contribution of icelandic ice caps to sea level rise trends and variability since the little ice age 1 in total icelandic ice caps contain 3600 km3 of ice which if melted would raise sea level by 1 cm here we present an overview of mass changes of icelandic ice masses since the end of the 19th century they have both gained and lost mass during this period changes in ice volume have been estimated both through surface mass balance measurements performed annually since 1990 and differencing of digital elevation models derived from various satellite and airborne observations while the glaciers showed little mass loss as the 20th century began losses increased rapidly after 1925 peaked in the 1930s and 1940s and remained significant until the 1960s after being near zero or even positive during the 1980s and early 1990s glacier mass budgets declined considerably and have since the mid 1990s shown an average annual loss of 9 5 1 5 gt a 1 contributing 0 03 mm a 1 to sea level rise since 1995 interannual variability in mass loss is high ranging from 2 7 to 25 3 1 5 gt a 1 corresponding to surface mass balances of 0 2 to 2 2 0 15 m we a 1 this variability is driven by climate fluctuations and also by transient reduction of albedo due to volcanic eruptions

thermal and mechanical properties of the cape verde rise the cape verde rise represents an extreme case of the process of oceanic midplate volcanism the regional depth and geoid anomalies relative to the plate model are the largest observed peaking at 2 4 km and 12m respectively the late tertiary volcanoes rest on mesozoic aged seafloor indicating that the lithosphere was about 130 m y old when loaded depth and geoid anomalies in the cape verde region are examined using both forward filtering and admittance techniques to determine the thermomechanical properties of the swell the effective elastic thickness for the plate supporting the volcanoes is 28 4 km and the compensation depth for the broad swell is 69 10 km these two results combined with heat flow observations from the rise are consistent with a model for reheating of the lithosphere beneath cape verde such that the average thermal gradient is increased by 35 while there is no doubt that the cape verde lithosphere has been reheated it is unlikely that the entire depth anomaly is explained by thermal anomalies confined to the lithosphere a linear programming inversion of the geophysical data places an upper bound of 1 7 km on the portion of the depth anomaly due to lithospheric reheating there are no permissible thermal models fitting all of the geophysical data that do not require either dynamic support from or elevated temperature within the asthenosphere

compensation of swells and plateaus in the north pacific no direct evidence for mantle convection at intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation a low geoid topography ratio 8 m km in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection after subtracting a reference geoid from the observed geoid previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000 4000 km since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid topography ratios 10 20 m km they were believed to reflect the planform of mantle convection we show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10 since the geoid spectrum is red the residual geoid is dominated by degree 11 when the harmonics of the reference geoid are rolled off gradually the regular pattern of geoid highs and lows disappears in the northeast pacific the new residual geoid reflects the lithosphere age offsets across the major fracture zones in the northwest pacific the residual geoid corresponds to isostatically compensated swells and plateaus we have calculated the geoid topography ratio for 10 swells and plateaus and have found a range of compensation depths the highest geoidaopography ratio of 5 5 m km occurs on the flanks of the hawaiian swell intermediate ratios occur in four areas including the midway swell these intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of airy compensated seamounts low geoid topography ratios occur over the remaining five areas e g emperor seamounts reflecting the absence of a thermal swell our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid we see only indirect evidence of thermal plumes reheating the lower lithosphere

small wavelength geoid and topography anomalies in the south atlantic ocean a clue to new hot spot tracks and lithospheric deformation numerous small wavelength elongated features are visible both on filtered geoid maps on topography maps of the south atlantic ocean a number of them are oriented along the directions of absolute plate motion between 80 ma b p and 30 ma b p n50 degree e for the african plate and n63 degree w for the south american plate magmatic traces left over fixed convective plumes more numerous than the classical hot spots best explain these observations other traces especially in the area of the sandwich subduction zone and on the african plate are oriented in a n50 degree w direction and seem to indicate large scale deformation of the oceanic plates

cooling of the oceanic lithosphere evidence from geoid anomalies across the udintsev and eltanin fracture zones abstract the often step like variation in the marine geoid across fracture zones is believed to reflect the change in age of the ocean floor and hence in temperature structure of the plate across the fracture zone the rate of change of the geoid with age has been estimated as a function of age from geoid offsets across fracture zones and used to constrain thermal models of lithospheric cooling results from three studies of the geoid offsets across the mendocino fracture zone are consistent with a thermal plate cooling model having a plate thickness between 100 and 125 km this is in contrast to results obtained from the combined measurements from fracture zones in the south pacific which are not consistent with either the plate model or boundary layer model of cooling to examine this inconsistency we have measured geoid offsets on seasat profiles across the udintsev and eltanin fracture zones in the south pacific and tested the robustness of the measurement technique we used a second order polynomial that included a step function centered on the fracture zone to estimate the regional geoid and the geoid offset the polynomials were fit to the geoid profiles using least squares minimization criteria data within a certain distance of the fracture zone were excluded from the fitting procedure in order to minimize the effects of horizontal cooling and lithospheric deformation near the fracture zone we find that the estimate of the geoid offset is very sensitive to the length of data excluded and that generally an exclusion width of 100 200 km is needed to avoid anomalies unrelated to the regional geoid offset near the fracture zone exclusion widths of less than 100 km generally lead to unpredictable errors in the geoid offset estimates using exclusion widths of between 150 and 200 km on most profiles we constructed geoid slope geoid offset divided by the age offset versus average age plots for each of the limbs of the two fracture zones similar trends were found on both the east and west limbs of the eltanin and the east limb of the udintsev the trends were inconsistent with a single thermal plate thickness and instead showed an initial decrease in slope between 0 and 35 m y followed by an abrupt increase followed again by a decrease in slope in contrast the west branch of the udintsev shows an increase in geoid slope values between 20 and 35 m y followed by a decrease the difference in observed trends across the western limb of the udintsev may be due to the effects of the louisville ridge north of the fracture zone on the regional geoid the similarity in trends on the remaining three limbs argues against the effects of isolated thermal or bathymetric anomalies and appears instead to reflect a general feature of the geoid slope versus average age relationship across fracture zones although the thermal plate cooling model is successful in predicting both seafloor depths and heat flow values out to ages of at least 80 m y b p it cannot explain the observed geoid slope values for these two fracture zones it is not clear at this point whether this is due to inadequacies in the cooling model or to peculiarities in fracture zone evolution possibly caused by small scale convection driven by the horizontal temperature gradients beneath fracture zones

circulation from a joint gravity field solution determination of the general ocean with the development of satellite altimetry it is possible to infer the geostrophic velocity of the surface ocean currents if the geoid and the position of the satellite are known accurately errors in current geoid models and orbit computations both due primarily to errors in the earth s gravity field model have limited the use of altimeter data for this purpose the objective of this investigation is to demonstrate that altimeter data can be used in a joint solution to simultaneously estimate the quasi stationary sea surface topography and the model for the gravity field satellite tracking data from twelve satellites were used along with seasat altimeter data for the solution the estimated model of compares well at long wavelengths with the hydrographic model of covariance analysis indicates that the geoid is separable from up to degree 9 at which point geoid error is comparable to the signal of

on the source of cross grain lineations in the central pacific gravity field subtle lineations in the marine gravity field of the central pacific derived from seasat altimeter data were observed by haxby and weissel 1986 they suggested that these cross grain lineations were evidence of small scale convection beneath the pacific plate we have examined these features by comparing multiple collinear gravity and bathymetry profiles in the fourier transform domain our nine gravity profiles were each obtained by stacking averaging three or more individual repeat geosat erm altimeter passes prior to stacking the individual geosat passes were fit to a cubic spline and then differentiated along track to produce along track deflections of the vertical or horizontal gravity corresponding bathymetric profiles were produced by projecting onto geosat ground tracks bathymetric observations from six r v thomas washington legs and three r v conrad legs that virtually coincide with these geosat tracks after fourier transforming the resulting gravity and bathymetry profiles we estimate admittances of gravity to bathymetry these admittances are generally low they also tend to be negative at very short wavelengths 50 km they are consistent with models of flexural isostatic compensation by a very thin lithosphere approximately 2 km they are not consistent with models of dynamic compensation we suggest therefore that either 1 these cross grain lineations began to form very near the east pacific rise or 2 they formed on older anomalously weak lithosphere we also suggest that the gravity lineations result primarily from loads beneath the seafloor in combination with secondarily loads on the seafloor depths of these subseafloor loads appear not to exceed significantly typical moho depths

revised history of early tertiary plate motion in the south west pacific reanalysis of early tertiary magnetic anomalies on the pacific plate south of the campbell plateau indicates that in early tertiary time there was a previously unrecognized triple junction of the pacific the antarctic and a third plate now beneath the bellingshausen sea corresponding revised parameters for pacific antarctica motion do not require early tertiary displacements within antarctica or new zealand but suggest a change in direction of the pacific plate over the hotspots of approximately the same sense and timing as the hawaiian emperor bend

a preliminary model for geosat altimeter data errors noise power spectral densities psds are derived from the analysis of geosat altimeter data from segments of nearly collinear repeat tracks the difference time series which are analyzed using autoregressive ar modeling techniques contain primarily time varying oceanographic signals and altimeter noise since geoid content is canceled by the differencing process an average noise psd obtained from eight independent repeat track psds is fitted closely by a first order markov model with a white noise floor the geosat average noise psd is compared with analogous spectra derived earlier for geos 3 and seasat in general the geosat noise psd is the same as the seasat noise spectrum at wavelengths greater than about 100 km but has a lower noise level at shorter wavelengths

evolution of the southwest indian ridge from the late cretaceous anomaly 34 to the middle eocene anomaly 20 abstract the determination of the motion of antarctica relative to africa is particularly important when considering the breakup of gondwana two models have been proposed that describe the pattern of seafloor spreading between africa and antarctica during the late cretaceous starting at chron 34 84 ma through to the middle eocene chron 20 46 ma in the first model the motion of antarctica relative to africa can be simply described by a rotation about a single pole of rotation in the second model which we favor the relative motion of antarctica and africa is more complex and a major change in spreading direction between chron 32 74 ma and chron 24 56 ma times is required in this paper we present ten plate tectonic reconstructions of the southwest indian ridge that were produced using a new compilation of magnetic bathymetric and satellite altimetry data in combination with interactive computer graphics these reconstructions illustrate that spreading directions started to change at chron 32 74 ma between chrons 31 and 28 69 64 ma spreading was very slow

high accuracy high resolution gravity profiles from 2 years of the geosat exact repeat mission satellite altimeter data from the first 44 repeat cycles 2 years of the geosat exact repeat mission geosat erm were averaged to improve accuracy resolution and coverage of the marine gravity field individual 17 day repeat cycles two points per second were first edited and differentiated resulting in alongtrack vertical deflection i e alongtrack gravity disturbance to increase the signal to noise ratio 44 of these cycles were then averaged to form a single highly accurate vertical deflection profile the largest contributions to the vertical deflection error is short wavelength altimeter noise and longer wavelength oceanographic variability the combined noise level is typically 6 rad both types of noise are reduced by averaging many repeat cycles over most ocean areas the uncertainly of the average profile is less than 1 rad 0 206 arcsec which corresponds to 1 mgal of alongtrack gravity disturbance however in areas of seasonal ice coverage its uncertainty can exceed 5 rad to assess the resolution of individual and average geosat gravity profiles the cross spectral analysis technique was applied to repeat profiles individual geosat repeat cycles are coherent 0 5 for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths this emit of resolution is governed by the signal to noise ratio thus when many geosat repeat profiles are averaged together the resolution limit typically improves to about 20 km except in shallow water areas further improvements in resolution will be increasingly difficult to achieve because the short wavelength components are attenuated by upward continuation from the seafloor to the sea surface these results suggest that the marine gravity field can be completely mapped to an accuracy of 2 mgal and a half wavelength resolution of 12 km by a 4 5 year satellite altimeter mapping mission

observations on electromagnetic bias in radar altimeter sea surface measurements because the relative radar cross section of the sea surface increases below mean sea level and decreases above it the range measurements of satellite radar altimeters are biased toward the wave troughs published and unpublished direct measurements of this electromagnetic em bias are examined as well as the predictions of theoretical developments the em bias is predominantly a function of the radar frequency used averaging 1 2 percent of the wave height of ka band and 3 3 percent of the wave height at x band the airborne measurements present a consistent picture of the variation of the relative radar cross section as a function of deviation from mean sea level a technique to measure em bias at the ku and c band operating frequencies of the topex satellite altimeter is described

an abrupt change in ridge axis gravity with spreading rate the global mid ocean ridge system shows a marked change in morphology and isostatic compensation as a function of spreading rate fast spreading ridges have axial highs with little bathymetric relief and low amplitude gravity signatures indicating that they are nearly in local isostatic equilibrium slow spreading ridges have large axial valleys bounded by rugged topography macdonald 1982 and large axial gravity troughs indicating that they are dynamically maintained while this variation in ridge axis morphology with spreading rate has been observed it has not been analyzed in a comprehensive manner moreover it is not known whether the transition from axial valley to axial high is a continuous function of spreading rate or whether it occurs abruptly at a particular rate such observations would provide important constraints on models of ridge axis dynamics vertical deflection profiles collected by the geosat radar altimeter have sufficient accuracy and resolution to reveal the change in ridge axis gravity with spreading rate in this study we have analyzed 44 geosat profiles over ridges with spreading rates ranging from 14 to 155 mm yr in agreement with previous studies we find that slow spreading ridges 70 mm yr are characterized by low amplitude ridge axis highs 15 rad unexpectedly we find that the transition from axial trough to axial high occurs abruptly at a spreading rate of 60 70 mm yr ridge axis gravity signatures are highly variable for rates less than 65 mm yr and very uniform at higher rates the transition of the gravity signature appears to be more abrupt than the transition of the topographic signature suggesting an abrupt change in the style of isostatic compensation with spreading rate published models of ridge axis dynamics do not explain this sharp transition

global relationship between oceanic geoid and seafloor depth new results a new global transfer function admittance between oceanic geoid and seafloor depth has been derived for the wavelength band 1000 4000 km after removal of lithospheric cooling effects using the most recently published crustal ages of the oceans in this wavelength band the admittance is positive reaches a maximum of 5m km around 2750 km wavelength then decreases to 2 5 m km on the average at shorter wavelengths between 2500 km and 1000 km these values are systematically higher by 30 40 than those we derived in a previous study using crustal ages of poorer quality in view of these new results we cannot exclude that part of the signal at the 2750 km wavelength is of convective origin unlike the conclusions of the previous study on the other hand at shorter wavelengths the observed admittance can still be interpreted in terms of crustal thickening and sediment loading

high resolution inversion for south atlantic plate kinematics using joint altimeter and magnetic anomaly data we present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone fz and magnetic anomaly location data jointly we define misfit functions that incorporate properties unique to each data type in particular the fz misfit function does not depend upon alignment of conjugate fz traces in the same way as magnetic lineations under the finite rotations this property is useful for fz locations in which the signals on conjugate sides of the ridge may include systematic differences or where data from one side of the ridge are sparse or missing formal error bounds estimated for the pole parameters show that the magnetic and fz data are complementary in their information content error bounds computed for the joint inversion are substantially smaller than for either the fz or magnetics data used separately indicating that simultaneous use of the data in an inversion is crucial we apply this method to seasat altimeter data and magnetic anomaly picks in the south atlantic we solve for finite poles corresponding to magnetic anomalies 5 6 8 13 21 22 25 30 33 33r and 34 these define a smooth path over the past 84 m y indicating that the plate motions have not been as erratic as found previously

geography and history of the louisville hotspot chain in the southwest pacific the louisville ridge is a 4300 km long late cretaceous and cenozoic hotspot chain the south pacific equivalent of the hawaiian emperor chain its northwestern end is being consumed by the tonga kermadec subduction zone but seabeam and magnetic mapping shows that the chain still includes more than 60 major volcanoes of both normal and reversed polarity distributed along a 75 km wide band that can be fitted to small circles about three successive poles of pacific plate hotspot rotation this band obliquely crosses fracture zones of the eltanin system but there is little interaction and no evidence of any genetic connection between the two structures forty of the louisville volcanoes grew above sea level and are preserved as high latitude i e coral free guyots they are spaced less than 100 km apart along most of the chain but there are none within 750 km of the inferred present location of the hotspot beneath a swell at the southeast end of the chain the rate of volcano building by the louisville hotspot declined sharply about 20 m y ago after being remarkably constant at 3 4 103 km3 m y for the previous 50 m y and none of the louisville volcanoes built during the past 10 12 m y the time of most profuse hawaiian volcanism has reached sea level however a seamount from which pleistocene lavas were dredged rises to within 540 m of the sea surface from the crest of the hotspot swell at 50 5 s 139 2 w guyot heights demonstrate that a hotspot swell several hundred meters high has persisted throughout the known life of the chain and sometimes had an isostatic depression on its loaded crest the depths of guyot shelf breaks increase systematically to the northwest from 530 to 2100 m proportional to the square root of volcano age with some scatter and tilting caused by plate boundary tectonism and local isostatic subsidence and uplift

geoid anomalies at fracture zones and thermal models for the oceanic lithosphere geoid anomalies derived from the geosat altimeter data set have been used to model the change in the geoid height across four pacific fracture zones after accounting for geoid signals from the flexural response to differential subsidence and thermal bending stresses we find that the estimates of geoid steps across the fracture zones are compatible with existing conductive models for the oceanic lithosphere if flexure is not accounted for the geoid steps will be underestimated by 30 40 departures from the predicted conductive trends may be explained by small scale convection and uncertainties in the crustal ages

somali basin chain ridge and origin of the northern somali basin gravity and geoid low the northern somali basin located between chain ridge and the horn of africa north of 4 n is characterized by a distinct 5 m geoid low and by large negative gravity anomalies the boundaries of the basin are marked by steep gradients in both gravity and geoid basement in northern somali basin is 1 2 km deeper than on the carlsberg ridge flank to the southeast or on the sheba ridge flank to the north with a sharp discontinuity across the boundary of the basin the western somali basin to the south was created in the late jurassic and early cretaceous by the movement of madagascar away from africa reinterpretation of the magnetic anomalies in the western somali basin shows that they record both limbs of a mid ocean ridge that was active by m22 time kimmeridgian and died soon after mo aptian magnetic and gravity data allow the relict ridge crest to be traced from davie ridge near the african coast to the dhow vlcc ars fracture zone complex at 50 e davie ridge is a transform fault connecting the western somali basin spreading center with a similar age spreading center in the mozambique basin the dhow vlcc ars complex can be shown to continue north of the 4 n bathymetric high separating the northern and western somali basins and to intersect the african margin near 7 n the northern somali basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between east and west gondwanaland the original northern somali basin was split apart by the northward motion of india in the late cretaceous and chain ridge formed along the new boundary thermal and gravity modeling shows that the flexure resulting from differential subsidence across chain ridge combined with the difference in lithospheric thermal structure late jurassic vs early tertiary on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across chain ridge the geoid step up from the basin up to the african coast can be modeled as an edge effect between the juxtaposed jurassic oceanic and african continental crust thus the geoid and gravity low over the northern somali basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly

reconstruction of madagascar and africa evidence from the davie fracture zone and western somali basin new seismic reflection gravity and magnetic data from offshore east africa allow the davie fracture zone to be traced from 11 s to its intersection with the kenyan coast at 2 s constraining the relative motion of madagascar and africa seasat derived free air gravity anomalies and slope rise positive magnetic anomalies observed in shipboard data help to locate the continent ocean boundaries cob off the shore of east africa and madagascar seismic reflection data further document a diapirprovince off madagascar presumably conjugate to that observed off kenya and somalia the dhow and very large crude carrier vlcc basement ridges are complex features and do not appear to be simple fracture zones owing their existence entirely to the separation of madagascar and africa from these data we determine a predrift fit of madagascar and africa involving a 14 2 rotation of madagascar to africa about a pole at 10 n 150 e the geometry of the reconstruction adheres to seismic and potential field data indicating the oceanic nature and extent of the comoros basin and of the somali basin between kenya and the seychelles and it does not conflict with onshore or offshore stratigraphy timing of the opening of the western somali basin is constrained by mesozoic marine magnetic anomalies and extrapolation to the interpreted cob and occurred between approximately 165 and 130 ma

global distribution of seamounts from seasat profiles bathymetrie profiles and contour charts have been used to study the distribution of seamounts in the deep ocean basins but only a small fraction of the seafloor has been sampled by ships at the present exploration rate it will take several centuries to map significant portions of the seafloor topography satellite altimetry which maps the topography of the equipotential sea surface is a promising tool for studying the gravity fields of seamounts because all ocean basins can be sampled in a couple of years using a model of a gaussian shaped seamount loading a thin elastic lithosphere we develop a new technique for measuring basic characteristics of a seamount from a single satellite altimeter profile the model predicts that the seamount diameter is equal to the peak to trough distance along the vertical deflection profile and that the overall diameter of the signature reveals the age of the lithosphere when the seamount formed moreover the model suggests that these two measurements are relatively insensitive to the cross track location of the seamount we confirm these model predictions using seasat altimeter profiles crossing 14 well surveyed seamounts in the pacific we then apply the measurement technique to 26 106 million kilometers of seasat profiles resulting in a new global set of seamount locations approximately one quarter of the seamounts identified in seasat profiles were previously uncharted modeling suggests that there is no direct relationship between the size of a seamount and its signature in the geoid therefore the set of locations is not a straightforward sampling of the total seamount population but is weighted toward seamounts which are poorly compensated a preliminary analysis indicates considerable variations in population density and type across the oceans most notable among them are the absence of seamounts in the atlantic variations in population density across large age offset fracture zones in the pacific the prevalence of small signatures in the indian ocean and the existence of linear trends in the large seamounts of the west pacific

thermomechanical evolution of oceanic fracture zones a fracture zone fz model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere as the lithosphere cools by conduction thermal and mechanical boundary layers develop and increase in thickness as age 1 2 surface expressions of this development such as topography and deflection of the vertical i e gravity field are most apparent along major fz s because of the sharp age contrast a simple model including the effects of lateral heat transport but with no elastic layer predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the fz ages observations however show that both the fz topography and the deflection of the vertical remain sharp as the fz evolves these two observations as well as the observed asymmetry in deflection of the vertical profiles across the udintsev romanche and mendocino fz s are explained by including a continuous elastic layer in the model the asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the fz modeling also shows that the evolution of the fz topography is extremely sensitive to the initial thermal structure near the ridge transform intersection model geoid steps and their development with age are used to access techniques for measuring geoid offsets across fz s reasonable step estimation techniques will underestimate the overall step amplitude by up to 50 this implies that abnormally thin thermal boundary layers derived from studies of geoid height versus age are not required by the data

dynamic sea surface topography gravity and improved orbit accuracies from the direct evaluation of seasat altimeter data a gravitational model incorporating seasat altimetry surface gravimetry and satellite tracking data has been determined in terms of global spherical harmonics complete to degree and order 50 this model pgs 3337 uses altimeter data as a dynamic observation of the satellite s height above the sea surface a solution for the ocean s dynamic topography is recovered simultaneously with the orbit parameters gravity and ocean tidal terms the recovered dynamic topography reveals the global long wavelength circulation of the oceans with a resolution of 2 000 km and is very similar to the mean upper ocean dynamic height derived from historical ship observations the pgs 3337 geoid has an uncertainty of 60 cm rms globally but only 25 cm rms over the ocean because of the altimeter measurements seasat orbits determined in this solution have an estimated accuracy for the radial position of 20 cm rms the difference between the altimeter observed sea height and the geoid plus dynamic topography model is 30 cm rms contained in these residuals are the sea height variability as well as errors from the geoid orbits tidal models and altimeter range measurements this performance level is 2 to 3 times better than that achieved with previous goddard gravitational models

a directional analysis of the small wavelength geoid in the pacific ocean the geoid over the pacific ocean has been filtered in order to enhance wavelengths of the order of 200 km as noticed by previous authors lineations elongated in the direction of present plate motion are observed in the east central pacific elongated features as clearly visible with a larger amplitude and a totally different direction are also present south of the hawaiian chain and west of baja california the dominant directions visible on the filtered geoid map have been confirmed by spectral analyses near the californian coast the ne sw lineations seem to be related to extensional tectonics south of hawaii the nw se directions correspond to the various directions of the pacific plate prior to 40 ma magmas implaced in the lithosphere over fixed convective plumes may be the source of these geoid lineations

geoid height versus topography for oceanic plateaus and swells oceanic plateaus and swells are a major component of the seafloor topography yet they remain among the most poorly understood features this is especially true of the oceanic plateaus which show large variations in crustal thickness to determine the depth and mode of compensation for 53 of the largest plateaus and swells we analyzed the relationship between geoid height and topography in polygonal areas containing each feature both geoid height and topography were first band pass filtered 400 km i 4000 km to isolate the signal associated with local compensation from flexural and deep mantle signals the ratio of geoid height to topography was then determined by fitting a straight line to the data except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with airy and thermal compensation models eighteen features have high geoid topography ratios that cannot be explained by the airy compensation model of crustal thickening these features thermal swells are partially supported by thermal buoyancy forces in the lower half of the lithosphere the ratios are highest for active hot spot swells and decay with the thermal age of the swell to values consistent with airy compensation of the enduring volcanic edifice the remaining features plateaus have lower geoid topography ratios in agreement with the airy compensation model those plateaus with average height greater than 4 km are thought to be continental fragments the shorter plateaus tend to be volcanic features modified continental plateaus presumably small fragments of extended and intruded continental margin crust cluster around heights of n3 km overlapping the range associated with oceanic plateaus since the origin of many plateaus is poorly understood this global geoid topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells

geodetic and oceanographic results from the analysis of 1 year of geosat data the first year of the geosat exact repeat mission has been analyzed to obtain improved orbits gravity model and representation of sea surface topography the starting point for this study were the original geophysical data records merged with orbits by haines et al 1990 computed with the gem t1 gravity model a radial orbit error model following the development of engelis 1987a was modified to account for resonant effects above degree 36 twenty four 17 day arcs were analyzed separate 17 day and a single 1 year solution for a gravity model and a spherical harmonic representation of the sea surface topography to degree 10 were found the root mean square crossover discrepancy with the a priori i e gem t1 model was 1 1 m which was reduced to 0 22 m with the 1 year solution comparison of overlaps in the arcs showed discrepancies of 1 3 m with the a priori field which were reduced to 0 07 m with the 1 year solution the determination of the degree 1 0 term in the sea surface topography was not possible in this analysis because of the high correlation of this term with an orbit error term in addition the 1 1 terms showed serious incompatibility with that implied by historical oceanographic information the final solution used here incorporated the degree 1 coefficients from oceanographic data the problem with the 1 1 coefficients may imply a small center of mass offset in the tracking station positions used in the orbit calculations or may result from the poor distribution of the available tracking stations error analysis indicated a 10 cm sea surface topography commission error to degree 10 the geoid commission error to degree 36 was 9 cm in the ocean areas the new potential coefficient model was compared with terrestrial data in a manner to that done by engelis and knudsen 1989 to show the improvement of the new model

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

ionospheric effects on geosat altimeter observations the presence of free electrons and ions modifies the propagation speed of electromagnetic waves thus the apparent sea height measured by a radar altimeter such as that on board geosat is influenced by passage through the ionosphere we explore ionospheric effects using several methods a model ionosphere for westford massachusetts is constructed from two frequency observations of global positioning system satellites this model is compared with a theoretical calculation faraday rotation observations and the ionospheric correction furnished with the geosat geophysical data records we discuss the influence of the solar cycle which is important at the moment as a particularly active maximum is approached some general remarks on the ionospheric influence on the oceanographic signal observed by a satellite altimeter are also included

an investigation of small offset fracture zone geoid waveforms approximately 1 500 geoid cross sections across 15 small offset south atlantic fracture zones fz s are compiled from seasat altimeter data and organized according to crustal age these profiles provide a basis for the comparison of the different fz s and the evolution of each over geological time i use an empirical orthogonal function decompostion to investigate the dependence of profile shape and amplitude upon crustal age the geoid cross sections are found to be coherent in form down the length of each fz and possess amplitudes that are inversely related to the relative spreading rate at the time of formation this observation is consistent with a simple model in which the active portion of a fracture zone the transform fault remains a fixed spatial length yielding a variable age offset across the fz

geophysical applications of satellite altimetry publications related to geophysical applications of seasat and geosat altimetry are reviewed for the period 1987 1990 problems discussed include geoid and gravity errors regional geoid heights and gravity anomalies local gravity field flexure plate tectonics and gridded geoid heights gravity anomalies 99 refs

seasat altimetry and the south atlantic geoid 2 short wavelength undulations this paper is a discussion of the short wavelength 25 100 km undulations of the geoid recovered from seasat altimeter data in the south atlantic these undulations are known to be highly correlated to seafloor topography and they considerably improve our knowledge about global bathymetry of the south atlantic which remains poorly surveyed the orientation of fracture zones fz with respect to the seasat profiles ensures an efficient recovery of the fz pattern in the younger 65 ma parts of the south atlantic between 20 n and 50 s we used 22 well marked fzs regularly distributed between 12 n and 47 s to compute eulerian poles describing the finite relative movement between africa and south america from the present back to 35 ma fz trends south of the equator are well accounted for with a single rotation and standard statistical tests do not indicate a need for distinct and successive rotations however these tests indicate an improvement of the fit when distinct poles are adjusted to separate subsets of our 22 fzs either fzs north and south of the equator or fzs north and south of tristan da cunha this points out the possibility of the existence of either corridors in which slight relative movements occur or diffuse elastic deformation over the entire south atlantic although this is really a second order effect we also present and discuss a map of the roughness of the geoid for the 25 to 110 km wave band strong disagreements between the roughness map and the gebco maps exist in regions of sparse ship track coverage for example the rio grande rise has an associated roughness feature which appears elongated and more symmetric with respect to the walvis ridge than what is displayed on the gebco maps the roughness map also clearly documents a long wavelength 400 km segmentation of the mid atlantic ridge between the equator and 35 s

investigations of relative plate motions in the south atlantic using seasat altimeter data fracture zones in the world s oceans appear clearly in the seasat altimeter geoid data at wavelengths of about 100 km and less in the north and south atlantic these signatures are stable in form and amplitude from the ridge crest to crust tens of millions of years old and can be mapped across nearly the width of the basin the signatures diminish in amplitude markedly on crust older than 70 to 80 m y b p apparently when thickly covered by sediments because fracture zone trajectories closely approximate plate motion flow lines they may be used to infer the plate kinematic history of the ocean basin in this paper i describe a new method for estimating the relative poles of opening describing the history of motion between two plates by using fracture zone trajectories the method is an iterative inversion whereby the trajectories of synthetic flow lines are linearized with respect to the pole parameters when applied to the seasat altimeter track data a waveform inversion based upon this method is possible in which the altimeter data are modeled using a synthetic geoid fracture zone signal this waveform inversion is applied to 17 prominent fracture zones in the central and south atlantic to estimate the poles of opening between the south american and african plates altogether 1022 seasat fracture zone crossings are used for each of these a waveform comparison is made the new method is found to be robust and converges to a stable set of pole solutions even if the initial flow lines are small circles about a single pole that poorly approximates the observed trajectories the solution poles document a cenozoic change in plate motion near anomaly 13 time 36 m y b p similar to that in the north atlantic at anomaly 1 3 21 time 36 50 m y b p

tectonic fabric map of the ocean basins from satellite altimetry data abstract satellite altimetry data provide a new source of information on the bathymetry of the ocean floor the tectonic fabric of the oceans i e the arrangement of fracture zones ridges volcanic plateaus and trenches is revealed by changes in the horizontal gravity gradient as recorded by satellite altimetry measurements seasat and geosat altimetry data have been analyzed and a global map of the horizontal gravity gradient has been produced that can be used to identify a variety of marine tectonic features the uniformity of the satellite coverage provides greater resolution and continuity than maps based solely on ship track data this map is also the first global map to incorporate the results of the geosat mission and as a result new tectonic features are revealed at high southerly latitudes this map permits the extension of many tectonic features well beyond what was previously known for instance various fracture zones such as the ascension tasman and udintsev fracture zones can be extended much closer to adjacent coninental margins the tectonic fabric map also reveals many features that have not been previously mapped these features include extinct ridges minor fracture zone lineations and seamounts in several areas especially across aseismic plateaus or along the margins of the continents the map displays broad gravity anomalies whose origin may be related to basement structures

global mean sea surface based upon the seasat altimeter data a mean sea surface map has been computed for the global ocean areas between 70 n latitude and 62 s latitude based upon the 70 day set of seasat altimeter data recorded in 1978 a combination of regional crossover adjustment techniques and a global reference surface using the best available geodetic parameters for seasat has formed the basis for the computation the overall rms value of the crossover residuals in the open ocean areas was 13 cm after the adjustment a contour map of the crossover residuals is very similar in nature to earlier contour maps of the collinear pass results for mesoscale variability the mean sea surface is presented in the form of a global contour map a 0 5 0 5 grid and a computerilluminated image

geoid anomalies across ascension fracture zone and the cooling of the lithosphere twenty three seasat altimeter profiles crossing the ascension fracture zone are used together with seafloor ages inferred from a kinematic model to estimate a geoid slope age relation between ages of 5 and 50 ma the data do not definitely contradict a half space cooling model slope 9 5 3 cm ma if biases are accepted resulting in large scatter however the data would seem to support a puzzling explanation in terms of a plate cooling model involving two distinct thermal thicknesses one of 70 km for ages 35 ma although preliminary this result is similar to that found in the pacific with a much larger data set

seasat gravity undulations in the central indian ocean abstract in the southeast indian ocean geoid and gravity maps derived from the geos3 and seasat satellites data show clear evidence of a lineated pattern located between the southeast indian ridge and the southern branch of the ninetyeast ridge we have carried out two dimensional spectral analyses of the gravity data over four regions each of 10 sx 10 dimension three being located over the indo australian plate one over the antarctica plate we have also analysed 13 seasat geoid height profiles roughly parallel to the southeast indian ridge covering both sides of the ridge power spectra of the gravity and geoid data over the indo australian plate confirm the existence of undulations oriented n40 e of 200 km wavelength and of 10 mgal and 40 ma the wavelength remains constant with increasing plate age such undulations are not seen west of the southeast indian ridge i e over the antarctica plate where observed the geoid undulations have a sinusoidal signature quite different from the typical step like signature of fracture zones the lineations observed in the southeast indian ocean have characteristics comparable to those reported by haxby and weissel in the central pacific and interpreted as evidence of small scale convection occurring in a low viscosity layer just beneath the lithosphere

geoid anomalies over two south atlantic fracture zones abstract seasat altimetry profiles across the falkland agulhas fracture zone fz and the ascension fz in the south atlantic were examined for evidence of step like geoid offsets predicted from thermal modeling of the lithosphere the geoid profiles exhibit much short wavelength power and the step like offsets are often small making reliable estimation of the heights of the observed geoid offsets difficult the offsets were estimated by the least squares fitting of quadratic curves incorporating a step function to the altimetry profiles a preferred offset value was determined for each profile by taking the average of step heights computed with various distances around the fracture zone excluded from the fit the age of the crust surrounding the fracture zones necessary for computing a theoretical geoid offset was determined from surface ship magnetic anomaly data and from existing ocean floor age maps observed variations in geoid step height with age of the lithosphere are not consistent with those predicted from standard thermal plate models for ages less than 30 ma the step offsets across both fracture zones decrease in a manner appropriate for an unusually thin plate with a thickness of 50 75 km at greater ages the offsets show complex behavior that may be due to bathymetric features adjacent to the fracture zones similar geoid patterns on opposite branches of the falkland agulhas fz are indicative of processes that act symmetrically on both sides of the mid atlantic ridge this behavior of the geoid is consistent both with small scale convection occurring beneath the lithosphere and with bathymetric features originally produced along the ridge crest and now located symmetrically on opposite sides of the ridge the west flank of the ascension fz displays a regrowth in step height at about 40 ma consistent with small scale convection and in agreement with other studies of pacific and south atlantic fracture zones

depth and geoid anomalies over oceanic hotspot swells a global survey the broad depth and geoid anomalies associated with 23 hotspot swells in oceanic areas have been analyzed maximum height and geographical extent of the topographic swell and of the geoid anomaly as well have been measured for each hotspot the results indicate a clear increase of the topographic swell height with age of the underlying lithosphere from values in the range 300 500 m at young ages to values in the range 1 500 2 000 m at ages larger than 100 ma the geoid anomaly amplitude also increases with plate age from nearly zero close to mid ocean ridges to 6 8 m over old plates on the other hand the geographical extent of the swell does not show any clear relationship with plate age the mean lateral extent of swells range from 1 000 to 1 500 km swells located close to spreading ridges show a significant non zero depth anomaly but are associated with negligible geoid signal these results complete those of a previous study where the apparent compensation depth of oceanic hotspot swells was showed to increase linearly with the square root of plate age and coincide roughly with the base of the thermal lithosphere this trend may either be interpreted in terms ofmore lithospheric thinning or dynamical support besides both seem necessary to explain the observed bathymetry in proportion evolving with aging of the lithosphere less

constraints on thermal and mechanical structure of the oceanic lithosphere at the bermuda rise from geoid height and depth anomalies abstract the bermuda rise is a broad area of anomalously shallow seafloor presumed to be of thermal origin dating to the middle eocene this and other midplate swells have been modeled both by elevated temperature in a convecting layer at the base of the plate and as thermal expansion within the conducting portion of the lithosphere using recently developed techniques for the estimation of lithospheric flexure and temperature structure from geoid bathymetry and heat flow data we place constraints on the depth of the thermal load and limit the possible thermal mechanisms causing this swell our data include depth anomalies calculated from dbdb5c bathymetry data geoid anomalies using data from both seasat and geos 3 altimeter missions and published heat flow values the first technique we employ to determine the thermo mechanical properties of the swell is a forward filtering technique with which we determine flexural rigidity and compensation depth by successive fits to topography filtered to separate subsurface and surface loading the second method is an admittance technique with which we directly determine the geoid to topography ratios corresponding to the volcano and the swell both linear filtering and admittance approaches yield similar conclusions concerning the elastic plate thickness and compensation depth of the bermuda rise indicating that the effective elastic thickness for the plate supporting the volcanoes is 30 5 km and the compensation depth for the swell is 55 10 km finally we use the linear programming technique to obtain extremal bounds on temperature as a function of depth and time in the lithosphere we find that the depth and geoid anomalies heat flow and elastic thickness observations are inconsistent with a mechanism for formation of the bermuda rise which consists solely of thermal anomalies confined to depths greater than 50 km at the initiation of hotspot activity if we use thermal rejuvenation alone to model the bermuda rise the observations require that it be accompanied by substantial reheating to shallow depth in the lithosphere on the basis of the data presented in this paper we cannot distinguish between a reheating model and a convection model for the origin of the bermuda rise though based on our geothermal modeling we preclude convective support models that do not change temperatures within the thermal plate

marine gravity of the southern ocean and antarctic margin from geosat in november of 1986 the u s navy satellite geosat began collecting unclassified gravity altimeter data as part of its exact repeat mission erm for national security reasons the geosat orbit was arranged so that it closely follows the seasat satellite altimeter ground track however there are two advantages of the geosat data over the seasat data first because of improvements in altimeter design geosat profiles are about 3 times more precise than seasat profiles this corresponds to an accuracy of 2 3 rad i e 2 3 mgal for wavelengths greater than 20 km second the geosat altimeter data were collected when the antarctic ice coverage was minimal february 1987 to march 1987 while seasat was only active during an antarctic winter june 1978 to september 1978 these new data reveal many previously uncharted seamounts and fracture zones in the extreme southern ocean areas adjacent to antarctica seven large age offset fracture zones apparent in the geosat data record the early breakup of gondwana finally the new data reveal the detailed gravity signatures of the passive and active continental margins of antarctica these data are an important reconnaissance tool for future studies of these remote ocean areas

late permian paleomagnetic results from the lodeve le luc and bas argens basins southern france magnetostratigraphy and geomagnetic field morphology paleomagnetic results are presented from 271 stratigraphically ordered horizons at four locations in southern france our focus is mainly on the late permian 258 horizons but results from 13 horizons in the triassic buntsandstein are also reported we argue that the permian results extend magnetostratigraphic coverage up to the upper capitanian stage some 6 million years after the end of the permocarboniferous reversed superchron defined by the illawarra reversal in the wordian stage when combined with published data an overall mean paleopole at 49n 161 e a95 4 n 9 is obtained this is virtually identical to the upper permian pole obtained by bazhenov and shatsillo 2010 using the intersecting great circle method agreement between the two procedures which are based on entirely independent data supports the geocentric axial dipole gad model

seasat derived gravity constraints on stress and deformation in the northeastern indian ocean we use seasat derived gravity data to investigate crustal deformation in the ne indian ocean gravity highs reflecting crustal undulations vary in orientation from e w in the central indian basin to ne sw in the wharton basin the undulations vary in trend similarly to the variation of the maximum compression directions predicted by a plate driving force model cloetingh and wortel 1985 and are essentially restricted to the area of predicted compression for both principal horizontal stresses this agreement implies that the stress model describes the basic features of the deformation observed in the gravity as well as the seismicity the gravity data also provide insight into two enigmatic tectonic features the transition in the morphology of the 90 e ridge at 10 s from a continuous high to a complex blocky structure appears related to the deformation since undulations in the central indian basin can be projected eastward to blocks on the ridge the morphology formerly interpreted as a fossil feature may reflect the recent deformation the discrepancy between the trends of the southernmost 85 e ridge and the 90 e ridge previously thought to exclude similar hot spot track origins appears to result from treating a crustal undulation as part of the 85 e ridge earthquakes do not appear to be preferentially located with respect to the peaks and troughs of the undulations

evidence from en echelon cross grain ridges for tensional cracks in the pacific plate sea floor topography in the pacific is mainly aligned with original spreading directions1 but is overprinted by alignments created by mid plate processes spreading produces abyssal hills and fracture zones and mid plate volcanism generates seamounts isolated or in chains a different category of topography the cross grain discovered in geoid height data collected by the seasat radar altimeter2 comprises linear troughs and swells spaced 200 km apart oblique to fracture zones and abyssal hills but parallel to the hawaiian chain three models have been proposed for the cross grain small scale convection organized into longitudinal rolls by the shear of the pacific plate2 compressive buckling3 and lithospheric boudinage resulting from plate wide tensile stresses4 5 none of the previously available data ruled out any of these models here we report multi beam bathymetric data revealing long narrow en echelon ridges along the cross grain interpreted as evidence of tension cracks in the pacific plate

corections for the effects of significant wave height and attitude on geosat radar altimeter measurements range estimates from a radar altimeter have biases which are a function of the significant wave height swh and the satellite attitude angle based on results of pre launch geosat modeling and simulation by the johns hopkins applied physics laboratory a correction for swh and attitude was already applied to the sea surface height estimates from geosat s production data processing by fitting a detailed model radar return waveform to geosat waveform sampler data we can provide an independent estimate of the height bias due to swh and attitude angle the waveform fitting also produces independent estimates of the swh and of the attitude angle the waveform fitting has been carried out for 10 s averages of geosat waveform sampler data over a wide range of swh and attitude values our results confirm that geosat sea surface height correction is good to well within the original decimeter level specification but that an additional height correction can be made at the several centimeter level our results are expressed as additive corrections to the noaa distributed geosat estimates of sea surface height swh and attitude and we provide coefficients for the polynomial expressions for these corrections

anomalous elastic thickness of the oceanic lithosphere in the south central pacific to first order the elastic thickness te of the oceanic lithosphere under mid plate volcanoes increases linearly with the square root of plate age at the time of loading1 4 a large scatter around this simple relationship is observed however suggesting that factors other than age influence the elastic thickness and produce departure from the first order trend5 to look at second order effects we have conducted a worldwide analysis over the pacific atlantic and indian oceans and determined te under sixty volcanoes applying elastic flexure theory and using as observational constraints geoid height data from the seasat satellite we find that for the three main oceans the depth of the elastic layer under volcanoes increases with age of plate at loading time t according to the simple relationship te 2 70 0 15 t1 2 te in km t in myr except under all of the volcanic chains of the south central pacific where this relation does not apply the elastic layer is found to be much thinner there and indicates a broad regional anomaly presumably of thermal origin well correlated with several other geophysical anomalies reported in this region6 10

effects of islands in the geosat footprint the presence of land within a radar altimeter s footprint may contaminate the altimeter s tracking performance and perturb the nadir oceanographic signal an altimeter tracker tends to move towards the brighter more specular land reflections in the radar return waveforms researchers may unintentionally include tracking data over small mid ocean islands in their data sets if their goal is to obtain sea surface height ssh information at the subdecimeter level such inclusion is inappropriate because islands can locally affect ssh by more than 4 m altimeter signal strengths and wave height related measurements are also adversely affected the tainted data should be edited however if editing criteria near island areas are too conservative useful geoid tide current sea state and storm surge information will be excluded a study has been performed which examines the effects of small mid ocean islands on the geosat tracker the surface return waveforms as well as the altimeter measurements have been analyzed open ocean waveforms have been compared with waveforms containing land reflections they differ perceptibly computations could be performed based on a combination of waveform data altimeter signal strength and sea surface heights to detect contaminated ssh values for subsequent editing

estimating the compensation depth of the hawaiian swell with linear filters broad areas of anomalously shallow seafloor surround regions of active or recently active hot spot volcanism the cause of these midplate swells has been modeled both as elevated temperatures in a convecting layer and as thermal expansion within the conducting portion of the lithosphere according to the former explanation we would expect the low density material compensating the swell to lie beneath the conducting lid whereas the latter model allows compensation at shallower depths we present a technique based on the application of linear filters for determining the compensation depth of midplate swells given observations of topography and geoid or gravity anomalies our method does not require an a priori estimate of swell height or width nor does it assume complete spatial or spectral separation between the swell topography and the signal from the hot spot volcanoes which are compensated by flexure of the oceanic moho rather we exploit the difference in the predicted sign and amplitude of the moho deflection beneath elevated topography produced by loading above versus loading below the elastic plate in order to separate the two effects an application of these linear filters to residual topography and geoid data from the region surrounding the hawaiian hot spot yields a compensation depth of 70 10 km within the lower lithosphere

comparison of geos 3 and seasat altimeter resolution capabilities a comparison is made between geos 3 and seasat altimeter data with regard to noise power spectral density psd derived geoid psd and short wavelength resolution at wavelengths greater than 450 km the noise psds for geos 3 and seasat are essentially the same and are attributed to mesoscale oceanographic effects at shorter wavelengths the noise spectra diverge because of the higher geos 3 instrument noise the average along track geoid psds are nearly identical between wavelengths of 4500 and 72 km the spectral coherence between repeat tracks shows that the short wavelength resolution of seasat data is 32 km and for geos 3 it is 60 km

origin of the louisville ridge and its relationship to the eltanin fracture zone system we have combined shipboard and seasat altimeter derived data in an intergrated geological and geophysical study of the louisville ridge a 3500 km long seamount chain extending from the tonga trench to the eltanin fracture zone a break in the smooth trend of the ridge at latitude 37 5 s has been recognized in both bathymetric and altimetric data the 40ar 39ar dating of rocks dredged either side of the break suggest that it is analogous to the bend in the hawaiian emperor seamount chain although the general trend of the ridge can be fit by small circles about pacific absolute motion poles determined from other seamount chains the new bathymetric and age data allow us to refine pacific absolute motion poles the continuity in smooth trend of the ridge and the eltanin fracture zone suggests some relationship between them however a major offset developed on this transform between 60 and 80 ma prior to the oldest dated rocks from the ridge although magmatism was more or less continuous on the ridge during 28 60 ma it probably occurred on crust with little or no offset thus magmatism appears to have been little influenced by the developing fracture zone by 28 ma the distance between the magmatic source and the fracture zone had decreased sufficiently for a portion of the ridge to have been emplaced on crust with an offset after about 12 ma however volcanic activity on the louisville ridge apparently waned despite a possible influence on the magmatism of the fracture zone

oceanic tides from geosat altimetry direct tidal analysis of the altimetry from the geosat exact repeat mission s first year is used to derive estimates of the diurnal and semidiurnal oceanic tides the geoid is removed by collinear differences at 34 1 day separation and the orbit error is reduced by subtracting a slowly modulated 1 cycle revolution term a sequence of independent analyses at grid areas of 1 latitude 1 5 longitude using orthotide functions ensures complete definition of the diurnal and semidiurnal species global admittance maps for m2 and s2 within the latitudes 58 n and 59 s compare well with ground truth at 66 open ocean sites maps of differences between geosat and the naval surface weapons center model show important areas of the order of 10 15 cm the integral for the global power input from the moon gives 2 51 tw m2 0 44 tw s2 lower than many previous estimates but in agreement with figures deduced from a recent analysis of satellite orbits

altimetry ship gravimetry and the general circulation of the north atlantic gravity accelerations estimated from satellite altimetric mean sea surfaces seasat and geos 3 are compared to ship gravity measurements ship gravity are closer to an estimate based on least squares collocation orbit perturbations altimetry and terrestrial gravity than to an estimate based on fourier transforms orbit perturbations and altimetry only both altimetric estimates yield a smoothed picture of the geostrophic component of sea surface currents in the north atlantic when gravity acceleration data from only nine cruises are subtracted from the altimetric gravity

tectonic history and new isochron chart of the south pacific we have developed an internally consistent isochron chart and a tectonic history of the south pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data highly accurate vertical deflection profiles 1 2 rad derived from 22 repeat cycles of geosat altimetry reveal subtle lineations in the gravity field associated with the south pacific fracture zones these fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas the high density of geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map the tectonic fabric was then used to derive new poles of rotation for 12 selected times in the late cretaceous and cenozoic from our reconstructions we estimated the former location of the spreading centers in order to derive a new set of isochrons interpreted unes of equal age on the ocean floor we believe that the use of new geosat altimeter data in combination with a multi plate reconstruction has led to an improvement in our understanding of south pacific tectonics

mechanical and thermal isostatic response of the del cano rise and crozet bank southern indian ocean from altimetry data an analysis of the relationship between the bathymetry and the gravity field has been carried out both in the frequency domain admittance technique and in the space domain over the del cano rise and the crozet bank southern indian ocean a bathymetric map contoured from original shipboard data and a geoid anomaly map computed from a merged set of seasat and geos3 data were used the inversion of admittances for water depth crustal thickness crustal density and elastic plate thickness shows that the isostatic response of the lithosphere supporting the del cano rise differs very much from that of the one situated beneath the crozet bank the del cano rise appears to have been formed near the active southwest indian ridge before early eocene and is locally supported by a thickened crust shallow thermal processes contribute to the present topography of the crozet bank emplaced further away from the southeast indian ridge on older crust