on the interpretation of linear relationships between seafloor subsidence rate and the height of the ridge summary several studies have found that the subsidence rate of the oceanic lithosphere measured along flow lines is a linear function of the height of the ridge this might have very important implications concerning our understanding of the cooling of the lithosphere here we study the relationship between the subsidence rate and the height of the ridge in the south atlantic and east indian oceans from the regression slopes of this relationship it has been inferred that lithosphere created at shallower ridges subsides faster and to deeper levels than that generated at deeper ridges we show however that the topography at young ages for various seafloor segments stays positively correlated with the topography at older ages synthetic data sets are obtained by adding random synthetic signals with the same spectrum as the residual topography to the average depth versus age relationship they are also characterized by the same linear relationship between subsidence rates and the height of the ridge this relationship is therefore not proof that lithosphere formed at shallow ridges cools faster than lithosphere formed at deeper ridges random signals linked with crustal thickness variations hotspot bulges and perhaps large wavelength dynamical topography would also yield such a relationship

crustal and upper mantle structure of northern tibet imaged with magnetotelluric data 1 magnetotelluric mt data were collected in northern tibet along the amdo to golmud highway during the 1995 and 1999 project indepth international deep profiling of tibet and the himalaya surveys broadband and long period mt data were collected and the te mode tm mode and vertical magnetic field data were inverted to yield a minimum structure two dimensional resistivity model the model obtained from inverting all responses simultaneously shows that a pervasive midcrustal conductor extends from the kunlun shan to the bangong nuijiang suture the vertically integrated conductivity conductance of this crustal layer is greatest in the northern qiangtang terrane at latitude 34 n the electrical resistivity of the upper mantle is constrained by the mt data to be in the range of 10 30 m across the songpan ganze and qiangtang terranes this is lower than would be expected if asian lithosphere underthrusts northern tibet as far as the qiangtang terrane the mt responses are more consistent with a model in which asian lithosphere extends as far south as the kunlun shan and the upper mantle beneath the songpan ganze and qiangtang terranes is sufficiently hot to contain a small fraction of interconnected partial melt

the volcanic tectonic cycle of the famous and amar valleys mid atlantic ridge 36 47 n evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections amar 3 the 1978 amar expedition extended the investigation of the mid atlantic ridge that was begun by the 1973 1974 famous expedition from the northern end of the famous rift the original famous area to the narrow central and southern end of the famous rift narrowgate region and into the broad amar valley south of fracture zone b available field and geochemical data allow us to characterize in detail the volcanic tectonic cycle for these two segments of the mid ocean ridge system a dynamic model of the crust forming processes within these two rift valley segments is deduced from 1 variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins 2 composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution and 3 detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition the decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes the common evidence of crystal resorption and the apparent disequilibrium between megacrysts phenocrysts and their enclosing glass this indicates control by a crustal magma chamber liquid and crystal compositions and textural relations can best be explained by a three stage crystallization history 1 crystallization of olivine plagioclase and clinopyroxene onto the floor of a magma chamber 2 partial resorption of mineral phases by a superheated undersaturated liquid above the floor 3 intratelluric crystallization of olivine and plagioclase clinopyroxene during rise of melt to the surface through the conduit system above the magma chamber a magma chamber model is suggested for the amar famous rift valleys that satisfies both seismic and geological constraints we propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment amar and narrowgate regions but that these thin and terminate near the intersecting fracture zones expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization the famous narrowgate rift is currently in a contraction period so that in the northern famous region primitive liquid has erupted at mount venus and mount pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the narrowgate region

extreme isotopic homogeneity among basalts from the southern east pacific rise mantle or mixing effect basaltic lavas which have erupted along the great oceanic ridge system are characterized by their overall chemical and isotopic homogeneity nevertheless they show evidence for variability in the chemical and isotopic composition of the ocean ridge basalt orb source whose scale and distribution remain uncertain isotopic compositions of sr nd pb and hf and ratios of some highly incompatible elements have been used as tracers of orb source compositions usually permitting source characteristics to be distinguished from petrogenetic effects the mid atlantic ridge mar in particular shows correlations in the isotopic compositions of several elements between isotopic compositions and topography and between these parameters and incompatible element concentrations and ratios1 3 we report here isotopic measurements for basalts dredged between approximately 30 and 34 s on the east pacific rise and adjacent maddingly rise which show highly uniform sr and nd isotopic compositions possibly resulting in part from mixing associated with a high rate of magma generation and rapid spreading 170 mm yr 1 however a ridge segment of comparable length between 18 and 22 s and with only a slightly lower spreading rate shows much greater isotopic variability hence we are apparently sampling small but detectable regional differences in mantle composition the isotopic homogeneity among the basalts from 30 to 34 s implies that any heterogeneity in the underlying upper mantle is uniformly distributed on a scale much smaller than the volume sampled in individual magma batches and is thus averaged out in the resulting lavas

depth versus age new perspectives from the chemical compositions of ancient crust petrological data provide a new approach to an evaluation of the depth age problem for ancient seafloor the correlations among basalt chemical composition axial depth and mantle temperature at current ocean ridges allow the determination of initial depth and mantle temperature for any portion of ancient seafloor that was created at a spreading center provided the chemical composition of the ancient crust is determined it is then possible to calculate a petrologically constrained depth at any age which can be compared to observed depths and depths from the classical half space models we evaluate data from dsdp and odp drill holes on crust older than 80 ma considering chemical composition back tracked depth and crustal thickness the data are complex and interpretation of their chemical composition requires consideration of alteration absence of glass compositions data quality and the influence of off axis volcanism and near ridge hot spots to check and expand the data set we develop and use trace element proxies for major element compositions since many trace element ratios are less influenced by alteration and by variable proportions of phenocrysts the twenty drill holes for which reliable data can be obtained are well distributed around the globe and include multiple sites on old crust in the atlantic pacific and indian ocean basins comparison of the chemical and crustal distributions between ancient and current n morb show that the oceanic crust older than 80 ma has significantly lower na 8 0 z r y sm ybn and higher cao al2o3 f e 8 0 and crustal thickness quantitative modeling of these results suggests that the mantle was hotter in this time period by about 50oc that the crust was several hundred meters shallower and 1 2 km thicker these observations show that half to two thirds of the observed flattening relative to a half space model is due to the change in mantle temperature and crustal composition thus only a few hundred meters of flattening by plate reheating by hot spots or by other mechanisms is required these results are consistent with the existence of abundant oceanic plateaus even at fast spreading rates in the mesozoic and with the apparent thickening of ocean crust with time 1999 elsevier science b v all rights reserved

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

petrogenetic implications of tholeiitic basalt glasses from the east pacific rise and the galapagos spreading center basalt glasses dredged from the east pacific rise and the galapagos spreading center show a wide range of composition with fm values feototal feototal mgo from 0 49 to 0 72 analyses for major elements of 40 unaltered glasses and of 17 of these glasses for trace elements demonstrate an analogous line of differentiation over a considerable distance in the east pacific using data from the experiments of peridotite partial melting and their implication for primary melts the composition of the initial basalt magma in equilibrium with the residual phase is extrapolated to be fm 0 40 0 48 the importance of apatite introduced from the parental peridotite for the incompatible elements ree f and p2o5 within various basalt differentiation series is demonstrated

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

comparative volcanology of small axial eruptive centers in the mark area we describe selected examples of small volcanic features identified during ocean drilling program odp site surveys in the mark area for which additional observations and sample data have been obtained by manned submersible and or drilling based on both morphologic relations and similarities in chemistry and petrography serocki volcano the site of bare rock drilling on odp legs 106 and 109 appears to be a flat topped megatumulus a large rootless vent developed on a lava flow originating from an adjacent cone located above the true eruptive fissure a linear set of five apparent volcanic centers on the east side of the median valley the eastmark cones differ from one another in morphology and degree of tectonic degradation they formed at different times and from different vents but all were localized by damming of the flows against the steep walls of the linear valley containing them the two northernmost features are probably megatumuli at the terminal ends of lava flows descending the valley walls the other three vents are cones with breached craters and probably were more recently erupted in situ the snakepit hydrothermal area is located near the highest and widest part of a 35 km long ridge that appears to be a major relatively young fissure eruption the associated lavas are among the geochemically most primitive in the mark area but have their counterparts in several subsets of lavas associated with serocki volcano in spite of the morphologic contrasts and large geographic separation between the two features we conclude that a volcano like form geographic proximity and conformance to a linear geometry do not guarantee compositional or temporal coherence nor is the alignment of apparent volcanic centers a reliable guide to underlying tectonic patterns the chemistry and petrography of the serocki and eastmark basalts resemble older adjacent lavas and also conform to subtle along ridge variations this and the relatively old appearance of their flow surfaces indicate that these lavas represent the waning stage of an older magmatic event the more recent eruptions in the snakepit area seem to reflect a renewed influx of the same relatively primitive and presumably hotter magma that initiated the serocki eruptions residual pockets of this hotter magma in the shallow crust may provide the heat that drives the present hydrothermal system

mechanical control of oceanic plate boundary geometry we present a global analysis of oceanic plate boundary geometry based on the mechanics of relative plate motion at mid ocean ridges and transform faults if the observed geometry formed by the first order segmentation of oceanic plate boundaries represents a state of mechanical equilibrium we find the relative strength of spreading ridges and their bounding transform faults to be fundamental to its organisation a consideration of power dissipation along adjacent lengths of spreading ridge and transform fault leads to a simple relationship between spreading obliquity and relative strength increased spreading rate is found to decrease the strength of spreading segments relative to transform faults proximity to an active hotspot reduces the relative strength of spreading ridges

structural constraints in neotectonic studies of thrust faults from the magnetotelluric method kochkor basin kyrgyz republic 1 a continuous high resolution magnetotelluric mt profile 5 4 km long reveals a 1 to 2 km thick section of neogene strata thrust beneath a crystalline range front along a gently dipping decollement in the kyrgyz tien shan central asia the imaged decollement links the intrabasin akchop hills fault which currently accommodates 15 of the total shortening across the central tien shan to a crustal ramp that dips 45 beneath the range bounds from the mt data on the base of the sediments and on the dip of the crustal ramp suggest 2 5 7 5 km of total shortening across the basin margin of which at least 2 5 4 8 km clearly involves the decollement and the recently developed akchop hills fault thus a significant fraction of the total shortening across the margin has been accommodated by this young fault system combined with preliminary studies that suggest a mid miocene age for the base of the stratigraphic section results drawn from the mt section indicate that shortening rates have increased since the onset of deformation perhaps within the past 1 2 million years

volatiles in basaltic glasses from the east pacific rise at 21 n implications for morb sources and submarine lava flow morphology pillow rim glasses from a suite of moderately evolved lavas erupted along the axis of the east pacific rise epr at 21 n were analyzed by high temperature mass spectrometry for volatile content concentrations of h2o f and cl in the 21 n glasses are low and correlate inversely with mg yielding well defined trends these and other geochemical data indicate derivation of the 21 n glasses from similar parental magmas produced from a highly depleted nearly homogeneous source 1 total volatile h2o f and s contents are lower and co2 content is somewhat higher in these samples than in previously analyzed mid atlantic ridge mar glasses 21 n glasses are similar in volatile content to glasses from the galapagos spreading center gsc at 95 w except in cl lower and co2 higher unlike most mar and gsc glasses co2 is the dominant volatile in all of the 21 n glasses with mg 62 all of the epr and gsc glasses contain reduced carbon species co and ch4 unlike most previously analyzed mar samples these data indicate that sources for mid ocean ridge basalts are extremely volatile poor 0 10 wt and are probably dominantly reduced below quartz fayalite magnetite buffer sheet flow and pillow basalts contain identical volatile contents thus volatile abundance is not a factor controlling flow morphology extrusion rate and or surface topography are probably the most important factors influencing flow morphology in submarine basalts h2o release patterns are however related to flow morphology mass pyrograms for sheet flow and pillow basalt glasses both show bimodal h2o release behavior at the same temperatures 800 c and1000 c 50 c however the dominant h2o release peak for sheet flow glasses is at the lower temperature for pillow rim glasses it is at the higher temperature infrared spectroscopic studies indicate that h2o in these glasses is present only as hydroxyl groups thus the cause of differences in the bimodal h2o release and how it is related to flow morphology is unclear

topography and geoid due to lithospheric mass anomalies summary a model of lithospheric thickness and a recent compilation of moho depths are used to compute the earth s isostatic surface topography and associated gravity anomalies the results are strongly influenced by the uncertainties in lithospheric depth and crustal density profiles the preferred models explain most of the observed topography and are highly correlated with observed gravity anomalies for harmonic degrees larger than 10 comparisons of our residual topography with geodynamical calculations of dynamic topography based on mantle circulation are rather poor

s wave velocity basalt chemistry and bathymetry along the mid atlantic ridge major element chemistry of mid ocean ridge basalt s wave velocity and bathymetry along the mid atlantic ridge are correlated on an intermediate length scale and the large anomalies in these observations are associated with hotspot locations the best correlations are for at depths of 100 200 km and there is no correlation for depths of 300 km or deeper s wave velocities are low directly under the mid atlantic ridge above 100 km but the low velocity regions shift away from the ridge at greater depth and a sinuous strip of asthenospheric low velocity extends along the atlantic hotspots from the azores to tristan da cunha these features suggest that common physical processes in the upper mantle produce these anomalies an inspection of the history of the atlantic ocean suggests that the asthenospheric low velocity strip may record earlier ridge position or possible upwelling in the mantle possible interaction between hotspots and the ridge blurs the distinction between passive and active upwellings

correlations of mid ocean ridge basalt chemistry with the geoid abstract the major element composition of mid ocean ridge basalts morb shows geochemical variations controlled by the temperature pressure and source composition produced by convection in the mantle beneath the spreading centers the long wavelength signal of the geoid reflects deep mantle processes in this paper we characterize the correlations between the major element chemistry of morb and the geoid anomalies at different wavelengths to determine the extent to which the major element composition of the morb reflects deep mantle circulation the chemical effects of the low pressure fractionation have been corrected using a new method based on the feot mgo ratio the fractionation trends modeled by a linear trend in the geochemical parameter space normalized by the mgo denominator converge to a common point with 8 mgo the separation vector d between the fractionation trends and this common point defines a new parameter whose sodium component shows a negative correlation significant at the 99 level with the axial depth for both the mid atlantic ridge mar and the east pacific ridge epr and with the geoid only for the mar the latter correlation suggests that the major element composition of the morb is clearly related to deep mantle convection by material and thermal coupling between the lower and upper mantle the lack of correlation at long or medium wavelengths between the mantle enrichment inferred from the k2o tio2 ratio and the geoid indicates that either source composition does not influence morb chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle

submersible observations at the axis of the ultra fast spreading east pacific rise 17 30 to 21 30 s abstract manned submersible observations conducted with the diving saucer cyana at the axis of the east pacific rise at four sites near 21 30 21 10 18 30 and 17 30 s demonstrate that the ridge evolves between two end members the volcanic end member is characterized by a non rifted or only slightly rifted topographic ridge with widespread but diffuse water venting the tectonic end member is marked by a well defined axial graben whose floor can be the locus of intense sulfide deposition at one site a small stockwork mineralization exposed through normal faulting was observed at the base of the graben wall the dives illustrate the discontinuous nature of volcanic tectonic and hydrothermal activities even in the case of the fastest accretion found in the ocean

spatial and temporal evolution of magmatic systems beneath the endeavour segment juan de fuca ridge tectonic and petrologic constraints major and trace element data for a suite of lavas from fifty six dredges and alven dives on the ridge axis and adjacent abyssal hills have been used to investigate the geometry and evolution of magmatic systems beneath the endeavour segment juan de fuca ridge the morphology of the endeavour segment between the northward propagating cobb offset and the recently formed 0 2 m y endeavour offset is dominated by a shallow rifted elongate crestal volcano endeavour ridge that deepens along strike into a broad deep basin at each offset a set of ridges interpreted to be previous crestal volcanoes rifted apart by spreading flank the endeavour ridge and chronicle the dueling propagator history of the cobb offset the tectonic evidence strongly suggests that a large portion of the endeavour segment may be a failing rift segment at this time lavas from the current axis of the endeavour segment are moderately fractionated mgo 6 8 5 wt and have generally higher sio2 al2o3 na2o and k2o and lower feo man lavas from south of the cobb offset soco lavas incompatible trace element abundances and ratios indicate the endeavour lavas are primarily enriched e morbs and t morbs e g zr nb 7 24 zr y 2 5 5 9 and ba tio2 6 64 in contrast with the soco lavas which are more depleted in character thus the 30 km wide cobb offset appears to mark a major geochemical boundary beneath the juan de fuca ridge in contrast with the endeavour segment axial lavas samples from adjacent abyssal hills are more similar to the soco lavas in their major and trace element characteristics these observations suggest that the parental magmas of the endeavour segment exhibit temporal variability with more enriched material arriving only recently beneath the ridge axis pronounced compositional variability is observed at small spatial scales within the endeavour segment axial lavas which does not correlate with axial morphology this variability is interpreted to reflect ubiquitous small scale mantle heterogeneity and poor mixing of multiple parental magmas during migration from the melt region or within axial magma chambers highly enriched samples zr nb 10 are localized near the summit region of endeavour ridge whereas slightly less enriched samples occur along the length of the axis and on the older flanking ridges recent enrichment may result from diminishing extents of partial melting of a heterogeneous source in response to tectonic reconfiguration causing more fusible enriched domains to dominate the chemical signature of melts produced small scale heterogeneity along strike seems incompatible with models of centralized upwelling of melts beneath the summit region of the ridge axis with shallow lateral injection of melts to distal ends of the segment unless these spatial variations actually reflect temporal variations in the source composition and collapse of the shallow magmatic systems toward the summit region as rift failure has progressed

isotopic evidence for chaotic imprint in upper mantle heterogeneity 1 the intrinsic structure of the isotope data set of samples from the mid atlantic ridge and east pacific rise believed to represent the isotopic composition of their mantle source reveals a close relationship between sample spatial distribution and their geochemical features the spatial distribution of their isotopic heterogeneity is self similar on a scale between 5000 and 6000 km about 1 6 of earth s circumference suggesting a self organized structure for the underlying mantle this implies the imprint of chaotic mantle processes induced by mantle flow and likely related to an early phase of highly dynamic behavior of the earth s mantle the size of the identified self organized region reflects the large length scale of upper mantle chemical variability and it is likely frozen since the proterozoic the geochemical heterogeneity of the asthenosphere along the ridges is believed to record a transition in the thermal conditions of the earth s mantle and to be reflected in the l 6 peak expansion of several geophysical observables

radial orbit error reduction and mean sea surface computation from the geosat altimeter data using the altimeter data from the geosat exact repeat mission we have produced yearly averaged mean profiles and a global mean sea surface the radial error of each 6 day orbital arc computed with the gem t2 geopotential was first estimated by calculating the amplitude and phase of the nine dominant frequencies of the difference between the altimetric profiles and the mean sea surface obtained when adding the permanent sea surface topography computed from the levitus climatological atlas to the gem t2 geoid we show that this operation is little affected by the choice of the geoid or by its formal error the resulting correction has been subtracted from each individual arc yearly mean profiles were then obtained by averaging the corrected altimetric data of each repeat cycle on a yearly basis their noise level is 1 to 2 cm and their resolution is 20 km but the differences of the altimetric heights at crossovers of ascending and descending tracks are still 30 cm nns the latter can be reduced to 7 cm rms by a crossover analysis in addition to the mean values standard deviations were computed at each point of the repeat cycle this yearly along track variability is of the order of 10 cm rms and is dominated by the ocean mesoscale variability a global yearly mean sea surface has been derived by bilinear interpolation its resolution ranges approximately from 160 km to 80 km depending on the latitude it is shown to be much less noisy than those deduced from geos 3 and seasat data

petrology and geochemistry of basalts from the southern juan de fuca ridge controls on the spatial and temporal evolution of mid ocean ridge basalt three morphologically distinct regions within the neovolcanic zone of the cleft segment of the southern juan de fuca ridge were investigated and sampled in detail using the dsrv alvin additional along strike and off axis samples were recovered by dredge the southernmost region the southern cleft site is characterized by a 3 km wide axial valley floored by ponded sheet flows and bisected by a 30 to 50 m wide cleft farther north at the young sheet flow site the ridge axis is characterized by a distinct 500 m wide inner graben that is largely covered by distinctly younger looking sheet flows the northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the cleft segment well into the zone of overlap with the neighboring vance segment the pillowed lavas at the young pillow mound site represent the most recent episode of volcanism along the cleft segment strong correlations exist between degrees of fractionation relative ages of lavas and latitude lavas are progressively younger looking and more mafic to the north the compositional range of mid ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35 40 low pressure fractional crystallization of relatively primitive but not primary depleted n type melts scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions generated from small scale mantle heterogeneities furthermore the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern juan de fuca ridge the more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along axis flow in a continuous axial magma chamber or conduit rather the data suggest lavas were derived from discrete magma chambers or lenses each in a different stage of evolution the youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds the oldest stage 40 additional crystallization of the most mafic composition is associated with focused hydrothermal activity and tectonic extension whereas the youngest events are characterized by ridge inflation diking and dispersed hydrothermal activity geochemical and tectonomagmatic features observed along the cleft segment are similar to those recently documented along the east pacific rise from 9 30 n 10 n suggesting the scales processes and stages of magmagenesis are similar along medium to fast spreading ridge segments

residual geoid anomalies in atlantic ocean basin relationship to mantle plumes the relations of geoid data from radar altimeter measurements geos 3 to water depth and geochemical variations along the mid atlantic ridge are investigated eight geophysical lines across the ridge in the atlantic ocean were constructed approximately normal to the local ridge trend slopes of geoid per kilometer of topographic relief show an asymmetry between the two sides of the ridge that generally is not matched by topography which has better symmetry between the two flanks subtracting a regional field based on previous work produces residual geoid anomalies whose source must be in the earth s outer 600 km the residual geoid anomalies of the north atlantic show striking correlation with the axis of the mid atlantic ridge but those in the south atlantic do not instead major positive residual geoid anomalies appear over the rio grande and walvis ridges determination of slope s residual geoid per million years crustal age reveals that s increases with greater residual geoid anomaly and with higher shallower crestal ridge topography the slope of 22 cm m y west of the azores high near 38 n suggests that the anomalously high crestal residual geoid and depth anomalies may not be entirely of thermal origin which may normally produce slopes only of about 8 cm m y as found near 28 n away from shallow ridge topography to account for this difference in slopes simply by a variation in mantle temperature would require a difference of about 940 c between the two sites which appears excessive also the decrease in topographic height with crustal age away from the azores high is much greater than predicted by theoretical cooling curves correlation of residual geoid anomalies with depth geochemical variations along the length of the mid atlantic ridge and the chemistry of the rio grande rise and walvis ridge suggest that many regions of anomalous residual geoid and crestal depth values may be in large part a function of crustal thickness and density variations induced by plume intrusion and volcanism in addition to their transient thermal effects

volcanic activity and crust mantle exposure in the ultrafast garrett transform fault near 13 28 s in the pacific the 130 km long garrett transform fault which offsets two ultrafast 14 5 cm yr epr spreading segments is the site of volcanic and tectonic activity evidence for recent volcanic events was observed during several dives on oblique ridges 3100 3500 m high trending 040 alpha beta and gamma and on small constructional mounds located in the central transform basin and eastern valley at a depth of 3500 5000 m the constructional features show a fissured terrain suggesting a component of lithospheric opening the strike slip motion of the transform is inferred at several locations between the two epr transform intersection zones from the presence of abundant sediment free talus with crushed rock debris fresh scarps and well developed striations on outcrops of serpentinized peridotites and metamorphosed volcanic breccia the width of the active tectonic zone varies between 200 and 3000 m in the explored area near the eastern and western epr transform intersection zones along the e w trending median ridge walls in the eastern and western transform troughs of the central basin and at several sites on the south facing wall of the transform domain first escarpment the presence of recent volcanic activity implies that the garrett transform behaves as a leaky transform fault the most common volcanics consisting of plagioclase and olivine phyric lava erupted in the transform are compositionally more depleted k ti 0 08 and less evolved mg 60 70 than the basalts encountered on the southern epr segments mg 40 66 and k ti 0 05 0 25 this suggests that different magmatic conditions are operating in the transform domain the transform strike slip motion is responsible for the emplacement of a sliver of serpentinized peridotite located in an area about 2 km wide and 10 km long along the median ridge at the contact with outcrops of metamorphosed basaltic breccia the ultramafics consist of residual harzburgites often impregnated with irregular lenses and dykes of basaltic liquids plagioclase wehrlite and gabbro and are found associated with troctolites dunite and gabbros the emplacement of the ultramafics and their gabbroic inclusions has resulted in the presence of metamorphosed tectonic breccia forming part of the median ridge in the central transform basin

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

interaction between subsurface ocean waters and calving of the jakobshavn isbrae during the late holocene a marine sediment core from vaigat in disko bugt west greenland has been analysed in terms of lithology dinoflagellate cysts and foraminifera in order to evaluate the influence of oceanographic variability on west greenland glacier stability the data show that during the past 5200 years the atlantic foraminiferal abundance in the subsurface waters of the west greenland current wgc episodically increased indicating periods of increases in the inflow of subsurface warm atlantic water at 2000 1500 cal yr bp and 1300 cal yr bp as well as periods of less pronounced increased bottom water temperatures around 4700 4000 cal yr bp 3100 2800 2600 1000 800 500 400 and at 200 cal yr the sedimentological and dinoflagellate cyst data indicate that these episodes with enhanced advection of irminger sea derived waters are accompanied by increased iceberg rafting which we link to increased iceberg calving in relation to destabilization of the jakobshavn isbrae the long term trend in the data documents the end of a late holocene thermal maximum between 5200 and 4300 cal yr bp and a final onset of the neoglaciation at 3500 cal yr bp increased responses of the iceberg rafting after 3500 cal yr bp reflects a westward seaward advance of the glacier margin in relation to onset of neoglaciation and a development of the glacier into a floating tongue after 2000 cal yr bp a comparison of our record with a record from the eastern north atlantic indicates that a nao like anomaly pattern between subsurface waters in west greenland and atmospheric temperature in the eastern north atlantic may have been operating during most of the late holocene however during the past 1000 years the nao signal may have weakened as some other mode of climate variability overprints the anti phase climate signal in this region