transitions in axial morphology along the southeast indian ridge shipboard bathymetric and magnetic profiles across the southeast indian ridge seir were analyzed in order to examine the nature of along axis variations in axial morphology at this intermediate spreading rate ridge three types of axial morphology are observed along the seir an axial high a shallow 200 700 m deep axial valley and a deep 1000 m deep axial valley an axial high is found to the east of the australian antarctic discordance aad east of 128 e and between 82 e and 104 e a shallow rift valley is found from 104 e to 114 e and from 82 e westward past the amerstdam st paul hotspot asp to about 30 s 75 e deep rift valleys are found from 114 e to 128 e in the vicinity of the aad and from the indian ocean triple junction iotj at 25 s 70 e to about 30 s 75 e the transition near 30 s occurs in an area of constant zero age depth and does not appear to result from an increase in mantle temperature it could be the result of the rapid increase in spreading rate along that portion of the seir the most likely cause of the other transitions in axial morphology is variations in mantle temperature the transitions between the different types of axial morphology are well defined and occur over a limited distance transitions in axial morphology are accompanied by significant changes in ridge flank topographic roughness the transitions from axial valleys to axial highs are also accompanied by changes in the amplitude of the seafloor magnetic anomalies our observations suggest that there are distinct modes rather than a continuum of axial morphology on the seir and that there appears to be a threshold mechanism for a rapid change between different states of axial morphology the asp has only a limited influence on the seir the ridge axis is marked by an axial valley for the entire distance from the iotj up to and past the asp the ridge axis becomes shallower as the asp is approached from the northwest but only by about 300 m over a distance of 800 km in addition the ridge continues to become shallower away from amsterdam island toward the transition to an axial high at 82 e 350 km to the east of the asp the kerguelen hotspot appears to exert a major influence on the morphology of the seir by feeding asthenospheric material to the ridge axis a long narrow finger like gravity high extends ene away from the kerguelen plateau for a distance of 500 km shipboard data show that the gravity high results from a large volcanic ridge the ridge appears analogous to the rodriguez ridge extending from the reunion hotspot toward the central indian ridge a series of lower and broader lineated gravity highs extend from the volcanic ridge toward the seir in the ridge segment between the 81 e and 85 e transforms which is the westernmost segment with an axial high the only region of significant off ridge seismicity on the antarctic flank of the seir is a diffuse band of epicenters extending from kerguelen to the seir within the segment between the 81 e and 85 e fracture zones the along axis gradient in depth from 86 e to the aad and the transitions in axial morphology at 104 e and 114 e most likely reflect along axis variations in mantle temperature and melt production rate due to distance from the kerguelen hotspot and the influence of the aad

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

western scotia sea margins improved constraints on the opening of the drake passage 1 we present a revised tectonic interpretation from 28 ma to 3 2 ma of the western sector of the scotia sea incorporating new multichannel seismic reflection profiles and magnetic anomaly identifications for the continental margin off the tierra del fuego island and available complementary data for the conjugate margin of the northwestern flank of the south scotia ridge seismic profiles show a remarkable diversity of the pair of conjugate passive margins of the western scotia sea in both their morphology and structural framework the tierra del fuego continental margin can be related to a classic rifted passive margin while the southwestern margin of the scotia sea is characterized by steep slopes mostly generated by subvertical faults that abruptly separate the continental crust of the south scotia ridge from the oceanic crust of the western scotia sea this structural difference was caused by intense strike slip tectonism mostly concentrated along the modern south scotia ridge since the early development of the western scotia sea we find evidence for a previously unrecognized magnetic anomaly 10 28 ma at the foot of the tierra del fuego continental margin the same anomaly is present at the conjugate northern flank of the south scotia ridge the timing of events leading to the earliest development of the western scotia sea which determined the opening of the drake passage is important because this gateway opening had a profound effect on global circulation and climate the thickness and the distribution of the sedimentary cover overall in the abyssal plain off the two western scotia sea margins is different this is due to the different regimes of the bottom current flows which affected the western scotia sea both in the past and in the present time

back arc extension in the scotia sea the nature of back arc extension in the east scotia sea is re examined with the use of an enlarged geophysical data set well developed oceanic magnetic lineations confirm that the present spreading episode started about 8 ma ago that spreading is asymmetric and that the total rate increased from 50 to 70 m m a about 1 5 ma ago most of the currently active south sandwich volcanic island arc lies upon ocean floor only 6 8 m a old and generated at the current spreading ridge subsequent extension has not modified the curvature of the arc east west magnetic lineations of miocene age in the central scotia sea and contemporaneous low k arc tholeiites dredged from the eastern south scotia ridge discovery bank indicate a regime of coupled subduction and back arc extension preceding that occurring now a speculative model involving a series of collisions of parts of this earlier discovery trench with ridge crest sections of the south american antarctic plate boundary explains the transformation of this earlier regime into the present self contained sandwich plate regime the considerable small scale variability observed in the back arc region may be seen as an inevitable consequence of the action of the ridge trench collision mechanism the entire scotia sea could have formed by a similar kind of back arc extensional modification of the south american antarctic plate boundary

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

subduction influence on magma supply at the east scotia ridge despite a spreading rate of 65 70 km ma 1 the east scotia ridge has along most of its length a form typically associated with slower rates of sea floor spreading this may be a consequence of cooler than normal mantle upwelling which could be a feature of back arc spreading at the northern end of the ridge recently acquired sonar data show a complex rapidly evolving pattern of extension within 100 km of the south sandwich trench new ridge segments appear to be nucleating at or near the boundary between the south american and scotia sea plates and propagating southwards supplanting older segments the most prominent of these north of 56 30 s has been propagating at a rate of approximately 60 km ma 1 for at least 1 ma and displays a morphology unique on this plate boundary a 40 km long axial high exists at the centre of this segment forming one of the shallowest sections of the east scotia ridge beneath it seismic reflection profiles reveal an axial magma chamber or amc reflector similar to those observed beneath the east pacific rise and valu fa ridge simple calculations indicate the existence here of a narrow 1 km wide body of melt at a depth of approximately 3 km beneath the sea floor from the topographic and seismic data we deduce that a localised mantle melting anomaly lies beneath this segment rates of spreading in the east scotia sea show little variation along axis hence the changes in melt supply are related to the unique tectonic setting in which the south american plate is tearing to the east perhaps allowing mantle flow around the end of the subducting slab volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed a southward mantle flow may have existed beneath the axis of the east scotia ridge throughout its history

upper mantle seismic anisotropy beneath antarctica and the scotia sea region summary recent investigations on shear wave splitting from recordings of permanent and temporary antarctic seismological stations have lead to a greater understanding of the upper mantle dynamics of the scotia sea region and the continental margin in the eastern weddell sea in terms of their tectonic evolution the analysis of shear wave splitting from teleseismic core sks skks pks and direct s waves reveals the seismic anisotropy and the strain field of the upper mantle similar to the caribbean anisotropy structures in the antarctic peninsula and scotia sea regions are assumed to be influenced by mantle flows in easterly directions around the subducting nazca plate in general anisotropy polarization directions in the scotia sea do not contradict this hypothesis with polarizations oriented nearly e w and therefore aligning with the suggested mantle flow patterns anisotropy strengths decrease from delay times of dt 1 8 s pmsa palmer station in the west towards the east with delay times of dt 0 3 s beneath hope south georgia and cand candlemas south sandwich islands nevertheless a lithospheric and therefore fossil origin cannot be ruled out only the exceptionally high delay times at pmsa probably originate in part from recent asthenospheric flow around the subduction slab of the former phoenix plate beneath the northwestern margin of the antarctic peninsula the continental margin of western dronning maud and coats land plays a crucial role in understanding the early processes during the break up of gondwana upper mantle seismic anisotropy with delay times well over dt 1 s in this region gives new constraints on ancient deformation processes during break up and former episodes two layer modelling reveals archaean anisotropy in the upper layer corresponding well to polarization directions of the south african kaapvaal craton lower layer anisotropy is assumed to have been created during early gondwana rifting stages

viscous forces acting on subducting lithosphere subducting lithosphere is driven by gravitational forces and resisted primarily by viscous forces three independent types of mantle flow combine to create viscous tractions on the surface of a subducting slab which terminates in the upper mantle 1 flow induced by plate convergence and slab subduction 2 flow caused by trench migration and 3 regional upper mantle flow unrelated to convergence or trench migration the equilibrium dip of a slab reflects a balance of viscous tractions with gravitational forces and slab stiffness we present results from a two dimensional finite difference model of upper mantle flow for different slab geometries where mantle viscosity increases exponentially with depth and the slab terminating in the upper mantle is assumed to be infinitely viscous in the overlying mantle wedge the subducting slab creates a strong eddy which is largely unaffected by trench migration or regional flow trench migration and regional mantle flow produce tractions which tend to lift the slab and which are independent of slab dip for a slab terminating in the upper mantle for dips 22 5 the upward torquing force on the slab due to convergence is negligible compared to the pressures created by trench migration and regional mantle flow slabs dipping 87 5 which are in torquing equilibrium require regional flow in the same direction as subduction or trench migration in the opposite direction which simultaneously tends to eliminate most of the horizontal pressure difference across the subduction zone the force calculations based on this model suggest that viscosity at the base of the lithosphere is 2 1020 pa s assuming viscosity increases exponentially with depth to 2 8 1021 pa s just above the lower mantle for force balanced slabs which penetrate halfway into the upper mantle with dip in the range of 30 to 75 slab dip increases linearly with decreasing trench migration rate and decreasing regional mantle flow

a teleseismic shear wave splitting study to investigate mantle flow around south america and implications for plate driving forces summary closure of the pacific ocean basin by the convergence of its surrounding plates some of which have deep continental roots implies that there is net mass flux out of the mantle under the pacific here we report on a shear wave splitting study designed to test the prediction that there should be flow around its southern margin our results show no evidence for present day flow around the tip of southern south america instead the results suggest present day flow directions in the southern atlantic that parallel the south american absolute plate motion direction even under antarctica the results also provide evidence for absolute plate motion driven by the basal drag of ocean basin scale mantle flow and suggest that 200 km thick flow boundary layers exist under south america and antarctica and also demonstrate that mantle flow directions cannot be reliably inferred from present day plate morphology

hydrothermal activity on near arc sections of back arc ridges results from the mariana trough and lau basin 1 the spatial density of hydrothermal venting is strongly correlated with spreading rate on mid ocean ridges with the interesting exception of hot spot affected ridges evidently because spreading rate is a reliable proxy for the magma budget this correlation remains untested on spreading ridges in back arc basins where the magma budget may be complicated by subduction induced variations of the melt supply to address this uncertainty we conducted hydrothermal plume surveys along slow spreading 40 60 mm yr and arc proximal 10 60 km distant sections of the southern mariana trough and the valu fa ridge lau basin on both sections we found multiple plumes overlying 15 20 of the total length of each section a coverage comparable to mid ocean ridges spreading at similar rates these conditions contrast with earlier reported results from the two nearest arc segments of a faster spreading 60 70 mm yr back arc ridge the east scotia ridge which approaches no closer than 100 km to its arc there hydrothermal venting is relatively scarce 5 plume coverage and the ridge characteristics are distinctly slow spreading small central volcanic highs bookended by deep median valleys and axial melt lenses restricted to the volcanic highs two factors may contribute to an unexpectedly low hydrothermal budget on these east scotia ridge segments they may lie too far from the adjacent arc to benefit from near arc sources of melt supply and subduction aided migration of mantle from the bouvet hot spot may reduce hydrothermal circulation by local crustal warming and thickening analogous to the reykjanes ridge thus the pattern among these three ridge sections appears to mirror the larger global pattern defined by mid ocean ridges a well defined trend of spreading rate versus hydrothermal activity on most ridge sections plus a subset of ridge sections where unusual melt delivery conditions diminish the expected hydrothermal activity

evolution of 3 d subduction induced mantle flow around lateral slab edges in analogue models of free subduction analysed by stereoscopic particle image velocimetry technique abstract we present analogue models of free subduction in which we investigate the three dimensional 3 d subduction induced mantle flow focusing around the slab edges we use a stereoscopic particle image velocimetry spiv technique to map the 3 d mantle flow on 4 vertical cross sections for one experiment and on 3 horizontal depth sections for another experiment on each section the in plane components are mapped as well as the out of plane component for several experimental times the results indicate that four types of maximum upwelling are produced by the subduction induced mantle flow the first two are associated with the poloidal circulation occurring in the mantle wedge and in the sub slab domain a third type is produced by horizontal motion and deformation of the frontal part of the slab lying on the 660 km discontinuity the fourth type results from quasi toroidal return flow around the lateral slab edges which produces a maximum upwelling located slightly laterally away from the sub slab domain and can have another maximum upwelling located laterally away from the mantle wedge these upwellings occur during the whole subduction process in contrast the poloidal circulation in the mantle wedge produces a zone of upwelling that is vigorous during the free falling phase of the slab sinking but that decreases in intensity when reaching the steady state phase the position of the maximum upward component and horizontal components of the mantle flow velocity field has been tracked through time their time evolving magnitude is well correlated to the trench retreat rate the maximum upwelling velocity located laterally away from the subducting plate is 18 24 of the trench retreat rate during the steady state subduction phase it is observed in the mid upper mantle but upwellings are produced throughout the whole upper mantle thickness potentially promoting decompression melting it could thereby provide a source for intraplate volcanism such as mount etna in the mediterranean the chiveluch group of volcanoes in kamchatka and the samoan hotspot near tonga

scotia sea tectonics from high resolution satellite gravity the release of altimetric data from the geosat geodetic mission by the us navy 1 is leading to a much improved understanding of tectonics in the southern ocean a region in which remoteness and adverse physical conditions have limited the acquisition of geophysical data by research ships the scotia sea is an outstanding example of back arc spreading which is revealed in some detail by free air gravity anomaly maps derived from the latest release of data acquired south of 30 s 2 sea surface height data for this region have been reduced to a grid of free air gravity anomalies and are illustrated here by means of colour shaded relief and contour maps the new data confirm the existence of a number of inactive spreading ridges within the scotia sea and surrounding small basins the amplitudes and wavelengths of gravity anomalies over these ridges conform in general to the expected relationship with spreading rate except in the central scotia sea where a proposed miocene slow spreading ridge appears to have left no clear signature the spreading ridge axis in the east scotia sea comprises seven or more segments separated by small mainly sinistral offsets and exhibits a median valley with depths of 200 1000 m that is reflected in free air lows of 10 40 mgal near both its northern and southern termini the gravity signature of the ridge becomes less distinct with a less pronounced axial low the northernmost segments of the ridge are displaced in a right lateral sense by a feature which appears to represent a southward migrating non transform offset whereas the process of spreading in drake passage and the east scotia sea was comparable to mid ocean ridges that in the central scotia sea may have been disorganized as observed in some western pacific back arc basins

using geochemistry to map mantle flow beneath the lau basin geochemical tracers can be used to map mantle flow beneath the lau basin tonga region over the past 6 m y and constrain the extent and means by which different components contribute to the magmas erupted there helium isotope data track the samoan plume to show that it infiltrated into the northwestern part of the lau basin at 40 mm yr 1 through an opening tear in the pacific plate in contrast to other recent studies our analysis indicates that this plume does not contribute to the arc lavas a louisville plume component is present in the northern tonga arc lavas but this was added from volcaniclastic sediments transported into the arc on the subducting slab and does not require the presence of plume material within the mantle wedge the louisville tracer has been used to infer that the mantle wedge beneath the island arc is downwelling at the rate of 20 40 mm yr 1 owing to viscous drag against the downgoing pacific plate a third geochemical tracer provided by the distinction between indian and pacific mantle indicates that indian mantle has migrated eastward at rates of 45 65 mm yr 1 the inferred rates of migration indicate that the translation of geochemical signatures occurs by mantle flow rather than by the order of magnitude faster movement of partial melts except for the flow in the mantle wedge the directions of mantle flow in this region are unrelated to the overlying plate motions

the effect of arc proximity on hydrothermal activity along spreading centers new evidence from the mariana back arc 12 7 n 18 3 n back arc spreading centers bascs form a distinct class of ocean spreading ridges distinguished by steep along axis gradients in spreading rate and by additional magma supplied through subduction these characteristics can affect the population and distribution of hydrothermal activity on bascs compared to mid ocean ridges mors to investigate this hypothesis we comprehensively explored 600 km of the southern half of the mariana basc we used water column mapping and seafloor imaging to identify 19 active vent sites an increase of 13 over the current listing in the interridge database irdb on the bathymetric highs of 7 of the 11 segments we identified both high and low i e characterized by a weak or negligible particle plume temperature discharge occurring on segment types spanning dominantly magmatic to dominantly tectonic active sites are concentrated on the two southernmost segments where distance to the adjacent arc is shortest 48 mm yr and tectonic extension is pervasive re examination of hydrothermal data from other bascs supports the generalization that hydrothermal site density increases on segments 90 km from an adjacent arc although exploration quality varies greatly among bascs present data suggest that for a given spreading rate the mean spatial density of hydrothermal activity varies little between mors and bascs the present global database however may be misleading on both bascs and mors the spatial density of hydrothermal sites mapped by high quality water column surveys is 2 7 times greater than predicted by the existing irdb trend of site density versus spreading rate

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

mantle flow at a slab edge seismic anisotropy in the kamchatka region the junction of the aleutian island and the kamchatka peninsula defines a sharp turn in the boundary of the pacific and north american plates terminating the subduction zones of the northwest pacific the regional pattern of shear wave birefringence near the junction indicates that trench parallel strain follows the seismogenic benioff zone but rotates to trench normal beyond the slab edge asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge and may influence the shallowing dip of the benioff zone at the aleutian junction

magma supply in back arc spreading centre segment e2 east scotia ridge segment e2 is situated in the back arc east scotia ridge the segment is unusual in that it has an axial topographic high underlain by a seismically imaged melt lens the axis of the segment which is 70 km long was sampled at 2 km spacing there is strong correlation between compositions and bathymetry and there is no evidence that lateral flow of magmas along dykes within the segment was more than 25 km magmas are more evolved towards the summit indicating that magma fractionation took place within the imaged melt lens na8 0 is roughly constant at 2 6 implying uniform degree of partial melting except for some samples at the summit that have na8 0 2 2 compositions closest to n morb occur at the segment tips and lree hree ratios increase toward the summit none of the magmas were derived from depleted sub arc mantle nevertheless most compositions in the segment were modified by slab derived components the low na8 0 samples have high ba nb compared with the rest of the segment dredged lavas from the lateral flanks of the summit have the most extreme compositions including ones derived from plume mantle and are end members for magma mixing processes that controlled compositions

does subduction induced mantle flow drive backarc extension abstract backarc extension is a characteristic feature of many narrow subduction zones seismological and geochemical studies imply the occurrence of mantle flow around the narrow subducting slabs previous 3d models suggested that backarc extension is related to subduction induced toroidal mantle flow the physical viability of this mechanism however has never been tested using laboratory based geodynamic models in this work we present dynamic laboratory models of progressive subduction in three dimensional 3d space that were carried out to test this mechanism to achieve this we have used a stereoscopic particle image velocimetry spiv technique to map simultaneously overriding plate deformation and 3d subduction induced mantle flow underneath and around an overriding plate the results show that the strain field of the overriding plate is characterized by the localization of an area of maximum extension within its interior at 300 500 km from the trench the position of maximum extension closely coincides within 2 cm scaling to 100 km with that of the maximum trench normal horizontal mantle velocity and velocity gradient measured at a scaled depth of 15 25 km below the base of the overriding plate and the maximum horizontal gradient of the vertical mantle velocity gradient we propound that in narrow subduction zones backarc extension in the overriding plate is mainly a consequence of the trench normal horizontal gradients of basal drag force at the base of the overriding plate such shear force gradients result from a horizontal gradient in velocity in the mantle below the base of the lithosphere induced by slab rollback calculations based on our models indicate a tensional horizontal trench normal deviatoric stress in the backarc region scaling to 28 8 mpa while the overriding plate trench normal stress resulting from the horizontal component of the trench suction force is about an order of magnitude smaller scaling to 2 4 3 6 mpa

eastbound sublithosphere mantle flow through the caribbean gap and its relevance to the continental undertow hypothesis recent evidence indicates that beneath the caribbean a tongue of sublithosphere mantle is flowing from the pacific to the atlantic dragging the overlying lithosphere eastward i shear wave splitting results from beneath the andean subduction zone and venezuela suggest mantle flow eastward through the caribbean ii volcanic chemistry in central america indicates a slab source beneath nicaragua but a different source in costa rica above the proposed pacific outflow iii an extinct volcanic arc accreted to the margins of the caribbean swept eastward through the caribbean gap between north south america the 1982 continental undertow model requires shallow mantle flow through the caribbean gap from the pacific to the atlantic if continents have deep roots and if shallow mantle flow beneath oceans is decoupled from convection at deeper levels the new evidence from the caribbean is thus compatible with the continental undertow model and perhaps with other models involving decoupled shallow flow

anisotropy in kuril kamchatka subduction zone structure to constrain the orientation and extent of seismic velocity anisotropy in the kuril kamchatka subduction zone we have developed a technique that isolates anisotropy at shallow depths by differencing splitting observed in teleseismic shear wave phases olivine a axis orientations inferred from 29 ss s phase pairs and two local s phases vary between the major tectonic components of the trench back arc system although the ss paths do not uniquely constrain a single model of anisotropy a axes are consistent with finite strain patterns predicted by plate convergence and back arc spreading and shearing

anisotropy and flow in pacific subduction zone back arcs we have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the tonga southern kuril japan and izu bonin subduction zones using shear wave splitting in s phases from local earthquakes and in teleseismic core phases such as sks the observed splitting in all four subduction zones is consistent with a model in which the lower transition zone 520 660 km and lower mantle are isotropic and in which significant anisotropy occurs in the back arc upper mantle the upper transition zone 410 520 km beneath the southern kurils appears to contain weak anisotropy the observed fast directions indicate that the geometry of back arc strain in the upper mantle varies systematically across the western pacific rim beneath izu bonin and tonga fast directions are aligned with the azimuth of subducting pacific plate motion and are parallel or sub parallel to overriding plate extension however fast directions beneath the japan sea western honshu and sakhalin island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing models of back arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in tonga and izu bonin but further three dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern kurils and japan the fast directions in the southern kurils and japan may require strain in the back arc mantle that is driven by regional or global patterns of mantle flow

growth and mass wasting of volcanic centers in the northern south sandwich arc south atlantic revealed by new multibeam mapping new multibeam swath bathymetric sonar data acquired using an em120 system on the rrs james clark ross supplemented by sub bottom profiling reveals the underwater morphology of a not vert similar 12 000 km2 area in the northern part of the mainly submarine south sandwich volcanic arc the new data extend between 55 45 s and 57 20 s and include protector shoal and the areas around zavodovski visokoi and the candlemas islands groups each of these areas is a discrete volcanic center the entirely submarine protector shoal area close to the northern limit of the arc forms a 55 km long east west trending seamount chain that is at least partly of silicic composition the seamounts are comparable to small subaerial stratovolcanoes in size with volumes up to 83 km3 indicating that they are the product of multiple eruptions over extended periods zavodovski visokoi and the candlemas island group are the summits of three 3 3 5 km high volcanic edifices the bathymetric data show evidence for relationships between constructional volcanic features including migrating volcanic centers structurally controlled constructional ridges satellite lava flows and domes and mass wasting of the edifices mass wasting takes place mainly by strong erosion at sea level and dispersal of this material along chutes probably as turbidity currents and other mass flows that deposit in extensive sediment wave fields large scale mass wasting structures include movement of unconsolidated debris in slides slumps and debris avalanches volcanism is migrating westward relative to the underlying plate and major volcanoes are asymmetrical being steep with abundant recent volcanism on their western flanks and gently sloping with extinct eroded volcanic sequences to their east this is consistent with the calculated rate of subduction erosion of the fore arc

narrow subducting slabs and the origin of backarc basins abstract both tectonic and seismic data suggest that horizontally wide in the direction parallel to the axis of the trench dipping subducting slabs typically move stably at fixed angles to the horizontal and that their associated trenches migrate very slowly or not at all in contrast where backarc basins babs exist the associated trenches typically migrate at rates of several cm year this behavior may be related to the finite width of the slabs under babs analysis shows that hydrodynamic suction created by the downdip motion of a wide slab is sufficient to balance the pull of gravity on the heavy slab however for a narrow slab sideways asthenospheric flow into the slab overriding plate corner effectively eliminates the suction which results in the rapid sinking of the slab hydrodynamic modeling is used to quantify this hypothesis modeling is used also to show that wide slabs are stable in their inclined position not because of the gravitational wing like sliding but due to the lifting force produced by the negative pressure suction in the slab overriding plate corner finally the model results confirm that for relatively narrow slabs this lifting force is small and cannot counterbalance gravity therefore when bending moments in a narrow slab exceed the plastic yield limit the slab becomes unstable this instability can cause the rapid migration of the slab s hinge and the associated arc at rates of 10 20 cm year and thus form the bab

new constraints on antarctic plate motion and deformation from gps data four years 1995 1998 of continuous global positioning system gps data recorded by 11 permanent international gps service igs stations in antarctica and surrounding regions have been processed using an optimized method for regional geodetic networks the 4 year time series has allowed us to extract nonlinear variations and constant horizontal velocities for all the stations in december 1997 we have set up a permanent gps station at dumont d urville in terre adelie the resulting data along with those of the scientific committee on antarctic research scar campaigns 1995 and 1996 are included in our processing a coseismic displacement 1 to 2 cm is detected in the horizontal components of this station we relate this to the march 25 1998 mw 8 1 balleny islands earthquake for which a dislocation model yields prediction in the right direction for the six antarctic stations the horizontal rates are very consistent with the rigid plate rotation all the residual velocities are negligible less than 2 mm yr except at o higgins in the head of the peninsula 7 mm yr toward the continent the position of the rotation pole 62 0 n 146 7 w and its rate 0 264 ma are significantly different from the nnr nuvel la predictions for the antarctic tectonic plate but are consistent with the australian relative motion the high horizontal residual in the peninsula area where iceberg calving rates and ice shelf disintegration have recently increased can be interpreted as the horizontal component of the elastic crustal response to the load variations

cordillera formation mantle dynamics and the wilson cycle we propose that the andes cordillera formed by a collision between the trenchward moving south american plate and the nazca slab and subslab mantle resistance to trenchward motion in the form of horizontal compressive normal stress deforms south america9s leading edge the advancing continent also perturbs the subslab mantle flow field as mantle is displaced this flow field detected via shear wave splitting includes trench parallel flow away from a central stagnation point and corner flow around the northern and southern ends of the continent the corresponding spatial variation in applied stress accounts for the large scale characteristics of andean deformation buckling at the stagnation point maximum normal stress forming the central andes and bolivian orocline and corner flow associated with the eastward motion of the caribbean and scotia plates the high stresses required to form and maintain the andes imply that south america9s trenchward motion is primarily driven by deep mantle flow coupled to its base no other driving force of sufficient magnitude is available similarities between north and south american plate motion history and large scale cordillera structure indicate that the western north american laramide rockies formed in a similar manner and that the north american plate is also driven westward by deep mantle flow we infer that atlantic spreading itself is similarly driven by contrast slab rollback in a closing ocean basin opens marginal basins at convergent margins as in the western pacific we suggest that there is a causal link between the opening and closing of ocean basins and the formation of cordilleras and marginal basins cordillera formation occurs at the leading continental margin during the spreading phase of the wilson cycle whereas marginal basins form during the closing phase

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

seismic anisotropy beneath the shumagin islands segment of the aleutian alaska subduction zone lithospheric strain and mantle flow in subduction zone back arcs reflect the fundamental dynamics of lithospheric subduction in this study we inferred possible strain geometries in the mantle beneath the eastern aleutians by mapping seismic anisotropy with shear wave splitting we analyzed s phases of local events recorded by the shumagin seismic network and obtained splitting parameters fast direction and splitting time t and their 95 confidence regions to avoid complexity from converted phases data were restricted to paths at incidence angles less than 32 and to events deeper than 50 km the s phases sample the forearc and back arc of the shumagin segment from 159 7 w to 163 8 w at depths up to 216 km in our data set of 84 local events 44 of 127 s phases yielded constrained splitting parameters with consistent fast directions that are approximately parallel to the volcanic arc roughly 60 the remaining 83 s phases produced null measurements and the fast directions inferred from these phase polarizations agree with the fast axis alignment in the constrained measurements splitting times vary from 0 1 to 0 35 s and show an overall increase with hypocentral depth observed splitting parameters are well matched by predicted values for olivine rich mantle wedge models with 1 sv anisotropy where the olivine b axis is arc orthogonal and the a axis is vertical no arc parallel strain or arcparallel no vertical strain these results are consistent with mantle wedge strain models in which arc normal compression is accompanied by arc parallel or vertical shearing or extension

geological evidence for the geographical pattern of mantle return flow and the driving mechanism of plate tectonics tectonic features at the earth s surface can be used to test models for mantle return flow and to determine the geographic pattern of this flow a model with shallow return flow and deep continental roots places the strongest constraints on the geographical pattern of return flow and predicts recognizable surface manifestations because of the progressive shrinkage of the pacific averaging 0 5 km2 yr over the last 180 m y this model predicts upper mantle outflow through the three gaps in the chain of continents rimming the pacific caribbean drake passage australian antarctic gap in this model upper mantle return flow streams originating at the western pacific trenches and at the java trench meet south of australia filling in behind this rapidly northward moving continent and providing an explanation for the negative bathymetric and gravity anomalies of the australian antarctic discordance the long continued tectonic movements toward the east that characterize the caribbean and the easternmost scotia sea may be produced by viscous coupling to the predicted pacific outflow through the gaps and the caribbean floor slopes in the predicted direction if mantle outflow does pass through the gaps in the pacific perimeter it must pass beneath three seismic zones central america lesser antilles scotia sea none of these seismic zones shows foci below 200 km mantle material flowing through the caribbean and drake passage gaps would supply the mid atlantic ridge while the java trench supplies the indian ocean ridges so that deep mantle upwellings need not be centered under spreading ridges and therefore are not required to move laterally to follow ridge migrations the analysis up to this point suggests that upper mantle return flow is a response to the motion of the continents the second part of the paper suggests a possible driving mechanism for the plate tectonic process which may explain why the continents move this hypothetical driving mechanism has four aspects 1 the lower mantle convects 2 this convection drives the continents by drag on their deep roots 3 return flow in the upper mantle provides a volumetric balance for the motion of the continental masses without passing under them and 4 oceanic plates are effectively decoupled from the asthenosphere and are driven largely by slab pull this mechanism accounts for the opening of the atlantic the ability of spreading ridges to stay on the midlines of oceans and the penetration of india into asia following their collision continental motions strongly imply lower mantle upwelling and divergence beneath the atlantic and southeast indian oceans and convergence and subsidence along the tethyan belt this picture disagrees with the concept derived from hot spot studies of an undeforming absolute reference framework at depth but weaknesses in current hot spot theory would make a rejection of the present model on this ground premature if the present model is generally correct a fairly simple pattern of lower mantle convection with as few as four cells may explain much of the tectonic complexity of the earth

an early miocene ridge crest trench collision on the south scotia ridge near 36 w abstract preliminary interpretation of marine geophysical data and geochemical analyses of dredged rocks has led to the identification of a ridge crest trench collision zone southeast of the south orkney islands the existence of a series of such collision zones along the south scotia ridge had earlier been hypothesised to explain changes in scotia sea evolution the collision zone takes the form of a double ridge with dissecting trough aligned ne sw and about 200 km long oceanic magnetic lineations in the northern weddell sea young northwestward towards the zone with the youngest identified as anomaly 6 20 ma reaching the central trough volcanics dredged from jane bank the northwestern ridge are chemically very similar to the more siliceous of the low k tholeiite series which characterises the presently active south sandwich arc jane bank is interpreted as an in situ remnant of the arc and upper fore arc of an intra oceanic arc produced by subduction of south american oceanic lithosphere like the present south sandwich islands part of the upper fore arc and all of any accretionary wedge appear to have been subducted unlike ocean floor at collision zones off chile and the antarctic peninsula the seabed in the trough and on the ridge to the southeast does not conform to the normal oceanic age depth relationship the estimated collision time 20 ma is close to the time of onset of n s extension in the central scotia sea the limited data set described here however does not establish the synchroneity of the collision over the entire 200 km of jane bank so an assessment of the extent of any causal relationship between the two events must await the analysis of a more extensive survey

distribution of vertical velocity inferred from secondary flow in a curved tidal channel high resolution observations from the curved tidal channel of the otago harbour shows secondary flows up to 20 of the primary flow and vertical velocity inferred from secondary flow up to 1 of the primary flow this vertical velocity is inferred on a much finer scale than previous works the spatial pattern of this vertical velocity is upward on the inside and downward on the outside of the bend consistent with previous laboratory flume measurements linear regression rp 0 95 shows the cross channel distribution of the observed secondary flow can be resolved from the horizontal adcp measurements well enough to be consistent with the cross channel distribution of secondary flow derived from the observed primary flow using the model of kalkwijk and booij 1986 linear regression rp 0 80 shows the vertical velocity inferred from observed secondary flow is consistent with vertical velocity derived from the observed primary flow using the model of kalkwijk and booij 1986 this also shows the cross channel distribution of the observed secondary flow is resolved well enough from the horizontal adcp measurements to be able to infer the vertical velocity from mass continuity the required horizontal resolution is made possible by radial basis function rbf smoothing and spatial interpolation that allows for continuity of the spatial derivatives modeling trajectories using channel dimensions and velocity field values equivalent to the observations show that 3 dimensional secondary circulation forms a loose helical flow pattern

shear wave anisotropy in the mariana subduction zone to determine the location strength and orientation of seismic anisotropy in the mariana subduction zone beneath guam we evaluated shear wave splitting in local s regional s and scs and teleseismic core phases such as sks recorded at station gumo fast directions from the local s phases have an average azimuth of 45 and splitting times range from 0 1 s to 0 4 s for local s phases from events within the southeastern half of the subducting slab splitting parameters manifest minimal frequency dependence in both fast direction and splitting time however for the remaining local s phases in the data set fast directions vary with frequency content no well constrained splitting parameters were obtained for the regional and teleseismic phases but the particle motions of these unsplit phases are consistent with an average anisotropic fast direction of 45 anisotropy due solely to olivine oriented by slab entrained flow in the mantle wedge would produce local s fast directions at 66 and anisotropy due solely to fossil seafloor spreading in the subducting slab would yield fast directions at 20 to 30 neither of these predictions is consistent with the observed fast directions however the observed splitting including the frequency dependent fast directions can be explained by models containing anisotropy in both the slab and wedge and possibly although not necessarily anisotropy due to recent extension in the overriding philippine sea plate

u series isotope data on lau basin glasses the role of subduction related fluids during melt generation in back arc basins new thermal ionization mass spectrometry u series disequilibrium data are presented for 24 basaltic to dacitic glasses from active spreading centres in the back arc lau basin sw pacific together with additional inductively coupled plasma mass spectrometry trace element analyses and sr nd pb isotope data valu fa ridge samples adjacent to the arc front have high u th and 230th 238u 1 implying a recent 350 ka addition of a u rich slab derived fluid the valu fa data can be combined with existing 230th 238u data for the central tonga arc to infer a fluid addition event at 50 ka the similar sources and time scales for fluid transfer beneath the valu fa ridge and beneath the arc itself suggest that the valu fa ridge is propagating into the arc front region central lau basin samples further behind the arc have lower u th and 230th 238u 1 similar to typical mid ocean ridge basalts morb within the central lau basin a water rich subduction component is seen only in samples closest to the arc and this fluid does not have the high u th composition of the fluid at valu fa melt generation in the central lau basin appears to be dominated by normal ridge type processes but the relatively low 230th 238u for these shallow ridges compared with global morb could be a consequence of increased melt productivity as a result of the elevated water contents the transition from 238u to 230th excesses within the back arc basin is not a smooth function of decreasing addition of a u rich fluid moving away from the arc front but also reflects the complex dynamics between two major mantle domains within the mantle wedge pacific beneath valu fa ridge indian beneath the central lau basin

hydrothermal plumes above the east scotia ridge an isolated high latitude back arc spreading centre we have identified first evidence for the presence of submarine hydrothermal activity along the east scotia ridge an isolated back arc spreading centre located at 55 60 s in the atlantic sector of the southern ocean using a combination of in situ optical light scattering sensor data and total dissolvable mn concentrations we demonstrate the existence of hydrothermal plumes overlying two segments of this 500 km ridge crest both segments exhibit anomalous topography and at least one segment is also underlain by an axial magma chamber seismic reflector future investigation of the fauna that inhabit these remote hydrothermal environments may provide an important missing link between the distinct biogeographical provinces delimited from previous investigation of northern atlantic versus eastern pacific vent sites

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

heat flow determinations of basement age in small oceanic basins of the southern central scotia sea results are reported from seven heat flow stations in small basins of the southern part of the central scotia sea css undertaken in order to determine basement ages the basins are small which makes magnetic anomaly based ages ambiguous and preserves basin subsidence that may have been anomalous as a result of local factors the fact that these small basins formed in a back arc setting adds additional uncertainty to depth based age estimates the results confirm that basin extension commenced in the eocene and indirectly support a relatively young back arc origin for the northern css but do not affect previously published suggestions of the age of onset of the antarctic circumpolar current

teleseismic imaging of subaxial flow at mid ocean ridges traveltime effects of anisotropic mineral texture in the mantle summary deformation of peridotite caused by mantle flow beneath an oceanic spreading centre can result in the development of seismic anisotropy traveltime anomalies and shearwave splitting will develop as seismic energy propagates through such an anisotropic region thus providing a signature of the deformation field at depth in this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle the finite strain fields of the passive upwelling model versus the buoyancy enhanced upwelling model are quite different this suggests that mineral aggregates deform differently in the two models thus developing seismic signatures that are distinguishable numerical estimates of the corresponding mineral textures are made using polycrystal theory for olivine with four operative slip systems the activation of a slip system is determined for each grain on the basis of the local critical resolved shear stress the computed grain deformation reflects a balance between stress equilibrium for the aggregate as a whole and strain continuity between neighbouring grains within the aggregate this approach enables a direct link to be made between the model flow fields and the resulting texture development given these mineral orientation distributions elastic parameters are calculated and wavefronts are propagated through the anisotropic structure traveltimes for teleseismic body waves are computed using ray theory and amplitudes are estimated for an across axis profile extending 100 km from the ridge axis relative p wave residuals of up to 1 s are predicted for the buoyant model with on axis arrivals being earliest since near vertical velocities are fastest beneath the axis on axis p wave arrivals for the passive model are half a second earlier than arrivals 60 km off axis and relative delays continue to increase slowly as distance from the ridge increases s wave splitting of almost a second is predicted for the buoyant model whereas less than a half second of splitting is determined for the passive model

modeling anisotropy and plate driven flow in the tonga subduction zone back arc the goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions the tonga subduction zone in particular can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate we calculated two dimensional mantle flow models using known tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures shear wave splitting was predicted for the anisotropy due to lattice preferred orientation lpo of olivine and orthopyroxene in the flow model finite strain fields the predicted shear wave splitting provides a good match to the fast directions parallel to the azimuth of subducting plate motion and splitting times 0 5 1 5 s observed in tonga both for models where lpo anisotropy develops everywhere above 410 km and for models where lpo anisotropy is confined to regions of relatively high stress if lpo anisotropy does develop over the entire upper 410 km of the mantle the strength of anistropy induced by a given amount of shear strain must be relatively weak 4 for shear strains of 1 5 with a maximum value of 6 for very large strains the splitting observations are comparably fit by a wide range of different viscosity models anisotropy due to melt filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting

variations in lava composition associated with flow of asthenosphere beneath southern central america existing trace element and isotopic data as well as new u series data for nicaragua and costa rica are examined in light of the recent suggestion that asthenospheric mantle is flowing northward behind the subducting nazca plate around the northwest corner of south america lavas from central nicaragua have trace element and isotopic compositions suggesting that they are derived from relatively depleted mantle with a long history of interaction with the subducting cocos plate lavas from northern and central costa rica have compositions indicating a more enriched mantle source that has been influenced little by the subducting slab these compositional traits vary irregularly between the two regions suggesting that mantle compositions and subduction influences intermingle in a complex manner beneath northern costa rica the dip of the subducting slab decreases abruptly southward and accretion of sediment begins in this same vicinity these changes in composition and physical parameters may all be induced or influenced by an influx of asthenospheric material from behind the retreating nazca plate

mantle flow in the south sandwich subduction environment from source side shear wave splitting 1 the occurrence of seismic anisotropy in the earth s upper mantle is a global phenomenon related to subcrustal deformation and flow processes the shear wave splitting analysis method has led to a global set of anisotropy maps mainly derived from receiver side analysis remote places with few seismometers deployed remain unexamined source side splitting analysis allows mapping of mantle fabrics in these regions here we investigate seismic anisotropy in the south sandwich islands subduction environment core reflected scs waves recorded at the neumayer seismograph network are corrected for well constrained receiver anisotropy and then analysed for source anisotropy sub slab mantle minerals are aligned horizontally almost parallel to the trench indicating a westward flow around the subducting slab this is consistent with a model of horizontal mantle flow due to slab rollback that was previously inferred from marine and geochemical studies in the back arc region

ridge collision slab window formation and the flux of pacific asthenosphere into the caribbean realm mantle wedge derived arc volcanism ceased in southern costa rica after ca 8 ma because of subduction of the aseismic cocos ridge beneath the central american arc and the subsequent opening of a slab window geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5 8 and 2 ma identify a source derived from the galapagos plume the presence of this source is explained by an influx of pacific upper mantle into the caribbean mantle wedge through a slab window where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted cocos ridge by using geochemical and isotopic signatures we trace this upper mantle flow beneath central america from southern costa rica northward at a rate of 40 mm yr

morphology of the transition from an axial high to a rift valley at the southeast indian ridge and the relation to variations in mantle temperature the southeast indian ridge exhibits a transition in axial morphology from an east pacific rise like axial high near 100 e to a mid atlantic ridge like rift valley near 116 e but spreads at a nearly constant rate of 74 76 mm yr assuming that the source of this transition lies in variations in mantle temperature we use shipboard gravity derived crustal thickness and ridge flank depth to estimate the variations in temperature associated with the changes in morphological style within the transitional region seabeam 2000 bathymetry shows scattered instances of highs valleys and split volcanic ridges at the axis a comparison of axial morphology to abyssal hill shapes and symmetry properties suggests that this unorganized distribution is due to the ridge axis episodically alternating between an axial valley and a volcanic ridge axial morphology can then be divided into three classes with distinct geographic borders axial highs and rifted highs are observed west of a transform fault at 102 45 e rift valleys are observed east of a transform fault at 114 e and an intermediate style morphology which alternates between a volcanic ridge and a shallow axial valley is observed between the two one segment between 107 and 108 30 e forms an exception to the geographical boundaries gravity derived crustal thickness and flank depth generally vary monotonically over the region with the exception of the segment between 107 e and 108 30 e the long wavelength variations in these properties correlate with the above morphological classification gravity derived crustal thickness varies on average 2 km between the axial high and rift valley regions the application of previous models relating crustal thickness and mantle temperature places the corresponding temperature variation at 25 c 50 c depending on the model used the average depth of ridge flanks varies by 550 m over the study area for a variation of 25 50 c thermal models of the mantle predict depth variations of 75 150 m these values are consistent with observations when the combined contributions of crustal thickness and mantle density to ridge flank depth are considered assuming airy isostasy crustal thickness variations differ at the two transitions described above a difference of 750 m in crustal thickness is observed at the rift valley intermediate style transition suggesting small variations in crustal thickness and mantle temperature drive this transition at the axial high rifted high intermediate style transition crustal thickness variations are not resolvable suggesting that this transition is controlled by threshold values of crustal thickness and mantle temperature and is perhaps related to the presence of a steady state magma chamber

motion of the scotia sea plates earthquake data from the scotia arc to early 2002 are reviewed in the light of satellite gravity and other data in order to derive a model for the motion of plates in the scotia sea region events with magnitude greater than or equal to5 which occurred on or near the boundaries of the scotia and sandwich plates and for which centroid moment tensor cmt solutions are available are examined the newer data fill some of the previous sampling gaps along the boundaries of the scotia and sandwich plates and provide tighter constraints on relative motions variations in the width of the brunhes anomaly on evenly spaced marine magnetic profiles over the east scotia ridge provide new estimates of scotia sandwich plate spreading rates since there are no stable fracture zones in the east scotia sea the mean azimuth of sea floor fabric mapped by sidescan is used to constrain the direction of spreading 18 newrate estimates and four azimuths from the east scotia ridge are combined with 68 selected earthquake slip vectors from the boundaries of the scotia sea in a least squares inversion for the best fitting set of euler poles and angular rotation rates describing the present day motions of the scotia and sandwich plates relative to south america and antarctica our preferred model tlp2003 gives poles that are similar to previous estimates except for scotia plate motion with respect to south america which is significantly different from earlier estimates predicted rates of motion also differ slightly our results are much more robust than earlier work we examine the implications of the model for motion and deformation along the various plate boundaries with particular reference to the north and south scotia ridges where rates are obtained by closure

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

no net rotation model of current plate velocities incorporating plate motion model nuvel 1 we present nnr nuvel1 a model of plate velocities relative to the unique reference frame defined by requiring no net rotation of the lithosphere while constraining relative plate velocities to equal those in global plate motion model nuvel 1 demets et al 1990 differences between nnr nuvel1 and no net rotation plate motion model am0 2 minster and jordan 1978 are as large as 15 mm yr in nnr nuvel1 the pacific plate rotates in a right handed sense relative to the no net rotation reference frame at 0 67 m y about 63 s 107 e this rotation nearly parallels but is 32 slower than the pacific hotspot euler vector of hs2 nuvel1 which is a global model of plate hotspot velocities constrained to consistency with nuvel 1 gripp and gordon 1990 at hawaii the pacific plate moves relative to the no net rotation reference frame at 70 mm yr which is 25 mm yr slower than the pacific plate moves relative to the hotspots differences between nnr nuvel1 and hs2 nuvel1 are described completely by a right handed rotation of 0 33 m y about 49 s 65 e the 99 confidence ellipsoids for plate hotspot motion in model hs2 nuvel1 exclude the corresponding euler vectors from model nnr nuvell thus the no net rotation reference frame differs significantly from the hotspot reference frame if the difference between reference frames is caused by motion of the hotspots relative to a mean mantle reference frame then hotspots beneath the pacific plate move with coherent motion towards the east southeast alternatively the difference between reference frames may show that the uniform drag no net torque reference frame which is kinematically equivalent to the no net rotation reference frame is based on a dynamically incorrect premise possible exceptions to the assumption of uniform drag include a net torque on the lithosphere due to attached subducting slabs and greater resistance to plate motion beneath continents than beneath oceans

opposing trends in crustal thickness and spreading rate along the back arc eastern lau spreading center implications for controls on ridge morphology faulting and hydrothermal activity abstract along back arc ridges located near the arc volcanic front crustal thickness is observed to vary systematically with arc proximity independent of spreading rate this effect is especially well expressed by the 400 km long eastern lau spreading center elsc where approaching the arc volcanic front from 100 to 40 km crustal thickness nearly doubles 5 5 9 km as spreading rates decrease by more than half 97 39 mm yr the crustal thickness variations at the elsc appear to result primarily from changes in mantle wedge composition caused by subduction we investigated the effects of these large and opposed crustal thickness and spreading rate variations on ridge morphology faulting and hydrothermal activity as part of the first phase of ridge2000 integrated studies in the lau back arc basin we used deep towed side scan sonar instruments dsl120a and imi30 to continuously map the near axis region within broader coverage ship multibeam bathymetry and acoustic imagery swaths an array of optical sensors maprs concurrently surveyed the near bottom water for hydrothermal plume anomalies hydrocasts made at identified plume sites confirmed their hydrothermal origin the data show that as spreading rates increase and crustal thicknesses decrease along the elsc 1 morphology transitions from shallow peaked volcanic highs to a deeper flat axis 2 faults become larger and more widely spaced 3 hydrothermal activity as measured by plume incidence increases and appears to reach levels higher than the global mid ocean ridge trend with spreading rate the observations indicate that crustal thickness magma supply has a greater control on ridge morphology and faulting than spreading rate even at fast rates where the thermal lithosphere should be thin however heat input from mantle advection proportional to spreading rate perhaps in combination with increased fault permeability appears to have a greater control on hydrothermal activity than crustal thickness and the magmatic robustness of the ridge although subduction effects give rise to the opposed trends in crustal thickness and spreading rate at the elsc the observed effects may have generic implications for controls of seafloor spreading characteristics at mid ocean ridges

group velocity tomography in the subantarctic scotia sea region more than 150 events recently recorded by seven seismic broadband stations ogs iaa iris gsett3 idc have been collected and processed to obtain an overview of the crust and upper mantle shear wave velocities group velocities of the fundamental mode rayleigh and love waves in the period range from 15 s to 50 s are used to obtain tomographic maps of the scotia sea region the tip of antarctic peninsula and the tip of south america errors in the measurements estimated on clusters are larger for love waves than for rayleigh waves and their averages are 0 060 0 080 km s and 0 030 0 040 km s respectively from the regionalisation of the dispersion measurements we obtain smoothed local dispersion curves in correspondence with the main geological and tectonic features and from their nonlinear inversion the shear wave velocity versus depth profiles the correlation length of the heterogeneity which can be resolved by rayleigh waves varies between 200 and 400 km in most parts of the studied area but becomes greater near the periphery of the maps the spatial resolution of love waves 400 600 km is poorer than that of rayleigh waves due to the deteriorated path coverage and to the larger errors in the group velocity measurements models of the shear wave velocity in the crust and upper mantle for the tip of south america the falkland plateau the scotia sea the south sandwich oceanic spreading ridge the south sandwich trench the south scotia ridge the tip of antarctic peninsula the bransfield strait and the drake passage are presented our regional models and the existing large scale models e g crust5 1 help to define a 3 d velocity model of the scotia sea region to be further investigated by waveform inversion

sediment subduction subduction erosion and strain regime in the northern south sandwich forearc 1 the first swath bathymetry and side scan sonar imagery from the south sandwich forearc reveals detailed seafloor morphology tectonic fabric and sedimentary features in an area where plate convergence is approximately normal to the trench simultaneously collected seismic reflection gravity and magnetic data provide information about the structure and composition of the forearc crust interpretation of these data together with the marine magnetic record of seafloor spreading in the east scotia sea back arc basin since 15 ma has enabled quantitative estimation of sediment subduction and subduction erosion rates any accretionary prism present is constrained to be very small extending 6 km or less from the trench the contrast between this result and a time integrated estimate of potential accretionary prism volume suggests that 95 of the sediment which has entered the trench since 15 ma has been subducted the position of the south sandwich island arc in relation to back arc magnetic lineations implies that the arc has migrated 70 km westward relative to the sandwich plate since 15 ma taking account of published observations concerning the typical geometric evolution of arc trench systems the average rate of forearc slope retreat during this interval is inferred to be 3 1 4 7 km myr 1 gravity modeling suggests that the forearc crust averages 10 km in thickness within 110 km of the trench for 10 km thick forearc crust the rate of slope retreat implies an average rate of subduction erosion of forearc crust of 31 47 km3 km 1 myr 1 at the present convergence rate of 74 km myr 1 this erosion rate requires a 420 635 m thick layer of subducting material derived from the forearc crust a time integrated estimate of the potential volume of forearc derived trench fill sediments subducted since 15 ma implies that frontal erosion can only account for 40 of total subduction erosion therefore basal erosion must significantly exceed frontal erosion the new data also provide insights concerning forearc strain regime and forearc basin evolution in the area studied extensional faulting near the trench slope break is probably caused by gravitational instability of the steep lower forearc slope a lack of extensional strain indicators in the upper forearc and arc may be a consequence of the fact that the east scotia sea is a mature back arc basin contributing significant ridge push in the vicinity of the arc the data reveal no evidence of arc parallel extension or of large serpentinite seamounts as found in the mariana forearc and a causal link between these two observations is proposed seismic profiles across the forearc basin suggest that the balance between sediment accumulation and erosion is sensitive to changes in the elevation of the trench slope break it is hypothesized that the basin is a dynamic feature which goes through repeated cycles of growth and destruction controlled by cyclic uplift and collapse of the trench slope break

the influence of plate motions on three dimensional back arc mantle flow and shear wave splitting both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western pacific subduction zones the goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting we calculated three dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow induced strain predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain a parameter study shows that both finite strain and textural anisotropy developed within three dimensional plate coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere flow models incorporating plate motion boundary conditions for the tonga southern kuril and eastern aleutian subduction zones produce splitting parameters consistent with observations from each region trench parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge

volcanic evolution of the south sandwich volcanic arc south atlantic from multibeam bathymetry abstract new multibeam bathymetry data are presented for the south sandwich intra oceanic arc which occupies the small sandwich plate in the south atlantic and is widely considered to be a simple end member in the range of intra oceanic arc types the images show for the first time the distribution of submarine volcanic tectonic and erosional depositional features along the whole length of the 540 km long volcanic arc allowing systematic investigation of along arc variations the data confirm that the volcanic arc has a simple structure composed of large volcanoes which form a well defined volcanic front but with three parallel cross cutting seamount chains extending 38 60 km from near the volcanic front into the rear arc there is no evidence for intra arc rifting or extinct volcanic lines topographic evidence for faulting is generally absent except near the northern and southern plate boundaries most of the volcanic arc appears to be built on ocean crust formed at the associated back arc spreading centre as previously proposed from magnetic data but the southern part of the arc appears to be underlain by older arc or continental crust whose west facing rifted margin facing the back arc basin is defined by the new bathymetry the new survey shows nine main volcanic edifices along the volcanic front and ca 20 main seamounts the main volcanoes form largely glaciated islands with summits 3 0 3 5 km above base levels which are 2500 3000 m deep in the north and shallower at 2000 2500 m deep in the south some of the component seamounts are interpreted to have been active since the last glacial maximum and so are approximately contemporaneous with the volcanic front volcanism seven calderas all either submarine or ice filled have been identified adventure volcano a newly discovered submarine volcanic front caldera volcano is described for the first time all but one of the calderas are situated on summits of large volcanoes in the southern part of the arc and most are associated with current or historic volcanic or hydrothermal activity shallow shelves around the islands are generally 1 10 km wide submerged banks up to 1100 m deep are interpreted as subsided erosional surfaces seamounts and emergent volcanoes experienced a range of mass wasting processes including by landsliding and smaller mass flows

mantle anisotropy beneath northwest pacific subduction zones to assess the location strength and orientation of seismic anisotropy in the southern kuril japan and izu bonin subduction zones we analyzed shear wave splitting in local s phases and teleseismic sks phases fast directions from these phases are roughly parallel to the direction of absolute pacific plate motion wnw in izu bonin roughly parallel to the strike of the trench nne in central honshu and roughly n in the southern kuril back arc in the vicinity of sakhalin island assuming that the orientation and strength of anisotropy does not vary with depth modeling of splitting times from local s and teleseismic phases recorded at sakhalin island requires that the maximum depth of anisotropy lies between 480 km and 950 km in contrast splitting times for local s phases from events in izu bonin rule out anisotropy in the transition zone all the data are consistent with a model in which the lower transition zone 520 660 km and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone 410 520 km possibly due to preferred orientation of spinel assuming that splitting in the upper mantle is produced by preferred orientation of olivine the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones the relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow but that flow correlated with upper plate deformation must also exist

seafloor spreading in the weddell sea and southwest atlantic since the late cretaceous geosat radar altimetry defines a pattern of closely spaced curvilinear gravity anomalies in the northern weddell sea and adjacent parts of the southwest atlantic these anomalies are caused by a fracture zone pattern resulting from seafloor spreading mainly on the southern flank of a ridge system which now exists only to the east of the south sandwich trench flowlines derived from these anomalies are combined with isochrons resulting from an interpretation of all available magnetic anomaly profiles in the region to produce a new tectonic summary chart this chart reveals major changes in spreading direction during i the cretaceous magnetic superchron ii the latest cretaceous paleocene 65 ma and iii the mid eocene 50 ma the latter two changes enclose a period of very slow spreading and define a kink in the flowlines which is discernable throughout most of the weddell sea preliminary modelling indicates that trends from the western weddell sea to near the south atlantic triple junction can be explained by the separation of just two plates there is some disagreement between the observed flowlines and those predicted by rotations published for the south atlantic and southwest indian ridge but abrupt changes in spreading direction and a slowing of rates are observed on all three branches of the south america africa antarctica plate system between the times of anomalies c31 and c21 this suggests that the lack of closure results from errors in the calculated rotations and that the seafloor of the weddell sea and southwest atlantic was created as a direct consequence of south america antarctica plate motion the data do not support the existence of a late cretaceous malvinas plate within the weddell sea although microplates in the northernmost weddell sea do appear to have formed briefly as a result of interactions between the former spreading ridge and a subduction zone located at the south scotia ridge

a laboratory model of subduction zone anisotropy abstract measurements of split seismic shear waves near pacific subduction zones show that fast polarization directions are primarily trench parallel in the seaward side mantle beneath a subducting slab and variable in the mantle wedge beneath back arc basins it is generally accepted that the azimuth of the fast polarization direction defines the average orientation of the crystallographic a axis in olivine here we use laboratory experiments to determine the orientation of the olivine a axis in the upper mantle in the vicinity of a subducting slab a laboratory analog for olivine a axis motion during creep deformation is developed using small cylinders whiskers suspended in a viscous fluid subduction beneath a stationary overriding plate variable dip angles and realistic rollback and down dip plate kinematics are examined plate rollback produces strong plate parallel whisker alignments for all dip angles examined alignment in the wedge occurs in three layers a layer of dip parallel alignment above the downgoing plate a layer of subhorizontal alignment beneath the overriding plate and a core of intermediate orientations the amount of alignment beneath the overriding plate and the wedge core decreases with increasing dip angle our results indicate that olivine a axis orientation in the seaward side mantle is controlled by the amount of slab rollback and that orientations in the mantle wedge depend upon slab dip angle

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

enhanced magma supply at the southern east scotia ridge evidence for mantle flow around the subducting slab abstract bathymetric and seismic data show that the southernmost segment of the east scotia ridge segment e9 is anomalous in its curved plan form and in the presence of a large axial volcanic ridge avr spreading commenced only within the past 1 million years on this segment which appears to have propagated both northward and southward the presence of a caldera at the summit of the avr indicates that a shallow magma chamber of limited extent existed recently beneath the ridge eruption from this magma chamber probably within the past 0 1 million years may have led to the formation of the avr itself careful examination of seismic reflection profiles suggests that small pockets of magma may still exist beneath the avr magmatism on segment e9 may be enhanced as a result of the flow of shallow mantle around the southern end of the subducting slab beneath the south sandwich arc in a manner similar to that proposed for the northern end this mantle probably carries a slightly enriched hotspot signature and is affected by volatiles released from the edge of the slab both tending to increase the supply of magma to the back arc spreading centre

crustal and upper mantle structure of southernmost south america inferred from regional waveform inversion 1 we determine the crustal and upper mantle structure of southern south america by inverting regional seismograms recorded by the seismic experiment in patagonia and antarctica we present a waveform inversion method that utilizes a niching genetic algorithm the niching genetic algorithm differs from the traditional genetic algorithm in that the inversion is performed in multiple subpopulations thus generating a broader search of the model space and enabling us to examine alternative local error minima the vertical and transverse waveforms were used extending from the p arrival to the surface waves and the inversion was performed at either 0 005 0 06 hz larger events or 0 02 0 06 hz smaller events the inversion included anisotropy by solving for separate sv and sh structures in the upper mantle results indicate that crustal thickness varies from 26 to 36 km with thicker values toward the northeast suggesting that there is little crustal thickening beneath the austral andes the average upper mantle velocities are similar to the preliminary reference earth model prem except that the southernmost region shows velocities of 5 slower than prem the upper mantle has up to 5 polarization anisotropy between the moho and 120 km depth the strongest anisotropy is localized in a lithospheric lid shallower than 65 km depth which overlies a pronounced low velocity zone this shallow limit to anisotropy is consistent with the relatively small shear wave splitting values found in this region these results suggest that the anisotropy is limited to lithospheric depths and may imply the absence of a strong mantle flow pattern in the asthenosphere

slice of intraoceanic arc insights from the first multichannel seismic reflection profile across the south sandwich island arc we present the first multichannel seismic mcs reflection line that crosses the south sandwich trench south sandwich island arc and east scotia sea backarc basin the line is used in combination with earthquake catalogue data to interpret the strain distribution across the sandwich plate and the relationship of forearc structures to processes operating at the trench the mcs data reveal a 1 2 km high fault scarp associated with a 20 km wide arcward tilted block in the mid forearc these features indicate large scale gravitational collapse and earthquake data are consistent with trench normal extension at shallow depth in this area there is however little evidence of distributed extension within the interior of the sandwich plate the mcs data show a small frontal wedge that achieves its maximum thickness only 18 km from the trench backarc magnetic data mid forearc extension and the small size of the frontal wedge are all consistent with long term and ongoing subduction erosion earthquake data suggest that this erosion is taking place in an environment of low interplate stress

seismic anisotropy from local earthquakes in the transition region from a subduction to a strike slip plate boundary new zealand shear wave splitting is used to investigate anisotropy in the crust and upper mantle in a subduction zone lower half of the north island of new zealand and its transition to oblique transform faulting marlborough area northern south island in marlborough delay times show almost no increase with depth and it is most likely that the higher frequency phases used in this study respond mainly to lithospheric anisotropy in the central marlborough fault system mfs fast polarizations are subparallel to the faults anisotropy is attributed to the presence of metamorphosed schist eclogite of 30 10 km thickness and located 50 80 km beneath the mfs on the edges of the mfs fast polarizations are parallel to the maximum compressive stress direction consistent with crack induced anisotropy in the crust the shear zone which is as wide as the island in the mantle as inferred from sks phases seems to occur in a narrower zone in the crust in the lower half of the north island fast polarizations from events at all depths are oriented parallel to the strike of the hikurangi subduction zone as well as to the faults polarisations are similar to those of sks phases which mainly sample the mantle this suggests that the lithosphere and the upper mantle asthenosphere deform in a coherent strike slip shear we calculate 1 2 0 3 velocity anisotropy in the first 200 km of the mantle from increasing delay times with depth in order to match the sks delay times this result requires the presence of anisotropic material down to 580 100 km depth or a change in anisotropy with depth or frequency dependent splitting

back arc basin basalt systematics the mariana east scotia lau and manus back arc basins babs have spreading rates that vary from slow 100 mm yr and extension axes located from 10 to 400 km behind their island arcs axial lava compositions from these babs indicate melting of mid ocean ridge basalt morb like sources in proportion to the amount added of previously depleted water rich arc like components the arc like end members are characterized by low na ti and fe and by high h2o and ba la the morb like end members have the opposite traits comparisons between basins show that the least hydrous compositions follow global morb systematics and an inverse correlation between na8 and fe8 this is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures lau manus hotter than mariana scotia this interpretation accords with numerical model predictions that faster subduction induced advection will maintain a hotter mantle wedge the primary compositional trends within each bab a positive correlation between fe8 na8 and ti8 and their inverse correlation with h2o 8 and ba la are controlled by variations in water content melt extraction and enrichments imposed by slab and mantle wedge processes systematic axial depth as a proxy for crustal production variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate hydrous fluxing enhances decompression melting allowing depleted mantle sources just behind the island arc to melt extensively producing shallow spreading axes flow of enriched mantle components around the ends of slabs may augment this process in transform bounded back arcs such as the east scotia basin the re circulation by mantle wedge corner flow to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the east lau spreading center manus southern rifts and mariana trough and the very depleted lavas of east scotia segments e8 e9 the crust becomes mid ocean ridge mor like where the spreading axes further away from the island arc and subducted slab entrain dominantly fertile mantle

mantle wedge control on back arc crustal accretion at mid ocean ridges plate separation leads to upward advection and pressure release partial melting of fertile mantle material the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away in back arc basins the subducting slab is an important control on the pattern of mantle advection and melt extraction as well as on compositional and fluid gradients modelling studies predict significant mantle wedge effects on back arc spreading processes here we show that various spreading centres in the lau back arc basin exhibit enhanced diminished or normal magma supply which correlates with distance from the arc volcanic front but not with spreading rate to explain this correlation we propose that depleted upper mantle material generated by melt extraction in the mantle wedge is overturned and re introduced beneath the back arc basin by subduction induced corner flow the spreading centres experience enhanced melt delivery near the volcanic front diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away our model explains fundamental differences in crustal accretion variables between back arc and mid ocean settings