculiar lattice shrinking of calcite was identified as well as an anomalous value of the zero field splitting parameter from the epr measurements the interpretation of these anomalies is confirmed by the identification of calcifying cyanobacteria throughout the channel path our results point out that microbiological activity can play a significant role in travertine deposition from hot springs furthermore the proposed approach representing a tool to identify crystal chemical remnants of past microbiological activity could be applicable to fossil travertines

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

quantitative biostratigraphy of the taranaki basin new zealand a deterministic and probabilistic approach a quantitative biostratigraphic analysis of the paleocene to lower miocene of the taranaki basin has enabled high precision in correlation zonation and assessment of depositional history biostratigraphic range end events based on 493 taxa in cuttings samples from eight wells representing foraminifera nannofossils dinoflagellates and miospores were culled to 87 range top events that were then analyzed by deterministic constrained optimization conop and probabilistic ranking and scaling rasc techniques all except 16 of the events are found to have relatively good biostratigraphic reliability the rasc probable sequence and probabilistic zonation give the best estimate of the sequence of events and zones to be encountered in any new well in the basin and a precise biostratigraphic scale for future exploration the conop composite section which matches well with that derived by conventional graphic correlation graphcor is readily related to previous zonations based on maximum ranges of taxa but gives an order of magnitude greater precision conop provides a precise correlation framework and reveals marked variation in thickness of stages across the basin when the composite section is calibrated against the time scale basinwide changes in depositional rate are revealed the upper eocene and oligocene mark an interval of slow deposition whereas the miocene marks a sharp increase in deposition the time calibrated composite section enables unconformities and changes in depositional rate found in individual wells to be precisely estimated many new unconformities are indicated particularly in the paleocene and eocene

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

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

two dimensional stratigraphic forward modeling reconstructing high relief clinoforms in the northern taranaki basin clinoforms the basic large scale architectural form within which sediments are stored and eventually fed down depositional dip in clastic wedges exist in many shapes and sizes understanding how they form evolve and degrade is critical to understanding how transport mechanisms affect the shelf margin and sediment partitioning and distribution and their implications on the presence of a working petroleum system the neogene stratigraphic succession of the taranaki basin in new zealand contains clinoform packages that display a variety of architectures well imaged on seismic data quantitative characterization of this interval was performed to unravel the processes by which clinoforms evolve under the influence of tectonic and isostatic driven subsidence sea level change and sediment supply fluctuations nine different clinoform packages were identified on the basis of changes in their seismic stratigraphic characteristics two dimensional stratigraphic forward modeling was used to conduct a sensitivity analysis and determine the relative importance of different geologic controls on their genesis our results show that during the early to late pliocene clinoform architectures were influenced by the opening of a back arc rifting structure in the taranaki basin northern graben which controlled sediment redistribution and partitioning at the same time a drop in global sea level allowed sediment bypass to distal parts of the basin during the late pliocene changes in the australian pacific subduction zone forced rapid uplifting of the southern alps generating a significant increase in sediment supply model simulations suggest that clinoform architectures during the late pliocene were controlled by this increase in sediment supply and associated loading

horizontal lithosphere compression and subduction constraints provided by physical modeling physical modeling of the subduction is performed with a two layered mantle model elastico plastic lithosphere and low viscosity asthenosphere and is governed by the criteria of similarity compression of the lithosphere in the area of a passive continental margin has been shown to produce a buckling instability in the oceanic plate with wavelengths of 200 km on the average later a localization of deformation occurs in sagging at some distance from the margin where a strongly dislocated linear ridge is formed due to the thrusting the plate then experiences a failure along the inclined zone and subduction starts the inner trench slope which forms therefore has a scraped structure and include a block of crushed and dislocated oceanic crust and sediments in the lower section if there is an old inclined fault striking across the compression of an oceanic lithosphere it is on that fault that a subduction zone is initiated the inner trench slope has then a different structure and forms due to normal faulting in the frontal part of the overriding plate the formation of a subduction zone requires a compression that is smaller than that in the preceding case by a factor of 2 or 3 the subducting plate experiences an elastico plastic bending and under specific conditions thrusting along the zone dipping from under the overriding plate oceanward and crossing the entire lithosphere the best agreement between generalized relief in subduction zone in the model and nature is achieved when a shear yield limit s 1 3 108 pa modulus of elasticity e about a few times 1011 pa and a thickness h 60 km are adopted for the real lithosphere

continental scale geographic change across zealandia during paleogene subduction initiation data from international ocean discovery program iodp expedition 371 reveal vertical movements of 1 3 km in northern zealandia during early cenozoic subduction initiation in the western pacific ocean lord howe rise rose from deep 1 km water to sea level and subsided back with peak uplift at 50 ma in the north and between 41 and 32 ma in the south the new caledonia trough subsided 2 3 km between 55 and 45 ma we suggest these elevation changes resulted from crust delamination and mantle flow that led to slab formation we propose a subduction resurrection model in which 1 a subduction rupture event activated lithospheric scale faults across a broad region during less than 5 m y and 2 tectonic forces evolved over a further 4 8 m y as subducted slabs grew in size and drove plate motion change such a subduction rupture event may have involved nucleation and lateral propagation of slip weakening rupture along an interconnected set of preexisting weaknesses adjacent to density anomalies

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

pacific antarctic australia motion and the formation of the macquarie plate summary magnetic anomaly and fracture zone data on the southeast indian ridge seir are analysed in order to constrain the kinematic history of the macquarie plate the region of the australian plate roughly east of 145 e and south of 52 s finite rotations for australia antarctic motion are determined for nine chrons 2ay 3ay 5o 6o 8o 10o 12o 13o and 17o using data limited to the region between 88 e and 139 e these rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the seir an analysis of the synthetic flowlines shows that the macquarie plate region has behaved as an independent rigid plate for roughly the last 6 myr finite rotations for macquarie antarctic motion are determined for chrons 2ay and 3ay these rotations are summed with australia antarctic rotations to determine macquarie australia rotations we find that the best fit macquarie australia rotation poles lie within the zone of diffuse intraplate seismicity in the south tasman sea separating the macquarie plate from the main part of the australian plate motion of the macquarie plate relative to the pacific plate for chrons 2ay and 3ay is determined by summing macquarie antarctic and antarctic pacific rotations the pacific macquarie rotations predict a smaller rate of convergence perpendicular to the hjort trench than the pacific australia rotations the onset of the deformation of the south tasman sea and the development of the macquarie plate appears to have been triggered by the subduction of young buoyant oceanic crust near the hjort trench and coincided with a clockwise change in pacific australia motion around 6 ma the revised pacific australia rotations also have implications for the tectonics of the alpine fault zone of new zealand we find that changes in relative displacement along the alpine fault have been small over the last 20 myr the average rate of convergence over the last 6 myr is about 40 per cent smaller than in previous models

gebel kamil the iron meteorite that formed the kamil crater egypt abstract the 45 m in diameter kamil impact crater was formed 10 g inferred from the density map compiled in this study is 3400 kg field data indicate that the iron bolide approached the earth s crust from the northwest 305 340 n travelling along a moderately oblique trajectory upon hypervelocity impact the projectile was disrupted into thousands of fragments shattering was accompanied by some melting of the projectile and of the quartz arenite target rocks which also suffered shock metamorphism

tectonic history of northern new caledonia basin from deep offshore seismic reflection relation to late eocene obduction in new caledonia southwest pacific new high quality multichannel seismic reflection data from the western new caledonia offshore domain allow for the first time the direct continuous connection of seismic reflectors between the deep sea drilling project 208 drill hole on the lord howe rise and the new caledonia basin a novel seismic interpretation is hence proposed for the northern new caledonia basin stratigraphy which places the eocene oligocene unconformity deeper than previously thought and revisits the actual thickness of the pre oligocene sequences a causal link is proposed between the obduction of the south loyalty basin over new caledonia nc and the tectonic history of the northern new caledonia basin here it is suggested that as the south loyalty basin was being obducted during early oligocene times the nc basin subsided under the effect of the overloading and underthrusted to accommodate the compressional deformation which resulted in 1 the uplift of the northern fairway ridge and 2 the sinking of the western flank of new caledonia this event also had repercussions farther west with the incipient subsidence of the lord howe rise

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

shock metamorphism and impact melting in small impact craters on earth evidence from kamil crater egypt kamil is a 45 m diameter impact crater identified in 2008 in southern egypt it was generated by the hypervelocity impact of the gebel kamil iron meteorite on a sedimentary target namely layered sandstones with subhorizontal bedding we have carried out a petrographic study of samples from the crater wall and ejecta deposits collected during our first geophysical campaign february 2010 in order to investigate shock effects recorded in these rocks ejecta samples reveal a wide range of shock features common in quartz rich target rocks they have been divided into two categories as a function of their abundance at thin section scale 1 pervasive shock features the most abundant including fracturing planar deformation features and impact melt lapilli and bombs and 2 localized shock features the least abundant including high pressure phases and localized impact melting in the form of intergranular melt melt veins and melt films in shatter cones in particular kamil crater is the smallest impact crater where shatter cones coesite stishovite diamond and melt veins have been reported based on experimental calibrations reported in the literature pervasive shock features suggest that the maximum shock pressure was between 30 and 60 gpa using the planar impact approximation we calculate a vertical component of the impact velocity of at least 3 5 km s 1 the wide range of shock features and their freshness make kamil a natural laboratory for studying impact cratering and shock deformation processes in small impact structures

target projectile interaction during impact melting at kamil crater egypt abstract in small meteorite impacts the projectile may survive through fragmentation in addition it may melt and chemically and physically interact with both shocked and melted target rocks however the mixing mingling between projectile and target melts is a process still not completely understood kamil crater 45 m in diameter egypt generated by the hypervelocity impact of the gebel kamil ni rich ataxite on sandstone target allows to study the target projectile interaction in a simple and fresh geological setting we conducted a petrographic and geochemical study of macroscopic impact melt lapilli and bombs ejected from the crater which were collected during our geophysical campaign in february 2010 two types of glasses constitute the impact melt lapilli and bombs a white glass and a dark glass the white glass is mostly made of sio 2 and it is devoid of inclusions its negligible ni and co contents suggest derivation from the target rocks without interaction with the projectile 2 o 3 0 84 18 7 wt feo t 1 83 61 5 wt and nio 2 melt that eventually solidified as white glass behaved as an immiscible liquid and did not interact with the projectile during the excavation stage dark glass melt engulfed and coated the white glass melt target fragments and got stuck to iron meteorite shrapnel fragments this model could also explain the common formation of white and dark glasses in small impact craters generated by iron bodies e g wabar

formation of a small impact structure discovered within the agoudal meteorite strewn field morocco a relic impact structure was recognized within the strewn field of the agoudal iron meteorite the heavily eroded structure has preserved shatter cones in a limestone basement and remnants of autochthonous and allochthonous breccias fragments of iron incorporated into the allochthonous breccia have a chemical composition ni 5 16 wt ir 0 019 ppm similar to that of the agoudal meteorite supporting a syngenetic origin of the strewn field and the impact structure the total recovered mass of agoudal meteorite fragments is estimated at approximately 500 kg the estimated size of the se nw oriented strewn field is 6 2 km model calculations with minimal preatmospheric size show that a similar meteorite strewn field plus one small crater with observed shock effects could be formed by fragmentation of a meteoroid approximately 1 4 m in diameter with an impact angle of approximately 60 from the horizontal however the most probable is an impact of a larger 3 4 m diameter meteoroid resulting a strewn field with approximately 10 craters 10 30 m in diameter each plus numerous meteorite fragments the calculated scattering area of meteorite shrapnel ejected from these impact craters could completely cover the observed strewn field of the agoudal meteorite

corrosion of fe ni alloys by cl containing akaganeite beta feooh the antarctic meteorite case a suite of 12 iron and 8 stony meteorites from various environments and locations in antarctica was studied representing a range of compositions metallographic structures and degrees of weathering polished sections of meteoritic metal with adhering corrosion products were examined microscopically with the electron microprobe and sem and were used as samples for x ray diffraction examination the major corrosion products are cl containing akaganeite up to 5wtper thousand cl and goethite with minor amounts of lepidocrocite and maghemite cl is not native to the meteorites but owing to the electrochemical nature of the corrosion of fe ni alloys is attracted from the snow and ice or rocky soil environment that contains low amounts of cl akaganeite precipitates near the reaction front incorporating cl ions into ion exchange sites where they may be retained made available for further depassivating corrosive action or become flushed from the system with time and distance from the reaction surface akaganeite ages and transforms to mainly goethite and maghemite it is again argued that lawrencite fecl 2 is not a mineral in meteorites in the stormy antarctic wind blown terrestrial particles may become attached to the weathering products on the meteorite and in time themselves decompose delivering terrestrial ions to the meteorite corrosion system the ions may move a significant distance from the surface and in particular cl may be found along almost invisible cracks in the deep interior trace element work and age determinations 36 cl should take this problem into account

palynofacies organic geochemistry and depositional environment of the tartan formation late paleocene a potential source rock in the great south basin new zealand abstract detailed palynofacies analysis of sidewall core samples taken from below within and above the tartan formation thanetian late paleocene 58 7 55 8 ma a potential source rock in the epeiric great south basin shows that the formation is characterised by very high percentages of degraded brown phytoclasts rare marine algae and amorphous organic matter and thereby represents a mix of terrestrial and marine kerogen the results indicate that the formation was deposited in a marginally marine hyposaline proximal environment under bottom conditions that varied from anoxic to oxic along a nearshore offshore transect samples from the upper part of the underlying wickliffe formation indicate deposition in a marginal to normal marine proximal environment under anoxic to oxic bottom environments the lower part of the overlying laing formation was deposited in an open marine relatively distal setting under anoxic to oxic bottom environments the palaeodepositional changes observed through this sequence may be best explained as a result of base level changes in a relatively shallow water sea the tartan formation was deposited in the central southern and eastern parts of the great south basin during a peak regression in the thanetian that terminated an overall mid to late paleocene aggradational to regressive trend in the basin and gave way to a latest paleocene earliest eocene transgression its earliest and thickest known occurrence is in the pakaha 1 well in the centre of the basin the formation is absent from proximal offshore wells and absent to very thin in onshore otago drill cores and outcrops most probably due to sediment bypass or subaerial erosion the tartan formation is characterised by a combination of high organic carbon contents and 13c enrichment relative to the enclosing formations corg and toc values are mostly in the range 3 7 17 1 mean 8 0 and 13c values 21 4 to 15 8 up to c 9 heavier than in the under and overlying formations for sidewall core samples there is a strong correlation between toc and 13c values r2 0 91 the heavy 13c values can only partly be attributed to the global paleocene carbon isotope maximum and indicate that other organic matter source and or regional depositional factors affected carbon fractionation in the tartan kerogen the tartan formation displays very good excellent petroleum generative potential up to 31 mg hc g rock but hi values presented herein range only from 157 to 268 mg hc g toc mean 203 mg hc g toc indicating the potential is primarily for gas with only minor oil the relatively poor hi values are attributed to the large influx of degraded woody material rendering the tartan formation less oil prone than previously inferred the tartan formation is biostratigraphically coeval with the waipawa formation of the new zealand east coast basin which is a similar source rock unit also enriched in 13c samples from the waipawa formation taken 1000 km north of the great south basin share palynofacies and geochemical characteristics with the tartan formation indicating that a base level fall also controlled the deposition of the waipawa formation in the east coast basin the size of the area affected by the base level fall and the lack of evidence for contemporaneous large scale tectonic uplift imply that the base level fall was caused by eustasy this suggests that the tartan and waipawa formations may be more extensive in new zealand offshore basins than previously thought

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

terrane boundary reactivation barriers to lateral fault propagation and reactivated fabrics rifting across the median batholith zone great south basin new zealand prominent pre existing structural heterogeneities within the lithosphere may localise or partition deformation during tectonic events the ne trending great south basin offshore new zealand formed perpendicular to a series of underlying crustal terranes including the dominantly granitic median batholith zone which along with the boundaries between individual terranes exert a strong control on rift physiography and kinematics we find that the crustal to lithospheric scale southern terrane boundary of the median batholith zone is associated with a crustal scale shear zone that was reactivated during late cretaceous extension between zealandia and australia this reactivated terrane boundary is oriented at a high angle to the faults defining the great south basin we identify a large granitic laccolith along the southern margin of the median batholith expressed as sub horizontal packages of reflectivity and acoustically transparent areas on seismic reflection data the presence of this strong granitic body inhibits the lateral south westward propagation of ne trending faults which segment into a series of splays that rotate to align along the margin as they approach further we also identify two e w and ne sw oriented basement fabrics likely corresponding to prominent foliations which are exploited by small scale faults across the basin we show that different mechanisms of structural inheritance are able to operate simultaneously and somewhat independently within rift systems at different scales of observation the presence of structural heterogeneities across all scales need to be incorporated into our understanding of the structural evolution of complex rift systems

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

analysis of a prehistoric egyptian iron bead with implications for the use and perception of meteorite iron in ancient egypt tube shaped beads excavated from grave pits at the prehistoric gerzeh cemetery approximately 3300 bce represent the earliest known use of iron in egypt using a combination of scanning electron microscopy and micro x ray microcomputer tomography we show that microstructural and chemical analysis of a gerzeh iron bead is consistent with a cold worked iron meteorite thin fragments of parallel bands of taenite within a meteoritic widmanstatten pattern are present with structural distortion caused by coldworking the metal fragments retain their original chemistry of approximately 30 wt nickel the bulk of the bead is highly oxidized with only approximately 2 4 of the total bead volume remaining as metal our results show that the rst known example of the use of iron in egypt was produced from a meteorite its celestial origin having implications for both the perception of meteorite iron by ancient egyptians and the development of metallurgical knowledge in the nile valley

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

plume like upper mantle instabilities drive subduction initiation 1 the control of upper mantle lithosphere interactions mli on compressional tectonics is not well resolved this applies to the role of mli in triggering of subduction initiation or lithosphere mantle lm downwellings we present results of thermo mechanically thermo dynamically coupled numerical experiments that are consistent with an array of recent geophysical constraints on lithosphere and upper mantle rheology and structure we demonstrate that mli can lead to initiation of continental lithosphere subduction inducing its spontaneous downthrusting to depths of 300 500 km upon plume impingement of the lithosphere this downthrusting is pre conditioned by rheological stratification of visco elasto plastic lithosphere and its free surface the subsequent evolution of the slab is governed by phase changes and its interactions with the surrounding mantle we demonstrate that the mode of mli is strongly affected by the lateral heterogeneities and the presence of suture zones in the lithosphere

arsenic incorporation in natural calcite lattice evidence from electron spin echo spectroscopy abstract quaternary travertines of the middle pecora valley tuscany italy contain up to 257 ppm arsenic such a content is environmentally relevant but low enough to make the exact chemical speciation of arsenic difficult by applying conventional investigation techniques the task was addressed by use of the electron spin echo ese spectroscopy taking advantage from the modulation by the arsenic nucleus of the decay spectrum of the paramagnetic ion mn ii occurring as replacement of ca in the calcite lattice interpretation of the spectra suggests that arsenic occurs in the calcite lattice in the position of c through the substitution co2 3 aso3 3 this mechanism of arsenic incorporation by calcite may be an effective limit of arsenic mobility under conditions where immobilization through sorption by iron and or manganese oxyhydroxides is not operating

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

lithosphere delamination with foundering of lower crust and mantle caused permanent subsidence of new caledonia trough and transient uplift of lord howe rise during eocene and oligocene initiation of tonga kermadec subduction western pacific 1 we use seismic reflection and rock sample data to propose that the first order physiography of new caledonia trough and norfolk ridge formed in eocene and oligocene time and was associated with the onset of subduction and back arc spreading at the australia pacific plate boundary our tectonic model involves an initial cretaceous rift that is strongly modified by cenozoic subduction initiation hence we are able to explain 1 complex sedimentary basins of inferred mesozoic age 2 a prominent unconformity and onlap surface of middle eocene to early miocene age at the base of flat lying sediments beneath the axis of new caledonia trough 3 gently dipping variable thickness and locally deformed late cretaceous strata along the margins of the trough 4 platform morphology and unconformities on either side of the trough that indicate a phase of late eocene to early miocene uplift to near sea level followed by rapid oligocene and miocene subsidence of 1100 1800 m and 5 seismic reflection facies tied to boreholes that suggest absolute tectonic subsidence at the southern end of new caledonia trough by 1800 2200 m since eocene time the cenozoic part of the model involves delamination and subduction initiation followed by rapid foundering and rollback of the slab this created a deep 2 km enclosed oceanic trough 2000 km long and 200 300 km across in eocene and oligocene time as the lower crust detached with simultaneous uplift and local land development along basin flanks disruption of late cretaceous and paleogene strata was minimal during this cenozoic phase and involved only subtle tilting and local reverse faulting or folding basin formation was possible through the action of at least one detachment fault that allowed the lower crust to either be subducted into the mantle or exhumed eastward into norfolk basin we suggest that delamination of the lithosphere with possible mixing of the lower crust back into the mantle is more widespread than previously thought and may be commonly associated with subduction initiation such as cenozoic events in the mediterranean and western pacific

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

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

anatomy of a young impact event in central alberta canada prospects for the missing holocene impact record small impact events recorded on the surface of earth are signifi cantly underrepresented based on expected magnitude frequency relations we report the discovery of a 36 m diameter late holocene impact crater located in a forested area near the town of whitecourt alberta canada although undetectable using visible imagery the presence of the crater is revealed using a bare earth digital elevation model obtained through airborne light detection and ranging lidar the target material comprises deglacial quaternary sediments with impact ejecta burying a late holocene soil dated to ca 1100 14 c yr b p most of the 74 iron meteorites 0 1 1196 g recovered have an angular exterior morphology these meteorites were buried at depths 25 cm and are interpreted to result from fragmentation of the original projectile mass either at low altitude or during the impact event impact of the main mass formed the simple bowl shaped impact structure associated with an ejecta blanket and crater fi ll the increasing availability of lidar data for many terrestrial surfaces will serve as a useful tool in the discovery of additional small impact features

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

late eocene early miocene facies and stratigraphic development taranaki basin new zealand the transition to plate boundary tectonics during regional transgression eight latest eocene to earliest miocene stratigraphic surfaces have been identified in petroleum well data from the taranaki basin new zealand these surfaces define seven regional sedimentary packages of variable thickness and lithofacies forming a mixed siliciclastic carbonate system the evolving tectonic setting particularly the initial development of the australian pacific convergent margin controlled geographic stratigraphic and facies variability this tectonic signal overprinted a regional transgressive trend that culminated in latest oligocene times the earliest influence of active compressional tectonics is reflected in the preservation of latest eocene early oligocene deepwater sediments in the northern taranaki basin thickness patterns for all mid oligocene units onwards show a shift in sedimentation to the eastern taranaki basin controlled by reverse movement on the taranaki fault system this resulted in the deposition of a thick sedimentary wedge initially of coarse clastic sediments later carbonate dominated in the foredeep close to the fault in contrast oligocene active normal faulting in a small sub basin in the south may represent the most northerly evidence for rifting in southern zealandia related to emerald basin formation the early miocene period saw a return to clastic dominated deposition the onset of regional regression and the southward propagation of compressional tectonics

widespread compression associated with eocene tonga kermadec subduction initiation eocene onset of subduction in the western pacific was accompanied by a global reorganization of tectonic plates and a change in pacific plate motion relative to hotspots during the period 52 43 ma we present seismic reflection and rock sample data from the tasman sea that demonstrate that there was a period of widespread eocene continental and oceanic compressional plate failure after 53 48 ma that lasted until at least 37 34 ma we call this the tectonic event of the cenozoic in the tasman area tecta its compressional nature is different from coeval tensile stresses and back arc opening after 50 ma in the izu bonin mariana region our observations imply that spatial and temporal patterns of stress evolution during western pacific eocene subduction initiation were more varied than previously recognized the evolving eocene geometry of plates and boundaries played an important role in determining regional differences in stress state

earth impact effects program a web based computer program for calculating the regional environmental consequences of a meteoroid impact on earth we have developed a web based program for quickly estimating the regional environmental consequences of a comet or asteroid impact on earth www lpl arizona edu impacteffects this paper details the observations assumptions and equations upon which the program is based it describes our approach to quantifying the principal impact processes that might affect the people buildings and landscape in the vicinity of an impact event and discusses the uncertainty in our predictions the program requires six inputs impactor diameter impactor density impact velocity before atmospheric entry impact angle the distance from the impact at which the environmental effects are to be calculated and the target type sedimentary rock crystalline rock or a water layer above rock the program includes novel algorithms for estimating the fate of the impactor during atmospheric traverse the thermal radiation emitted by the impact generated vapor plume fireball and the intensity of seismic shaking the program also approximates various dimensions of the impact crater and ejecta deposit as well as estimating the severity of the air blast in both crater forming and airburst impacts we illustrate the utility of our program by examining the predicted environmental consequences across the united states of hypothetical impact scenarios occurring in los angeles we find that the most wide reaching environmental consequence is seismic shaking both ejecta deposit thickness and air blast pressure decay much more rapidly with distance than with seismic ground motion close to the impact site the most devastating effect is from thermal radiation however the curvature of the earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon

efficient disruption of small asteroids by earth s atmosphere accurate modelling of the interaction between the atmosphere and an incoming bolide is a complex task but crucial to determining the fraction of small asteroids that actually hit the earth s surface most semi analytical approaches have simplified the problem by considering the impactor as a strengthless liquid like object pancake models1 2 but recently a more realistic model has been developed that calculates motion aerodynamic loading and ablation for each separate particle or fragment in a disrupted impactor3 4 here we report the results of a large number of simulations in which we use both models to develop a statistical picture of atmosphere bolide interaction for iron and stony objects with initial diameters up to 1 km we show that the separated fragments model predicts the total atmospheric disruption of much larger stony bodies than previously thought in addition our data set of 1 000 simulated impacts combined with the known pre atmospheric flux of asteroids with diameters less than 1 km5 6 7 8 9 10 11 12 elucidates the flux of small bolides at the earth s surface we estimate that bodies 220 m in diameter will impact every 170 000 years

geochemistry of quaternary travertines in the region north of rome italy structural hydrologic and paleoclimatic implications in the tyrrhenian region of central italy late quaternary fossil travertines are widespread along two major regional structures the tiber valley and the ancona anzio line the origin and transport of spring waters from which travertines precipitate are elucidated by chemical and isotopic studies of the travertines and associated thermal springs and gas vents there are consistent differences in the geochemical and isotopic signatures of thermal spring waters gas vents and present and fossil travertines between east and west of the tiber valley west of the tiber valley 13c of co2 discharged from gas vents and 13c of fossil travertines are higher than those to the east to the west the travertines have higher strontium contents and gases emitted from vents have higher 3he 4he ratios and lower n2 contents than to the east fossil travertines to the west have characteristics typical of thermogene thermal spring origin whereas those to the east have meteogene low temperature characteristics including abundant plant casts and organic impurities the regional geochemical differences in travertines and fluid compositions across the tiber valley are interpreted with a model of regional fluid flow the regional mesozoic limestone aquifer is recharged in the main axis of the apennine chain and the groundwater flows westward and is discharged at springs the travertine precipitating waters east of the tiber valley have shallower flow paths than those to the west because of the comparatively short fluid flow paths and low normal heat flow the groundwaters to the east of the tiber valley are cold and have co2 isotopic signatures indicating a significant biogenic contribution acquired from soils in the recharge area and limited deeply derived co2 in contrast spring waters west of the tiber valley have been conductively heated during transit in these high heat flow areas and have incorporated a comparatively large quantity of co2 derived from decarbonation of limestone the elevated strontium content of the thermal spring water west of the tiber valley is attributed to deep circulation and dissolution of a triassic evaporite unit that is stratigraphically beneath the mesozoic limestone u series age dates of fossil travertines indicate three main periods of travertine formation ka 220 240 120 140 and 60 70 based on the regional flow model correlating travertine deposition at thermal springs and precipitation in the recharge area we suggest that pluvial activity was enhanced during these periods our study suggests that travertines preserve a valuable record of paleofluid composition and paleoprecipitation and are thus useful for reconstructing paleohydrology and paleoclimate

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

the kamil crater in egypt we report on the detection in southern egypt of an impact crater 45 meters in diameter with a pristine rayed structure such pristine structures are typically observed on atmosphereless rocky or icy planetary bodies in the solar system this feature and the association with an iron meteorite impactor and shock metamorphism provides a unique picture of small scale hypervelocity impacts on earth s crust contrary to current geophysical models ground data indicate that iron meteorites with masses of the order of tens of tons can penetrate the atmosphere without substantial fragmentation

linking active margin dynamics to overriding plate deformation synthesizing geophysical images with geological data from the norfolk basin the tonga kermadec subduction system in the southwest pacific preserves a series of crustal elements and sediments which have recorded subduction initiation rift and back arc basin formation the norfolk basin is the farthest landward of all back arc basins formed in the tonga kermadec region and may preserve the earliest record of subduction initiation regionally for the norfolk basin we use a set of multibeam bathymetry magnetic and seismic reflection and refraction data to constrain basin structure and the mode and timing of formation a structural interpretation reveals a two stage tectonic evolution 1 a convergent tectonic regime until 38 34 ma alternatively related to island arc collision or subduction initiation and 2 lithospheric extension after 34 ma these observations may help to constrain mechanical models that predict rapid extension following convergence of the overriding plate during subduction initiation or arc reversals

episodic formation of neotethyan ophiolites tibetan plateau snapshots of abrupt global plate reorganizations during major episodes of supercontinent breakup abstract neotethyan ophiolites along the bangong nujiang and yarlung tsangpo sutures tibetan plateau consist of mantle peridotite cumulates isotropic gabbro diabase plagiogranite and basaltic lava they comprise morb iab oib and boninitic rocks and mostly belong to supra subduction zone ssz type compositionally different rocks crystallized synchronously over a distance of 2000 km along the bangong nujiang and yarlung tsangpo sutures during the short periods of 190 182 and 132 120 ma respectively resembling the izu bonin mariana ibm proto arc the early jurassic bangong nujiang ophiolites formed in forearc settings during subduction initiation while the middle jurassic early cretaceous bangong nujiang ophiolites and the early cretaceous yarlung tsangpo ophiolites formed in forearc settings at pre existing subduction zones crystallization of the neotethyan ssz ophiolites from the eastern mediterranean middle east tibetan plateau to southeast asia and formation of the associated metamorphic rocks temporally coincided with major supercontinent breakup events 1 initial breakup of pangea in central atlantic and westernmost tethys 190 ma 2 separation of west and east gondwana 170 160 ma and initial opening of piemont ligurian tethys 175 164 ma 3 initial breakups of east and west gondwana 135 125 ma and 4 final breakup of gondwana 104 96 ma far field forces associated with abrupt global plate reorganizations may have triggered subduction initiation or transient interplate coupling decoupling at pre existing subduction zones leading to episodic formation of the neotethyan ssz ophiolites and rapid exhumation of metamorphic rocks in subduction zones the formation events of neotethyan ssz ophiolites and associated metamorphic rocks represent snapshots of global plate reorganizations during the major episodes of supercontinent breakup

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

mesozoic history of the fairway aotea basin implications for the early stages of gondwana fragmentation the fairway ridge is a buried continental structure that separates the fairway basin from the new caledonia basin the proposed cretaceous age of the fairway basin has remained highly hypothetical to date deep offshore petroleum exploration wells revealed well dated mesozoic carbonaceous sedimentary rocks in the taranaki basin at the southern end of the aotea basin in this paper we use geophysical data to confirm the continuity of the 2000 km long fairway aotea basin connecting new caledonia to new zealand and prove its early late cretaceous age analysis of seismic reflection profiles together with newly compiled gravity and magnetic maps reveals late cretaceous ne sw trending lineaments projecting northeastward from major tasman sea fracture zones and the bellona trough which demonstrate that the opening of the fairway aotea basin predates the opening of the tasman sea this result combined with observations of the mesozoic regional geology suggests that the lord howe fairway and norfolk ridges are part of a remnant late early cretaceous continental arc which was fragmented into three pieces by the late early to early late cretaceous this event might be contemporaneous with a plate motion change between the gondwana and pacific plates and or the arrival of the hikurangi plateau in the subduction zone around 105 ma which caused the cessation of subduction along this plate boundary we interpret either of those two events as being possible trigger events for the post early cretaceous fragmentation of the eastern gondwana margin in a slab retreat process

cosmic ray exposure and pre atmospheric size of the gebel kamil iron meteorite cosmogenic he ne and ar as well as the radionuclides 10be 26al 36cl 41ca 53mn and 60fe have been determined on samples from the gebel kamil ungrouped ni rich iron meteorite by noble gas mass spectrometry and accelerator mass spectrometry ams respectively the meteorite is associated with the kamil crater in southern egypt which is about 45 m in diameter samples originate from an individual large fragment individual as well as from shrapnel concentrations of all cosmogenic nuclides stable and radioactive are lower by a factor 3 4 in the shrapnel samples than in the individual assuming negligible 36cl decay during terrestrial residence indicated by the young crater age 85 cm i e a pre atmospheric mass 20 tons with a preferred radius of 115 120 cm 50 60 tons the analyzed samples came from a depth of approximately 20 cm individual and approximately 50 80 cm shrapnel the size of the gebel kamil meteoroid determined in this work is close to estimates based on impact cratering models combined with expectations for ablation during passage through the atmosphere folco et al 2010 2011

early oligocene marine canyon channel systems implications for regional paleogeography in the canterbury basin new zealand abstract the oligocene is a time of maximum submersion of the zealandia landmass it coincides with changes in oceanographic currents as well as with the inception of the cenozoic plate boundary through new zealand our analysis of offshore wells and seismic reflection lines in the canterbury basin reveals condensed oligocene strata

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

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

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