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

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

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

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

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

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

methane driven late pleistocene δ13c minima and overflow reversals in the southwestern greenland sea a core transect across the southwestern greenland sea reveals coeval events of extremely negative planktic and benthic 1 3 c excursions between 40 and 87 ka the most pronounced event event 1 began at peak dansgaard oeschger stadial 22 85 ka with a duration of 18 k y during this episode incursions of atlantic intermediate water caused a bottom water warming of up to 8 c the amplitude timing and geographic pattern of the 1 3 c events suggest that this bottom water warming triggered clathrate instability along the east greenland slope and a methane induced depletion of 1 3 c d i c dic dissolved inorganic carbon since 1 3 c event 1 matches a major peak in atmospheric ch 4 concentration this clathrate destabilization may have contributed to the rise in atmospheric ch 4 and thus to climate warming over marine isotope stage 5 1

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

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

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

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

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

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

an early proterozoic three stage rift system kaapvaal craton south africa abstract a three stage model that constitutes a fully evolved rift system is proposed for a number of superimposed early proterozoic basins on the kaapvaal craton based on interpreted depositional systems and syndepositional tectonics this model consists of three superimposed basins 1 1 a pre graben stage that influenced the dominion group witwatersrand supergroup and klipriviersberg group of the ventersdorp supergroup 2 2 graben development that superimposed the platberg group bothaville and allanridge formations of the ventersdorp supergroup 3 3 post graben thermal subsidence that superimposed the black reef quartzite formation and the chuniespoort ghaap group of the transvaal supergroup oblique collision of the kaapvaal craton and zimbabwe craton is interpreted to have generated stresses that initiated the three stage system at different time intervals compressional deviatoric stress appears to have replaced tensional stress and kept the system in operation tectonically the system is analogous to the baikal rift system and the pannonian basin system while the general configuration of the superimposed basins is comparable to the north sea basin or the north china basin

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

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

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

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

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