isostasy of the southern mid atlantic ridge long wavelength and short wavelength effects the isostatic transfer function for the southern mid atlantic ridge was computed from 25 profiles of gravity and topography normal to the ridge trend as follows from the comparison with previous studies the transfer function for the mid atlantic ridge is insensitive to the tectonic setting of profiles used the experimental transfer function was employed to remove all coherent short wavelength features in the gravity and topography from 17 long profiles across the southern mid atlantic ridge the profiles were then recomputed as a function of t age of the lithosphere and averaged over equal ages the observed regional topography regional gravity anomaly and altimeter geoid anomaly agree well with conductive cooling models of the spreading lithosphere an observed slope of the topography versus t1 2 and of the geoid versus t can be explained with a single set of model parameters comparison with the estimates by haxby for the northern mid atlantic and reykjanes ridges suggests a systematic decrease in the temperature of the mantle from 1500 1600 c near iceland to 1000 1100 c at the southern mid atlantic ridge the theoretical gravity anomaly deduced by the analytical method of haxby and turcotte is sensitive to the thermal parameters only in the age range 0 20 m y the fit of the theoretical to the observed gravity anomaly is good outside the axial strip 0 3 m y the poor fit at the axis is attributed to inadequacy in the boundary condition of the model

isostasy of aseismic tectonic units in the south atlantic ocean and geodynamic implications abstract mechanical isostasy of the lithosphere in the south atlantic ocean was studied using information on gravity anomalies and bathymetry with additional constraints imposed by the altimetric geoid the isostatic responses admittances over the walvis ridge rio grande rise and trindade seamount chain were computed using a three dimensional algorithm the eastern walvis ridge and the rio grande rise have the same response which is well explained by an airy model of isostasy the other features are regionally supported a variation in the thickness of the elastic plate was found along the western walvis ridge a high value of the elastic plate thickness 20 km was found under the trindade chain geodynamic implications are discussed in the light of these results

seasat altimetry and the south atlantic geoid 1 spectral analysis this paper presents a discussion of the spectral characteristics of geoid profiles derived from seasat alitmetry in the south atlantic individual profile spectra are computed and averaged in seven separate geodynamic provinces mid atlantic ridge flanks walvis and rio grande aseismic ridges and argentine and cape basins three distinct wavebands can be identified in most spectra for wavelengths shorter than 35 km the spectra are typical of white noise with a 7 cm rms amplitude typical of seasat altimeter measurement noise for wavelengths between 300 and 800 km our uppermost limit since original profiles are 1700 km long the spectra are in reasonable agreement with kaula s empirical rule and are compatible with a pratt thermal boundary layer compensation mechanism with an ill defined compensation depth of 30 60 km the paper centers on the description of the 35 300 km part of the spectra computation of an impedance for each province requires good knowledge of its bathymetry which is far from satisfactory as yet in the south atlantic we show that for wavelengths longer than 80 km the squared amplitude spectrum of bathymetry varies as the inverse of wave number we propose that this relationship might also hold at shorter wavelengths although it is not observed due to presently insufficient data using these spectral estimates and correcting for the mean depth of the topography we derive impedance actually response function estimates separately for each province the data are well accounted for by a regional compensation scheme based on elastic flexure models elastic thickness the best resolved parameter ranges from 2 to 5 km for the ridge and flanks and the crustal thickness from 4 to 13 km crustal thickness appears to be larger on the east flank demonstrating a general lack of symmetry between east and west in the south atlantic which appears to hold at all wavelengths and is also expressed in the bathymetry the rio grande rise involves low but nonzero elastic thickness and large crustal thickness 25 35 km in agreement with previously proposed models of local airy compensation the walvis ridge appears to imply a larger elastic thickness in excess of 8 km if the crustal thickness is less than 20 km which may be due to emplacement on older lithosphere therefore despite a lack of sufficient bathymetric control geoid data can be used at the scale of the entire south atlantic to provide reasonable information on thermomechanical compensation in the various provinces the spectral structure emphasizes the 35 300 km spectral window and should be used in constructing band pass filters that can separately enhance features related to plate kinematics or sublithospheric convection

isostasy of fracture zones in the atlantic ocean the cross spectral method of studying the isostasy was applied to profiles of gravity and topography which run across fracture zones in the atlantic ocean between 40 sup 0 n and 40 sup 0 s 30 section in total were used each section 400 km long in wavelengths 25 to 80 km the coherence of the free air gravity anomaly and topography attains 0 8 while the phase is close to zero which indicates that the disturbances in gravity are controlled by the topography and its isostatic compensation the experimental isostatic response function admittance deduced in this study is compared with previous results which relied on different spatial distribution of data a single mechanism of isostasy can explain well the gravity anomalies across fraction zones and across the median valley of the mid atlantic ridge a basic feature of this mechanism is the elastic lithosphere 6 to 9 km in thickness we show that the topography and gravity anomalies over the mid atlantic ridge crest are essentially three dimensional nevertheless the admittance is reliable computed from a set of profiles by using the method of d mckenzie and c bowin this possibility is provided by the favorable pattern of the topography spectrum in the area under consideration

carlsberg ridge northern indian ocean gravity and isostasy summary gravity and bathymetry profiles across the carlsberg ridge in the indian ocean are analysed to investigate the isostatic compensation of this part of the indian ocean spreading ridge system the ridge has a rough flanking topography and a median valley which are the characteristics of slow spreading ridges we use the crossspectral technique to obtain an admittance function for isostatic studies admittance estimates in the long wavelength region 125 500 km indicate that the elastic plate thickness supporting the long wavelength topography is in the range 5 10 km we have also examined the isostasy of the axial region synthetic topography and gravity profiles computed for a cooling plate model are subtracted from the observed to estimate the median valley signatures the residual gravity anomaly over the ridge axis is mostly explained by the median valkey topography with a uniform crustal thickness of 6 km indicating that the median valley is not in isostatic equilibrium

a model study of ocean circulation beneath filchner ronne ice shelf antarctica implications for bottom water formation 1 an isopycnic coordinate ocean circulation model has been applied to the southern weddell sea including the cavity beneath filchner ronne ice shelf with the aim of investigating the buoyancy forced circulation on the continental shelf buoyancy forcing is associated with both the annual growth and decay of sea ice and the interaction between ice shelf and ocean in the model a generally anti cyclonic circulation develops beneath the ice shelf so that new shelf waters entering the cavity in the west emerge colder and fresher in the east the outflow contributes to a dense current that spills off the continental shelf and descends the slope oceanographic observations from the region are consistent with this picture and highlight the overflow as a major source of weddell sea bottom water

the response of ice shelf basal melting to variations in ocean temperature abstract a three dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean warming scenarios the model predicts that the total ice shelf basal melt increases quadratically as the ocean offshore of the ice front warms this occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf both of which are found to increase linearly with ocean warming the behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature this study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results particularly the claim that a single melt rate sensitivity to warming is universally valid the hypothesized

modeling the evolution of antarctic ice sheet over the last 420 000 years implications for altitude changes in the vostok region a new thermomechanical three dimensional model designed to simulate the evolution of the antarctic ice sheet over long time periods is presented this model incorporates the various types of ice flow found in antarctica relatively slow inland ice flow that is essentially due to ice deformation fast ice flow in the regions with ice streams and ice shelf flow by coupling these three types of flow it is possible to predict grounding line migration simulations covering four glacial interglacial cycles have been conducted by forcing this model with a temperature record from vostok and a sea level record from marine cores our findings indicate that grounding line migration induced by sea level changes is the primary factor governing the evolution of the antarctic ice volume on the other hand the altitude of the ice sheet surface at vostok is driven by accumulation rate variations the amplitude of the altitude change does not exceed 150 m and is very similar for all the sites located on the antarctic plateau

ocean circulation beneath filchner ronne ice shelf from three dimensional model results a high resolution three dimensional ocean circulation model is applied to the cavity beneath filchner ronne ice shelf fris the model predicts predominantly barotropic currents which form a series of cyclonic gyres in the deep basins and anticyclonic circulations around the islands the surface circulation can be such that the water moves in the direction of decreasing or increasing ice thicknesses in the former case leading to freezing while melting at the ice shelf base results in the latter case the pattern of melting and freezing is consistent with known distributions of marine ice and melting areas beneath fris an anticyclonic circulation around the korff and henry ice rises with melting west of korff ice rise and freezing on the eastern side and north of henry ice rise is the main source for an ice pumping mechanism that produces the observed large marine ice body in the central filchner ronne ice shelf the estimates for net melting for realistic conditions at the open ocean boundary are 40 50 km 3 yr 1 indicating that ice shelf ocean interaction is an important contribution to the mass balance of the ice shelf

a primitive equation ocean circulation model using a general vertical coordinate transformation 1 description and testing of the model the ocean general circulation model of the geophysical fluid dynamics laboratory has been modified to accommodate a generalized vertical coordinate transformation the model with a slightly modified transformation z h is applied to three simple test cases a clear improvement of barotropic and baroclinic topographic rossby waves has been achieved compared with the z coordinate model a somewhat more typical time independent problem for the overflow of dense water masses over a zonal ridge exhibits the potential benefits as well as the flaws of the new model vertically integrated mass transport is very sensitive to changes in bottom slope in regions where planetary and topographic are of comparable magnitudes this translates into large errors in the z model due to the crude approximation to the actual topography the deep flow in the z model does not follow the bottom exactly horizontal overshoots of dense water lead to convective mixing on the southern side of the ridge this can be a serious problem for modeling more realistic overflow situations with the z model increased diapycnal diffusion has been identified as the most serious problem with the terrain following coordinate system for this specific application

basin formation in the high himalaya by arc parallel extension and tectonic damming zhada basin southwestern tibet 1 the late miocene pleistocene zhada basin in southwestern tibet provides a record of subsidence and basin formation within an active collisional thrust belt the 800 m thick basin fill is undeformed and was deposited along an angular unconformity on top of tethyan strata that were previously shortened in the himalayan fold thrust belt modal sandstone petrographic data conglomerate clast count data and detrital zircon u pb age spectra indicate a transition from detritus dominated by a distal northern source to a local southern source this transition was accompanied by a change in paleocurrent directions from uniformly northwestward to basin centric at the same time the depositional environment in the zhada basin changed from a large braided river to a closed basin lake sedimentation in the zhada basin was synchronous with displacement on the qusum and gurla mandhata detachment faults which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin respectively these observations indicate that accommodation for zhada basin fill was produced by a combination of tectonic subsidence and damming as midcrustal rocks were evacuated from beneath the zhada basin in response to arc parallel slip on crustal scale detachment faults

thermohaline circulation below the ross ice shelf a consequence of tidally induced vertical mixing and basal melting the warmest water below parts of the ross ice shelf resides in the lowest portion of the water column because of its high salinity vertical mixing caused by tidal stirring can thus induce ablation by lifting the warm but dense water into contact with the ice shelf a numerical tidal simulation indicates that vertically well mixed conditions predominate in the southeastern part of the sub ice shelf cavity where the water column thickness is small basal melting in this region is expected to be between 0 05 and 0 5 m yr and will drive a thermohaline circulation having the following characteristics high salinity shelf water at 1 8oc formed by winter sea ice production in the open ross sea flows along the seabed toward the tidal mixing fronts below the ice shelf and meltwater at 2 2oc produced in the wellmixed region flows out of the sub ice shelf cavity along the ice shelf bottom sensitivity of this ablation process to climatic change is expected to be small because high salinity shelf water is constrained to have the sea surface freezing temperature

future sea level rise due to projected ocean warming beneath the filchner ronne ice shelf a coupled model study a general ocean circulation model is coupled with a 3d thermodynamical ice sheet shelf model to simulate the response of the filchner ronne ice shelf fris antarctica and coastal parts of its catchment basin to a postulated inflow of warm deep water into the ice shelf cavity on a 1000 yr timescale prescribed ocean warming based on climate projections enters the ice shelf cavity in the up to 1500 m deep filchner trough and penetrates deep into the sub ice cavity increasing basal melt rates induce geometry changes of the cavity which in turn have an impact on the ocean circulation and therefore the modelled melt rates highest melt rates of about 20 m yr 1 follow the up to 180 km retreating grounding line basal mass loss reaches about 250 km3 yr 1 doubling the present day value the most vulnerable areas below the fris are the bailey ice stream and the area between the institute and moeller ice streams where the increased melting accounts for about 80 km of the modelled grounding line retreat on the backward sloping bedrock the potential additional contribution to the eustatic sea level rise due to the grounded ice loss simulated in an ensemble approach against a transient control experiment is about 0 05 mm yr 1 during the first 500 yr and about 0 17 mm yr 1 thereafter

modeling the effects of frazil ice crystals on the dynamics and thermodynamics of ice shelf water plumes the seawater that comes into contact with the base of a floating ice shelf is modified as a result of the phase changes that occur melting is prevalent in the deepest parts of the subice cavity and this drives a buoyant flow of ice shelf water isw along the sloping ice shelf base the ascent of the isw toward the surface of the ocean causes supercooling because the freezing point rises with the falling pressure and this induces a change from melting to freezing assuming that seed crystals exist the isw now fulfills the condition for the rapid growth of disc shaped frazil ice crystals which may subsequently settle upward out of suspension under the action of gravity a simple numerical model of these processes has been developed based on the theory of inclined plumes the isw is treated as a turbulent particle laden gravity current ascending a reactive boundary and containing a suspended crystal load which evolves in response to the supercooling of the water and the inverted sedimentation of the crystals the frazil ice has two important effects on the behavior of the isw plume because the generation of crystals through out the plume provides such a large area over which phase changes can occur the conversion of supercooling into ice happens much more readily than is possible through turbulent transfer of heat and mass at the ice shelf base a suspended crystal load reduces the bulk density of the isw so that a growing suspension causes the plume to accelerate while deposition of crystals onto the ice shelf effects a deceleration there are positive feedbacks in that an acceleration of the plume induces more rapid crystal growth while a deceleration allows suspended crystals to settle out more easily

isostatic response of the central indian ridge western indian ocean based on transfer function analysis of gravity and bathymetry data abstract the central indian ridge cir is a part of the major active mid ocean ridge system trending north south between 2 n and the triple junction at 25 s previous investigations suggest that the half spreading rate varies from 1 6 to 2 5 cm yr between the northern and southern ends of the ridge and the diffuse plate boundary dividing the indian and australian plates intersects the central indian ridge near the equator in the present study i have attempted to investigate the nature of isostatic response of young oceanic topography created by spreading along the central indian ridge gravity and bathymetric profiles four in northern part of the ridge within or close to the diffuse boundary zone and six in southern part outside the zone have been utilized separately to obtain their isostatic response based on transfer function analysis the admittance estimates for the long wavelengths suggest that the elastic plate model fits the data for the compensation of sea floor topography near the ridge crest an effective elastic thickness ranging from 5 to 10 km with an average t e of 7 2 km fits best to the data for the northern part of the central indian ridge and a value from 10 to 17 km with an average t e of 13 3 km fits the data for the southern part of the ridge isostatic response function estimates confirm that the difference in elastic plate thicknesses between the northern and southern segments is real and significant the observed low effective elastic thickness for the data in the northern part of the central indian ridge close to the diffuse boundary zone is interpreted as due to higher than average temperatures beneath the ridge these higher temperatures have been inferred due to the perturbations of the regional heat flow anomaly associated with the diffuse plate boundary

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

how iceberg calving and grounding change the circulation and hydrography in the filchner ice shelf ocean system the formation of bottom water in the southern weddell sea is strongly influenced by the flow of ice shelf water isw out of the filchner ronne ice shelf cavity the breakout of three giant icebergs in 1986 and their grounding on the shallow berkner bank modified the circulation and water mass formation in the filchner trough and the adjacent sea areas hydrographic measurements along the filchner ice shelf front carried out with rv polarstern in 1995 show significant changes in the water mass characteristics and flow patterns in the filchner trough in comparison to measurements from the early 1980s changes in the trough will affect the flow over the sill to the deep weddell abyssal plain we combine a three dimensional ocean circulation model with conductivity temperature depth and stable isotope measurements to investigate the details of the circulation in front of and beneath the filchner ice shelf we assess the impact of stranded icebergs and a more southerly ice shelf front position caused by a 1986 iceberg calving event on the circulation and observed water mass properties results indicate variations of the flow pattern in the filchner trough and on berkner bank where high salinity shelf water the feedstock for isw is produced the calving and grounding impacts illustrate the sensitivity of the ice shelf ocean system to perturbations in local bathymetric settings

sensitivity of the amery ice shelf antarctica to changes in the climate of the southern ocean abstract coupled ocean atmospheric general circulation models indicate that warming of up to 3 c may occur over the next century in waters immediately to the north of the amery ice shelf the impact of this warming on the ocean cavity under the amery ice shelf and the mass exchange at the interface between the ocean cavity and the ice shelf is investigated using a three dimensional ocean model warming of between 0 25 and 3 0 c is applied along the ice front in a series of model runs rather than in a single transient run changes in salinity are also considered for larger amounts of warming the model results show that the circulation in the ocean cavity changes as warming increases particularly in the gyres that dominate the horizontal circulation the changes in the heat flux from the warming increase the melt rates from the base of the amery ice shelf from the present day mean melt rate and net mass loss estimates of 0 28 m yr 1 and 14 2 gt yr 1 respectively by approximately 0 55 m yr 1 c 1 and 2

the tectonic evolution of the indian ocean triple junction anomaly 6 to present the tectonic evolution of the indian ocean ridge ridge ridge triple junction the junction of the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir is inferred by mapping structures along its traces on the three plates with a long range side scan sonar geological long range inclined asdic gloria the sonar images to the west of the triple junction show two different styles of tectonic evolution some sections of the trace on the african plate are marked by a swir facing scarp containing fine lineaments that are probably normal faults these correspond with sections of the trace on the antarctic plate of the same age that have a sharp intersection of swir and seir abyssal hills that is to be expected from a simple ridge ridge ridge triple junction evolution a herringbone pattern of isochrons in contrast there are other sections of the trace on the african plate that have a blocky structure with no definite termination of cir abyssal hills against those of the swir the corresponding regions of the antarctic plate show large 10 to 30 km long overlapping fault scarps tapering to the northeast and ending in asymmetric valleys to the southwest these major normal faults are each offset by 1 6 km forming an en echelon series the two different styles of tectonic structures indicate two different modes of evolution although why the triple junction has evolved with two different modes is unclear periods of asymmetric spreading in the cir magnetic anomaly sequence collected on the african plate may possibly indicate that the cir and seir have been offset by a fracture zone at the triple junction during one of these two modes echo sounder traverses on the antarctic and african plates commonly show a steady shallowing of the cir and seir seafloor toward the traces a steep scarp facing the swir and a broad marginal deep at the base of this scarp this rift flank shape resembles that of the swir valley s rift shoulders close to the present triple junction mitchell 1991a suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes if the shape of the rift shoulder uplift along the trace on the antarctic plate is interpreted with a simple flexural model involving an end loaded elastic plate the inferred rigidity 1019 1021 n m or 2 6 km effective elastic thickness is consistent with the suggestion that the isostatic uplift occurred in young weak lithosphere close to the triple junction interestingly the observed 1 km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load 2 5 1011 n m that is equivalent to half the present cross sectional area of the swir valley 35 km from the triple junction

circulation beneath the filchner ice shelf antarctica and its sensitivity to changes in the oceanic environment a case study we investigate the sensitivity of the ocean circulation in the filchner trough to changes in the large scale oceanic environment and its impact on the mass balance of the filchner ice shelf antarctica three experiments with a three dimen sional ocean model describe i the current situation ii a scenario with increased ocean temperatures and iii a scenario with reduced sea ice formation rates on the adjacent continental shelf in the final discussion brief results of a combined scenario with increased ocean temperatures and reduced sea ice formation are presented the changes from the current situation affect the circulation in the filchnert rough and melting and freezing processes beneath the ice shelf the latter affect the amount and propert ies ofice shelf water isw a component of antarctic bottom water net basal melt rates provide an overall measure for the changes while the control run yields 0 35 m a i net melting averaged over the filchner ice shelf area the warming scenario results in a more than twofold increase in ice shelf mass loss reduced production of high salinity shelf water due to smaller sea ice formation rates in the second scenario leads on the other hand to a decrease in basal mass loss because the deep cavity is less well ventilated than in the con trol run isw is cooled and the ice shelf is stabilized under this scenario which is arguably the more likely development in the southern weddell sea

thermohaline circulation and interaction between ice shelf cavities and the adjacent open ocean the circulation system in an ice shelf cavity is driven by buoyancy fluxes due to melting and freezing of ice and horizontal pressure gradients at the interface between the cavity and the open ocean hence the inflow and outflow pattern and the hydrography in the open ocean influence the general hydrographic condition in the cavity which at least provides the potential for melting and freezing processes applying a three dimensional ocean general circulation model to an idealized ice shelf cavity geometry coupled with an open ocean at a topographic ice shelf barrier we found an important parameter controlling the interaction between these two systems idealized studies for different ice shelf and sea bottom topographies and forcing mechanisms for the open ocean show that the ice shelf edge represents a natural barrier for barotropic interaction because of the sudden decrease in water column thickness since the water column thickness and the coriolis force determine the characteristics for geostrophic flow separated circulation systems arise for the open ocean and the ice shelf cavity only in areas where constant water column thickness and from the oceanographic point of view constant f h contours can be observed across the barrier an increased barotropic current can surmount the ice edge and ventilate the water mass beneath the ice shelf this is only the case at lateral sloping sidewalls or at deep depressions which can be found for example in the southern weddell sea in all other cases the circulation in the ice shelf cavity is closed and almost unaffected by the hydrography outside the barrier

ocean circulation and ice ocean interaction beneath the amery ice shelf antarctica simulations of the ocean dynamics in the cavity under the amery ice shelf antarctica were carried out using a three dimensional numerical ocean model two different boundary conditions were used to describe the open ocean barotropic exchange at the ice front the simulations show that the circulation in the ocean cavity is predominantly barotropic and is generally steered by the cavity topography the circulation is driven by the density gradient in the cavity which is strongly influenced by the heat and salt fluxes from melting and freezing processes at the ice ocean interface and by the horizontal exchange of heat and salt across the open ocean boundary at the ice front the interaction at the ice ocean interface allows the basal component of the mass loss of the amery ice shelf to be estimated in the two simulations the computed losses were 5 8 gt yr 1 and 18 0 gt yr 1 values consistent with observations the bulk of the melting occurred near the southern grounding line of the ice shelf although substantial melting also occurred in areas where heat transport by horizontal circulation was large accretion was restricted to areas where water from upstream melting became supercooled as it ascended the ice shelf base

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

seasonal ventilation of the cavity beneath filchner ronne ice shelf simulated with an isopycnic coordinate ocean model 1 the ocean cavity beneath filchner ronne ice shelf is observed to respond to the seasonal cycle of water mass production on the continental shelf of the southern weddell sea here we use a numerical model to investigate the propagation of newly formed shelf waters into the cavity we find that the model reproduces the most distinctive features of the observed seasonality and offers a plausible explanation for those features the most saline shelf waters are produced in the far west where the inflow to the cavity peaks twice each year the major peak occurs during the short period around midwinter when convection reaches full depth and the densest waters are generated once the surface density starts to decline dynamic adjustment of the restratified water column leads to a gradual fall in the salinity at depth and a secondary peak in the inflow that occurs in summer at the western coast beneath the ice shelf the arrival of the wintertime inflow at the instrumented sites is accompanied by a rapid warming while the slower decline in the inflow leads to a more gradual cooling water brought in by the secondary summer peak flows mainly to the eastern parts of the cavity here the seasonality is suppressed because the new inflows mix with older waters that recirculate within a topographic depression this pooling of waters in the east where the primary outflow of ice shelf water is generated dampens the impact of seasonality on the local production of weddell sea bottom water

sea level changes at the lgm from ice dynamic reconstructions of the greenland and antarctic ice sheets during the glacial cycles abstract new experiments were performed with three dimensional thermomechanical models of the greenland and antarctic ice sheets to simulate their behaviour during the glacial cycles to reconstruct their thickness and extent at the last glacial maximum lgm and to estimate their glacio eustatic contribution to the global sea level stand the calculations used improved ice dynamic and isostatic treatments updated datasets on higher grid resolutions and refined climatic treatments based on newly calibrated transfer functions between ice core records and climatic perturbations results are discussed from a reference run with standard parameters that is compared with available glacial geological observations and from a series of sensitivity experiments focusing on isostatic adjustment thermomechanical coupling climatic forcing mass balance changes and basal melting rates and viscosity changes of antarctic ice shelves for the antarctic ice sheet we find that volume changes are closely linked with grounding line changes of the west antarctic ice sheet at the lgm the grounding line extended close to the continental shelf break almost everywhere ice over central east antarctica was generally thinner than today and varied mainly in accordance with accumulation fluctuations for the greenland ice sheet melting is important only during interglacial periods and the most sensitive period concerns the size of the ice sheet during the eemian at the lgm the greenland ice sheet extended beyond the present coastline to cover at least the inner continental shelf and thinned by up to several hundred meters in central areas for a plausible range of parameters the experiments indicate that at the time of maximum sea level depression 21 kyr bp the antarctic ice sheet contributed 14 18 m to the sea level lowering and the greenland ice sheet 2 3 m significantly less than the older climap reconstructions whereas both ice sheets were at 21 kyr bp close to their maximum extent the experiments also indicate that their maximum volumes were reached only by 16 5 kyr bp greenland and 10 kyr bp antarctica equal to an additional sea level lowering of respectively 0 4 and 3 7 m holocene retreat was essentially complete by 5 kyr bp in greenland but is found to still continue today in west antarctica before reversing to growth during the next millenium the models were found to reproduce gross features of the ice sheet s history since the lgm in reasonably good agreement with available glacial geological data although observational control on ice thickness changes remains very limited

recent atmospheric warming and retreat of ice shelves on the antarctic peninsula in 1978 mercer1 discussed the probable effects of climate warming on the antarctic ice sheet predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the antarctic peninsula analyses of 50 year meteorological records have since revealed atmospheric warming on the antarctic peninsula2 3 and a number of ice shelves have retreated4 8 here we present time series of observations of the areal extent of nine ice shelves on the antarctic peninsula showing that five northerly ones have retreated dramatically in the past fifty years while those further south show no clear trend comparison with airtemperature data shows that the pattern and magnitude of ice shelf retreat is consistent with the existence of an abrupt thermal limit on iceshelf viability the isotherm associated with this limit having been driven south by the atmospheric warming ice shelves therefore appear to be sensitive indicators of climate change

an application of the response function technique to profiles of bathymetry and gravity in the gulf of aden summary the relationship between bathymetry and gravity in the eastern and the western gulf of aden is analysed by computing response functions the form of the response functions indicates that the short wavelength topography is isostatically compensated the most appropriate mechanism of compensation is one in which the topography is not locally compensated but regionally compensated by flexure of the lithosphere though the sets of profiles from the eastern and the western gulf of aden are separated by only 500 750 km the observed response functions are very different the lithosphere in the eastern gulf of aden can be modelled by an elastic plate 15 3 km thick whereas an elastic plate 3 2km thick is probably an appropriate model for the lithosphere in the western gulf of aden a different mechanism of isostasy the pratt operates in the compensation of the long wavelength topography of the mid ocean ridge in the gulf of aden a larger negative broad bouguer gravity anomaly across the western gulf of aden than across the east indicates that upper mantle temperatures increase from east to west the long wavelength shallowing of the gulf of aden and increasing elevation of the adjacent continents from east to west are therefore probably mainly the result of thermal uplift the subdued nature of the median valley and consequently other short wave length topography in the west can be explained by hot low viscosity asthenosphere and the inferred thin lithosphere in the west also suggests high temperature mantle finally a magnetic anomaly superimposed on the axial anomaly which is probably related to the depth of the curie temperature is more common in the western gulf of aden than in the east and this is also consistent with upper mantle temperatures increasing to the west there are a number of similarities between the mid ocean ridge in the western gulf of aden and the reykjanes ridge it is suggested that the western gulf of aden is anomalous because of its proximity to the afar hot spot in the same way that the reykjanes ridge is often considered to be anomalous because of its proximity to the iceland hot spot

a new analysis of gravity and topography data over the mid atlantic ridge non compensation of the axial valley abstract topographic and associated gravity signals often result from the interference of several phenomena in the present paper an admittance function for the superposition of several physical mechanisms acting in the same waveband is derived if the processes are not phase related the resulting admittance function is the average of the individual admittance functions weighted by the squared amplitude of the topography for each process we use this concept of superposition of different processes to understand the compensation of the atlantic ridge valley classical transfer function studies applied to slow spreading ridges have concluded that the topography was emplaced on an elastic plate about 10 km thick these analyses of gravity and topography seem to contradict the physical models explaining the median rift valley which is thought to be due to dynamical effects and thus to be uncompensated we obtain an average axial valley gravity and topography profile by stacking several profiles perpendicular to the mid atlantic ridge and subtracting a long wavelength thermal effect we find that the gravity over topography spectral ratio of the average axial valley signal is consistent with an uncompensated process our study thus confirms that the mode of formation of the axial valley of a slow spreading ridge involves an uncompensated mechanism the presence of an additional process characterized by low admittance values uncorrelated from one profile to the other is also suggested in order to explain the observed admittance function the study of the long wavelength 300 km gravity and topography signal leads us to invoke the cooling of the upper portion of the crust by water circulation and to exclude the presence of a large amount of partial melt at depth more than 5 over a 20 km thick layer at a mean depth of 60 km

negative gravity anomaly over spreading rift valleys mid atlantic ridge at 26 n abstract a pronounced negative free air gravity anomaly commonly occurs over the median valley of slow spreading ocean ridges previous results using wiener filtering and cross spectral analysis techniques for the mid atlantic ridge obtained estimates of the elastic plate thickness in the range of 7 13 km and the existence of a residual negative gravity anomaly over the median rift valley suggesting that the rift valley has a response function different than the remainder of the spreading ridge in this paper we have improved the derivation of the topography gravity admittance function for spreading ocean crust by carefully avoiding several sources of spectral splattering when processing the data 1 selecting data from a cruise that followed a flowline of central north atlantic relative plate motion and hence is least corrupted by fracture zones and 2 accounting for the difference in distance between the gravity meter and the regional variation in elevation as the ridge crest is traversed improvements of lesser importance include the use of cubic splines to interpolate to equally spaced data rather than linear interpolation and correction of the free air anomaly values for long wavelength variations of the indirect effect comparison of the resulting admittance function to elastic flexure response functions suggests an elastic plate thickness of about 8 km the improved admittance function when convolved with the ridge topography provides a predicted gravity profile that accounts very well for the negative anomaly over the rift valley therefore the isostatic response function for the rift valley is similar to that for the topography away from the rift valley

crustal structure of atlantic fracture zones i the charlie gibbs fracture zone summary a seismic refraction experiment was carried out in 1982 over and in the vicinity of the southern transform valley of the charlie gibbs double fracture zone to measure the rate of crustal thinning normal to the southern transform valley axis three reversed lines were shot with obs two were east west one parallel to the fault on normal crust and one along the fault the third line was north south normal to and intersecting the first two lines travel times were modelled by ray tracing thin 4 km thick anomalously low velocity crust was detected in the fracture zone along the n s line normal crust thinned over a distance of 40 km from 8 to 5 km towards the fracture zone where the belt of anomalous crust was 12 km wide several publications demonstrate that such thinning is not due primarily to the effect of cold lithosphere on accretion at the ridge transform intersection the thinning may be related to the supply and distribution of magma from an injection zone situated roughly at the centre of each spreading segment a gravity profile along the n s line was also analysed the bouguer anomaly was modelled subject to the constraints of the seismic ray tracing after applying corrections for the three dimensional bathymetry near the profile and for the lithospheric anomaly due to the age contrast across the northern fracture zone the gravity models supported the seismic interpretation and also indicated low densities within the fracture zones two alternative models were obtained for the structure under hecate bank an east west ridge which lies within the crust generated at the short central spreading segment of the cgfz one model features dense material within the crust the other normal but uplifted crust the evidence favours the latter mode but the cause of the uplift is uncertain the uplift may have

rough versus smooth topography along oceanic hotspot tracks observations and scaling analysis some hotspot tracks are topographically smooth and broad nazca carnegie cocos galapagos walvis iceland while others are rough and discontinuous easter sala y gomez tristan gough louisville st helena hawaiian emperor smooth topography occurs when the lithospheric age at emplacement is young favoring intrusive magmatism whereas rough topography is due to isolated volcanic edifices constructed on older thicker lithosphere the main controls on the balance of intrusive versus extrusive magmatism are expected to be the hotspot swell volume flux qs plate hotspot relative speed v and lithospheric elastic thickness te which can be combined as a dimensionless parameter r qs v 1 2 te which represents the ratio of plume heat to the lithospheric heat capacity observational constraints show that except for the ninetyeast ridge r is a good predictor of topographic character for r 3 they are smooth and dominated by intrusion