observations of the kuroshio s barotropic and baroclinic responses to basin wide wind forcing 1 observations show that the kuroshio in the east china sea ecs kuroshio responds to the large scale wind stress curl field at two time scales it is argued that these two responses are related to barotropic and baroclinic modes that reach the ecs via different waveguides variability in the ecs kuroshio is assessed by comparing satellite altimetry historical hydrography and the pacific decadal oscillation pdo index with the latter used as a proxy for the large scale wind stress curl forcing sea level difference across the ecs kuroshio is positively correlated with pdo at zero lag and negatively correlated at 7 year lag in contrast pycnocline steepness and pdo are uncorrelated at zero lag and negatively correlated at 7 year lag these signals in the ecs kuroshio considered together with wind stress curl anomalies in the open ocean are consistent with a barotropic response to the wind at zero lag the barotropic response is likely forced in the central north pacific by wind stress curl anomalies of opposite sign one of which is centered at ecs latitudes 27 n while the other sits further north this suggests that in general the absolute transport at a given latitude is not simply that predicted by the sverdrup balance along the latitude this is a consequence of waveguides that can steer the barotropic mode across latitude lines in contrast the signals that lag pdo by 7 years are consistent with a baroclinic mode which represents the ocean s time integrated response to the wind stress curl along a single latitude band between 24 n and 27 n

variability in the location of the antarctic polar front 90 20 w from satellite sea surface temperature data the path of the antarctic polar front pf is mapped using satellite sea surface temperature data from the noaa nasa pathfinder program the mean path and variability of the pf are strongly influenced by bathymetry meandering intensity is weaker where the bathymetry is steeply sloped and increases in areas where the bottom is relatively flat there is an inverse relationship between meandering intensity and both the width of the front and the change in temperature across it there is a persistent large separation between the surface and subsurface expressions of the pf at ewing bank on the falkland plateau

new insights on the tectonics along the new hebrides subduction zone based on gps results 1 at the new hebrides nh subduction zone ridges born by the subducting australia plate enter the trench and collide with the overriding margin results from gps surveys conducted on both sides of the trench and new bathymetry maps of the nh archipelago bring new light on the complex tectonics of this area convergence vectors present large variations that are not explained by australia pacific a p poles and that define four segments vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors slow convergence i e 30 40 mm yr is found at the central segment facing the d entrecasteaux ridge the southern segment moves faster than a p motion predicts 89 to 124 mm yr relatively to a western north fiji basin wnfb reference the northern and southern segments rotate in opposite directions consistently with the extension observed in the troughs east of both segments both rotations combine in central vanuatu into an eastward translation that bulldozes the central segment into the wnfb at 55 mm yr that model suggests that the motion of the central segment forced by the subduction collision of the d entrecasteaux ridge influences the motion of the adjoining segments the new caledonia archipelago is motionless with respect to the rest of the australia plate despite the incipient interaction between the loyalty ridge and the nh margin southeast of the interaction area convergence is partitioned into a 50 mm yr trench normal component accommodated at the trench and a 90 mm yr trench parallel component close to the a p convergence and presumably accommodated by a transform boundary at the rear of the nh arc

contributions of wind forcing waves and surface heating to sea surface height observations in the pacific ocean the dominant processes affecting sea surface height ssh variability observed by the topex poseidon altimeter vary regionally in the pacific baroclinic rossby waves equatorially trapped kelvin waves steric response to seasonal heating and the response to wind stress curl forcing are all important the steric response to surface heating dominates seasonal ssh variability in the subpolar gyre and the eastern subtropical gyre south of the kuroshio extension and south of 20 n in the eastern pacific the dominant contribution to ssh is from near annual period rossby waves to quantify the wave energy observed ssh was assimilated into a kinematic model of westward propagating waves these waves account for 70 of ssh variance between 10 s and 10 n but only 30 between 10 n and 30 n although wave energy in the eastern pacific is correlated with ssh anomalies at the equator the much larger wave energy in the western pacific is correlated with wind stress curl suggesting that the rossby waves there are locally forced in addition to these planetary waves the ocean response to wind forcing via ekman pumping is observed in several places specifically in the north equatorial current a quasi steady topographic sverdrup balance is detectable over most of the north pacific at latitudes as low as 10 15 n as well as in the south pacific where it is seen north of 50 s the decomposition of the ssh signal into propagating waves an ekman pumping response and sverdrup transport is consistent with the results from an isopycnal numerical model

modal variability in the southeast pacific basin energetics of the 2009 event abstract we study the barotropic variability in the southeast pacific basin in particular focusing on the extreme event during the fourth quarter of 2009 a 3 year integration of a barotropic shallow water model forced with wind stress anomalies generates localized variability that is similar in spatial extent and amplitude as the observed anomalous event an eigenmode analysis of the same model shows the presence of several free modes in the southeast pacific but projection of the modal patterns on the model output shows that their amplitudes are low instead the mode is interpreted as an almost free mode the modal excitation accounts for a considerable fraction 23 on average of the kinetic energy input by the wind stress in the southeast pacific basin increasing to 38 for the anomalous event in 2009 surprisingly a similar but weaker event during the third quarter of 2008 appears to have been more significant from an energetics point of view with almost 50 of the energy being input into the mode key areas of energetic dissipation appear to be the eltanin fracture zone the crest of the east pacific rise and the chile rise east pacific rise intersection

the global distribution of f h and the barotropic response of the ocean the barotropic response of the ocean over smoothly varying topography is directly related to the potential vorticity gradient f h a global estimate of f h and f h have been constructed from a high resolution digital topographic data base four different regimes of the barotropic response of the ocean are suggested by these maps 1 in the tropical latitudes from 20 s to 20 n f h h the response will be similar to a flat bottom sverdrup regime 2 in mid latitudes 20 to 70 north of 30 s f h h the response will be quasi stationary weak and dominated by nonlocal forcing flow along isolines of f h 3 in a number of small regions throughout the mid latitudes in both hemispheres f h h in these regions the local response flow across isolines of f h should be amplified and wavelike motions may occur over a broader bandwidth of frequencies 4 south of 30 s the mid ocean ridge system becomes zonal and isolates several large regions within closed lines of f h in these regions principally the abyssal plains of the southern ocean the low frequency barotropic response of the ocean may be wavelike in character and much stronger than the sverdrup response consequently this analysis suggests that the southern ocean may have a stronger barotropic response to wind forcing than the other five major ocean basins

propagation of back arc extension into the arc lithosphere in the southern new hebrides volcanic arc new geophysical data acquired during three expeditions of the r v southern surveyor in the southern part of the north fiji basin allow us to characterize the deformation of the upper plate at the southern termination of the new hebrides subduction zone where it bends eastward along the hunter ridge unlike the northern end of the tonga subduction zone on the other side of the north fiji basin the 90 bend does not correspond to the transition from a subduction zone to a transform fault but it is due to the progressive retreat of the new hebrides trench the subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the new hebrides arc and toward the south of the hunter ridge so that the direction of convergence remains everywhere orthogonal to the trench in the back arc domain the active deformation is characterized by propagation of the back arc spreading ridge into the hunter volcanic arc the n s spreading axis propagates southward and penetrates in the arc where it connects to a sinistral strike slip zone via an oblique rift the collision of the loyalty ridge with the new hebrides arc less than two million years ago likely initiated this deformation pattern and the fragmentation of the upper plate in this particular geodynamic setting with an oceanic lithosphere subducting beneath a highly sheared volcanic arc a wide range of primitive subduction related magmas has been produced including adakites island arc tholeiites back arc basin basalts and medium k subduction related lavas

modal decay in the australia antarctic basin abstract the barotropic intraseasonal variability in the australia antarctic basin aab is studied in terms of the excitation and decay of topographically trapped barotropic modes the main objective is to reconcile two widely differing estimates of the decay rate of sea surface height ssh anomalies in the aab that are assumed to be related to barotropic modes first an empirical orthogonal function eof analysis is applied to almost 15 years of altimeter data the analysis suggests that several modes are involved in the variability of the aab each related to distinct areas with almost closed contours of potential vorticity second the dominant normal modes of the aab are determined in a barotropic shallow water sw model these stationary modes are confined by the closed contours of potential vorticity that surround the eastern aab and the crest of the southeast indian ridge for reasonable values of horizontal eddy viscosity and bottom friction their decay time scale is on the order of sever

effect of three dimensional slab geometry on deformation in the mantle wedge implications for shear wave anisotropy shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench parallel fast directions three dimensional flow may give rise to trench parallel stretching and provide an explanation for these patterns of seismic anisotropy along strike variations in slab geometry produce trench parallel pressure gradients and are therefore a possible mechanism for three dimensional flow in this study we quantify the effects of variable slab dip curved slabs oblique subduction and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones temperature dynamic pressure velocity and strain are calculated with high resolution three dimensional finite element models these models include temperature and stress dependent rheology and parameterized slab and trench geometry thick layers 20 60 km with strong trench parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15 or strong curvature in the slab in these cases strong trench parallel stretching develops when flow lines have an oblique to trench normal orientation this suggests that trench parallel seismically fast directions may not indicate trench parallel flow lines in systems with large along strike variations an oblique component of stretching is confined to a 20 30 km layer above the slab in systems with oblique subduction the effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench parallel flow that extends 50 100 km into the core of the mantle wedge

response of north atlantic ocean circulation to atmospheric weather regimes a new framework is proposed for investigating the atmospheric forcing of north atlantic ocean circulation instead of using classical modes of variability such as the north atlantic oscillation nao or the east atlantic pattern the weather regimes paradigm was used using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the nao using ocean only historical and sensitivity experiments the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated the results suggest that the atlantic ridge ar negative nao nao and positive nao nao regimes induce a fast monthly to interannual time scales adjustment of the gyres via topographic sverdrup dynamics and of the meridional overturning circulation via anomalous ekman transport the wind anomalies associated with the scandinavian blocking regime sbl are ineffective in driving a fast wind driven oceanic adjustment the response of both gyre and overturning circulations to persistent regime conditions was also estimated ar causes a strong wind driven reduction in the strengths of the subtropical and subpolar gyres while nao causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies nao induces a southward shift of the gyres through the southward displacement of the wind stress curl the sbl is found to impact the subpolar gyre only via anomalous heat fluxes the overturning circulation is shown to spin up following persistent sbl and nao and to spin down following persistent ar and nao conditions these responses are driven by changes in deep water formation in the labrador sea

interannual bottom pressure signals in the australian antarctic and bellingshausen basins abstractanalyses of large scale 750 km ocean bottom pressure pb fields derived from the gravity recovery and climate experiment grace and from an estimating the circulation climate of the ocean ecco state estimate reveal enhanced interannual variability partially connected to the antarctic oscillation in regions of the australian antarctic basin and the bellingshausen basin with pb magnitudes comparable to those of sea level and good correlation between the grace and ecco pb series consistent with the theory of gill and niiler the patterns of stronger pb variability are partly related to enhanced local wind curl forcing and weakened gradients in h f where h is ocean depth and f is the coriolis parameter despite weaker h f gradients motions against them are sufficiently strong to play a role in balancing the local wind input topographic effects are as or more important than changes in f additionally and contrary to the dominance of barotropic processes at subannual time scales barocl

wind forced changes in labrador current transport three years of current data from the offshore branch of the labrador current are examined the meters were at least 400 m deep to avoid being struck by icebergs and positioned over the 1000 m isobath on the eastern flank of hamilton bank the currents are predominantly directed along isobath at periods of 1 8 days the motion is bottom intensified at longer periods 10 40 days it is essentially independent of depth one of the objectives of this paper is to determine whether or not the 10 to 40 day fluctuations which are o 5 cm s 1 can be explained by large scale wind forcing over the north atlantic monthly variations in the transport of the north atlantic are calculated by means of the topographic sverdrup relationship the sverdrup return flow along the coast of labrador w is found to have a well defined seasonal cycle with a range of 6 sv the maximum southward transport generally occurs in february which is also the month when the standard deviation of w is a maximum 9 sv if we assume that the monthly changes of the return flow are barotropic and confined to a boundary current of say 100 km width and 1 km depth the mooring depth then 1 sv corresponds to a speed of 1 cm s 1 we would then expect the large scale wind over the north atlantic to cause the flow along the labrador coast to vary with a seasonal range of 6 cm s 1 and exhibit monthly variations in winter of 9 cm s 1 however a comparison of the w and current records shows no similarity we therefore speculate with some corroborative evidence from an analysis of nain sea level that these energetic variations in the topographic sverdrup return flow along the labrador coast probably occur in deeper water offshore of the current meter mooring the cause of the barotropic current variations remains unclear it is possible that they do not reflect genuine transport changes but are due to a meandering of the labrador current further analysis and perhaps measurements of bottom pressure are required to explore this possibility

reconciling surface plate motions with rapid three dimensional mantle flow around a slab edge a central axiom of the theory of plate tectonics is that the surface plates are coupled to and carried along by convection in the earth s mantle the situation is rather different at subduction zones however margarete jadamec and magali billen now present calculations that quantify this variance they use three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system to determine the surrounding mantle flow field they show that near a subduction zone edge mantle flow velocities can have magnitudes of greater than ten times the surface plate motions and that the complex mantle flow field is consistent with observations from seismic anisotropy these results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally implying large scale coupling between the mantle and surface plates but the fit is poor at subduction zones here three dimensional numerical models of buoyancy driven deformation for the alaska subduction transform system are used to find the origin of the regional decoupling of flow the results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally suggesting large scale coupling between the mantle and the surface plates1 2 the fit is typically poor at subduction zones however where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to3 4 5 6 7 and may be several times faster than6 plate motions here we present three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system and use them to determine the origin of this regional decoupling of flow we find that near a subduction zone edge mantle flow velocities can have magnitudes of more than ten times the surface plate motions whereas surface plate velocities are consistent with plate motions8 and the complex mantle flow field is consistent with observations from seismic anisotropy5 the seismic anisotropy observations constrain the shape of the eastern slab edge and require non newtonian mantle rheology the incorporation of the non newtonian viscosity9 10 results in mantle viscosities of 1017 to 1018 pa s in regions of high strain rate 10 12 s 1 and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates these results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab pull force available to drive subducting plates

location and dynamics of the antarctic polar front from satellite sea surface temperature data the location of the antarctic polar front pf was mapped over a 7 year period 1987 1993 within images of satellite derived sea surface temperature the mean path of the pf is strongly steered by the topographic features of the southern ocean the topography places vorticity constraints on the dynamics of the pf that strongly affect spatial and temporal variability over the deep ocean basins the surface expression of the pf is weakened and the pf meanders over a wide latitudinal range near large topographic features width and temperature change across the front increase and large scale meandering is inhibited elevated mesoscale variability is seen within and downstream of these areas and may be the result of baroclinic instabilities initiated where the pf encounters large topographic features the strong correlations between topography and pf dynamics can be understood in the context of the planetary potential vorticity ppv or f h field mean ppv at the pf varies by more than a factor of 2 along its circumpolar path however at the mesoscale the pf remains within a relatively narrow range of ppv values around the local mean away from large topographic features the pf returns to a preferred ppv value of 25 10 9 m 1 s 1 despite large latitudinal shifts the mean paths of the surface and subsurface expressions of the pf are closely coupled over much of the southern ocean

analogue modeling of arc and backarc deformation in the new hebrides arc and north fiji basin in most backarc basins extension is perpendicular to the arc thus individual spreading ridges extend approximately parallel to the arc in the north fiji basin however several ancient and active spreading ridges strike 70 90 to the new hebrides arc these high angle spreading ridges relocated southward during the asymmetric opening of the north fiji basin we have simulated the structural development of the north fiji basin and the new hebrides arc with scaled analogue models and the results have inspired us to come to several tentative conclusions we interpret the orientation of the high angle spreading ridges to be related to the asymmetric opening of the backarc basin around a hinge where they form close to the hinge relocation of these spreading ridges is most likely related to subduction of the west torres plateau along the new hebrides trench this resulted in localized collision retarded rollback of the subducting slab along the northwest corner of the trench and reduced extension and shearing in the northwest corner of the north fiji basin backarc extension continued in the rest of the north fiji basin owing to continued rollback of the southern part of the subducting slab here active extension was separated from the slightly or nonextending northwest corner by a zone striking at high angle to the new hebrides arc i e the hazel holme extensional zone moreover impingement of the d entrecasteaux ridge into the overriding plate led to local deformation and fragmentation of the arc

the relationship between deep ocean currents and winds east of barbados abstract observations of currents collected at the polymode array iii cluster c 16 n 54 w are compared with radiosonde winds measured at barbados 13 n 62 w using a linear response analysis the winds and the currents are coherent throughout the water column 5400 m over the subinertial frequency range of 0 025 to 0 25 cpd the coherence is highest between the east west wind stress and north south currents over smooth topography the wind related ocean currents have a rms of 2 5 cm s 1 at 500 m and 1 1 cm s 1 at 4000 m and account for approximately one third of the total eddy kinetic energy the wind current phase is depth independent and does not vary significantly over the 200 km horizontal scale of the array the response amplitude is surface intensified and increases with decreasing frequency which is consistent with a deterministic theoretical model the coherence values and response estimates suggest an off resonant barotropic response to large scale wind forcing

an ocean resonance in the southeast pacific 1 satellite altimeter observations of sea surface height show an extensive region of high variability in the southeastern pacific which previous studies have suggested is a rossby wave trapped by topography here we investigate the feature using results from a high resolution global ocean model we calculate the empirical orthogonal modes and find that 29 of the ssh variability is due to a single mode we correlate the time series of the mode with the local ekman pumping and find that the correlation is largest when the wind curl is averaged over a region roughly the size of the mode we calculate the response function relating the two time series and find that it is dominated by a resonance at zero frequency with a decay time of about two days we conclude that the feature is a highly damped geostrophic mode of the ocean

causes of large scale sea level variations in the southern ocean analyses of sea level and a barotropic model 1 we analyze a decade of sea surface height ssh measurements in the southern ocean from the topex poseidon and ers altimeters with a focus on the variability at timescales 2 years among the different processes contributing to large scale ssh variations the barotropic response to the winds dominates poleward of 50 s while thermosteric processes dominate equatorward except for resonant basins for the barotropic modes and regions of intense eddy activity a finite element barotropic model has been developed to analyze the vorticity budget the ssh from the model agrees well with observations the leading barotropic mode which is annular and is confined near antarctica is responsible for most of the barotropic circumpolar transport it is coherent with the zonally integrated eastward wind stress consistent with a free mode response although previously evidenced in bottom pressure data this mode is only partially seen in altimeter data because of ice coverage it nevertheless distinctly appears above the pacific ridges where it expands meridionally up to midlatitudes in the rest of the domain several regions coherent with the local wind stress curl are found these are regions isolated by f h contours mostly deep basins an analysis of the vorticity budget shows that generally topographic sverdrup balance is the leading process for periods 50 days but in some regions resonant basins diffusive and nonstationary terms are important a model experiment shows that transients redistribute energy along f h waveguides contributing to drain resonant regions as was hypothesized in previous works

topographic coupling of the atlantic overturning and gyre circulations abstractthe vorticity dynamics associated with the mean and time varying gyre and overturning circulations of the atlantic ocean are examined in a realistic ocean model hindcast simulation of the late twentieth century abyssal flow interaction with sloping bottom bathymetry gives rise to the bottom pressure torque bpt term of the vertically integrated vorticity equation the dominance of this term in the closure of the barotropic gyre circulation noted in previous studies is corroborated here for both non eddy resolving and eddy resolving versions of the parallel ocean program pop model this study shows that bpt is also a dominant term in the vorticity balance of the atlantic meridional overturning circulation amoc and therefore represents a key dynamical link between the overturning and gyre streamfunctions the interannual variability of the atlantic circulation over the last several decades viewed in terms of time varying integral vorticity balances demonstrates the fundamental role played by

deep earthquakes in the southwest pacific a tectonic interpretation this paper examines the spatial distribution of deep earthquakes in the tonga fiji new hebrides region of the southwest pacific our interpretation emphasizes the complex cenozoic tectonics of the pacific indo australian plate boundary as a primary control on the distribution and deformation of subducted lithosphere most deep earthquakes in the interarc region are associated with the contorted pacific plate lithosphere subducted at the tonga trench however anomalous groups of deep earthquakes located west of the tonga zone are unrelated to the present plate configuration tectonic reconstructions of the region to 8 m y b p provide circumstantial evidence that 1 the anomalous events west of the tonga zone occur in two pieces of detached lithosphere subducted at the vitiaz and proto new hebrides trenches during the late miocene 2 the flattening of the inclined seismic zone in northernmost tonga is related to the rapid opening of the lau basin since 4 m y b p and 3 the sharp westward curvature of the tonga seismic zone in this area coincides with a preexisting bend in the late miocene vitiaz arc the sharpness of its present curvature appears to be a secondary effect of shear flow in the lower mantle and compression between detached vitiaz and attached tonga lithosphere thus much of the contortion of the subducted lithosphere beneath tonga fiji may be produced by local tectonic interactions rather than collision of the slab with an impenetrable boundary in the midmantle

the barotropic electromagnetic and pressure experiment 1 barotropic current response to atmospheric forcing during 1986 1987 an array of horizontal electrometers which measure the subinertial barotropic velocity was deployed for 11 months in the central north pacific to study atmospherically forced motions in a low eddy kinetic energy region significant coherence is found to be ubiquitous between the barotropic velocity and one or more surface atmospheric variables over the north pacific the maximum squared coherence between currents and wind stress curl at all periods greater than 3 days is typically 0 6 rarely below 0 4 and occasionally above 0 8 and is dominantly nonlocal although local coherence is seen at some sites especially at the shorter periods both the coherence and the intersite pattern similarity are strongest at the periods of relative maxima in the barotropic current autospectra e g at 6 8 10 14 and 20 50 days the locations at which the wind stress curl forces the motions at each site and for each period have been determined while there is substantial intersite variability some of the observed patterns of squared coherence over the north pacific are very similar to those predicted by simple linear smooth bottomed quasi geostrophic models of atmospherically forced barotropic currents the forcing location is occasionally found to be the same spot for all oceanic instruments especially at the autospectral maxima in barotropic current more commonly distinct forcing locations are found for each instrument even when closely spaced suggesting topographic influence in other cases multiple forcing locations are implicated for the motions at a single station all of this site and period dependent variability of the forcing location is incompatible with the simple models and is due either to the known spatial inhomogeneity of the curl forcing field or to topographic effects the importance of the former is emphasized using simple arguments based on analytic transfer functions between the atmospheric and oceanic variables and on observed wavenumber spectra for wind stress curl quantitative assessment of topographic effects must await suitable numerical models in addition maximum likelihood wavenumber spectra of the barotropic currents have been estimated in those bands with an adequate number of coherent sensors the wavenumbers at the spectral peaks are compatible with the dispersion relation for rossby waves modified by the observed large scale linearly sloping topography which shoals to the northeast the observed group velocities are in general quite consistent with the inferred forcing locations based on the ocean atmosphere coherences

geosat and advanced very high resolution radiometer observations of oceanic planetary waves adjacent to the hawaiian islands geosat observations of the central north pacific ocean show westward propagating mesoscale features with length scales 500 km time scales 105 155 days and observed zonal phase speeds commensurate with those of nearly nondispersive baroclinic rossby waves advanced very high resolution radiometer derived sea surface temperatures observed during april 1988 showed a cusp shaped pattern north of the hawaiian islands congruent with concurrent geosat derived sea level height anomalies it is believed that the cusp shaped pattern is the surface manifestation of a baroclinic rossby wave the observed wavelength of the cusp shaped pattern was 560 km it had an observed meridional extent horizontal amplitude of 200 300 km and an inferred zonal propagation speed of 0 052 m s 1 the westward propagating sea level height structure was observed to amplify as it approached 150 w reaching amplitudes of 0 06 0 08 m in the vicinity of the cusp shaped feature these waves are believed to be locally forced by the topography either as the result of eastward flow over the submerged island ridge or as a result of baroclinic instability during periods of westward flow

reduction of icesat systematic geolocation errors and the impact on ice sheet elevation change detection 1 the ice cloud and land elevation satellite icesat has been acquiring unprecedented observations of our planet s varied surfaces with impressive precision however systematic errors remain in the data including orbital variation and long term bias trend pointing errors these errors are particularly troublesome because they cannot be separated from true surface elevation change and confound instrument range bias determination we present a method to correct these systematic pointing errors to the sub arcsecond level we demonstrate the impact of these errors and their corrections on greenland and antarctica ice sheet surface elevation change observations and sea surface anomaly observations as well as instrument range bias determination while further improvements in the icesat data will certainly be made the analysis presented here demonstrates a major error source can be significantly reduced resulting in improved surface elevation accuracies for ice sheet elevation change detection

geochemistry vs seismo tectonics along the volcanic new hebrides central chain southwest pacific abstract compositional variations major and trace element of pleistocene to present rocks along the volcanic new hebrides central chain southwest pacific are presented and discussed using a new set of analyses we focus in particular on the anomalous mg k lile and lree rich and al and si poor character of the basalts from aoba and santa maria volcanoes and to a lesser extent from ambrym volcano all located in the area where the d entrecasteaux zone collides with the arc in addition we note a compositional transition from normal arc basalts to boninite related high mg andesites at the southern termination of the nhcc the peculiar composition of the rocks of the volcanoes facing the d entrecasteaux zone has traditionally been related more or less directly to the collision subduction occurring in this area we present an alternative hypothesis of a strong geochemical anomaly under santa maria and aoba volcanoes unrelated to this collision in this model a westward and upward invasion of dupal type enriched mantle corroborated by the pattern of intermediate seismicity occurs under this part of the arc in strong contrast to the usual pacific type morb mantle source present under all other volcanoes of the nhcc such a dupal type source may explain the anomalous geochemistry of the resulting volcanic products

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

current tectonics of the tonga new hebrides region abstract this paper proposes a present day tectonic map of the tonga new hebrides region the result of combining data from bathymetry satellite derived gravity seismicity magnetic measurements and gps observations focal mechanisms and a large amount of bathymetric data collected during the last decade in the lau and north fiji back arc basins indicate a deformation distributed over numerous spreading ridges rather than diffuse and shear dominated as was previously thought the region is also characterised by a large variation of both consumption rate along the arcs and opening rate along the back arc basins spreading centres geometry and rate of spreading in the lau and north fiji back arc basins and segmentation of the tonga and new hebrides arcs are closely related to and significantly influenced by the subduction of aseismic ridges namely the louisville d entrecasteaux and loyalty ridges where subducting aseismic ridges enter the trenches we observe slower convergence arc transverse strike slip faulting thrusting at the rear of arcs and slow or absent back arc spreading rapid subduction correlates with fast back arc opening sometimes parallel spreading ridges in the back arc domain are required to accommodate the fast motion of convergence

multiple oscillatory modes of the argentine basin part ii the spectral origin of basin modes in this paper the spectrum of barotropic basin modes of the argentine basin is shown to be connected to the classical rossby basin modes of a flat bottom constant depth rectangular basin first the spectrum of basin modes is calculated for the argentine basin by performing a normal mode analysis of the barotropic shallow water equations then a homotopy transformation is performed that gradually morphs the full bathymetry geometry through a flat bottom configuration into a rectangular basin following the eigenmodes through this transition establishes a connection between most of the basin modes and the classical rossby basin modes of a rectangular geometry in particular the 20 day mode of the argentine basin is identified with the lowest order mode of classical theory sensitivity studies show that the decay rate of each mode is controlled by bottom friction but that it is insensitive to lateral friction lateral friction strongly impacts the oscillation frequency in addition the modes are found to be only slightly sensitive to the presence of a background flow

large scale sea level variations and associated atmospheric forcing in the subtropical north east atlantic ocean abstract the large scale sea level variations in the subtropical north eastern atlantic ocean are characterised by a predominant seasonal fluctuation analysis of 6 years 1993 98 of combined topex poseidon and ers 1 2 altimetric data together with data of concurrent oceanic surface geophysical fields ecmwf net heat fluxes ecmwf winds and reynolds sea surface temperature reveals that the dominant physical forcing of sea level variations on an annual timescale is the air sea net heat fluxes this local forcing generates a seasonal ocean steric response whose amplitude of the order of 4 5 cm varies spatially within the studied area and temporally over the 6 year period the residual variability is mainly characterised by a non seasonal signal which is observed northwards of the azores current 34 n and its time variation resembles that of the north atlantic oscillation nao a combination of steric effects wind induced heat fluxes and wind effects barotropic sverdrup response appear to explain this signal some evidence of a local ekman pumping response is also found regionally both in the seasonal and non seasonal variations of the observed sea level at the interannual timescale an overall sea level trend with a mean rate of 0 5 cm year is detected however the magnitude of the trend varies locally so that the entire sea surface appears to undergo a general tilt which extends beyond the studied area superimposed on this long term trend a sharp temporal sea level rise occurs in 1995 mainly between the azores and madeira islands all together these low frequency sea level variations seem to co vary with the large scale sea surface temperature sst variations this implies a comparable evolution of the sea level with the upper ocean thermal content which suggests a contribution from steric effects a weaker situation is observed in the northern area indicating a possible low frequency wind forcing enhanced contribution

sea level comparison between topex poseidon altimetric data and a global ocean general circulation model from an assimilation perspective the topex poseidon altimetric sea level observations during 1993 are used to validate a simulation made by the laboratoire d oceanographie dynamique et de climatologie ocean general circulation model ogcm at global scales the forcing fields are provided by the european centre for medium range weather forecasts reanalysis project the ogcm is a coarse horizontal resolution model 2 longitude by 1 5 latitude with 31 levels in the vertical mainly used for climate studies horizontal mixing is done along isopycnal surfaces whereas the parameterization of vertical mixing is based on a local estimate of the turbulent kinetic energy the comparison is quantitative in the sense that topex poseidon observations are interpolated onto the model grid by an objective interpolation in view of the assimilation case the model simulation is compared to the observations at large spatial scales of 500 km or more the basic features of the variability are simulated well by the model in particular with respect to the annual cycle major discrepancies are found in the vicinity of western boundary currents and in the southern ocean due in part to the inadequate horizontal resolution in anticipation of a future assimilation of the observed sea level a decomposition of the model sea level into baroclinic barotropic components diagnosed with some dynamical variables of the model is then examined it is shown that the baroclinic component explains the large seasonal variations confined in the upper layers whereas the barotropic component is associated with intraseasonal variability the results of the present study confirm that the model sea level is consistent with the observed large scale variability providing a basis for future assimilation of the data into the ogcm to improve the representation of the ocean state

high mg andesites from the southern termination of the new hebrides island arc sw pacific abstract in the southern new hebrides arc magmatic and tectonic processes are closely linked between 21 s and 22 s a normal broadly arc tholeiitic magmatic suite is essentially similar to those occurring in the main part of the arc whereas south of 22 s at the southern termination of the arc a high mg andesite suite appears with the more mafic endmembers having mineralogical and compositional affinities to high ca boninites during the last 2 ma this southern termination propagated southwards and played a continuing role in the transient transform junction between the southern tip of the trench and the n s spreading axis of the north fiji basin in this region which has been dominated for several million years by fast transform motions the low magma production rates in this section of the arc and the unusual boninite related affinities of the arc volcanics may be due to a combination of a subducting slab torn by hinge zones abnormally small depth to slab distances beneath volcanoes and an unusually hot ambient geotherm due to rising diapirs supplying the intersecting back arc spreading axis the on going collision between the loyalty ridge and the arc may also contribute to these unusual magmatic characteristics as presently only a very small amount of convergence occurs along the southernmost segment of the trench in such a complex tectonic setting with a spreading axis propagating into an intra oceanic arc and major transform plate motions generation of high ca boninitic magmas occurs by melting of a refractory hydrated mantle at a shallow level under the hunter ridge the necessary extra heat being supplied by the rising diapirs supplying magmas to the intersecting spreading ridge axis the high mg andesite suite probably results via a combination of fractional crystallization from such high ca boninitic parental magmas and subsequent magma mixing processes operative all along the southern termination of the arc

when slabs collide a tectonic assessment of deep earthquakes in the tonga vanuatu region an unusual group of more than 100 earthquakes is located 600 km beneath the north fiji basin in the southwest pacific ocean these earthquakes are attributed to seismicity within detached segments of the subducted australian plate one of these detached slab segments has collided with and now impinges on the subducted pacific plate at a depth of 500 km the region in the mantle where the two slab segments meet is also characterized by an unparalleled abundance of large magnitude mw 7 0 earthquakes for this depth the folded shape of the pacific slab beneath fiji as well as the abundance of earthquakes is interpreted to result from deformation and deformation enhanced phase transformations as the two slabs collide and settle on the 660 km discontinuity detachment of the slab segments is interpreted to have occurred at ca 5 ma but collision between the eastern segment and the west dipping pacific slab is interpreted to have occurred at ca 4 ma coincident with initial opening of the overriding lau basin

deep earthquakes beneath the fiji basin sw pacific earth s most intense deep seismicity in stagnant slabs abstract previous work has suggested that many of the deep earthquakes beneath the fiji basin occur in slab material that has been detached and foundered to the bottom of the transition zone or has been laid down by trench migration in a similar recumbent position since nowhere else in the earth do so many earthquakes occur in slabs stagnated in the transition zone these earthquakes merit closer study accordingly we have assembled from historical and modern data a comprehensive catalogue of the relocated hypocenters and focal mechanisms of well located deep events in the geographic area between the bottoms of the main vanuatu and tonga wadati benioff zones two regions of deep seismogenesis are recognized there i 163 deep shocks have occurred north of 15 s in the vityaz group from 1949 to 1996 these seismological observations and the absence of other features characteristic of active subduction suggest that the vityaz group represents deep failure in a detached slab that has foundered to a horizontal orientation near the bottom of the transition zone ii another group of nearly 50 outboard deep shocks occur between about 450 and 660 km depth west of the complexly buckled and offset western edge of the tonga wadati benioff zone their geometry is in the form of two or possibly three small circle arcs that roughly parallel the inferred motion of tonga trench migration earthquakes in the southernmost of these arcs occur in a recumbent high seismic wavespeed slab anomaly that connects both to the main inclined tonga anomaly to the east and a lower mantle anomaly to the west van der hilst r 1995 complex morphology of subducted lithosphere in the mantle beneath the tonga trench nature vol 374 pp 154 157 both groups show complexity in their focal mechanisms the major question raised by these observations is the cause of this apparent temporary arrest in the descent of the tonga slab into the lower mantle we approach these questions by considering the effects of buoyant metastable peridotite in cold slab material that was detached and rapidly foundered or was buckled segmented and laid out in the transition zone

a geologic perspective on the miocene evolution of the antarctic circumpolar current system abstract the dominant geologic feature of pelagic sediment accumulation patterns at intermediate water depths in the southern ocean is the sequential northward migration and replacement of a calcareous ooze sedimentary facies by a siliceous ooze facies following the late oligocene opening of the drake passage and the subsequent initiation of the antarctic circumpolar current closely spaced whole core measurements of bulk density are evaluated for four sites drilled in the southern ocean during ocean drilling program odp legs 113 sites 689 and 690 and 120 sites 747 and 751 these sites are all located on submarine rises in open ocean pelagic depositional environments south of the polar front a major oceanographie and sedimentologic boundary high resolution profiles of bulk density are used to correlate between adjacent drill holes on the maud rise where multiple overlapping sedimentary sequences were recovered using the advanced hydraulic piston core barrel apc stratigraphie age models have been applied to the bulk density data to provide a temporal framework for a discussion of the sedimentologic basis for paleoceanographic interpretations and also to constrain future acoustic research studies the early to middle miocene intervals at sites 689 and 690 are marked by distinct sequences of meter scale alternating carbonate and siliceous units separated by several proposed stratigraphie hiatuses at least six dissolution productivity events are identified in the middle to late miocene on the maud rise and others are noted at the odp sites on the kerguelen plateau the downhole profiles are used to identify the density signature and position of sediment facies changes and proposed stratigraphie hiatuses in sediment cores of late oligocene to late miocene age based on the reversals inflections and discontinuities in the downhole bulk density trends an analysis of the observed bulk density trends at these sites is combined with a review of other tectonic sedimentologic oceanographie and modeling studies to develop a hypothesis for explaining the observed sediment accumulation patterns at intermediate water depths of the southern ocean

interseismic and coseismic motions in gps series related to the ms 7 3 july 13 1994 malekula earthquake central new hebrides subduction zone on july 13 1994 an earthquake ms 7 3 occurred at malekula in the new hebrides archipelago the gps data collected across the new hebrides trench between 1990 and 1996 were processed in order to separate the interseismic and coseismic motions from the drifts related to the convergence with the australian plate the gps derived coseismic displacements at the gps site in malekula are 49 15 mm southward 230 30 mm westward and 170 37 mm downward when the cmt derived displacements are 50 mm southward 210 mm westward and 150 mm downward taking into account the interseismic strain accumulation 25 mm yr at the source established from historical seismicity 7 5 mm yr at the gps site the strain free convergence rate at malekula is 49 3 mm yr other gps derived convergence rates are 95 1 mm yr at efate and 37 2 mm yr at santo these rates imply a regional right lateral motion between the efate and the santo malekula segments in contrast the focal mechanism of the earthquake mostly indicates a left lateral motion therefore we hypothesize that the earthquake is related to variations in the interplate coupling along the converging boundary of the santo malekula segment

a local time dependent sverdrup balance in the eastern north pacific ocean direct observations of the dynamic balance between time dependent winds and deep ocean currents are described for the eastern north pacific ocean at 42 n 152 w currents from 150 to 4000 meters below the surface at frequencies from 0 01 to 0 1 cycle per day are significantly correlated with the wind stress curl derived from u s navy operational wind fields the horizontal currents are depth independent below 300 meters and they flow parallel to the potential vorticity gradient derived from the earth9s rotation and the large scale bottom topography these characteristics are expected for such periodic motions with horizontal scales larger than 500 kilometers and represent a generalized sverdrup balance between the atmospheric forcing and the oceanic response

stochastically forced current fluctuations in vertical shear and over topography the effects of constant vertical shear of zonal velocity uniform meridional bottom slope and a deep meridional wall representing the mid atlantic ridge on the response of a linear two layer quasi geostrophic ocean to stochastic wind stress curl forcing are investigated the model is motivated by discrepancies in energy levels and distributions between recent observations and a similar model without shear or topography that predicted remote coherences relatively well the results indicate that the presence of mean shear can significantly enhance the wind generation of surface intensified oceanic kinetic energy at frequencies above the zero shear cutoff for free baroclinic waves the primary effect of the sloping topography is enhanced barotropic response near the barotropic topographic sverdrup resonance where topographic and planetary potential vorticity gradients cancel the wall inhibits the barotropic response and increases coherences on the mooring side of the wall remote coherences for 49 day period fluctuations are due to long barotropic waves as in the previous model so the present model only partially reconciles the previous discrepancies

an ocean model for climate variability studies abstract models of the time dependent ocean circulation can be simplified considerably by filtering out all short term small scale motions which are unimportant for climatic processes for time scales large compared with a day and space scales large compared with the internal rossby radius of deformation 50 km the currents in most of the interior ocean can be determined diagnostically as quasi equilibrium fields so that only the salinity and temperature fields need be treated prognostically regions of closed f h contours however represent exceptions here trapped vorticity gyres exist as free flow solutions without external forcing and in the presence of forcing the barotropic velocity field must therefore be determined prognostically through a potential vorticity equation for the gyres lateral boundary layers and the equatorial regions also require separate treatment these were not considered specifically but it is suggested that integrated parametrical models analogous in structure to mixed layer models or the integrated boundary layer models of aerodynamics may be the most appropriate technique for coupling these regions to the interior ocean in a comprehensive ocean model suitable for climate studies a coupled multi region model of the global ocean circulation based on these scale considerations could be sufficiently cost effective to permit systematic investigation of the role of the oceanic heat storage and transport in climate variability studies over a wide spectrum of space and time scales the analysis of the seasonal variations of the interior ocean circulation represents a simple example in which the filtered model yields considerably simpler and more readily interpretable results than a fully three dimensional unfiltered model

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

observations of wind forced deep ocean currents in the north pacific this study examines the atmospheric response of the deep ocean using 200 instrument years of moored current measurements from across the breadth of the mid latitude north pacific we have found evidence of a seasonal modulation in the ocean eddy kinetic energy beneath the thermocline at several locations north of 35 n this modulation is often in phase with the local atmospheric forcing function at frequencies between 0 1 and 0 01 cpd correlations between the local wind stress curl and the deep currents have been found for a few locations the local forced response is parallel to the potential vorticity gradient f h and consistent in amplitude with a local topographic sverdrup balance nonlocal forcing in the form of flow along isolines of potential vorticity has been estimated and it is comparable to the observed flow at some locations but the modeled flow is not correlated with the observations throughout the mid latitude north pacific the bottom slope tends to enhance the effect this suggests that topography narrows the available bandwidth for forced barotropic rossby waves facilitating a quasi steady topographic sverdrup response over most of the basin however the enhanced effect reduces the magnitude of the ocean s response to wind forcing nonlocally forced sverdrup currents flow along isolines of f h are the dominant component but are only of the order of 1 cm s 1 locally forced sverdrup currents flow across isolines of f h are typically of the order of 0 1 cm s 1 rms and are not commonly measured hence these wind forced variations are a background level signal in the mid latitude eddy kinetic energy in a few isolated areas the bottom slope reduces the effect in these regions the local sverdrup response is amplified and has been observed

a petrologic anomaly accompanying outboard earthquakes beneath fiji tonga corresponding evidence from broadband p and s waveforms 1 using high resolution triplicate body waveforms we investigate strong lateral variations in p and sh wave speeds vp and vs beneath the tonga subduction zone where cold lithosphere is subducting rapidly at 200 mm yr 1 under these conditions the lack of prominent anomalies of high vp and vs in the lower mantle implies that a large amount of recently subducted material remains above the lower mantle this inference is also evident from a subhorizontal swath of outboard earthquakes in the transition zone that is up to 1000 km away from the wadati benioff zone beneath the back arc we find a sharp east west trending demarcation north of which outboard earthquakes occur and vp and vs abruptly drop by 3 thus the effect of cold temperature to raise vp and vs as indicated by the presence of outboard earthquakes must be counteracted by petrologic variations we interpret that outboard earthquakes occur where impounding of subducted metastable olivine or volatiles reduces seismic wave speeds and triggers deep earthquakes considering the density of these candidates and the strong effect of partial melt on vs volatiles in the form of hydrous phases or melt induced by dehydration are not consistent with observed reductions in vp and vs meanwhile metastable olivine remains a viable candidate as a cold slab passes below the 410 km discontinuity olivine becomes metastable and makes the tip of the slab buoyant such a mechanism could explain why the leading end of a fast subducting slab may be successively sequestered in the transition zone

high mg adakites from kadavu island group fiji southwest pacific evidence for the mantle origin of adakite parental melts we report here the compositions of primitive high mg adakite lavas from the kadavu island group fiji two distinctive high mg adakite compositions are present on kadavu and both are strongly enriched in sr and la with high sr y 120 and la yb 30 values the east kadavu high mg adakite is relatively enriched in k2o and depleted in tio2 and nb our results from kadavu suggest that there is an adakite magma series that is the result of complex open system magmatic processes including magma mixing between relatively low sio2 mantle derived melts of high mg adakite composition and high sio2 low mg adakite magma the results from kadavu island suggest that adakite magma suites worldwide are likely to include primitive high mg compositions despite the lack of fi eld evidence in many cases

the barotropic response of the subpolar north pacific to stochastic wind forcing in a recent observational study koblinsky et al 1989 attempted to demonstrate that the topographic sverdrup balance can account for mesoscale current fluctuations with periods between 10 and 100 days in the abyssal north pacific however the topographic sverdrup balance proved difficult to verify systematically over the north pacific in this paper the conditions required to obtain the topographic sverdrup balance are investigated through numerical experiments with a stochastically forced barotropic quasi geostrophic numerical model of the subpolar north pacific in the numerical experiments the topographic sverdrup balance emerges from area averages of the terms in the vorticity equation but only for periods longer than about 40 days when the averages are taken over 4 4 region this period increases when averages are taken over progressively smaller regions an examination of vorticity balances at individual points along sections through the model domain indicates that the topographic sverdrup balance is not obtained at individual points even for very long period motions the possibility of verifying topographic sverdrup dynamics with oceanic data is considered it is argued that topographically generated small scales in the velocity field reduce the likelihood that the topographic sverdrup balance can be systematically observed in velocity measurements from individual current meter moorings the results suggest that the vorticity balance is complex and that topographic sverdrup dynamics cannot be expected to provide an adequate explanation for the seasonal modulation of the abyssal eddy kinetic energy that is observed with moored instrumentation in the north pacific

the role of heating winds and topography on sea level changes in the north atlantic seasonal and interannual to decadal variations of large scale altimetric sea surface height ssh owing to surface heating and wind forcing in the presence of topography are investigated using simplified models the dominant forcing mechanisms are time scale dependent on the seasonal time scale locally forced thermosteric height explains most of the ssh variance north of 18 n first mode linear long baroclinic rossby waves forced by changes in the winds and eastern boundary conditions explain most of the variance between 10 n and 15 n and are also important east of greenland on interannual to decadal time scales local thermosteric height remains important at several locations in the middle and high latitudes a topographic sverdrup response explains interannual to decadal ssh between 53 n and 63 n east of greenland farther south the linear rossby wave model explains ssh variations on interannual to decadal time scales between 30 n and 50 n from mid basin to the eastern boundary propagation of the eastern boundary condition into the interior dominates the interannual to decadal ssh signals south of 30 n the effect from nao related heat flux on ssh is small but forcing the topographic sverdrup models with nao regressed winds gives slightly better agreement with the observed ssh in the subpolar gyre on interannual to decadal time scales than using the full winds

evidence for wind driven current fluctuations in the western north atlantic two subsurface current meter moorings were deployed for 20 months along 70 w in a low energy portion of the western north atlantic the resulting data were compared with estimates of the wind stress curl in an attempt to seek a relationship between currents below the mixed layer and meteorological forcing at subinertial frequencies currents and temperature tend to be significantly coherent with the curl and are most coherent with the curl at remote locations usually more than 500 km away from the mooring and generally to the east coherences generally decrease with increasing frequency the current response is surface intensified a linear stratified model for stochastically wind forced rossby waves in a flat bottomed ocean is presented as an attempt to rationalize the results the effort was not entirely successful so that it appears that a more realistic model perhaps including bottom topography and mean currents will be needed to rationalize these observations fully

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

seismological evidence for tearing of the pacific plate at the northern termination of the tonga subduction zone we present seismological evidence for tearing of the pacific plate at the northern termination of the tonga subduction zone seismicity cross sections indicate 1 abrupt termination of intermediate depth seismicity along an east west trending zone coincident with the northern termination of the subduction zone and 2 shoaling of the subducted slab at its northern termination earthquake distribution and source mechanism determinations indicate progressive downwarping and tearing of the pacific plate as it enters the northernmost segment of the tonga subduction zone source mechanisms for 57 events located along a narrow belt of high seismicity in northernmost tonga indicate dip slip faulting along near vertical planes oriented 285 coinciding with the observed direction of plate convergence precisely determined depths for 21 events 18 57 km suggest that the plate is downwarped 15 km prior to initiation of tearing depths of the 7 april 1995 m s 8 event and aftershocks suggest tearing of the plate may extend through the entire thickness of the oceanic lithosphere

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle

a near uniform fluctuation of ocean bottom pressure and sea level across the deep ocean basins of the arctic ocean and the nordic seas across the arctic ocean and the nordic seas a basin wide mode of ocean bottom pressure and sea level fluctuation is identified using satellite and in situ observations in conjunction with a global ocean circulation model and its adjoint the variation extends across the interconnected deep ocean basins of these semi enclosed arctic seas collectively called the arctic mediterranean with spatially near uniform amplitude and phase the basin wide fluctuation is barotropic and dominates the region s large scale variability from sub monthly to interannual timescales the fluctuation results from bifurcating coastally trapped waves generated by winds along the continental slopes of the arctic mediterranean and its neighboring seas including the north atlantic ocean the winds drive ekman transport across the large bathymetric gradients forcing mass divergence between the shallow coastal area and the deep ocean basins and creating ocean bottom pressure anomalies of opposite signs in the two regions the anomalies rapidly propagate away as barotropic coastally trapped waves with the coast and continental slope as respective boundaries the waves subsequently bifurcate at the shallow straits connecting the arctic mediterranean with the rest of the globe the straits transmit the shallow anomalies but not the deep variations thereby inhibiting the anomalies mutual cancelation by geographically separating the two anomalies that enter the deep arctic basins equilibrate uniformly across the domain characterized by a homogeneous depth integrated planetary potential vorticity distribution the potential vorticity s steep gradient that borders the basins shields the region from neighboring shallow variations giving rise to the observed spatially confined fluctuation compensating anomalies outside the arctic adjust similarly across the rest of the globe but are comparatively negligible in amplitude because of the global ocean s larger area relative to that of the deep arctic mediterranean the study from a technical perspective illustrates the utility of a model s adjoint in identifying causal mechanisms underlying a complex system