deformation and hydrothermal metamorphism of gabbroic rocks within the godzilla megamullion parece vela basin philippine sea abstract microstructural and petrologic analyses of 7 gabbroic rocks sampled from the medial area of the godzilla megamullion site kh07 02 d18 located along the parece vela basin spreading ridge parece vela rift philippine sea reveal the development of a high temperature ductile shear zone associated with hydrothermal metamorphism in the lower crust the deformed gabbroic rocks are petrographically classified into mylonites and an ultramylonite and are characterized by porphyroclastic textures consisting mainly of coarse plagioclase and clinopyroxene amphibole porphyroclasts in a fine grained matrix plagioclase crystallographic preferred orientations vary from 010 100 and 001 100 patterns in the mylonites to a weak 001 100 pattern in the some mylonites and ultramylonite suggesting a change in the deformation mechanism from dislocation creep to grain size sensitive creep with increasing intensity of deformation the chemical composition of matrix plagioclase is generally more sodic than that of porphyroclasts secondary amphibole is ubiquitous consisting mainly of pargasite and magnesiohornblende brown hornblende and actinolite green hornblende the mineral assemblage is consistent with the hydrothermal metamorphic reaction clinopyroxene calcic plagioclase fluid amphibole sodic plagioclase compared with deformed gabbroic rocks from the breakaway and termination areas of the godzilla megamullion the samples record ductile shearing under high temperature conditions possibly related to the development of the godzilla megamullion although hydrothermal activity in the medial area appears to have been less intense than in both the breakaway and termination areas

wind drag coefficient over first year sea ice in the bering sea profiles of relative wind speed and temperature were collected from four levels over rough sea ice in the marginal ice zone of the bering sea and used to compute surface wind drag coefficients based upon 138 profiles for near constant ice conditions northeast winds of 3 15 m s 1 and near neutral stability the mean near neutral 10 m drag coefficient was 0 00309 0 00049 this is among the largest drag coefficients measured for neutral atmospheric conditions over sea ice sixty five percent of the variance of this estimate is explained by orientation of the ice floe to the wind an additional 5 of the variance is explained by wind speed dependence provided that diabatic atmospheric stability over the bering sea ice pack is well correlated with low wind speed the significant ice advecting wind stress on the ice is determined from the neutral value of the drag coefficient a function of ice roughness length alone the ice forecaster thus may use remote measurements or estimates of wind speed and surface roughness to assess the wind component of ice advection

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

boron isotope geochemistry and u pb systematics of altered morb from the australian antarctic discordance odp leg 187 abstract boron and pb isotopic compositions together with b u th pb concentrations were determined for pacific and indian mantle type mid ocean ridge basalts morb obtained from shallow drill holes near the australian antarctic discordance aad boron contents in the altered samples range from 29 7 to 69 6 ppm and are extremely enriched relative to fresh morb glass with 0 4 0 6 ppm b similarly the 11b values range from 5 5 to 15 9 in the altered basalts and require interaction with a 11b enriched fluid similar to seawater 39 5 and or boron isotope fractionation during the formation of secondary clays positive correlations between b concentrations and other chemical indices of alteration such as h2o co2 k2o p2o5 u and 87sr 86sr indicate that b is progressively enriched in the basalts as they become more altered interestingly 11b shows the largest isotopic shift to 16 in the least altered basalts followed by a continual decrease to 5 6 in the most altered basalts these observations may indicate a change from an early seawater dominated fluid towards a sediment dominated fluid as a result of an increase in sediment cover with increasing age of the seafloor the progression from heavy 11b towards lighter values with increasing degrees of alteration may also reflect increased formation of clay minerals e g saponite a comparison of 238u 204pb and 206pb 204pb in fresh glass and variably altered basalt from site 1160b shows extreme variations that are caused by secondary u enrichment during low temperature alteration modeling of the u pb isotope system confirms that some alteration events occurred early in the 21 5 m y history of these rocks even though a significant second pulse of alteration happened at 12 ma after formation of the crust the u pb systematics of co genetic basaltic glass and variably low temperature altered basaltic whole rocks are thus a potential tool to place age constraints on the timing of alteration and fluid flow in the ocean crust

observations of longitudinal rolls in a near neutral atmosphere abstract aircraft and satellite data an used to study the structure of longitudinal roll vortices in a nearly neutral zi l 1 2 where zi is the inversion height and l is the monin obukhov length boundary layer over the ice covered bering sea during february steam fog formed over cracks and leads in the ice was used as a tracer to delineate the various scales of roll motion seen in satellite images the satellite information combined with aircraft data collected by the noaa p 3 indicated the presence of a hierarchy of roll vortex motions it is suggested that interactions of the various scales of motion resulted in certain scales dominating in one area and other scales dominating in another two kilometer wavelength variations an attributed to the inflection point instability mechanism while 12 15 km variations seen to have been reinforced by the upstream topography on the chukotka peninsula organization of the fog banks on scales of 30 km was also present and may be attributable to resonant subharm

anomalous opening of the equatorial atlantic due to an equatorial mantle thermal minimum abstract the geology of the equatorial atlantic is dominated by a broad east west megashear belt where a cluster of large fracture zones offsets anomalously deep segments of the mid atlantic ridge mar the origin and evolution of this megashear region may lie ultimately in an equatorial mantle thermal minimum the notion of a mantle thermal minimum in the equatorial atlantic is supported by an equatorial minimum of zero age topography a maximum in mantle shear waves seismic velocity and a minimum in the degree of melting indicated by the chemistry of mar basalts and peridotites this thermal minimum has probably been a stable feature since before the cretaceous separation of africa from south america it caused a pre opening equatorial continental lithosphere thicker and colder than normal the cretaceous benue trough in western africa and the amazon depression in south america are interpreted as morphostructural depressions created or rejuvenated by strike slip transpressional and transtensional tectonics ducing extension of the cold thick equatorial lithosphere the oceanic rift propagating northward from the south atlantic impinged against the equatorial thicker colder and therefore stronger than normal continental lithosphere that consequently acted as a locked zone this and a low magmatic budget due to the cold upper mantle caused a lower than normal rate of propagation of the oceanic rift into the equatorial belt with diffuse deformation during mostly amagmatic extension the thick cold lithosphere prevented major cretaceous igneous activity from the st helena plume eventually initial weak isolated nuclei oceanic lithosphere were emplaced separated by e w continent continent transforms opening occurred largely by strike slip motion along these initial transforms the consequences were that the equatorial atlantic opened prevalently along an e w direction in contrast to the n s opening of the north and south atlantic and that sheared continental margins are particularly well developed in the equatorial atlantic after further continental separation the cold equatorial mantle caused a low degree of melting with na rich morb and alkali basalt rather than normal morb and with undepleted mantle peridotities thin crust depressed ridge segments and a prevalence of amagmatic extension similar conditions still exist today long transforms offsetting short ridge segments kept sea floor spreading unstable and dominated by transform tectonics with transform migration transpression and transtension causing strong vertical motion emersion and subsidence of lithospheric blocks development of deep pull apart basins and preservation of relict slivers of old lithosphere occasionally even of continental lithosphere within younger crust the equatorial transforms are caused ultimately by a long lived thermal minimum in the upper mantle and not vice versa however they then create second order rebound thermal effects that help maintain the thermal minimum in the upper mantle it can be speculated that mantle thermal minima at the earth s equator might be related to true polar wander triggered by subduction of dense masses into the mantle

a correspondence of altimetric gravity texture to abyssal hill morphology along the flanks of the southeast indian ridge 1 we examine the relationship between altimetric gravity and abyssal hill textures along the flanks of southeast indian ridge where previous studies observed a progressive west to east increase in abyssal hill roughness and scales such a relationship is often considered unlikely because abyssal hills are typically smaller than upward continuation filter scales however our results which exclude ridges fracture zones and large pseudofaults show that altimetric gravity roughness and characteristic scales in this region also increase from west to east and at the largest scales gravity texture exhibits ridge parallel lineaments synthetic profiles also demonstrate how the texture parameters of a filtered field are related to those of the unfiltered field particularly where the characteristic scale of the latter is smaller than the filter length

variations in upper crustal structure due to variable mantle temperature along the southeast indian ridge 1 there is a systematic variation in axial morphology and axial depth along the southeast indian ridge seir with distance away from the australian antarctic discordance an area of cold uppermost mantle since spreading rate 72 76 mm yr and mantle geochemistry appear constant along this portion of the seir the observed variations in axial morphology and axial depth are attributed to a gradient in mantle temperature in this study we report results from a multichannel seismic investigation of on axis crustal structure along this portion of the seir three distinct forms of ridge crest morphology are found within our study area axial highs rifted axial highs and shallow axial valleys axial highs have a shallow 1500 m below seafloor bsf magma lens and a thin 300 m layer 2a along the ridge crest rifted axial highs have a deeper 2100 m bsf magma lens and thicker 450 m layer 2a on axis beneath shallow axial valleys no magma lens is imaged and layer 2a is thick 450 m there are step like transitions in magma lens depth and layer 2a thickness with changes in morphology along the seir the transitions between the different modes of axial morphology and shallow structure are abrupt suggesting a threshold type mechanism variations in crustal structure along the seir appear to be steady state persisting for at least 1 m y portions of segments in which a magma lens is found are characterized by lower relief abyssal hills on the ridge flank shallower ridge flank depths and at the location of along axis mantle bouguer anomaly mba lows the long wavelength variation in ridge morphology along the seir from axial high segments to the west to axial valley segments to the east is linked to the regional gradient in mantle temperature superimposed on the long wavelength trend are segment to segment variations that are related to the absolute motion of the seir to the northeast which influence mantle melt production and delivery to the ridge

global prediction of abyssal hill root mean square heights from small scale altimetric gravity variability 1 abyssal hills which are pervasive landforms on the seafloor of the earth s oceans represent a potential tectonic record of the history of mid ocean ridge spreading however the most detailed global maps of the seafloor derived from the satellite altimetry based gravity field cannot be used to deterministically characterize such small scale 10 km morphology nevertheless the small scale variability of the gravity field can be related to the statistical properties of abyssal hill morphology using the upward continuation formulation in this paper i construct a global prediction of abyssal hill root mean square rms heights from the small scale variability of the altimetric gravity field the abyssal hill related component of the gravity field is derived by first masking distinct features such as seamounts mid ocean ridges and continental margins and then applying a newly designed adaptive directional filter algorithm to remove fracture zone discontinuity fabric a noise field is derived empirically by correlating the rms variability of the small scale gravity field to the altimetric noise field in regions of very low relief and the noise variance is subtracted from the small scale gravity variance suites of synthetically derived abyssal hill formed gravity fields are generated as a function of water depth basement rms heights and sediment thickness and used to predict abyssal hill seafloor rms heights from corrected small scale gravity rms height the resulting global prediction of abyssal hill rms heights is validated qualitatively by comparing against expected variations in abyssal hill morphology and quantitatively by comparing against actual measurements of rms heights although there is scatter the prediction appears unbiased

propagating rift model for the v shaped ridges south of iceland 1 we present new marine geophysical data which constrain the seafloor spreading history of the reykjanes ridge near iceland and the origin of its flanking v shaped topographic and gravity ridges contrary to the geometry assumed in pulsing plume models the v shaped ridges are not symmetric about the reykjanes ridge axis and seafloor spreading has not been symmetric about a stable axis thus existing models must at least be modified to include an additional asymmetry producing mechanism the best understood and documented such mechanism is rift propagation one possibility is that plume pulses drive the propagators however rift propagation also produces v shaped wakes with crustal thickness variations suggesting the possibility that a pulsing iceland plume might not be necessary to explain the reykjanes v shaped ridges scarps and troughs

a three dimensional radially anisotropic model of shear velocity in the whole mantle summary we present a 3 d radially anisotropic s velocity model of the whole mantle saw642an obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on non linear asymptotic coupling theory nact the model is parametrized in level 4 spherical splines which have a spacing of 8 the model shows a link between mantle flow and anisotropy in a variety of depth ranges in the uppermost mantle we confirm observations of regions with vsh vsv starting at 80 km under oceanic regions and 200 km under stable continental lithosphere suggesting horizontal flow beneath the lithosphere we also observe a vsv vsh signature at 150 300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast spreading segments in the transition zone 400 700 km depth regions of subducted slab material are associated with vsv vsh while the ridge signal decreases whilethemid mantlehasloweramplitudeanisotropy vsv in the lowermost 300 km which appears to be a robust conclusion despite an error in our previous paper which has been corrected here the 3 d deviations from this signature are associated with the large scale low velocity superplumes underthecentralpacificandafrica suggestingthatvsh vsv isgeneratedinthepredominant horizontal flow of a mechanical boundary layer with a change in signature related to transition to upwelling at the superplumes

waveform tomography reveals channeled flow at the base of the oceanic asthenosphere understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists using full waveform seismic tomography we imaged a pattern of horizontally elongated bands of low shear velocity most prominent between 200 and 350 kilometers depth which extends below the well developed low velocity zone these quasi periodic fingerlike structures of wavelength 2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers below 400 kilometers depth velocity structure is organized into fewer undulating but vertically coherent low velocity plumelike features which appear rooted in the lower mantle this suggests the presence of a dynamic interplay between plate driven flow in the low velocity zone and active influx of low rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer

contrasted hydrothermal activity along the south east indian ridge 130 e 140 e from crustal to ultramafic circulation using a combined approach of seafloor mapping mapr and ctd survey we report evidence for active hydrothermal venting along the 130 140 e section of the poorly known south east indian ridge seir from the australia antarctic discordance aad to the george v fracture zone fz along the latter we report eh and ch4 anomalies in the water column above a serpentinite massif which unambiguously testify for ultramafic related fluid flow this is the first time that such circulation is observed on an intermediate spreading ridge the ridge axis itself is characterized by numerous off axis volcanoes suggesting a high magma supply the water column survey indicates the presence of at least ten distinct hydrothermal plumes along the axis the ch4 mn ratios of the plumes vary from 0 37 to 0 65 denoting different underlying processes from typical basalt hosted to ultramafic hosted high temperature hydrothermal circulation our data suggest that the change of mantle temperature along the seir not only regulates the magma supply but also the hydrothermal activity the distribution of hydrothermal plumes from a ridge segment to another implies secondary controls such as the presence of fractures and faults along the axis or in the axial discontinuities we conclude from these results that hydrothermal activity along the seir is controlled by magmatic processes at the regional scale and by the tectonics at the segment scale which influences the type of hydrothermal circulation and leads to various chemical compositions such variety may impact global biogeochemical cycles especially in the southern ocean where hydrothermal venting might be the only source of nutrients

evidence for variations in magma production along oceanic spreading centers a critical appraisal recent geologic geophysical and geochemical studies of the oceanic lithosphere near fracture zones have resulted in the proposal that the magma budget defined as the amount of magma delivered to magma chambers or conduits beneath a spreading center for a given amount of spreading decreases as fracture zones are approached seismic reflection and refraction studies indicate that the crust becomes anomalously thin near fracture zones but reference must be made to the boundary between residual upper mantle peridotites and overlying cumulate rocks in order to assess potential variations in the magma budget the position of this interface however generally is not constrained by geophysical studies geochemical variations in basaltic glasses collected near fracture zones are consistent with a decrease in partial melting as fracture zones are approached but they could also be produced by variations in open system mag matic processes with no change in the extent of partial melting although a decrease in the magma budget as fracture zones are approached is consistent with these data so are alternative models that incorporate a constant magma budget

fault rotation and core complex formation significant processes in seafloor formation at slow spreading mid ocean ridges mid atlantic ridge 13 15 n 1 the region of the mid atlantic ridge mar between the fifteen twenty and marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension normal faults show large amounts of rotation dome shaped corrugated detachment surfaces core complexes intersect the seafloor at the edge of the inner valley floor and extinct core complexes cover the seafloor off axis we have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments 13 and 15 n segments steep outward facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30 the rotation occurs very close to the ridge axis as much as 20 within 5 km of the volcanic axis and is complete by 1 my producing distinctive linear ridges with roughly symmetrical slopes this morphology is very different from linear abyssal hill faults formed at the 14 n magmatic segment which display a smaller amount of rotation typically 15 we suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults if faults are long lived a dome shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex a single ridge segment can have several active core complexes some less than 25 km apart that are separated by swales we present two models for multiple core complex formation a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales either model can explain the observed morphology

ice banding as a response of the coupled ice ocean system to temporally varying winds in this study we model formation of ice bands in the marginal ice zones a one dimensional coupled ice ocean model is used in which the ice model is coupled to a reduced gravity ocean model through interfacial stresses the internal ice stresses are important only at high ice concentrations 90 100 otherwise the main balance for the ice motion is between the air ice stress and the ice water stress i e free drift the drag coefficients were chosen so that the air ice momentum flux is 3 times greater than the air ocean momentum flux thus the ekman transport is larger under the ice than in the open water so that winds parallel to the ice edge with the ice on the right produce upwelling the upwelling simulation was extended to include temporally varying forcing which was chosen to vary sinusoidally with a 4 day period this forcing resembles successive cyclone passings perpendicular to the ice edge when the oceanic upper layer was thin which means that the dynamics are strongly nonlinear the ice bands were formed the up downwelling signals do not disappear in wind reversals because of nonlinear advection this leads to convergences and divergences in oceanic and ice velocities that manifest themselves as ice banding at least one wind reversal is needed to produce one ice band

inferring upper mantle structure by full waveform tomography with the spectral element method summary mapping the elastic and anelastic structure of the earth s mantle is crucial for understanding the temperature composition and dynamics of our planet in the past quarter century global tomography based on ray theory and first order perturbation methods has imaged long wavelength elastic velocity heterogeneities of the earth s mantle however the approximate techniques upon which global tomographers have traditionally relied become inadequate when dealing with crustal structure as well as short wavelength or large amplitude mantle heterogeneity the spectral element method on the other hand permits accurate calculation of wave propagation through highly heterogeneous structures and is computationally economical when coupled with a normal mode solution and applied to a restricted region of the earth such as the upper mantle sem importantly sem allows a dramatic improvement in accounting for the effects of crustal structure here we develop and apply a new hybrid method of tomography which allows us to leverage the accuracy of sem to model fundamental and higher mode long period 60 s waveforms we then present the first global model of upper mantle velocity and radial anisotropy developed using sem our model semum confirms that the long wavelength mantle structure imaged using approximate semi analytic techniques is robust and representative of the earth s true structure furthermore it reveals structures in the upper mantle that were not clearly seen in previous global tomographic models we show that semum favourably compares to and rivals the resolving power of continental scale studies this new hybrid approach to tomography can be applied to a larger and higher frequency data set in order to gain new insights into the structure of the lower mantle and more robustly map seismic structure at the regional and smaller scales

air ice drag coefficients for first year sea ice derived from aircraft measurements turbulent flux measurements were made at four levels 42 90 195 and 340 m by the noaa p 3 aircraft over first year sea ice in the northern bering sea during february 1982 three profiles of momentum flux and mean wind were used to calculate an air ice drag coefficient cd of 3 0 6 10 3 referenced to a 10 m anemometer height the boundary layer was slightly unstable zi l 1 2 where zi was the inversion height of 660 m and l the monin obukhov length the mean wind speed at the 42 m height was 17 m s and the air temperature was 20 c from turbulent heat flux measurements the value of the bulk heat transfer coefficient ch was 0 73 1 6 10 3 giving a ch cd of 0 24 the bowen ratio was greater than 2 8 comparison of the present turbulent flux and variance profiles with those collected over the ocean shows agreement which increases confidence in the calculations the geostrophic drag coefficient u g where u is the friction velocity and g is the geostrophic wind was 0 047 the turning angle of the surface wind measured by an anemometer at a height of 3 m on the ice was 32 and the ratio of surface wind speed to the geostrophic wind speed was 0 76

geologic evidence for the plate driving mechanism the continental undertow hypothesis and the australian antarctic discordance investigations of the pattern of flow in the mantle have been carried out almost exclusively by theoretical geophysicists mainly fluid dynamicists because mantle flow has been considered too deep and too disconnected to leave direct traces in surface tectonic geology this geophysical approach has not yet provided a detailed understanding of mantle flow the assumption of invisibility of mantle flow from the surface should be reassessed the tectonic history of gaps in the circum pacific ring of continents caribbean scotia sea australian antarctic discordance suggests that upper mantle return flow is escaping from the pacific hemisphere as the pacific ocean contracts and is avoiding the regions beneath continents major continental masses may thus have the deep roots suggested by some seismologists if these roots are coupled to the lower mantle plates that include continents may be driven by continental undertow in this case continental motions may tell us the pattern of lower mantle flow and the resulting four celled lower mantle picture is remarkably simple a key area for testing the continental undertow hypothesis is the australian antarctic discordance a low saddle on the spreading ridge between these two continents in the continental undertow view upper mantle flow escaping the pacific is met in the discordance by upper mantle flow originating at the java sumatra trench with the bathymetric low representing the sink toward which this converging flow is moving two new lines of evidence support this interpretation 1 isotope geochemistry of ridge basalts from the discordance shows that mantle sources with distinctive pacific ocean and indian ocean characters meet precisely at the discordance 2 seismic tomography reveals cool upper mantle under the discordance which argues for feeding of this ridge segment laterally rather than from beneath hot spot tracks in and just east of australia appear to argue against the continental undertow hypothesis but do not allow an unambiguous test of the continental undertow hypothesis mantle flow patterns may in fact be partly visible from the surface and careful evaluation of key regions by tectonic geologists may provide a fruitful approach to understanding mantle dynamics

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

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

water rich basalts at mid ocean ridge cold spots although water is only present in trace amounts in the suboceanic upper mantle it is thought to play a significant role in affecting mantle viscosity melting and the generation of crust at mid ocean ridges the concentration of water in oceanic basalts1 2 has been observed to stay below 0 2 wt except for water rich basalts sampled near hotspots and generated by wet mantle plumes3 4 5 here however we report unusually high water content in basaltic glasses from a cold region of the mid ocean ridge system in the equatorial atlantic ocean these basalts are sodium rich having been generated by low degrees of melting of the mantle and contain unusually high ratios of light versus heavy rare earth elements implying the presence of garnet in the melting region we infer that water rich basalts from such regions of thermal minima derive from low degrees of wet melting greater than 60 km deep in the mantle with minor dilution by melts produced by shallower dry melting a view supported by numerical modelling we therefore conclude that oceanic basalts are water rich not only near hotspots but also at cold spots

drag measurements for first year sea ice over a shallow sea for a free drift case a method is developed for simultaneously determining air ca and water cw drag coefficients by solving the vertically integrated stress balance equation for sea ice the method allows the algebraic determination of ca and cw from measurements of relative wind and relative current at single reference levels from estimates of the densities of all three fluids and ice thickness and from estimates of the accelerations caused by the nonsurface stress terms derived from position measurements the method treats the ice as a natural drag plate and therefore includes contributions from both skin and form drag one shortcoming of the technique is that if the relative wind and current are colinear then only the ratio of ca and cw can be determined another is that the accelerations caused by other forces need to be independently determined an experiment was conducted on a single floe in the eastern bering sea over the continental shelf during march 1981 the site was about 80 km from the ice edge and was occupied for approximately 3 days following the passage of a storm which had broken the pack into 10 20 m diameter floes both profile and slab inversion estimates of drag were made from the current meter and anemometer measurements at the site ca at 3 m was 4 55 10 3 by the profile method compared to 3 63 4 39 10 3 for slab depending on assumptions cw at 1 1 m by profile was 24 2 10 3 compared to 18 3 22 0 10 3 for slab the two methods gave results which were not statistically separable and which were among the highest drags observed for sea ice the instantaneous stress balance on the floe included contributions from material acceleration coriolis force the two surface stresses and sea surface tilt relative to mean current and tide

widespread active detachment faulting and core complex formation near 13 n on the mid atlantic ridge oceanic core complexes are massifs in which lower crustal and upper mantle rocks are exposed at the sea floor1 2 3 they form at mid ocean ridges through slip on detachment faults rooted below the spreading axis2 3 4 5 6 to date most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes here we present a survey of the mid atlantic ridge near 13 n containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis the detachment faults are apparently all currently active and at various stages of development a field of extinct core complexes extends away from the axis for at least 100 km our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity preliminary examination of seismicity and seafloor morphology farther north along the mid atlantic ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow spreading ridge than previously suspected

miocene detachment faulting predating epr propagation southern baja california at the southern tip of the baja california peninsula we characterize the onshore structures and kinematics associated with crustal necking leading up to the pliocene breakup and early east pacific rise seafloor spreading from a combination of tectonic field investigations k ar and cosmogenic isotope dating and geomorphology we propose that the los cabos block represents the exhumed footwall of a major detachment fault this north trending detachment fault is marked by a conspicuous low dipping brittle ductile shear zone showing a finite displacement with top to the se ending to the ese this major feature is associated with fluid circulations which led to rejuvenation of the deformed cretaceous magmatic rocks at a maximum of 17 5 ma the detachment footwall displays kilometer scale corrugations controlling the present day drainage pattern this major detachment is synchronous with the development of the san jose del cabo basin where syntectonic sedimentation took place from the middle miocene to probably the early pliocene we propose that this seaward dipping detachment fault accommodates the proximal crustal necking of the baja california passive margin which predates the onset of formation of the east pacific rise spreading axis in the cabo puerto vallarta segment our data illustrate an apparent anticlockwise rotation of the stretching direction in baja california sur from 17 ma to the pliocene

modulation of hurricane activity in the gulf of mexico by the madden julian oscillation the madden julian oscillation mjo is a large scale episodic modulation of tropical winds and precipitation that travels eastward from asia to america with a characteristic repeat time of 30 to 60 days here it is shown that when mjo wind anomalies in the lower troposphere of the eastern pacific are westerly gulf of mexico and western caribbean hurricane genesis is four times more likely than when the mjo winds are easterly accurate predictions of the mjo may lead to improved long range forecasts of tropical cyclone activity

development of the australian antarctic depth anomaly the oceanic australian antarctic discordance aad contains two unusual features 1 n s trending anomalously deep bathymetries and 2 rough basement morphologies in young 45 spreading obliquities

shearing within lower crust during progressive retrogression structural analysis of gabbroic rocks from the godzilla mullion an oceanic core complex in the parece vela backarc basin abstract microstructural and petrological analyses of gabbroic rocks sampled from the godzilla mullion located along the parece vela basin spreading ridge parece vela rift philippine sea reveal the development of a ductile shear zone in the lower crust the shear zone is interpreted to represent a detachment fault within an oceanic core complex microstructures indicative of intense deformation characterized by porphyroclastic textures consisting dominantly of coarse plagioclase porphyroclasts and lesser clinopyroxene porphyroclasts in a fine grained matrix are observed within samples of gabbroic rocks dredged near the breakaway area of the godzilla mullion dredge site d6 samples are classified into three types based upon the grain size of fine grained plagioclase in the matrix coarse 80 130 m medium 25 m and fine 10 m although the chemical composition of plagioclase porphyroclasts is consistently an 40 50 among all sample types the compositions of fine grains in the matrix vary with decreasing grain size being an 40 50 for the coarse type an 30 40 for the medium type and an 5 30 for the fine type this finding implies that the composition of fine grained plagioclase in the matrix is related to the following retrograde reaction that occurred during deformation clinopyroxene plagioclase fe ti oxide fluid hornblende plagioclase plagioclase crystal preferred orientations also show a gradual change with grain size varying from a 010 100 pattern for the coarse type 010 100 and 001 100 patterns for the medium type and a weak 001 100 pattern or random orientations for the fine type these patterns are interpreted to result from a change in the deformation mechanism of plagioclase from dislocation creep to grain size sensitive creep with decreasing temperature thereby leading to strain softening and localization during cooling although secondary amphibole occurs ubiquitously within all samples the chemical composition of amphibole varies from pargasitic hornblende i e brown hornblende to actinolite i e green hornblende within each of the sample types however amphibole in the coarse type shows no evidence of deformation whereas brown hornblende in the medium and fine types is plastically deformed as a consequence we argue that the microstructural development of the gabbroic rocks occurred during uplift related cooling of the gabbro body and that a primary shear zone developed near the breakaway area at depth under anhydrous conditions at high temperatures above 850 c the shear zone subsequently evolved during progressive retrogression in association with hydration of the shear zone possibly resulting in the development of the detachment fault that gave rise to the godzilla mullion

sr nd hf isotopes along the pacific antarctic ridge from 41 to 53 s 1 major trace element and sr nd hf isotope data in basalts collected along the pacific antarctic ridge par axis between 53 and 41 s far from any hotspot influence reveal tight coherent geochemical variations within the depleted morb mantle all samples are located below the pacific reference line defining two sub oceanic mantle domains on each side of the easter microplate the data extend the par 66 53 s field towards more radiogenic sr 0 70264 less radiogenic nd 7 7 and hf 11 4 values the along ridge geochemical variability is closely related to the morphological segmentation of the ridge anomalous geochemical features are attributed to the atypical morphology of two segments due to the presence of off axis magmatism the first order ridge discontinuity defined by the menard transform fault separates two slightly different mantle domains each with its own history

upper mantle heterogeneities and dynamics the width of geochemical and residual elevation anomalies along the mid ocean ridge system decrease as both the distance between migrating ridge segments and related hotspots and the local geochemical dispersion in the anomalies increase these observations provide further evidence that flows of mantle plumes in the upper mantle are deflected by migrating ridge sinks once they override hotspots

development of oceanic detachment and asymmetric spreading at the australian antarctic discordance 1 the largest known oceanic detachment terrains occur in segment b3 of the australian antarctic discordance aad using newly collected bathymetry magnetic and gravity data we show that segment b3 is divided into two contrasting second order segments the western subsegment b3w is characterized by well ordered ridge parallel abyssal hills and low mantle bouguer gravity anomalies the eastern subsegment b3e displays rough chaotic morphology and includes several megamullions characterized by high mantle bouguer gravity anomaly values the crust is estimated to be thinner by a maximum of 3 km in southern b3e the combination of chaotic morphology with thinner crust supports the idea that the megamullions are exposed footwalls of oceanic detachments megamullion terrains are characterized by higher magnetization than adjacent terrains most likely as a result of serpentinization of peridotite exposed at the detachment surfaces detachment surfaces constitute up to 70 of the total area of both ridge flanks younger than 2 ma in b3e indicating that oceanic detachments have played a major role in its development spreading in b3e has been extremely asymmetric with higher apparent rates associated with the large detachment surfaces where up to 75 of the total extension occurred similar asymmetric spreading on oceanic detachments is also recognized in segment b4 suggesting that this is the dominant mode of extension associated with cold mantle and low magma supply in this deepest part of the aad where it is confined to a mere 100 km long section of the aad spreading axis

sea ice drift near bering strait during 1982 six argos ice stations were deployed in the northeastern bering sea in january and february 1982 four moved north through bering strait and made between one and five passes through the strait before they sank or were crushed by the ice and two remained in western norton sound and shpanberg strait net winds in the region were directed toward the southwest and were relatively uniform geographically net currents were northward and both mean velocity and mean speed were greater at bering strait than at shpanberg strait norton sound contributed ice northward through bering strait in the mean during winter and spring there was also a net divergence of ice near cap lisburne a theory for open shelf shallow water ice drift is developed that adds terms for bottom drag and stress feedback from the water column a stress analysis for a station that remained in western norton sound and shpanberg strait showed that bottom drag did not contribute significantly but that stress feedback from the accelerated shallow water current can be of the same order as the coriolis force however the analysis contained a residual possibly associated with internal ice stress coastal setup or unknown measurement error which was of roughly the same order as the wind and current stress

global equivalent magnetization of the oceanic lithosphere abstract as a by product of the construction of a new world digital magnetic anomaly map over oceanic areas we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the world s ocean this map reveals consistent patterns related to the age of the oceanic lithosphere the spreading rate at which it was formed and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere as for the age the equivalent magnetization decreases significantly during the first 10 15 myr after its formation probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration then increases regularly between 20 and 70 ma reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization as for the spreading rate the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate with a threshold at 40 km myr in agreement with an independent global analysis of the amplitude of anomaly 25 this result combined with those from the study of the anomalous skewness of marine magnetic anomalies allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate finally specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures such as the cold australian antarctic discordance marked by a lower magnetization and several hotspots marked by a high magnetization

chaotic topography mantle flow and mantle migration in the australian antarctic discordance oceanic crust formed over the past 30 million years at the australian antarctic discordance aad is characterized by chaotic sea floor topography reflecting a weak magma supply from an unusually cold underlying mantle during the past 3 4 million years however a source of increased magma supply coinciding with the known indian pacific mantle isotopic boundary has propagated into the eastern aad displacing the chaotic terrain and replacing it with normal sea floor pacific mantle reached the eastern boundary of the aad at least 7 million years ago but it was not until 3 4 million years ago that lavas derived from pacific mantle were first erupted within the aad this long hiatus combined with the ridge transform geometry across the aad boundary constrains the locus of mantle migration to a narrow relatively shallow region directly beneath the spreading axis of the southeast indian ridge

evidence for variable upper mantle temperature and crustal thickness in and near the australian antarctic discordance abstract the southeast indian ridge seir in and near the australian antarctic discordance aad exhibits at a constant spreading rate almost the full range of the many geophysical and geochemical parameters characteristic of the slow mid atlantic ridge and fast east pacific rise we used satellite derived gravity data in combination with seamarc ii bathymetry in and near the aad to examine regional density variations in the upper mantle beneath the aad through three dimensional gravity analysis we found that at least two end member models satisfy the gravity observations regional crustal thickness variations of at least 3 km along the seir near the aad or a temperature anomaly of the order of 150 c in the upper mantle beneath the seir these new observations combined with other geophysical and geochemical characteristics of the australian antarctic discordance provide further evidence that the temperature structure of a mid ocean ridge is a controlling factor in addition to spreading rate in the crustal accretionary process numerical models of mantle flow beneath mid ocean ridges offer one means of investigating the dynamic effect of a variable upper mantle temperature on the accretionary process our results indicate that temperature is important especially at intermediate and slower spreading rates where thermal effects can dominate mantle flow beneath a mid ocean ridge and result in increasing crustal production with decreasing spreading rate at the constant intermediate spreading rate of 37 mm yr characteristic of the seir in and near the aad our numerical models show that significant crustal thinning 2 4 km can occur with relatively small variations in upper mantle temperature all else being equal thus combined with our end member gravity models these observations and results suggest that both anomalously cool upper mantle and thin crust exist beneath the aad

westward migration of pacific ocean upper mantle into the southern ocean region between australia and antarctica the southern ocean south of the great australian bight hosts a globally anomalous region of mid ocean ridge the australian antarctic discordance in addition to its recognized morphological and geophysical anomalies the discordance represents the current on axis location of a proposed isotopic boundary between indian ocean and pacific ocean upper mantle convective regimes previously defined from analyses of 4 ma mid ocean ridge basalts morb pb sr and nd isotope analyses of 36 66 ma seafloor dredged from either side of the northward extrapolation of the australian antarctic discordance show that 36 ma seafloor to the east has an indian ocean morb isotopic signature this indicates that the indian pacific isotopic boundary does not extend directly north of the ridge toward the southern margin of australia progressive westward migration of an arcuate shaped front of pacific ocean upper mantle therefore appears to be a consequence of australian antarctic rifting and southern ocean opening suggesting that the current location of the isotopic boundary within the discordance may be entirely coincidental

fault systems at hyper extended rifted margins and embryonic oceanic crust structural style evolution and relation to magma abstract the study of seismic sections derived from the magma poor australia antarctica rifted margins enables to investigate the structural style and evolution of fault systems and their relation to magmatic additions during final rifting and lithospheric breakup the aim of this study is thus to better characterize fault systems their organization in time and space as well as their relationship to magmatic systems our observations show that extensional detachment faults related to mantle exhumation form out of sequence and initiate as normal upward concave faults the observations can be best explained by an alternation between a pure shear mode development of symmetric arrays of normal faults and a simple shear mode development of detachment faults with a continuous delocalization and re localization of the deformation in the footwall of the previous exhumation fault this model suggests that the asthenospheric uplift is slow and decoupled from the deformation in the brittle and serpentinized upper basement layer the presence of such a decollement level possibly formed by the serpentinisation front has a major control on the style and organization of faulting the polyphase out of sequence organization of faults can explain that despite mantle exhumation is accommodated along multiple asymmetric exhumation faults the overall exhumed mantle domain is mostly symmetric at the scale of both margins

detailed tectonic evolution of the reykjanes ridge during the past 15 ma 1 we present a new detailed tectonic model of the reykjanes ridge which examines the rift propagation hypothesis for the v shaped ridges and its asymmetric lithospheric accretion four major southward rift propagations extend through our entire survey area and several additional small scale rift propagations are observed including northward propagators if plume pulses only drive southward propagators then two different driving mechanisms for propagators must exist there is a major difference in the crustal accretion asymmetry between the area immediately off the iceland shelf and that farther south both in rift propagation pattern and free air gravity lineations furthermore we identify two small offset features coined ponsu transforms from which rift propagation is both initiated and stopped the pattern of the v shaped ridges on the reykjanes ridge is not symmetric about the reykjanes ridge and the v shaped ridges are not linear continuous features our rift propagation model produces excellent fits to magnetic data and provides a self consistent model for the evolution of the reykjanes ridge during the past 15 ma

geophysical signatures of oceanic core complexes author posting the authors 2009 this article is posted here by permission of john wiley sons for personal use not for redistribution the definitive version was published in geophysical journal international 178 2009 593 613 doi 10 1111 j 1365 246x 2009 04184 x

seafloor spreading on the southeast indian ridge over the last one million years a test of the capricorn plate hypothesis abstract plate motions in the indian ocean are inconsistent with a rigid indo australian plate an equatorial diffuse boundary dividing the plate into separate indian and australian plates significantly improves the fit of kinematic plate models to the spreading rates transform azimuths and earthquake slip vectors on the spreading center boundaries an additional boundary further dividing the australian plate into australian and capricorn plates has been proposed to account for much of the remaining inconsistency and the pattern of intraplate earthquakes j y royer r g gordon science 277 1997 1268 1274 the proposed boundary is 2000 km wide where it intersects the southeast indian ridge several recent geophysical cruises to the southeast indian ridge including a cruise within the proposed boundary provide many new data for investigating the validity of the capricorn plate model these new observations strongly support the hypothesis that the capricorn plate exists statistical tests of the data from the southeast indian ridge alone are not sufficient to confirm it but motion about the rodriguez triple junction rtj suggests some non rigidity in the antarctica australia somalia circuit inferred deformation with enforced closure about the rtj leads to an estimate of plate motion consistent with the capricorn plate model however the diffuse capricorn australia boundary does not extend south of the st paul fracture zone 800 km narrower than the previously proposed boundary

toward a minimum change model for recent plate motions calibrating seafloor spreading rates for outward displacement summary we use seafloor spreading distances derived from reconstructions of more than 7000 crossings of young magnetic anomalies along seven plate boundaries to study outward displacement a source of systematic bias in estimates of seafloor spreading rates which is caused by a combination of processes that shift magnetic polarity transition zones away from their idealized locations linear regressions of 81 independent sequences of seafloor opening distances as a function of their magnetic reversal ages for anomalies younger than c3n 1 4 19 ma yield 75 positively valued intercepts for zero seafloor age confirming the ubiquitous outward shift of magnetic reversals reported by previous authors grouping these data into 29 locally consistent clusters yields better constrained zero age intercepts that are uniformly positive and average 2 2 0 3 km globally these values which are 1 3 km at most locations and are significantly larger 3 5 km along the well surveyed reykjanes and carlsberg ridges agree well with published magnetic polarity zone transition widths which are estimated directly from nearbottom seafloor magnetic measurements significant variations in outward displacement along the southeast indian ridge are strongly correlated with changes in axial morphology and axial depth however a similar correlation is not observed along other ridges forward magnetic anomaly modelling suggests that variations in outward displacement can be explained by differences in the magnetic source layers that are assumed to characterize different spreading centres if not corrected for outward displacement the implied systematic upward biases in seafloor spreading rates which are averaged over the width of anomaly 1 the youngest reversal that is used for plate reconstructions range from 6 mm yr 1 along the reykjanes ridge where outward displacement is 4 5 km to 3 mm yr 1 along ridges where outward displacement approximates the global average of 2 km

do the 1998 antarctic plate earthquake and its aftershocks delineate a plate boundary plate motion data along the eastern southeast indian ridge seir are not fit by a rotation pole consistent with data along the western seir demets et al 1988 looked for but did not find evidence of a microplate north of the seir to explain the discrepancy we test the hypothesis that the large mw 8 2 strike slip 1998 antarctic earthquake lies on the boundary of a microplate here termed balleny south of the seir that explains the plate motion discrepancy a small circle about an antarctic balleny euler pole determined by plate closure fits the east west band of aftershocks consistent with a strike slip boundary however the antarctic balleny pole predicts right lateral slip while the seismicity exhibits left lateral strike slip motion the opposite sense of slip not only rules out the balleny plate hypothesis but requires deformation elsewhere to accommodate both the plate motion differences along the seir and the strain accumulated from the antarctic event

thermal regime of the southeast indian ridge between 88 e and 140 e remarks on the subsidence of the ridge flanks 1 the flanks of the southeast indian ridge are characterized by anomalously low subsidence rates for the 0 25 ma period less than 300 m ma 1 2 between 101 e and 120 e and less than 260 m ma 1 2 within the australian antarctic discordance aad between 120 e and 128 e the expected along axis variation in mantle temperature 50 c is too small to explain this observation even when the temperature dependence of the mantle physical properties is accounted for we successively analyze the effect on subsidence of different factors such as variations in crustal thickness the dynamic contribution of an old detached slab supposedly present within the mantle below the aad and depletion in m a parameter here defined as the ubiquitously distributed melt fraction within the asthenosphere these effects may all contribute to the observed anomalously low subsidence rate of the ridge flanks with the most significant contribution being probably related to the depletion in m however these effects have a deep seated origin that cannot explain the abruptness of the transition across the fracture zones that delineate the boundaries of the aad near 120 e and near 128 e respectively

residual depth anomalies and the origin of the australian antarctic discordance zone a new high resolution depth anomaly map covering the anomalously deep and rough australian antarctic discordance aad has been constructed using crustal ages derived from a detailed aeromagnetic survey the map shows a large arcuate shaped negative depth anomaly that is centered on the southeast indian ridge and trends nne across the australian plate sse across the antarctic plate within this broad scale feature two prominent depth anomaly lows are observed at 45 s 128 e the northern flank and 54 s 125 e the southern flank both lows are associated with 15 ma oceanic crust the observed depth anomaly patterns are compared with the distinctive patterns predicted by coldspot downwelling limbs of convection cells and thin crust models of the discordance source the observed depth anomaly does not result from absolute plate motions over a fixed coldspot source because the predicted ene trend on the antarctic plate is not in agreement with the sse trend observed the symmetric arrangement of the large scale depth anomaly and prominent lows about the ridge axis suggests instead a source that has varied in strength but remained located at the ridge axis as the ridge migrated northeastward in the absolute reference frame the organized pattern of elongated depth anomaly highs and lows predicted for upper mantle convection cells is not evident in the observed depth anomaly map thus a convergence of downwelling limbs of convection cells beneath the discordance is not indicated if the source of cooler upwelling that produces less magma and hence thin crust has not varied over time nor migrated along the ridge then the predicted depth anomaly would persist unchanged with distance from the ridge axis and trend in the direction of relative plate motion parallel to fracture zones the observed depth anomaly trends obliquely across fracture zones and changes in both amplitude and location relative to the ridge axis and is therefore not consistent with cool upwelling producing thin crust to explain the features of the depth anomaly map we propose that asthenospheric material flowing from the amsterdam hotspot in the west and the balleny and tasmantid hotspots in the east collides within the discordance propagating rifts converging on the aad provide evidence for such asthenospheric flow attenuated shear velocities beneath the george v fracture zone complex and lavas geochemically identical to those from propagating rifts associated with hotspots suggest a thermal anomaly producing additional asthenospheric flow east of the aad the increased flow and greater proximity of the discordance to the thermal anomaly and hotspots to the east produce a higher pressure gradient and hence greater driving force which results in a westward migration of the collision zone with time seafloor spreading over the westward moving collision zone has produced the observed arcuate shaped anomaly with the accompanying oblique depth anomaly trends we cannot decipher from depth anomalies alone whether the converging flows downwell within the aad or simply mix with upwelling materials