origin and emplacement of mariana forearc seamounts large seamounts occur on the outer half of the mariana forearc they represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material the degree of metamorphism in this material depends on the amount of water available and the pressure temperature regime of the forearc wedge the major source for the water involved in the metamorphism is most likely the descending slab theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the mariana forearc thermal models indicate that higher grade greenschist facies should occur farther from the trench seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the mariana forearc

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

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

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

relevance of silicon isotopes to si nutrient utilization and si source assessment in antarctic waters 1 we analyzed 29si of dissolved silicate for eight water column profiles across the southern ocean south of australia in spring 2001 from the seasonal ice zone siz north to the subantarctic zone saz including the first isotopic compositions measured for si depleted seawaters all profiles display mixed layer enrichments in heavy si isotopes relative to deep water in accordance with preferential uptake of the light isotope by diatoms as silicate levels decrease from the siz northward across the polar front zone pfz to the saz surface and mesopelagic 29si signatures generally become progressively heavier but the most si depleted saz waters do not exhibit 29si values heavier than in the pfz this intricacy appears to derive from variations in the vertical and horizontal supply of silicate to surface waters and by applying a steady state open system model we estimate a fractionation factor 29 between diatoms and seawater of 0 45 0 17 independently of zones and phytoplankton community though encouraging these results are related to latitudinal changes in mesopelagic 29si values complexity in surface silicate 29si correlations and differences from previous studies which underline the need for caution in the use of silicon isotopes in paleoceanographic studies until systematic efforts have been undertaken to better understand modern variations

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

southern ocean control of silicon stable isotope distribution in the deep atlantic ocean the fractionation of silicon si stable isotopes by biological activity in the surface ocean makes the stable isotope composition of silicon delta si 30 dissolved in seawater a sensitive tracer of the oceanic biogeochemical si cycle we present a high precision dataset that characterizes the delta si 30 distribution in the deep atlantic ocean from denmark strait to drake passage documenting strong meridional and smaller but resolvable vertical delta si 30 gradients we show that these gradients are related to the two sources of deep and bottom waters in the atlantic ocean waters of north atlantic and nordic origin carry a high delta 30 sisignature of 1 7 parts per thousand into the deep atlantic while antarctic bottom water transports si with a low delta si 30 value of around 1 2 parts per thousand the deep atlantic delta si 30 distribution is thus governed by the quasi conservative mixing of si from these two isotopically distinct sources this disparity in si isotope composition between the north atlantic and southern ocean is in marked contrast to the homogeneity of the stable nitrogen isotope composition of deep ocean nitrate delta n 15 no3 we infer that the meridional delta si 30 gradient derives from the transport of the high delta si 30 signature of southern ocean intermediate mode waters into the north atlantic by the upper return path of the meridional overturning circulation moc the basin scale deep atlantic delta si 30 gradient thus owes its existence to the interaction of the physical circulation with biological nutrient uptake at high southern latitudes which fractionates si isotopes between the abyssal and intermediate mode waters formed in the southern ocean

silicon isotope fractionation during nutrient utilization in the north pacific the distribution of silicon in the north pacific is controlled by the utilization of silicic acid by diatoms a process that fractionates silicon si isotopic compositions silicon isotope variations are presented for six water column profiles from the surface mixed layer down to the deep waters of the north pacific ocean although the observed si isotopic variations display an apparently simple inverse relationship with dissolved nutrient concentrations in fact they reflect mixing of surface waters undergoing active si isotope fractionation and deep waters with more uniform concentrations and isotope compositions samples from the surface of the subtropical gyre have the lowest dissolved si concentrations and heaviest si isotope compositions of marine waters measured thus far fractionation in the surface waters follows a typical rayleigh type distillation curve for a closed surface water reservoir resulting from stratification of the surface layer in the subarctic region in contrast an open system prevails within the subtropical gyre where there is significant recycling of silicic acid in the upper water column and lateral transport of silicon within surface currents for deep waters the si isotope composition distinguishes between the northern north pacific deep water npdw and the southerly derived bottom water the relatively light si isotope compositions measured from waters within the subarctic gyre provides evidence for isolation of the nutrient pool in the north pacific

factors controlling the silicon isotope distribution in waters and surface sediments of the peruvian coastal upwelling we present the first systematic study of the silicon isotope composition in the water column 30sisi oh 4 and in diatoms 30sidiatom from the underlying surface sediments in a coastal upwelling region the surface waters upwelling on the shelf off peru are mainly fed by southward flowing subsurface waters along the coast which show a mean 30sisi oh 4 of 1 5 the concentration of dissolved silicic acid si oh 4 increases towards the south in these waters and with increasing water depth suggesting lateral mixing with water masses from the south and intense remineralisation of particulate biogenic silica bsio2 in the water column and in the surface sediments surface waters in the realm of the most intense upwelling between 5 s and 15 s have only marginally elevated 30sisi oh 4 values 30sisi oh 4 1 7 with respect to the source si isotope composition whereas further north and south where upwelling is less pronounced surface waters are more strongly fractionated 30sisi oh 4 up to 2 8 due to the stronger utilisation of the smaller amounts of available si oh 4 the degree of si oh 4 utilisation in the surface waters along the shelf estimated from the si oh 4 concentration data ranges from 51 to 93 the 30sidiatom values of hand picked diatoms in the underlying surface sediments vary from 0 6 to 2 0 which is within the range of the expected fractionation between surface waters and diatoms the fractionation signal in the surface waters produced during formation of the diatoms is reflected by the 30sidiatom values in the underlying sediments with the lowest 30sidiatom values in the main upwelling region the silicon isotope compositions of bsio2 30sibsio2 from the same surface sediment samples are generally much lower than the 30sidiatom signatures indicating a significant contamination of the bsio2 with biogenic siliceous material other than diatoms such as sponge spicules this shift towards lighter 30sibsio2 values by up to 1 3 compared to 30sidiatom signatures for the same surface sediment samples potentially biases the interpretation of 30si paleorecords from sediments with low bsio2 concentrations and thus the reconstruction of past si oh 4 utilisation in surface waters

forearc structures and tectonic regimes at the oblique subduction zone between the hikurangi plateau and the southern kermadec margin the convergent southern kermadec margin is examined to study the structural development and mechanical parameters of the inner trench slope in response to the oblique subduction of the oceanic hikurangi plateau swath bathymetric data have been combined with other geophysical data to reveal that the south kermadec inner trench wall is segmented into distinct upper middle and lower margins the lower margin is a nongrowing compressionally unstable accretionary wedge that is dissected by transcurrent faults the wedge has been thinned and narrowed as a result of tectonic erosion the middle margin is stable by comparison and separated from the lower margin by the 220 km long awanui fault this fault is a landward dipping strike slip reverse fault the middle upper margin including east cape ridge ecr forms a deforming backstop with a sharp along strike change in tectonic regime south of 36 40 s transpression characterizes the ecr shear zone north of 36 40 s the ecr is marked by uplift by extension and farther north by subsidence it is suggested that subsidence and subsequent tectonic erosion of the margin north of 35 50 s in the wake of hikurangi plateau subduction have broken the link between once continuous segments of the ecr tonga platform the unstable nature of the southern kermadec wedge under compression together with a comparison of its taper characteristics with those of modern convergent margins implies that the wedge deforms under a relatively high internal pore pressure but high effective basal friction regime strike slip deformation in the wedge is attributable to a relatively strong interplate coupling related to the buoyancy and ruggedness of the hikurangi plateau

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

deep scientific dives in the japan and kuril trenches abstract in the summer of 1985 during the french japanese kaiko program ten dives to depths of 6000 m in the japan and kuril trenches were made in the newly launched submersible nautile the sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving seabeam mapping geomagnetic and gravimetric measurements and single channel seismic profiling the results of the dives provide new constraints on the geodynamics of these subduction zones in the japan and kuril trenches huge slump scars were observed on the landward slopes of the trenches slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material the slump scars allowed us to observe the internal structure of the margin the monoclinal structure on the northern japan trench margin deduced from the seismic profiles and dsdp drilling was confirmed several dives on kashima seamount confirmed that this volcano has recently been split into two parts by a normal fault system comparisons of lithology and paleontology on the two separated parts of the seamount were made deep sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches it can be concluded that the whole margin is venting fluids from depths of 2 3 km which is consistent with the indications of overpressure observed in drill sites on the japan trench margin the fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor

geophysical constraints on washington convergent margin structure gravity and magnetic maps of western washington reveal the lateral structure and fabric of the washington coast range puget basin and southern washington cascade range the magnetic and gravity maps show large amplitude positive anomalies associated with the shallow but largely buried section of washington coast range mafic rocks which are separated by negative anomalies over deep sedimentary basins the positive anomalies indicate that the coast range mafic basement extends farther east than previously thought at least as far east as the longitude of seattle linear and steep gravity and magnetic gradients indicate many unmapped often buried faults in the washington coast range province magnetic highs are also associated with mapped batholiths in the cascade arc several magnetic highs observed east of the coast range rocks and west of these batholiths may be associated with buried tertiary plutons or ophiolites two dimensional gravity and magnetic modeling constrained with geological and other geophysical data indicate that the coast range province rocks are about 1 km thick at the coast thickening to as much as 30 km near their postulated eastern edge a maximum boundary on the average density of the upper 15 20 km of the rocks that compose the coast range province of 2920 kg m3 was established by the modeling suggesting a composition largely of basalt and gabbro with little interbedded sediments under these rocks may be mantle or a subduction complex composed of dense mafic ultramafic and sedimentary rocks like that proposed to underlie vancouver island previous gravity models of the washington margin include lower densities for the proposed subduction complex than for vancouver island suggesting a lower component of mafic and ultramafic rocks than the rocks underlying vancouver island however my washington model requires that the proposed subduction complex be more dense than the trench sediments and therefore that material denser than sediments be incorporated within it the absence of continental mantle and the modeled wedge shape of the coast range province upper crust suggest that erosion of the bottom of the overriding plate by subduction processes may have occurred

episodic development of a convergent margin inferred from structures and processes along the southern chile margin seismic reflection data acquired in the vicinity of isla mocha across the southern coast of chile image structures formed along the continental margin and reveal an episodic history of accretion nonaccretion and possibly erosion structures formed at the toe of the continental slope suggest frontal accretion of to 1 km of trench fill seismic images also reveal that a small accretionary wedge 20 30 km wide abuts the truncated continental metamorphic basement that extends seaward from beneath the shelf the small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style as current rates of frontal accretion could have accumulated all of the existing accretionary wedge in less than 1 2 m y this is a small fraction of convergence history along this margin and the current accretionary mode has not been consistently maintained in the past the isla mocha region is located between the temperate climate of central chile and the glacial climate of southern chile and climatic conditions in this region have likely fluctuated sufficiently to cause significant variation in trench sediment supply accretionary and nonaccretionary or erosional episodes are probably linked to temporal variations in trench sediment thickness as suggested by observations along the chile margin currently thick trench sediment correlates with accretion along the southern chile margin and thin trench sediment correlates with nonaccretion tectonic erosion as near the chile ridge and from the juan fernandez ridge to northern chile the isla mocha region also lies 900 1000 km north of the chile triple junction and the chile ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near isla mocha this part of the precollision zone provides an excellent reference to examine the effects of chile ridge collision in the development of the chile margin the most apparent effect of subduction of the buoyant young crust of the chile ridge is a shallow trench that is nearly devoid of sediment consequently the triple junction is undergoing nonaccretion or erosion and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypassed the triple junction region the interplay of subduction zone processes such as trench sedimentation and ridge collision has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge

what controls silicon isotope fractionation during dissolution of diatom opal abstract the silicon isotope composition of opal frustules from photosynthesising diatoms is a promising tool for studying past changes in the marine silicon cycle and indirectly that of carbon dissolution of this opal may be accompanied by silicon isotope fractionation that could disturb the pristine silicon isotope composition of diatom opal acquired in the surface ocean it has previously been shown that dissolution of fresh and sediment trap diatom opal in seawater does fractionate silicon isotopes however as the mechanism of silicon isotope fractionation remained elusive it is uncertain whether opal dissolution in general is associated with silicon isotope fractionation considering that opal chemistry and surface properties are spatially and temporally i e opal of different age diverse in this study we dissolved sediment core diatom opal in 5 mm naoh and found that this process is not associated with significant silicon isotope fractionation since no variability of the isotope effect was observed over a wide range of dissolution rates we can rule out the suggestion that back reactions had a significant influence on the net isotope effect similarly we did not observe an impact of temperature specific surface area or degree of undersaturation on silicon isotope partitioning during dissolution such that these can most likely also be ruled out as controlling factors we discuss the potential impacts of the chemical composition of the dissolution medium and age of diatom opal on silicon isotope fractionation during dissolution it appears most likely that the controlling mechanism of silicon isotope fractionation during dissolution is related to the reactivity or potentially aluminium content of the opal such a dependency would imply that silicon isotope fractionation during dissolution of diatom opal is spatially and temporally variable however since the isotope effects during dissolution are small the silicon isotope composition of diatom opal appears to be robust against dissolution in the deep sea sedimentary environment

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

structural styles of an accretionary wedge south of the island of sumba indonesia revealed by seamarc ii side scan sonar the accretionary wedge south of sumba island indonesia is in the early stages of continent island arc collision australian continental shelf sediments are being actively accreted to the sunda arc at the timor trough we have correlated seamarc ii side scan images and bathymetry with 3 5 khz and seismic reflection profiles to construct a structural interpretation of this zone deformation is concentrated on the lower slope of the accretionary wedge within 15 to 25 km of the thrust front above which the strain rate appears to decrease abruptly three distinct structural styles are developed in the survey area the characteristics of the easternmost region are 1 deformation in a fold and thrust geometry 2 presence of mud volcanoes and mud ridges parallel to the thrust front indicating elevated fluid pressure and 3 the development of a long decollement extending 10 to 15 km from the thrust front in contrast the central region shows a more homogeneous thickening resulting mainly from closely spaced faulting seaward verging thrust faults and conjugate strike slip faults are the main structures in this segment of the accretionary wedge mud volcanoes and a very long decollement are absent in the central region we present a hypothesis in which structural style is controlled by the length of the decollement developed in front of the thrust front short thick thrust sheets may favor uniform shortening whereas long thin sheets may be unstable to buckling and deform by folding high fluid pressure may facilitate propagation of the decollement in contrast to the accretion occurring in the other two regions the western part of the accretionary wedge is in the process of being indented and redeformed by a basement ridge

a new type of active margin the convergent extensional margin as exemplified by the middle america trench off guatemala the results of dsdp legs 67 and 84 during which a transect of holes was drilled across the middle america trench off guatemala show no accretion at the front of that subduction zone since early eocene time the tectonic evolution of the trench includes extensional structure on the landward as well as the seaward trench slopes the guatemalan margin is proposed as the model of a new type of active margin the convergent extensional active margin ce margin

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

empirical reflection travel time versus depth and velocity versus depth functions for the deep sea sediment column numerous correlations of one way reflection travel times and depths to reflecting horizons have been proposed in the initial reports of the deep sea drilling project dsdp having carefully reviewed proposed correlations we selected 233 correlated depths ranging from 27 m to 1 4 km and one way travel times ranging from 0 02 to 0 7 s from 154 dsdp sites worldwide assuming that the velocity depth function for deep sea sediments can be approximated by an exponential form we have used these data to establish statistically well constrained global models relating depth to travel time and velocity to depth there is remarkably little scatter in the data based on nonlinear least squares fits the best depth time function is z km 3 03 1n 1 0 52t s the rms error in depth is 26 m the corresponding average velocity depth function is v km s 1 59exp 0 33z we estimate that the real site to site variability of velocities at depth is less than 0 20 km s any dependence of velocity on composition is below the resolution of the data and available evidence suggests that age has no significant effect these results imply that to a depth of 1 4 km the physical state of sediments depends on in situ overburden pressure and temperature which are related to depth of burial and that chemical and mechanical equilibrium is achieved in a period of time that is geologically short our results are consistent with laboratory measurements of p wave velocities in deep sea sediments but velocity depth functions for terrigenous and calcareous sections derived from sonobuoy wide angle reflection surveys are incompatible with the time and depth data and overestimate velocities by as much as 10 and 30 respectively these systematic errors may result from the fact that the marked anisotropy of deep sea sediments is neglected in the interpretation of reflection data

numerical study of the ocean circulation on the east china sea shelf and a kuroshio bottom branch northeast of taiwan in summer using the regional ocean model system the ocean circulation on the east china sea ecs shelf was examined by a fine resolution model which was nested in a coarse resolution pacific ocean model the high resolution simulation shows an accurate volume transport of 2 70 sv sv 10 6 m 3 s 1 through the tsushima strait which is more consistent with the previous 5 5 year observation value 2 64 sv than former model results for the taiwan strait it also shows a close volume transport 1 03 sv to a recent estimate 1 20 sv at the same time the model results reproduced almost all of the known circulation structure on the ecs shelf in addition the hindcast of 2009 shows a kuroshio bottom branch current to the northeast of taiwan kbbcnt the kbbcnt is confirmed by the observational bottom high salinity water from 15 august to 2 september 2009 whose distribution is also reproduced by the model results tracer and particle experiments were carried out to elucidate the formation of the high salinity water and the pathway of the kbbcnt in light of the field observation and numerical experiments a new pathway of the kbbcnt is proposed bifurcated from the subsurface water of kuroshio northeast of taiwan it upwells northwestward gradually from 300 to 60 m then turns to northeast in the region around 27 5 degrees n 122 degrees e and finally reaches 31 degrees n off the mouth of the changjiang river along similar to 60 m isobaths forming the bottom saline water off the coast of zhejiang province china

the balance between silica production and silica dissolution in the sea insights from monterey bay california applied to the global data set silicon isotope tracers were used to examine the relative magnitude of silica dissolution and silica production in the monterey bay california upwelling system a diatom bloom dominated by skeletonema costatumand chaetoceros spp was encountered under conditions of moderate upwelling profiles of silica production and dissolution rates were obtained at seven stations that sampled both inside and outside the bloom integrated silica production rates ranged from 5 4 to 108 mmol si m 22 d 21 averaging 42 8 mmol si m 22 d 21 integrated silica dissolution rates were considerably lower than production rates with values between 0 63 and 6 5 mmol si m 22 d 21 mean 5 2 90 mmol si m 22 d 21 the mean ratio of integrated silica dissolution to integrated silica production d p between the surface and the 0 1 light depth was 0 075 omitting one station with an unusually high d p of 0 61 indicating that on average 93 of silica production was supported by new silicic acid the f ratio for diatom nitrogen use estimated from silicic acid and nitrate depletion curves and the mean d p ratio was found to be 0 83 indicating that silica was being regenerated at a rate that was only slightly slower than that for particulate organic nitrogen these data provide direct evidence confirming earlier hypotheses that the silica pump is weak in monterey bay analysis of the global data set on d p in the surface ocean leads to the hypothesis that low d p 0 10 or less are typical of diatom bloom events with d p rising to values in excess of 0 50 during nonbloom periods this pattern is shown to be consistent with previous estimates that the annual mean d p ratio in the upper 200 m of the global ocean is 0 5 0 6 a regional analysis reveals that the fraction of silica production supported by new silicic acid varies as a hyperbolic function of the level of gross silica production similar to the variation in the f ratio for n use with primary productivity these trends suggest that diatom blooms especially those occurring in more productive waters are the main vectors of silica export in the sea with the majority of the silica produced during nonbloom periods being recycled in the euphotic zone

geophysical characteristics of the southern mariana trough 11 50 n 13 40 n despite slow opening rates generally inferred for the mariana trough the southernmost part of the basin has fast spreading geophysical and morphologic characteristics that are unlike the features of the basin to the north a side scan sonar and geophysical survey maps the eastern part of the basin and the seafloor spreading center between 11 50 n and 13 40 n and identifies the following characteristics the ridge across axis profile forms a triangular to rounded high with relief of 100 to 500 m and cross sectional area variations of 1 to 7 km2 the along axis mantle bouguer gravity gradient is 0 2 mgal km axial segmentation occurs as overlapping axes and small deviation in trend no transform fault offsets exist despite significant changes in the trend of the spreading center characteristics of the surrounding basin include shallower overall depth than in the north no well developed frontal arc high in the southernmost trough the close proximity of submarine arc type volcanoes to the spreading center and tectonic fabric that is at a high angle to the trend of the spreading center on the eastern flank but is concordant on the western flank these characteristics imply different tectonic and magmatic conditions in the southern trough from the rest of the basin we propose that these effects are related to 1 the geometry of trench rollback in the southern trough leading to trench parallel extension generating inward radiating extensional faults 2 decoupling of the trench parallel extensional strain by the spreading center so that it primarily affects the eastern flank of the basin and 3 augmentation of the spreading center s magmatic budget by arc magmatic sources contributing to its fast spreading character although these effects may be accentuated in the southern mariana trough by the geometry of trench rollback and position of the slab which here underlies the spreading center they reflect distinct volcanic and tectonic processes which are varyingly expressed in back arc systems but are not normally found at mid ocean ridges

marine geology of the sodwana bay shelf southeast africa abstract the geostrophic current controlled northern zululand shelf displays a unique assemblage of interesting physical sedimentological and biological phenomena the shelf in this area is extremely narrow 3 km and is characterised by submarine canyons coral reefs and steep gradients on the continental slope three submarine canyons occur in the study area and are classified as mature or youthful phase canyons depending on the degree to which they breach the shelf these canyons originated as mass wasting features which were exploited by palaeo drainage during sea level regressions shelf lithology is dominated by a series of coast parallel patch coral reefs which have colonised beachrock and aeolianite sequences that extend semi continuously from 5 to 95 m and delineate late pleistocene palaeocoastline events the unconsolidated sediment on the shelf is either shelf sand mainly terrigenous quartz grains or bioclastic sediment large scale subaqueous dunes commonly form in the unconsolidated sediment on the outer shelf due to the agulhas current flow these dunes occur as two distinct fields at depths of 35 to 70 m the major sediment transport direction is towards the south but occasional bedload parting zones exist where the bedform migration direction changes from south to north

high rates of arc consumption by subduction processes some consequences new estimates of long term consumption of japanese and peruvian continental fore arc margins together with recent drilling data ocean drilling program on some other erosional island arcs such as izu bonin mariana or tonga allow the calculation of a mean rate of 7 3 km m y landward migration of the arc trench system on both east and west sides of the pacific basin during cenozoic time such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the atlantic ocean retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the pacific basin mass transfers in subduction zones must be reconsidered in the light of these new estimates especially the balance between net growth and loss of continental crust in recent geologic time a 50 m y period is long enough to erode the initial width of the volcanic arc and fore arc massif of any subduction zone assuming a mean trench volcanic arc landward migration of 5 km m y and a mean trench volcanic arc distance of 250 km this circumstance thus raises the question of whether the izu bonin mariana and tonga arcs are older than their reported eocene age

mechanisms of subduction accretion along the central aleutian trench we have used migrated 24 fold seismic records to study mechanisms of subduction accretion along the central aleutian trench two major structural units underlie the landward trench slope and are separated by an acoustically defined decollement surface that can be traced at least 20 km landward of the trench axis the upper unit consists of three to four structural blocks of deformed trench fill each block possesses a consistent internal structure and is bound by major landward dipping thrust faults the structures within the blocks include monoclinal sequences antiforms synforms and faulted folds beneath the decollement nearly undeformed trench fill 1 km thick is being under thrust along with the underlying oceanic lithosphere this basal section is an intervening unit between the offscraped sediment and the rough igneous oceanic basement and may prevent the oceanic crust from being incorporated into the subduction complex at least within 20 km of the front of the subduction zone owing to bending stresses the oceanic crust fails along normal faults beneath and at least 15 km landward of the trench axis these normal faults although poorly constrained are believed to propagate upwards producing offsets within both the subducting and the accreting sediment in addition normal faulting enhances the irregular surface of the oceanic crust structural associations imaged on all of the seismic reflection profiles although possibly fortuitous suggest that the subduction of these irregular basement highs results in the temporary displacement of the overlying fault bound sediment packets and promotes the development of the structures found within the accretionary prism tectonic riffling of the accreted sediment by the passage of the basement relief may also provide a mechanism by which underthrusting sediments are decoupled from the subducting lithosphere and underplated to the base of the aleutian accretionary complex

coulomb wedge model applied to the subduction of seamounts in the japan trench results from the french japanese kaiko exploration program of the japan trench and complementary data are used to propose a model of subduction of seamounts examples in two seamount chains one at the junction between the japan and kuril trenches and the other in the southern part of the japan trench off inubo cape illustrate the stages of subduction the japan trench margin evolved as a homogeneous wedge of deformable noncohesive coulomb material during seamount subduction this is illustrated by the subduction of the daiichi kashima seamount in the southern japan trench our model predicts that the first consequence of seamount subduction is a compressive thickening of the toe of the wedge above the advancing flank of the seamount because of the oversteepening of the rigid base of the wedge this agrees with our observations in the daiichi kashima area this thickening shifts landward across the wedge with the advancing flank of the seamount erosion dominates the wedge above the trailing flank of the seamount because of the corresponding understeepening of the rigid base of the wedge a reentrant of the toe of the wedge is thus created it is largest when the oceanic flank of the seamount coincides with the base of the landward slope at this time the trench axis will probably have an abnormally large amount of sediments as illustrated at the junction between the japan and kuril trenches after the subduction of the seamount the edge of the wedge should progressively move back to its initial location frontal accretion is absent or very limited

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

aeromagnetic evidence for a buried early cretaceous magmatic arc northeast japan positive aeromagnetic anomalies recent drilling and models constructed from these data delineate the plutonic roots of the early cretaceous kitakami magmatic arc in northeast japan buried plutons mostly offshore produce belts of positive magnetic anomalies these anomalies and magnetotelluric data suggest that the plutons form a batholith 70 120 km wide nearly 800 km long and 10 15 km thick the batholith may mark the location of the main kitakami arc most of the exposed kitakami plutons are 2 20 km in diameter some are 3 km thick the small plutons line up along nw trending faults some may have been satellite vents that tapped into the magma supply of the main arc the batholithic roots of the main arc now compose almost half of the modern japan forearc basement steep magnetic gradients offset anomalies and basin stratigraphy portray extensive faulting of the kitakami batholith during oblique subduction in the late cretaceous and rifting in the miocene the eastern boundary of the kitakami batholith lies between 90 and 140 km west of the modern trench much closer than the 300 km distance between the active arc and trench the early cretaceous forearc basin and accretionary prism may underlie the modern forearc basin east of the batholith but clear evidence is lacking much of the early cretaceous margin including most of its forearc therefore is missing how the material was removed is unknown it could have been strike slip faulted eroded by subduction related processes or both

tectonic erosion at the front of the japan trench convergent margin abstract the imaging of a multichannel seismic record was improved by reprocessing using pre stack techniques the reprocessed record shows structures that indicate tectonic erosion and gravity collapse at the front of the japan trench margin much of the lower slope appears to be underlain by a detached coherent block of continental crust the lower slope has failed by mass wasting and the resulting apron of slump debris at the base of the slope has become involved in thrust faulting at the front of the subduction zone slumping continues as long as debris is removed from the front of the margin by subduction and the apron cannot build up sufficiently to stabilize the failing lower slope truncated beds at the base of the upper plate indicate subcrustal erosion as well this probably being the main cause of massive subsidence of the margin subsidence was the cause of oversteepening destabilization and subsequent gravity collapse of the leading edge of the upper plate

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

sediment accretion subduction and dewatering at the base of the trench slope off costa rica a seismic reflection view of the decollement sea beam bathymetric and water gun seismic reflection data off the west coast of costa rica image the base of the trench slope its underlying structure and the behavior of a decollement surface in the presence of relief on the subducting plate a low angle landward dipping reflection marks the master decollement above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope a small horst block at least 9 km long 1 5 km wide and 300 m high is being obliquely subducted beneath the landward trench slope on five seismic lines the decollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level the undulating geometry of the decollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front the dewatering may favor development of bedding parallel zones of low shear strength within the subducting sediments and the master decollement may seek one of these zones

constraining the oceanic barium cycle with stable barium isotopes highlights we present a ba isotope data set of seawater river waters and biogenic particles ba isotope signatures of upper ocean waters are heavier than river and deep waters adsorption of lighter ba isotopes on biogenic particles induces the fractionation ba isotopes trace land sea interactions and ocean mixing processes decoupling of ba from major nutrients confirms ba to be a biointermediate element abstract the distribution of barium ba concentrations in seawater resembles that of nutrients and ba has been widely used as a proxy of paleoproductivity however the exact mechanisms controlling the nutrient like behavior and thus the fundamentals of ba chemistry in the ocean have not been fully resolved here we present a set of full water column dissolved ba dba isotope 137badba profiles from the south china sea and the east china sea that receives large freshwater inputs from the changjiang yangtze river we find pronounced and systematic horizontal and depth dependent 137badba gradients beyond the river influence characterized by generally light signatures 0 0 to 0 3 0 3 the 137badba values in the upper water column are significantly higher 0 9 0 9 than those in the deep waters 0 5 0 5 moreover 137badba signatures are essentially constant in the entire upper 100 m in which dissolved silicon isotopes are fractionated during diatom growth resulting in the heaviest isotopic compositions in the very surface waters combined with the decoupling of dba concentrations and 137badba from the concentrations of nitrate and phosphate this implies that the apparent nutrient like fractionation of ba isotopes in seawater is primarily induced by preferential adsorption of the lighter isotopes onto biogenic particles rather than by biological utilization the subsurface 137badba distribution is dominated by water mass mixing the application of stable ba isotopes as a proxy for nutrient cycling should therefore be considered with caution and both biological and physical processes need to be considered clearly however ba isotopes show great potential as a new tracer for land sea interactions and ocean mixing processes

numerical study on the pattern and origins of kuroshio branches in the bottom water of southern east china sea in summer pattern and origins of kuroshio branches in the bottom water of southern east china sea ecs were carefully examined by numerical simulations based on the regional ocean modeling system roms together with observations model results show that in the bottom water of ecs the intrusion pattern of kuroshio is mainly composed of an offshore kuroshio branch current okbc which bifurcated from the kuroshio northeast of taiwan flows nearly along the isobath of similar to 100 m and a nearshore kuroshio branch current nkbc which originated from the kuroshio northeast of taiwan upwells northwestward gradually from similar to 250 m to similar to 60 m then turns to northeast around 27 5 degrees n 122 e thereafter flows northeastward along the isobath of similar to 60 m and finally reaches at 30 5 degrees n where it turns to east furthermore we found that the nkbc mostly originated in the kuroshio subsurface water 120 250 m east of taiwan whereas the okbc mainly stemmed from the kuroshio water 60 120 m east of taiwan this pattern and origins of okbc and nkbc well addressed the observational phenomena that off the coast of zhejiang province china there were colder less saline and more phosphate rich bottom water near the isobath of similar to 60 m rather than near the isobath of similar to 100 m in august 2009 finally it is proposed that on southern ecs continental shelf kuroshio exhibits its intrusion branches by an anticyclonical stair structure bottom stair nkbc middle stair okbc and top stair kuroshio surface branch kbc

subduction hinge migration abstract it has been argued that back arc tectonics is controlled principally by the absolute motion of the overriding plate and the migration of the hinge of the downgoing plate thus hinge migration rollback is fundamentally important to understanding the geodynamics of back arc regions but even rates of hinge migration are unknown the rate of hinge migration can be estimated if the rate of back arc extension and the motion of the overriding plate are known rates of rollback in a hotspot reference frame range from near zero in the case of the new hebrides arc to 50 70 mm a at the south sandwich arc and may be as high as 100 mm a at the tonga trench but the hinge is actually advancing along the izu bonin trench as evidenced by the fact that there is no significant active extension behind the izu bonin arc the rate of hinge advance increases from north to south along the izu bonin trench and correlates well with the dip of the subducted slab active spreading in the mariana trough may be attributed to a rate of hinge advance which is lower than the rate of retreat of the philippine plate all of the possible motions of overriding plates and subduction hinges which might give rise to back arc opening are represented among the examples considered thus there is no unique circumstance in which back arc extension occurs finally there appears to be no clear correlation between hinge motion and the age of the subducted lithosphere

structure and eastward extension of the changjiang river plume in the east china sea 1 the hydrographic structure and offshore extension of freshwater plume discharged from the changjiang also known as the yangtze river in the northern east china sea were investigated by analyzing conductivity temperature depth ctd data and drifter trajectories collected during the summers of 1997 and 1998 from june to early september when southerly winds prevail the plume tends to move northeast in the chinese coastal area and then separates from the coastal zone to travel eastward over 400 km offshore across the western shelf of the northern east china sea during other seasons when northerly winds prevail the plume is confined to the chinese coast in the summer the plume in the midshelf confined to a thin surface layer 10 to 15 m thick extends eastward in the form of patches of low salinity water rather than spreading as a tongue shaped pattern from the changjiang mouth the eastward movement of patches in the western shelf is primarily due to upwelling favorable southerly winds upon reaching the vicinity of cheju do an island in the middle of northern east china sea the patches are advected to the korea tsushima strait by either the cheju warm current or a northward flowing mean current of the kuroshio branch current and then finally flow into the east japan sea

the effects of weathering variability and anthropogenic pressures upon silicon cycling in an intertropical watershed tana river kenya abstract we present the first study of the si isotopic composition of dry season river waters from the tana river kenya data encompasses the entire river basin with samples collected from headwaters to the estuary thereby capturing a salinity gradient in the headwaters the isotopic signature is affected by climate as a result of its control on soil drainage and weathering the 30 si signatures in the basin range from 0 69 up to 2 23 signatures are clearly affected by dams an increase in 30 si ratios of 0 54 and a decrease in the dissolved si dsi concentration by 41 were observed downstream of the masinga dam the largest of a succession of 5 hydroelectric dams this reduction in si load is most likely due to increased diatom productivity as the corresponding change in 30 si signature is consistent with the known fractionation by these organisms the 30 si composition of waters entering the estuary is ca 2 and dsi concentrations are 349 m in the estuary the dsi concentrations decrease linearly following the salinity while the 30 si ratio remains stable indicating the absence of processes affecting the dsi pool for the studied range of salinity

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

fractionation of silicon isotopes during biogenic silica dissolution abstract silicon isotopes have been investigated for their potential to reveal both past and present patterns of silicic acid utilization primarily by diatoms in surface waters of the ocean interpretation of this proxy has thus far relied on characteristic trends in the isotope composition of the dissolved and particulate silicon pools in the upper ocean as driven by biological fractionation during the production of biogenic silica bsio2 or opal by diatoms however other factors which may influence the silicon isotope composition of diatom opal particularly post formational aging and maturation processes remain largely uninvestigated here we report a consistent fractionation of silicon isotopes during the physicochemical dissolution of diatom bsio2 suspended in seawater under closed conditions this fractionation acts counter to that occurring during bsio2 production and at about half its absolute magnitude with dissolution discriminating against the release of the heavier isotopes of silicon at an enrichment factor edsi bsi of 0 55 corresponding to a fractionation factor 30 28 of 0 99945 the enrichment factor did not vary with source material indicating the lack of a significant species effect or with temperature from 3 to 20 c thus the dissolution of bsio2 produces dissolved silicon with a 30si value that is 0 55 more negative than its parent bsio2 an effect that must be accounted for when interpreting oceanic 30si distributions the 30si values of both the dissolved and particulate silicon pools increased linearly as dissolution progressed implying a measurable 0 1 change in the relative 30si of opal samples whenever the difference in preservation efficiency between them is 20 this effect could account for 10 30 of the difference in diatom 30si values observed between glacial and interglacial conditions it is unlikely however that the inferred maximum possible change in b30sio2 of 0 55 would be manifested in situ as a high mean percentage of dissolution would include complete loss of the more soluble members of the diatom assemblage

arc evolution a magnetic perspective from the antarctic peninsula the antarctic peninsula is a mesozoic cenozoic magmatic arc built on palaeozoic and younger basement it was formed by processes related to the subduction of pacific ocean floor at its western margin although subduction has now ceased along most of its length the peninsula features all the tectonic components commonly associated with a developing arc system basement accretionary complex magmatic arc arc related basins intra arc extension and post subduction volcanism seventeen thousand kilometres of high resolution aeromagnetic data have recently been collected in a transect across part of the arc covering an area 290 by 230 km and incorporating examples of most of the above tectonic components the new map reveals distinct magnetic signatures which can now be related to each of these components in a way that was not possible with reconnaissance data sets a characteristic magnetic anomaly pattern for each component is described and comparisons drawn with magnetic studies of other arc regions

the iceland plume and its influence on the evolution of the ne atlantic the papers printed below were contributed by scientists who attended the second arthur holmes european research meeting held in reykjavik iceland in early july 1994 the igneous rocks produced by mantle plumes and at seafloor spreading centres provide a powerful probe for investigating the state of the mantle beneath the lithosphere iceland is a particularly good window into the convecting mantle because the north atlantic spreading centre crosses directly above the mantle plume the plate divergence allows decompression melting to occur to depths as shallow as 15 km below the surface in other mantle plumes such as hawaii the thick lithospheric lid above the plume prevents the convecting mantle rising shallower than the base of the lithosphere which lies at a depth of many tens of kilometres a further feature of the ne atlantic is that decompression melting beneath the spreading centres adjacent to iceland samples the asthenospheric mantle consistently at all distances from the mantle plume this allows the spatial and temporal extent and variability of the plume to be investigated the arthur holmes meeting was held early july 1994 in reykjavik iceland fittingly enough at a location almost on top of the european north american plate boundary and directly above the iceland plume the presentations covered six main themes melt generation the relative importance of plume depleted mid ocean ridge basalt and other magma sources breakup processes and effects on marginal basins the timing and expression of volcanic and tectonic events and geochemical and geophysical manifestations of the plume through space

flood discharges of a small river into open coastal waters plume traits and material fate abstract the dynamics of plumes from large rivers are relatively well known many estuaries are small however and discharge directly onto exposed open shorelines and presumably produce smaller plumes that may have different properties therefore this study measured traits of a small estuary on the australian east coast as a model system focusing on a plume size b distinctness of plume edges and c imprints on the seafloor although plumes were found to be limited in spatial extent ca 1 km offshore 2 4 km longshore and were constrained near the shore by onshore winds they exported high nutrient loads from an urbanised watershed the small plumes were shallow

silicon isotope constraints on sources and utilization of silicic acid in the northern south china sea the stable silicon isotopic composition 30si of waters and diatoms has increasingly been used to investigate the biogeochemical cycling of si in the major ocean basins here we present the first si isotope data set from the northern south china sea nscs a large marginal sea system in the western north pacific to examine sources and utilization of silicic acid si oh 4 during two cruises in july august 2009 summer and january 2010 winter samples for isotope measurements of dissolved si oh 4 30sisi oh 4 and of biogenic silica 30sibsi in suspended particles were collected along a transect perpendicular to the coast from the inner shelf to the deep water slope as well as at the south east asian time series study seats station located in the nscs basin surface 30sisi oh 4 generally increased from values 2 3 on the inner shelf to 2 8 above the deep basin suggesting an increasing utilization of dissolved si oh 4 reflecting the transition from eutrophic to oligotrophic conditions the 30sibsi values were systematically lower than the corresponding 30sisi oh 4 in the euphotic zone above 100 m on the shelf and slope in contrast at station seats in the nscs basin 30sibsi signatures in both seasons were within error equal to 30sisi oh 4 in the surface mixed layer above 50 m and 30sibsi in waters below were significantly higher than the corresponding 30sisi oh 4 by comparing the field data with the si isotope fractionation revealed by the rayleigh or the steady state models we demonstrate the existence of variable si oh 4 origins in different areas of the nscs surface waters on the inner shelf were largely fed by nutrients from the pearl river input while the primary source of si oh 4 for the euphotic zone on the outer shelf and slope was upwelling or vertical mixing from underlying waters the si oh 4 in the surface mixed layer of the nscs basin might have originated from horizontal mixing with other highly fractionated surface waters as a consequence the si isotope dynamics in the nscs are largely controlled by variable biological fractionation of si in waters from different sources with different initial si isotopic compositions rather than any single source water

dissolved silicon isotopic compositions in the east china sea water mass mixing vs biological fractionation we present the first set of dissolved silicon isotope data in seawater delta si 30 si oh 4 from the east china sea a large and productive marginal sea significantly influenced by the kuroshio current and freshwater inputs from the changjiang yangtze river in summer august 2009 the lowest surface delta si 30 si oh 4 signatures of 2 1 parts per thousand corresponding to the highest si oh 4 concentrations similar to 30 0 mu mol l 1 were observed nearshore in changjiang diluted water during advection on the east china sea inner shelf surface delta si 30 si oh 4 increased rapidly to 3 2 parts per thousand while si oh 4 became depleted indicating increasing biological utilization of the si oh 4 originating from the changjiang diluted water this is also reflected in the water column profiles characterized by a general decrease of delta si 30 si oh 4 and an increase of si oh 4 with depth on the east china sea mid shelf and slope in winter december 2009 january 2010 however the delta si 30 si oh 4 was nearly constant at 1 9 parts per thousand throughout the water column on the east china sea shelf beyond the nearshore which was a consequence of enhanced vertical mixing of the kuroshio subsurface water horizontal admixture of kuroshio surface water which is highly fractionated in si isotopes was observed only beyond the shelf break significant seasonal differences in delta si 30 si oh 4 were detected in the surface waters beyond the changjiang diluted water influenced region on the east china sea shelf where the winter values were similar to 1 0 parts per thousand lower than those in summer despite the same primary si oh 4 supply from the kuroshio subsurface water during both seasons this demonstrates significantly higher biological consumption and utilization of si oh 4 in summer than in winter

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

extension in the northern mariana inner forearc seismic reflection seamarc ii acoustic imagery and bathymetric data across the northern mariana island arc reveal dense high angle normal faulting of the arc and inner forearc faults with throws up to 780 m cut a thick sequence of stratified but otherwise undeformed sediments the major faults display a sinuous or zigzag trace that is commonly associated with rifting this pattern can be indicative of a single event of triaxial strain that requires two contemporaneous conjugate fault pairs in orthorhombic symmetry interpretation and statistical analyses of the data show activity since the last sedimentary deposition and concurrent fault development of two distinct fault populations one associated with the backarc basin s east boundary fault and the other with extension in the forearc extensional faults along the eastern side of the backarc basin near the active arc at 22 n where active rifting is occurring have approximately the same trend as the east boundary fault in the survey area we estimate the direction of extensional minimum principal strain for the east boundary fault at 22 n to be 39 1 orthogonal to the extensional strain direction measured in the inner forearc the change in strain is distinct and abrupt occurring across a 20 km wide region along the arc massif within this narrow region of transition the intermediate and minimum principal strain directions are nearly equal in magnitude with orientations midway between the dominant forearc and backarc basin principal strain directions statistical analyses show that the strain from either backarc or forearc regime is not transmitted across this narrow region the two opposing strain regimes coexist virtually juxtaposed the coexistence of these two orthogonal extensional fault regimes within the overriding plate between the mariana backarc basin and inner forearc may be explained by assuming that a weak transitional region exists along the east boundary fault zone where stresses can be more easily relieved this allows active basin wide extension in the backarc basin while the forearc data are consistent with models that require radial forearc deformation resulting from increasing arc trench curvature of the northern half of the mariana island arc

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

the isotope composition of inorganic germanium in seawater and deep sea sponges abstract although dissolved concentrations of germanium ge and silicon si in modern seawater are tightly correlated uncertainties still exist in the modern marine ge cycle germanium stable isotope systematics in marine systems should provide additional constraints on marine ge sources and sinks however the low concentration of ge in seawater presents an analytical challenge for isotopic measurement here we present a new method of pre concentration of inorganic ge from seawater which was applied to measure three ge isotope profiles in the southern ocean and deep seawater from the atlantic and pacific oceans germanium isotopic measurements were performed on ge amounts as low as 2 6 ng using a double spike approach and a hydride generation system coupled to a mc icp ms germanium was co precipitated with iron hydroxide and then purified through anion exchange chromatography results for the deep i e 1000 m depth pacific ocean off hawaii nearby loihi seamount and the deep atlantic off bermuda bats station showed nearly identical 74 70ge values at 3 19 0 31 2sd n 9 and 2 93 0 10 2sd n 2 respectively vertical distributions of ge concentration and isotope composition in the deep southern ocean for water depth 1300 m yielded an average 74 70ge 3 13 0 25 2sd n 14 and ge si 0 80 0 09 mol mol 2sd n 12 significant variations in 74 70ge from 2 62 to 3 71 were measured in the first 1000 m in one station of the southern ocean near sars seamount in the drake passage with the heaviest values measured in surface waters isotope fractionation by diatoms during opal biomineralization may explain the enrichment in heavy isotopes for both ge and si in surface seawater however examination of both oceanographic parameters and 74 70ge values suggest also that water mass mixing and potential contribution of shelf derived ge also could contribute to the variations combining these results with new ge isotope data for deep sea sponges sampled nearby allowed us to determine a ge isotope fractionation factor of 0 87 0 37 2sd n 12 during ge uptake by sponges although ge has long been considered as a geochemical twin of si this work underpins fundamental differences in their isotopic behaviors both during biomineralization processes and in their oceanic distributions this suggests that combined with si isotopes ge isotopes hold significant promise as a complementary proxy for delineating biological versus source effects in the evolution of the marine silicon cycle through time

sources and biological fractionation of silicon isotopes in the eastern equatorial pacific abstract silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4 n and 3 s latitude at 110 w longitude in the eastern equatorial pacific silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation the value of e estimated by applying a steady state isotope fractionation model to data from all stations between 4 n and 3 s was 0 77 0 12 std err when the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the equatorial undercurrent euc the resulting value of e was 1 08 0 27 std err similar to the value established in culture studies 1 1 when the non steady state rayleigh model was applied to the same restricted data set the resulting value of e was significantly more positive 0 61 0 16 std err to the extent that the equatorial system approximates a steady state these results support a value of 1 1 for the fractionation factor for isotopes of si in the sea without the assumption of steady state the value of e can only be constrained to be between 0 6 and 1 1 silicic acid in equatorial pacific deep water below 2000 m had a near constant 30 si of 1 32 0 05 that value is significantly more positive than obtained for north pacific deep water at similar depths at stations to the northwest of our study area 0 9 1 0 and it is slightly less positive than new measures of the 30 si of silicic acid from the silicic acid plume centered over the cascadia basin in the northeast pacific si oh 4 180 m 30 si 1 46 0 12 sd n 4 we show that the data from the equator and cascadia basin fit a general trend of increasing 30 si oh 4 with increasing silicic acid concentration in the deep sea but that the isotope values from the northeast pacific are anomalously light the observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution confirmation of such high variability in deep water 30 si oh 4 within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea

physical mechanisms for the offshore detachment of the changjiang diluted water in the east china sea 1 physical mechanisms for the summertime offshore detachment of the changjiang diluted water cdw into the east china sea are examined using the high resolution unstructured grid finite volume coastal ocean model fvcom the model results suggest that isolated low salinity water lens detected west of cheju island can be formed by 1 a large scale adjustment of the flow field to the changjiang discharge and 2 the detachment of anticyclonic eddies as a result of baroclinic instability of the cdw front adding the changjiang discharge intensifies the clockwise vorticity of the subsurface current originating from the taiwan warm current flowing along the 50 m isobath and thus drives the low salinity water in the northern coastal area of the changjiang mouth offshore over a submerged plateau that extends toward cheju island given a model horizontal resolution of less than 1 0 km the cdw front becomes baroclinically unstable and forms a chain of anticyclonic and cyclonic eddies the offshore detachment of anticyclonic eddies can carry the cdw offshore this process is enhanced under northward winds as a result of the spatially nonuniform interaction of wind induced ekman flow and eddy generated frontal density currents characteristics of the model predicted eddy field are consistent with previous theoretical studies of baroclinic instability of buoyancy driven coastal density currents and existing satellite imagery the plume stability is controlled by the horizontal ekman number in the changjiang this number is much smaller than the criterion suggested by a theoretical analysis

the influence of water mass mixing on the dissolved si isotope composition in the eastern equatorial pacific silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon si in this study the distribution of the si isotope composition of dissolved silicic acid 30si oh 4 was analyzed in the water column of the eastern equatorial pacific eep where one of the globally largest oxygen minimum zones omzs is located samples were collected at 7 stations along two meridional transects from the equator to 14 s at 85 50 w and 82 00 w off the ecuadorian and peruvian coast surface waters show a large range in isotope compositions 30si oh 4 2 2 to 4 4 with the highest values found at the southernmost station at 14 s this station also revealed the most depleted silicic acid concentrations 0 2 mol kg which is a function of the high degree of si utilization by diatoms and admixture with waters from highly productive areas samples within the upper water column and the omz at oxygen concentrations below 10 mol kg are characterized by a large range in 30si oh 4 which mainly reflects advection and mixing of different water masses even though the highly dynamic hydrographic system of the upwelling area off peru does not allow the identification of clear si isotope signals for distinct water masses therefore we cannot rule out that also dissolution processes have an influence on the 30si oh 4 signature in the subsurface water column deep water masses 2000 m in the study area show a mean 30si oh 4 of 1 2 0 2 which is in agreement with previous studies from the eastern and central pacific comparison of the new deep water data of this study and previously published data from the central pacific and southern ocean reveal substantially higher 30si oh 4 values than deep water signatures from the north pacific as there is no clear correlation between 30si oh 4 and silicic acid concentrations in the entire data set the distribution of 30si oh 4 signatures in deep waters of the pacific is considered to be mainly a consequence of the mixing of several end member water masses with distinct si isotope signatures including lower circumpolar deep water lcdw and north pacific deep water npdw

tectonic control of the subducting juan fernandez ridge on the andean margin near valparaiso chile near the latitude of valparaiso chile a fundamental change in configuration of the benioff zone volcanic arc activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two degree segment offshore valparaiso extending from the shelf edge seaward across the eastern end of juan fernandez ridge near the o higgins group of seamounts the juan fernandez ridge strikes northeast rather than continuing its east west trend across the pacific basin the ridge uplifts the upper plate and sediments of the valparaiso basin are deformed against its southern flank this deformation is consistent with the southward migration required by an oblique trending ridge and the nearly trench normal vector of plate convergence in the trench axis the ridge forms a basement barrier behind which sediments 2 5 km deep have ponded the lower slope over the ridge appears eroded whereas the margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide nazca plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust its relation to deeper processes segmenting the andean orogen appears to involve prior tectonic events

228ra 226ra and 226ra ba ratios to track barite formation and transport in the water column abstract we measured 228 ra ex 226 ra ex and 226 ra ex ba ex ratios in suspended and sinking particles collected at the oceanic flux program ofp time series site in the western sargasso sea and compared them to seawater ratios to provide information on the origin and transport of barite baso 4 in the water column the 228 ra ex 226 ra ex ratios of the suspended particles down to 2000 m are nearly identical to those of seawater at the same water depth these ratios are much lower than expected if suspended barite was produced in surface waters and indicate that barite is produced throughout the mesopelagic layer the 228 ra ex 226 ra ex activity ratios of sinking particles collected at 1500 and 3200 m varied mostly between 0 1 and 0 2 which is intermediate between the seawater ratio at these depths ex ba total ba lithogenic considered to be mainly barite present in the sinking flux is a mixture of crystals formed recently in the upper water column formed several years earlier in the upper water column or formed recently in deeper waters we observe a sizeable temporal variability in the 228 ra ex 226 ra ex ratios of sinking particles which indicates temporal variability in the relative proportion of barite crystals originating from surface with a high 228 ra ex 226 ra ex ratio and mesopelagic with a low 228 ra ex 226 ra ex ratio sources however we could not discern a clear pattern that would elucidate the factors that control this variability the 226 ra ba ratios measured in seawater are consistent with the value reported from the geosecs expeditions 2 3 dpm mol 1 below 500 m depth but are significantly lower in the upper 500 m high 226 ra ex ba ex ratios and elevated sr concentrations in suspended particles from the upper water column suggest preferential uptake of 226 ra over ba during formation of srso 4 skeletons by acantharians which must contribute to barite formation in shallow waters deeper in the water column the 226 ra ex ba ex ratios of suspended particles are lower than those of seawater since 228 ra ex 226 ra ex ratios demonstrate that suspended barite at these depths has been produced recently and in situ their low 226 ra ex ba ex ratios indicate preferential uptake of ba over ra in barite formed in mesopelagic water

interaction between the chile ridge and chile trench geophysical and geothermal evidence geophysical and geothermal data are examined from the three southernmost sections of the chile ridge starting at 44 s and continuing south to the triple junction of the nazca antarctic and south america plates at 47 s these sections represent three progressively younger stages in a ridge trench collision event corresponding to 3 m y before the collision 1 m y before the collision and culminating in an ongoing collision at the triple junction magnetic and bathymetric data across the chile rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench in the collision zone a normal looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope there is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the pacific farallon spreading center collided with north america in contrast the overriding plate is greatly affected by the ridge collision the landward trench slope steepens and narrows as the collision zone is approached at the southern end of the collision zone a ridge associated with the taitao fracture zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope geothermal measurements were made along three transects of the margin corresponding to the time 3 m y before the collision during the collision and 6 m y after the collision the heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge values as high as 345 mw m2 were recorded on the lower trench slope a prominent bottom simulating reflector bsr observed over a wide area of the landward trench slope north of the triple junction and more locally south of the triple junction is used to expand the grid of heat flow observations excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the bsr the heat flow measurements compare favorably with a theoretical model assuming conductive heat flow we estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion periodic ridge collisions in the past may account for the apparent truncation of the andean forearc region

subduction of a late cretaceous seamount of the louisville ridge at the tonga trench a model of normal and accelerated tectonic erosion the hotspot generated louisville ridge is a 4000 km chain of seamounts typically 2 2 5 km high and 10 40 km in diameter and an underlying crustal swell 1 5 km high and 100 km wide trending nnw across the southwestern pacific the northwest end of the ridge collides with the north trending tonga trench 26 s which just north of that point is exceptionally deep 10 8 km and lacks both a turbidite wedge and a bordering accretionary complex the collision zone is moving rapidly southward multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2 3 km beneath the lower western trench slope which is interpreted as the flat summit of a subducted guyot motuku of the louisville chain projected eastward the summit plain passes 1 2 km above the trench floor dredging of the nearby inner trench wall recovered uppermost cretaceous maestrichtian oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted louisville seamount the principal effects of hotspot ridge collision with a sediment starved trench are 1 the impacting seamounts are subducted rather than accreted and 2 although some seamount rocks are temporarily accreted the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km m y accelerated tectonic erosion is related to 1 fracturing shearing and general weakening of arc substrate rocks as they are lifted by the swell penetrated by impacting seamounts and left to collapse as the ridge moves away 2 a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell 3 a possible elevation of the subduction decollement to account for the removal of as much as 30 000 km of material from a 400 km sector of the trench in 1 million years and 4 a reduction in supply of arc derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge normal tectonic erosion in the tonga trench is apparently minor and we conclude that the bulk of the 37 000 km of material which fills subducting grabens each million years is arc derived volcanic and pelagic sediment