dynamics of the mid ocean ridge plate boundary recent observations and theory the global mid ocean ridge system is one of the most active plate boundaries on the earth and understanding the dynamic processes at this plate boundary is one of the most important problems in geodynamics in this paper i present recent results of several aspects of mid ocean ridge studies concerning the dynamics of oceanic lithosphere at these diverging plate boundaries i show that the observed rift valley to no rift valley transition globally due to the increase of spreading rate or locally due to the crustal thickness variations and or thermal anomalies can be explained by the strong temperature dependence of the power law rheology of the oceanic lithosphere and most importantly by the difference in the rheological behavior of the oceanic crust from the underlying mantle the effect of this weaker lower crust on ridge dynamics is mainly influenced by spreading rate and crustal thickness variations the accumulated strain pattern from a recently developed lens model based on recent seismic observations was proposed as an appealing mechanism for the observed gabbro layering sequence in the oman ophiolite it is now known that the mid ocean ridges at all spreading rates are offset into individual spreading segments by both transform and nontransform discontinuities the tectonics of ridge segmentation are also spreading rate dependent the slow spreading mid atlantic ridge is characterized by distinct bulls eye shaped gravity lows suggesting large along axis variations in melt production and crustal thickness whereas the fast spreading east pacific rise is associated with much smaller along axis variations these spreading rate dependent changes have been attributed to a fundamental differences in ridge segmentation mechanisms and mantle upwelling at mid ocean ridges the mantle upwelling may be intrinsically plume like 3 d beneath a slow spreading ridge but more sheet like 2 d beneath a fast spreading ridge

application of neural networks for the prediction of the unconfined compressive strength ucs from equotip hardness abstract this paper presents the application of a neural network for the prediction of the ucs from hardness tests on rock samples to investigate the suitability of this approach the results of the network are compared to predictions obtained by conventional statistical relations the network was trained to predict the ucs based on the hardness porosity density grain size and rock type information of a rock sample a dataset containing 194 rock sample records ranging from weak sandstones to very strong granodiorites was used to train the network with the levenberg marquardt training algorithm two sets each containing 17 rock samples were used to validate the generalization and prediction capabilities of the network

the origins of shallow water carbonate lithofacies thickness distributions one dimensional forward modelling of relative sea level and production rate control the observation that shallow marine carbonate strata often have exponential lithofacies thickness distributions is one of the most fundamental results in carbonate stratigraphy in recent years because it is an observation that can be tested for its repeatability in various outcrop and subsurface examples and also because it raises the question of what sedimentary processes and climatic and oceanographic settings might lead to formation of a particular lithofacies thickness distribution this observation in turn links to the significant issue of how carbonate strata record climatic and oceanographic change through geological time this study applies a simple one dimensional numerical stratigraphic forward model of carbonate platform strata dougal to study how relative sea level oscillations could control lithofacies distribution dougal records platform top carbonate accumulation influenced by water depth dependent sediment production in euphotic oligophotic and aphotic production profiles with a lag depth controlling onset of production results from five single model runs highlight the issue of non stationary behaviour where statistical properties of the strata change with elevation up the section and show that exponential lithofacies thickness distributions can be generated from an entirely deterministic model results of multiple model runs 27 200 in total spanning a range of production and accommodation creation rates demonstrate that the three main controls on carbonate lithofacies distribution in this one dimensional model are i complex variations in the rate of creation of accommodation here due to high frequency glacio eustatic oscillations ii rate of sediment production dominated in these model runs by euphotic production rates and iii operation of autocyclic oscillations in deposition driven by a lag depth effect and formed during certain high frequency rising limbs on the glacio eustatic curve in these multiple model runs only ca 13 of the total runs created exponential distributions compared with 28 in the documented outcrop examples suggesting that other processes not included in this model play an important role more work is required to determine what these processes might be obvious candidates would be the various sedimentary processes that occur in three dimensions on platform tops such as sediment transport and variations in rate and type of sediment production related to biological variability and other process that occur in other different depositional settings for example sediment transport on slopes

gravity field determination and characteristics retrospective and prospective gravimetry has had a long history using pendulums torsion balances and static spring gravimeters relative accuracy adequate for many geophysical problems was already attained by 1900 but it took another half century to build readily portable gravimeters calibration and datum definition remained problems until the 1970s when free fall absolute gravimeters were developed that now have a precision of 10 3 mgal the problems of geographic inaccessibility and field party costs notably in areas of greatest tectonic interest are now being overcome by airborne gravimetry that has already achieved accuracies of 1 3 mgal with resolutions of 10 to 20 km satellite techniques are the best way to determine the long wavelength variations of the gravity field the resolution of the models has steadily improved with the number of satellites and the precision of the observations the best current model includes tracking data from more than 30 satellites satellite altimetry and surface gravimetry and has a resolution of about 290 km harmonic degree 70 with the most recent improvements coming from doppler orbitography and radiopositioning integrated by satellite doris tracking of the spot 2 satellite and satellite laser ranging slr doris and global positioning system gps tracking of the topex poseidon satellite meanwhile radar altimetry has become the dominant technique to infer the marine geoid with a resolution of tens of kilometers or shorter similarly the gravity fields of the moon venus and mars have been determined to harmonic degrees 70 75 and 50 respectively although tracking limitations result in variations of spatial resolution modeling earth s gravity field from the abundance of precise data has become an increasingly complex task with which the development of computer capacity has kept pace contemporary solutions now entail about 10 000 parameters half of them for effects other than the fixed gravity field of earth temporal variations arising from tides have long been well modeled and nontidal changes are now being identified the improvement in gravitational models engendered corresponding advances in geophysical interpretation isostatic models were refined and expanded to account for regional thermal and tectonic histories interpretation of the long wavelength gravity field determined by satellite techniques has been mainly in terms of plate tectonics as a manifestation of mantle convection gravity has been significant in inferring that there must be a large increase in viscosity with depth most strongly from the apparent slow sinking of subducted slabs the prospects for increasing accuracy and resolution in the determination of earth s gravity field rest primarily with the development of new measurement systems airborne gravimetry is taking promising new steps using gps but significant global model improvement awaits a dedicated satellite gravimetry system and future satellite altimeter missions will do more for ocean dynamics studies than geoid improvement advances in interpretation will occur through the development of other data such as seismic tomography and larger scale computer modeling of tectonics and convection

resolving marine gravity with ers 1 satellite altimetry to estimate how well ers 1 data from the 35 day repeat mission can resolve fine structure in the marine geoid or gravity field we compare along track gravity and gridded gravity fields derived from ers 1 fast delivery altimeter data to those computed from geosat geodetic mission altimeter observations in the southern ocean we find that single passes of ers 1 data can resolve along track gravity anomalies with wavelengths as short as 28 30 km i e comparable to geosat however the ers 1 gridded gravity field can only resolve anomalies as short as 66 km versus 18 km for geosat this two dimensional resolution is limited by the larger ground track spacing 35 km at 60 s of ers 1 to resolve fine structure in the marine gravity field it is essential to have altimeter data along more closely spaced ground tracks such as those planned for the ers 1 176 day repeat mission in 1994

a detailed gravity field over the reykjanes ridge from seasat geosat ers 1 and topex poseidon altimetry and shipborne gravity the method of least squares collocation lsc was used to construct a detailed gravity field over the reykjanes ridge using seasat geosat ers 1 topex poseidon altimeter data and shipborne gravity data mean altimeter derived geoid gradients along with their standard deviations were used in order to avoid cross over adjustment of sea surface heights the ship data were adjusted to a satellite only gravity field using a quadratic polynomial in time and then merged with the satellite data to produce a combined gravity field the necessary covariance functions were derived using the law of covariance propagation in a test case a comparison between a combined gravity field and ship data not included in the calculation yielded an rms difference of 2 31 mgals the combined gravity field contains a wealth of short wavelength information and clearly reveals the detailed tectonic structures associated with the reykjanes ridge

tectonic evolution of the pacific margin of antarctica 2 structure of late cretaceous early tertiary plate boundaries in the bellingshausen sea from seismic reflection and gravity data 1 interpretations of multichannel seismic mcs reflection and potential field data suggest that some prominent gravity anomalies in the bellingshausen sea are associated with plate boundaries that were active during the late cretaceous and early tertiary between 83 and 93 w a belt of negative anomalies extends along the west antarctic continental slope which we term the continental slope gravity anomaly csga mcs profiles show that the csga coincides with an acoustically opaque structural high imaged beneath the lower slope we interpret this structure as the upper part of an accretionary prism which formed during southward subduction of the phoenix and charcot plates before chatham rise separated from west antarctica mcs profiles crossing the same margin to the northeast show no evidence of an extensive buried accretionary prism but instead reveal an abrupt northeastward steepening of the continental slope near 78 w we attribute this change in tectonic style at least in part to subduction erosion resulting from subduction of rough oceanic basement which formed at the antarctic phoenix ridge after an abrupt decrease in spreading rate at chron 23r 52 ma near 95 w the bellingshausen gravity anomaly bga consists of a prominent low high gravity couple which crosses the west antarctic continental shelf slope and rise the bga corresponds to a buried asymmetric basement trough where cretaceous oceanic basement dips beneath more elevated basement to the east the trough probably formed after subduction of charcot plate ocean floor stalled at the nearby antarctic peninsula margin near the end of the cretaceous normal superchron ocean floor to the east of the bga became attached to the antarctic peninsula and the bga trough subsequently accommodated a small amount of convergent motion between the antarctic peninsula and the ocean floor to the west initially part of the marie byrd land plate and later part of the bellingshausen plate tectonism probably ceased at the bga at chron 27 61 ma as a result of a general plate reorganization in the south pacific

refined spreading history at the southwest indian ridge for the last 96 ma with the aid of satellite gravity data summary the spreading history of the oceans is modelled mostly by using magnetic anomalies and the fracture zone geometry the high quality satellite derived gravity data that became available in recent years reveal the details of fracture zones which can be used as flow lines to control spreading models we have applied this approach to the southwest indian ridge swir in order to refine its spreading history this is particularly useful for the period of complex spreading between magnetic anomalies 33 and 23 where the magnetic anomalies alone cannot resolve the detailed spreading history we find four main stages in the spreading history of the swir since 96 ma including two that were not noted previously between 96 ma and anomaly 33 76 3 ma and between anomalies 23o 51 7 ma and 18o 40 1 ma o denotes old boundaries of normal magnetization period we also find that the start of the period of complex spreading was at anomaly 33 somewhat earlier than previously proposed we discuss the characteristics of the extension that the old transform faults underwent during the complex spreading phase in response to the counterclockwise rotation of spreading new transform faults appeared at that time considerably widening the transform zones

gravity of the arctic ocean from satellite data with validations using airborne gravimetry oceanographic implications 1 precise mappings of sea surface topography slope and gravity of the arctic ocean are derived from altimeter data collected by envisat and icesat both altimeters measured instantaneous sea surface height at leads in the sea ice to reduce contamination by ice freeboard signal and tracker noise in envisat height data a retracking of the waveform data was performed analogous reprocessing of icesat data was also done arctic mean sea surfaces msss were computed from envisat data spanning 2002 2008 and icesat data spanning 2003 2009 farrell et al 2012 used these icen msss to estimate mean dynamic topography mdt these same envisat and icesat data are used in sea surface slope form to compute the arctic satellite only arcs 2 altimetric marine gravity field arcs 2 extends north to 86 n and uses grace goce gravity data goco02s for its long wavelength 260 km components use of envisat data improves the spatial resolution over that of existing arctic marine gravity fields in many areas arcs 2 s spatial resolution aids in tracing tectonic fabric e g extinct plate boundaries over broad areas of the arctic basin whose tectonic origin remains a mystery arcs 2 s precision is validated using nasa 2010 2011 operation icebridge oib airborne gravimetry arcs 2 and oib gravity along with icen mss results are employed to locate short wavelength errors approaching 1 m in current arctic marine geoids egm2008 precise oib airborne gravity corroborates that such errors in current geoid gravity models are widespread in arctic areas lacking accurate surface gravity data these geoid errors limit the spatial resolution at which mdt can be mapped

on the accuracy of digital bathymetric data the global 5 arcmin gridded topography data etopo 5 are based on contour maps rather than original soundings and have large errors the artificial statistical distribution generated by digitizing contours makes these data unsuitable for use in regression models for depth age variations their amplitude spectrum is bounded by a frequency 4 power law so that they should not be used when the gravity topography transfer function is important such as in studies of flexural isostasy and mantle convection i assess the accuracy of 14 491 069 digital ship soundings in 2253 cruise surveys collected between 1955 and 1992 in the lamont doherty earth observatory on line data base by analyzing 329 058 crossover errors coes at intersecting ship tracks five percent of cruises with internal coes yield root mean square coe amplitudes exceeding 500 m all of these have errors in digitizing two way travel time from analog precision depth recorder traces twenty eight cruises were found which had errors caused by misinterpretation of the nominal sound velocity used when travel times were reported as nominal depths two nominal sound velocities in common use differ by 2 5 an amount which is often undetectable producing uncertainties in depth of this magnitude ship data have been acquired at different rates over time with the peak of activity in the early 1970s although present technologies can yield very accurate data these are acquired at a rate which is small with respect to the total available data the cumulative median global coe has remained constant at 26 m since the late 1970s most recent data acquisition has been in the northern hemisphere oceans and the oldest and least accurate data are in the southern oceans where median coes are 100 250 m the majority of the data in the south pacific were acquired before the advent of satellite navigation

gravity field of the southeast central pacific from geosat exact repeat mission data efficient techniques for mapping the marine gravity field with geosat exact repeat mission erm data are demonstrated geosat has completed more than fifty two 17 day global erm repeat cycles and has collected roughly 50 million observations of geoid undulations or more precisely sea surface topography because the global ground track repeats every 17 days the erm observations are highly redundant by averaging the repeat cycles very precise profiles of alongtrack geoid undulations are obtained for a region of the southeast central pacific without any crossing arc orbit adjustments these profiles are shown to be repeatable to within 10 cm over their entire 3000km length i e to three parts in 108 this southeast central pacific region contains a portion of the east pacific rise as well as some cross grain gravity field lineations and is therefore of considerable geophysical interest in addition to profiles of alongtrack deflections of the vertical regularly gridded gravity anomalies are derived for this region and portrayed in color the procedure used for recovering gravity has two stages in stage 1 alongtrack deflections of vertical are derived by fitting cubic splines to alongtrack heights computing alongtrack slopes at standardized latitudes and then averaging slopes for coincident passes in stage 2 gridded band limited gravity anomalies are recovered from deflections by using fast fourier transform techniques to accomplish the necessary inverse vening meinesz transformation high precision gem t1 orbits are used and crossover adjustments are not performed the operational navy astronautics group orbits are shown to have systematic errors which may be as large as 10 m

small ocean basin development along the scotia antarctica plate boundary and in the northern weddell sea abstract the oceanic crust of the northern weddell sea and scotia sea contains the following principal morphostructural elements 1 the south scotia ridge and the south orkney microcontinent both with continental crust 2 the oceanic powell and jane basins and 3 jane bank which belongs to an island arc the analysis of mcs profiles and of gravimetric and magnetic data from russian italian and spanish cruises supplemented with satellite gravimetric data has enabled us to determine the relationship between these elements and to propose a model for the main stages of cenozoic evolution in the area during the early cenozoic the weddell sea oceanic crust was subducted under the southern margin of the south orkney microcontinent the subduction probably ended westwards at the south powell ridge a submarine extension of the antarctic peninsula a major transcurrent fault zone is identified in the northwestern weddell sea bounding oceanic crust of mesozoic and cenozoic ages this fault zone was probably active at least to the miocene the drifting of the south orkney microcontinent from the antarctic peninsula during the late eocene to early miocene originated the powell basin jane basin developed as a backarc related to the subduction of the weddell sea oceanic crust below jane bank the seismic stratigraphy of the depositional sequences in these two basins indicates that spreading in jane basin started simultaneously with the end of the opening in powell basin the active spreading ridge of the weddell sea collided with the trench and was subducted below jane bank at 15 20 ma drifting in jane basin and subduction below jane bank ended shortly thereafter in the middle miocene and the boundary between the antarctic scotia plates migrated north of the south orkney microcontinent along the south scotia ridge present tectonic activity in the region is minor

scotia sea tectonics from high resolution satellite gravity the release of altimetric data from the geosat geodetic mission by the us navy 1 is leading to a much improved understanding of tectonics in the southern ocean a region in which remoteness and adverse physical conditions have limited the acquisition of geophysical data by research ships the scotia sea is an outstanding example of back arc spreading which is revealed in some detail by free air gravity anomaly maps derived from the latest release of data acquired south of 30 s 2 sea surface height data for this region have been reduced to a grid of free air gravity anomalies and are illustrated here by means of colour shaded relief and contour maps the new data confirm the existence of a number of inactive spreading ridges within the scotia sea and surrounding small basins the amplitudes and wavelengths of gravity anomalies over these ridges conform in general to the expected relationship with spreading rate except in the central scotia sea where a proposed miocene slow spreading ridge appears to have left no clear signature the spreading ridge axis in the east scotia sea comprises seven or more segments separated by small mainly sinistral offsets and exhibits a median valley with depths of 200 1000 m that is reflected in free air lows of 10 40 mgal near both its northern and southern termini the gravity signature of the ridge becomes less distinct with a less pronounced axial low the northernmost segments of the ridge are displaced in a right lateral sense by a feature which appears to represent a southward migrating non transform offset whereas the process of spreading in drake passage and the east scotia sea was comparable to mid ocean ridges that in the central scotia sea may have been disorganized as observed in some western pacific back arc basins

evidence for a volcanic rifted margin and oceanic crustal structure for the falkland plateau basin marine geophysical data from the western falkland plateau show strongly lineated magnetic anomalies and a coincident linear gravity high that identify the southeastern margin of a falkland islands block as a volcanic rifted continental margin characteristics of onshore and offshore dykes suggest this margin is most probably slightly younger than 190 ma together with these data seismic reflection profiles and sonobuoy seismic refraction data from the basin province of the falkland plateau east of the islands indicate an elevated oceanic origin for the major part of the falkland plateau basin most probably within the karoo hotspot province these conclusions are compatible with but do not directly indicate the independent rotation of a falkland islands block and closely constrain models of southwest gondwana evolution

arctic ocean gravity field derived from icesat and ers 2 altimetry tectonic implications a new detailed marine gravity field for the persistently ice covered arctic ocean derived entirely from satellite data reveals important new tectonic features in both the amerasian and eurasian basins reprocessed geoscience laser altimeter system glas data collected by nasa s ice cloud and land elevation satellite icesat between 2003 and 2005 have been combined with 8 years worth of retracked radar altimeter data from esa s ers 2 satellite to produce the highest available resolution gravity mapping of the entire arctic ocean complete to 86 degrees n this arctic satellite only arcs marine gravity field uniformly and confidently resolves marine gravity to wavelengths as short as 35 km arcs relies on a gravity recovery and climate experiment grace only satellite gravity model at long 580 km wavelengths and plainly shows tectonic fabric and numerous details imprinted in the arctic seafloor in particular in the enigmatic amerasian basin ab for example in the makarov basin portion of the ab two north south trending lineations are likely clues to the highly uncertain seafloor spreading history which formed the ab

an optimal procedure for deriving marine gravity from multi satellite altimetry an optimal procedure is developed for deriving gravity anomalies from sea surface height measurements obtained by a number of different satellites the inclinations of which may vary we begin by recasting the problem of the conversion of the deflection of the vertical to gravity in the frequency domain we show that a deterministic approach based on the vening meinesz integral is equivalent in the frequency domain to the stochastically based method of least squares collocation a new method for gridding the deflection of the vertical is developed that uses the fact that satellite tracks of the same type ascending or descending are nearly parallel so that values at grid points can be estimated by first performing along track interpolations followed by one cross track interpolation both using akima s spline this gridding method is very efficient compared with methods such as that of fitting minimum curvature surfaces especially for dense data such as for the 168 day cycles of ers 1 a weighted least squares method is then employed to obtain the north and east components of deflection of the vertical components using the gridded along track components from all of the individual satellite missions finally gravity anomalies are computed from the two deflection of the vertical components in the frequency domain with truncated kernel functions the use of which is related to the prior removal of a high degree reference gravity field this new procedure is more than 100 times faster than least squares collocation and yields gravity anomalies with errors that are comparable to those derived by least squares collocation and smaller than those derived by other recent applications of spectral techniques as judged by comparisons with ship gravity measurements the case of gravity computation for a single altimetric satellite is handled separately and a method is given to estimate the noise spectra of deflection of the vertical and reduce the numerical problems caused by satellites with high inclination angles the new procedures make possible quick yet accurate global updates of the marine gravity field

new global seamount census from altimetry derived gravity data summary recent revisions to the satellite derived vertical gravity gradient vgg data reveal more detail of the ocean bottom and have allowed us to develop a non linear inversion method to detect seamounts in vgg data we approximate vgg anomalies over seamounts as sums of individual partially overlapping elliptical polynomial functions which allows us to form a non linear inverse problem by fitting the polynomial model to the observations model parameters for a potential seamount include geographical location peak vgg amplitude major and minor axes of the elliptical base and the azimuth of the major axis the non linear inversion is very sensitive to the initial values for the location and amplitude hence they are constrained by the centre and amplitude of the uppermost contours obtained with a 1 eotvos contour interval with these initial conditions from contouring we execute a step wise and fully automated inversion and obtain optimal model estimates for potential seamounts these are statistically evaluated for significance using the akaike information criterion and f tests a logarithmic barrier technique is applied to ensure positivity of all seamount amplitudes after automatic and manual inspections of the model parameters we estimate actual heights and basal ellipses of the inspected potential seamounts directly from the predicted bathymetry grid in this study we find globally 24 643 potential seamounts h 0 1 km that are located away from continental margins 8458 potential seamounts are taller than 1 km although our global estimate is significantly lower than predictions from previous studies a first order reconciliation of the size frequency statistics obtained from those studies reveals that the previous counts are systematically overestimated because of the ambiguity of gravity signals due to small seamounts of h 1 km and the overlap with abyssal hills we estimate the global seamount census to lie in the 40 000 55 000 range the seamount data from this study are accessible from http www soest hawaii edu pt smts

gravity fields over mid ocean ridges from geosat gm data variations as a function of spreading rate mean gravity profiles have been obtained from gravity fields constructed using geosat geodetic mission altimeter data for a number of areas over mid ocean ridges the technique of averaging many profiles reduces the effects of along axis variability and allows systematic changes as a function of spreading rate to be seen more clearly for spreading rates larger than about 60 65 mm yr an axial gravity high is observed with an almost constant amplitude 12 mgal that narrows as the spreading rate increases for spreading rates lower than this an axial gravity low is observed whose amplitude decreases rapidly with spreading rate while the width reduces only slowly the disappearance of a median valley occurs abruptly on any one ridge but occurs at slightly different spreading rates on the southeast indian ridge 68 mm yr and the pacific antarctic ridge 60 mm yr these observations are compared with the predictions of current models of mid ocean ridge processes but are not fully explained by them

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

resolution of the scripps noaa marine gravity field from satellite altimetry the july 1995 declassification of the entire geosat gm satellite altimeter data set enabled a joint scripps noaa effort to compute a new version 7 2 marine gravity field on a 2 minute grid this gravity field covers the world oceans between 72 n and 72 s and is derived from a combination of ers 1 and geosat gm and erm data an earlier noaa geosat only gravity field solution was confined to the southern latitudes because the 1992 declassification was limited to gm data south of 30 s a simple coherence analysis between accurately navigated ship gravity profiles and comparable gravity profiles obtained from the gravity grids reveals that the scripps noaa gravity field is coherent with ship gravity down to 23 30 km this slight increase in resolution over the previous noaa geosat only gravity field short wavelength resolution of 26 30 km implies that the increased spatial coverage provided by the ers i altimeter when combined with geosat improves the solution coherence analyses between satellite gravity and ship topography and ship gravity and ship topography show that even shorter wavelength gravity anomalies 13 km are present in sea surface measurements made by ship even so the scripps noaa marine gravity field does an excellent job of resolving most of the short wavelength gravity anomalies covering the world oceans

continental fragmentation along the south scotia ridge transcurrent plate boundary ne antarctic peninsula abstract the study of the south scotia ridge on the basis of swath bathymetry multichannel seismic and magnetometry profiles obtained during the hesant92 93 cruise and complemented with satellite gravimetry and seismicity data illustrates the tectonics of the region the thinned continental crust fragments of the ridge are bounded by oceanic crust of the scotia sea to the north and powell basin to the south the northern boundary represents the contact between the scotia and antarctic plates this boundary is a sinistral transpressional fault with transtensional segments and moderate recent tectonic activity another fault located at the southern boundary appears inactive and does not reveal any features that would enable the kinematics to be determined both faults have associated steep scarps since they separate oceanic and continental crust types the most significant active deformation lies in the axial depression of the ridge within a band delineated by fault systems with wsw ene and sw ne strikes these faults develop pull apart basins which separate the northern and southern blocks of the ridge the northern block is being fragmented from the antarctic plate by a zone of transtensive faults and is probably a crustal element independent of the antarctic plate the axial depression which crosses the ridge slightly obliquely is characterized by deep basins locally more than 5000 m deep and associated high seismicity the fault geometry and earthquake focal mechanisms indicate an active sinistral transtensive regime for the fault system although it may locally have transpressive regimes depending on the fault plane and the stress field orientations the internal basins are characterized by an asymmetric development showing itself as depositional wedges generally thickening northward deposits onlap the southern margins and are affected by normal faults in the northern margins the seismicity around the scotia plate shows that the present stresses are compressive along the northern boundary with the south american plate 1 sw ne and subhorizontal and along the western boundary with the antarctic plate 1 wnw ese and subhorizontal for the south scotia ridge however 1 is steeply inclined and 3 is subhorizontal with a nw se trend the stress distribution in bransfield strait is similar to the ridge and the recent extension could be partially explained by the westward continuation of the active fault system of the central south scotia ridge the fragmentation of continental crustal blocks due to the tectonic activity along the transcurrent plate boundaries is a mechanism that contributes to the deformation of the northeastern end of the antarctic peninsula this area appears appropriate for the analysis of continental plate fragmentation processes

antarctic marine gravity field from high density satellite altimetry summary closely spaced satellite altimeter profiles 5 km collected during the geosat geodetic mission geosat gm and those planned for the extended ers 1 mission are easily converted to grids of vertical gravity gradient and gravity anomaly as profile spacing decreases it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross track gradients if one is only interested in the horizontal and vertical derivatives of the gravitational potential however adjustment of the profile is unnecessary the long wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along track derivative of each profile ascending and descending slope profiles are then interpolated onto separate uniform grids these two grids are summed and differenced to form comparable grids of east and north vertical deflection using laplace s equation the vertical gravity gradient is calculated directly from the vertical deflection grids fourier analysis is required to construct gravity anomalies from the two vertical deflection grids these techniques are applied to high density 2 km profile spacing geosat gm profiles in antarctic waters 60 s to 72 s gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean the resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles after adjustment of a dc shift in the shipboard gravity profiles 5 mgal the rms difference between the ship and satellite gravity is 5 5 mgal many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large leaky transform along the pacific antarctic rise

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

active crustal fragmentation along the scotia antarctic plate boundary east of the south orkney microcontinent antarctica abstract the structure of the scotia antarctic plate boundary is poorly known east of the south orkney microcontinent new multichannel seismic profiles together with magnetic gravity and swath bathymetry data obtained during the scan97 cruise show a complex relief of raised blocks and elongated depressions that may reach more than 6000 m in depth these depressions develop in relation with extensional active structures and constitute an uncommon feature in the oceans where most of the trenches are formed in subduction contexts the main crustal elements of the area include the oceanic crust of the scotia plate the discovery bank composed of continental crust a tectonic domain with intermediate features between continental and oceanic crusts that includes the southern bank and the oceanic crust of the northern weddell sea representing the antarctic plate the intermediate domain was probably developed during the late cenozoic subduction of the weddell sea oceanic crust below the discovery bank the fault zone associated with the plate boundary is characterized at present by sinistral transcurrent and transtensional slips which develop a ne sw elongated deep pull apart basin with extreme crustal thinning and mantle uplift the complex bathymetry and structure of the plate boundary are consequences of the presence of continental and intermediate crusts where the deformations are concentrated between the two stable oceanic domains the location of a major part of the plate boundaries around the scotia arc is probably determined by the position of the continental and intermediate crustal fragments surrounded by oceanic crust due to the differential behavior experienced during deformation

a detailed view of the south pacific geoid from satellite altimetry images of sea surface undulations in the south pacific have been constructed from geos 3 and seasat altimeter data height discrepancies at crossover points associated with long wavelength radial orbit error were suppressed by taking along track derivatives of the ascending and descending profiles these geoid slopes were then rotated and scaled to produce the north and east components of the deflection of the vertical finally the results are displayed by using the hill shading technique where gray tone images represent the innner product of the deflection vector with an assigned sun vector less apparent sea surface undulations can be enhanced by varying the sun s zenith and azimuth shorterwavelength sea surface undulations reflect seafloor topography for instance fracture zones fz s appear as elongated sharp steps in the sea surface while seamounts produce circular bumps since large areas of the south pacific are unsurveyed many previously undetected features appear on the images comparisons with bathymetric charts reveal 72 uncharted seamounts having geoid expressions greater than or equal to easter island s expression the dominant features in the images however are the large age offset fz s such as the eltanin and udintsev fz s the images reveal that the eltanin fz is connected to the louisville ridge combined they produce a continuous geoid signature across most of the south pacific this supports the hypothesis of hayes and ewing 1968 that the louisville ridge is the northwest extension of the eltanin fz

late cretaceous to early tertiary subduction history of the antarctic peninsula quantitative estimates of the rates and azimuths of phoenix plate convergence with the antarctic peninsula have been derived from plate rotation calculations for two periods in the late cretaceous and early tertiary 83 0 67 7 and 61 1 53 4 ma published marine magnetic anomaly identifications and flow lines interpreted from gravity anomaly maps were used in simple spherical geometry calculations to derive phoenix pacific stage rotations these were combined with published pacific antarctic rotation data to determine contemporaneous phoenix antarctic stage rotations the results indicate a significant change in azimuths of relative motion between the late cretaceous and early tertiary late cretaceous and perhaps earlier oblique subduction probably caused migration of fore arc slivers along the margin resulting in variations in width of the accretionary prism comparison between synthetic magnetic profiles and a 900 km long magnetic profile across ocean floor produced at the antarctic phoenix ridge during the early tertiary establishes the time of a major decrease in spreading rate and hence also in convergence rate as chron c23r 52 3 ma the associated change in subduction dynamics may have caused the dextral transtensional deformation observed in the george vi sound region and initiated uplift of blueschist facies rocks now exposed on smith island the calculated convergence history does not provide a simple explanation for the occurrence of high mg andesite lavas on alexander island which were erupted more than 20 ma before the antarctic phoenix ridge arrived at the margin the existence of these lavas implies either earlier subduction of another ridge or slab break up beneath alexander island

forearc tectonic evolution of the south shetland margin antarctic peninsula the main provinces of the south shetland margin antarctic peninsula characterized on the basis of multichannel seismic long range side scan sonar and swath bathymetry data include from northwest to southeast 1 the oceanic crust of the former phoenix plate flexed down toward the margin and affected by normal faulting in the upper crust 2 a narrow trench with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge 3 an accretionary prism with a complex internal structure the toe of which is overthrust above the youngest trench deposits 4 a midslope forearc basin with an asymmetric synformal structure and 5 the continental shelf which includes two distinct tectonic provinces calculations of the late cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6 7 ma from about 60 mm yr resulting from the slowing and eventual cessation of spreading on the antarctic phoenix ridge once spreading had completely ceased 3 5 2 4 ma the convergence rate at the trench equalled the rate of extension in bransfield strait which was probably less than 10 mm yr at first but may have increased since 1 3 ma and this in turn would imply a corresponding increase in convergence rate above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges laterally and vertically segmented by normal faults most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles this suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope the forearc basin records a long history of subsidence during which the depocenter migrated landward paralleling retrogradational erosion of the distal margin of the basin subduction of the hero fracture zone at approximately the same location since 10 ma may have enhanced tectonic erosion of the accretionary prism in the forearc basin a synform is coincident with a bathymetric reentrant but its axis is oblique to the trends of both the basin and the fracture zone the origin of this synform and the final phase of uplift of the blueschist facies terrane on which smith island stands are probably related to their position above the subducting fracture zone ridge the present elevation of smith island is thought to result from uplift which followed a ridge crest trench collision directly southwest of the hero fracture zone at 5 5 ma the northwestern edge of the mesozoic to early cenozoic arc batholith trending northeast southwest on the shelf is proposed to have acted as a backstop to the accretionary prism during cenozoic subduction

variations in ridge morphology and depth age relationships on the pacific antarctic ridge adjacent segments of the pacific antarctic ridge display significantly different morphologies and depth age relationships over seafloor younger than 36 ma the spreading corridor southwest of fracture zone xii is characterized by a rift valley and an usually small subsidence constant of 226 13 m m y while the two spreading corridors immediately northeast of fracture zone xii have an axial high and a subsidence constant consistent with the global average this abrupt variation in ridge morphology is not usually characteristic of medium rate spreading centers nor is such an abrupt variation expected of adjacent ridge segments that are spreading at the same rate we suggest that a thermal anomaly beneath the ridge may influence the first order effects of spreading rate and lithospheric cooling enough to produce the observed rift valley and axial high and the different subsidence constants although we are not certain what would produce the thermal anomaly here we speculate that when the spreading rate on the pacific antarctic ridge increased from slow to intermediate rates since 20 ma so did the need for materials for accretion which may be supplied in part by along axis asthenospheric flow from hotspots or a hot region to the northeast a sufficient supply of hot asthenosphere may still be lacking in the ridge segment with the axial valley to the southwest leaving it cooler and starved for accretionary materials

mesozoic seafloor spreading in the southern weddell sea a re evaluation of weddell sea magnetic anomaly data in the light of satellite gravity maps provides important constraints on plate kinematics prior to chron c34 83 ma although m series anomalies are difficult to identify the evidence favours younger ages than previously suggested based on the assumption that these anomalies formed at the south america antarctica plate boundary geosat derived free air gravity maps show a distinct anomaly anomaly t associated with the southern termination of a herring bone pattern of flow lines the associated magnetic anomaly has been identified previously as anomaly m20 on the basis of a model of very slow spreading but is interpreted here as an edge effect associated with an abrupt decline in spreading rate which occurred between m4 126 ma and m0 118 ma and which induced the fine segmentation pattern to the north although anomalies are present to the south of anomaly t they are of reduced amplitude and are difficult to correlate over distances of more than c 100 km nevertheless they appear to strike approximately w e sub parallel to the younger anomalies to the north and show a resemblance to anomalies m4 m12 modelled at a rate of 15 km ma 1 flow line modelling based on published rotations for the southwest indian ridge and southern mid atlantic ridge gives a reasonable fit to the observed trends of weddell sea gravity ridges two alternative models for the m series reconstruction of africa and antarctica are employed corresponding to the two m2 fits suggested by bergh although it is not possible to reject either of the models the tighter fit of the two gives slightly better agreement with weddell sea trends in both models a drop in rates occurs at m0 time anomalies farther south correspond with previously identified features such as the orion explora and andenes anomalies and the polarstern bank all characterized by magnetic anomaly highs attributed to excess volcanism during early break up

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

three dimensional gravity inversion model of the deep crustal structure of the central drake passage shackleton fracture zone and west scotia ridge antarctica 1 gravity and bathymetric data collected by the spanish r v hesperides over the shackleton fracture zone sfz and the west scotia ridge wsr were used to invert for the three dimensional 3 d structure of the deep crust data from the global gravity grid and global seafloor topography ggsft were also employed to enlarge the cruise area the merged data were analyzed to determine 3 d deep structure by numerical inversion water layer contribution to the gravity anomaly was eliminated taking into account the bathymetry spectral analysis of the reduced data yielded mean crust mantle interface cmi depths of 10 5 1 2 km inversion of the regional anomaly gave a 3 d detailed geometry of the cmi which generally agrees with the 2 d models established along profiles where gravity data and multichannel seismic lines are available the wsr shows an asymmetrical structure with a reverse fault located southeastward to the central valley this fault was developed probably as a consequence of the nw se compressive deformations which occurred following the spreading the 3 d view shows that the sw end of the spreading axis was affected more intensely by the compression the sfz represents an active sinistral transpressive fault zone of the scotia antarctica plate boundary and shows crustal thickening related to bathymetric highs the crustal thinning detected at the intersection with the inactive wsr suggests a complex interaction between these two structures

new gravity data in the arctic ocean comparison of airborne and ers gravity new gravity fields from airborne gravimetry and from ers 1 and 2 satellite altimetry cover extensive portions of the arctic ocean these two data sets may constitute as much as 60 of the data contributions to the arctic gravity project compilation here we evaluate the accuracy and resolution of these data and quantify their impact on the compilation both gravity determinations compare favorably with geological survey of canada surface measurements in the beaufort sea airborne 1 86 2 09 mgal rms ers 2 64 3 11 mgal rms comparisons between the airborne and ers data over the chukchi borderlands reveal a 4 38 mgal rms difference over the smoother region of the field and 7 36 mgal rms over the rugose field generated by the shallow ridges and deep troughs coherency between the two data sets in the chukchi region implies a resolution of 19 km comparison with science ice expedition submarine measurements over chukchi plateau suggests that the ers field resolves even shorter wavelength signal than the airborne data whereas in the beaufort sea the airborne data showed better coherence to ground truth data long wavelength differences exist between the two data sets expressed as a 2 3 mgal offset over the chukchi region this study highlights the respective strengths of the two data sets the ers gravity field has the advantage of ubiquitous coverage of the ocean south of 81 5 degreesn a denser sampling of the gravity field and a recovery of signal down to similar to 15 km the airborne data cover a significant portion of the polar hole in the satellite coverage have lower measurement noise and recover somewhat higher anomaly amplitudes in the 25 100 km wavelength range

gravity anomalies derived from seasat geosat ers 1 and topex poseidon altimetry and ship gravity a case study over the reykjanes ridge summary we have employed least squares collocation to derive a gravity field over the reykjanes ridge using seasat geosat erm ers 1 and topex poseidon altimeter data combined with ship gravity to avoid a crossover adjustment to correct for orbital errors we used mean geoid gradients which were obtained by averaging gradients over 60 10 and 36 repeat cycles for geosat erm ers 1 and topex poseidon respectively the average standard deviations for the geosat erm ers 1 and topex poseidon mean gradients are 1 12 3 25 and 2 01 rad respectively the standard deviations of the ship gravity which were assigned based on weightings derived from an analysis of crossing differences range from 6 72 to 20 17 mgal the necessary covariances for the least squares collocation computations were derived using the law of covariance propagation before merging with the altimeter data the ship gravity for each leg was adjusted by removing a quadratic polynomial in time in order to match a satellite only gravity field in a least squares sense the result of the adjustment suggests that most of the ship gravity data collected in the 1960s and the 1970s have average offsets of about 14 mgal which is close to the offset of the potsdam datum the rms difference between the satellite only gravity derived using least squares collocation and the adjusted ship gravity is 7 10 mgal smaller than the rms difference of 12 47 mgal between the satellite only gravity derived by a fourier transform method and the adjusted ship gravity the rms difference between the combined gravity field derived from both altimetry and ship gravity and the adjusted ship gravity is 2 65 mgal suggesting that the former has successfully absorbed the high frequency component of the gravity signal provided by the latter the combined gravity field thus features a regionally uniform medium resolution from altimetry and a locally high resolution from ship gravity the average accuracy estimate given by least squares collocation for the combined gravity is 5 76 mgal the combined gravity field reveals detailed tectonic structures over the reykjanes ridge related to the interaction of the iceland hot spot and sea floor spreading at the ridge

gravity fields of the southern ocean from geosat data in august 1990 the u s navy declassified all geodetic mission gm radar altimeter data acquired by the geosat satellite over oceanic regions south of 60 s we have used these gm data in conjunction with the unclassified lower resolution geosat exact repeat mission erm altimeter data to construct high resolution gravity fields on a 5 km grid covering the annular region of the southern ocean which lies between 60 s and 72 s and encircles antarctica during the gm a complete mapping of the marine geoid between 72 and 72 n was accomplished the gm produced more densely spaced ground tracks typically 2 or 3 km at 60 s than those of either the erm or seasat consequently we were able to use the gm data to map the marine gravity field at a higher resolution than was previously possible using satellite altimeter data this paper describes the techniques we used to derive these gravity fields and image them these techniques involve 1 computing along track sea surface height slopes 2 gridding of these ascending and descending slopes 3 converting the slopes to conventional deflections of the vertical 4 transforming the deflections to gravity anomalies in the frequency domain and 5 imaging the resulting images of the marine gravity field reveal much that is new about the seafloor and the tectonic fabric of the southern ocean a region which includes large expanses of seafloor that have never been surveyed by ships

high resolution mean sea surface computed with altimeter data of ers 1 geodetic mission and topex poseidon the remote sensing satellite ers 1 launched in 1991 to study the earth s environment was placed on a geodetic 168 day repeat orbit between 1994 april and 1995 march to map through altimetric measurements the gravity field over the whole oceanic domain with a resolution of 8 km at the equator in both along track and cross track directions we have analysed the precise altimeter data of the geodetic mission and by also using one year of topex poseidon altimeter data we have computed a global high resolution mean sea surface the various steps involved in pre processing the ers 1 data consisted of correcting the data for environmental factors editing and reducing through crossover analyses the radial orbit error which directly affects sea surface height measurements for this purpose we adjusted sinusoids at 1 and 2 cycle rev 1 along the ers 1 profiles in order to minimize crossover differences between ers 1 and yearly averaged topex poseidon profiles in effect the orbit of topex poseidon is very accurately determined within 2 3 cm for the radial component so topex poseidon altimeter profiles can serve as a reference to reduce the ers 1 radial orbit error the ers 1 residual orbit error was further reduced through a second crossover analysis between all ascending and descending profiles of the geodetic mission the along track ers 1 and topex poseidon data were then interpolated over the whole oceanic domain on a regular grid of 1 16 1 16 size the mapping of the gridded sea surface heights reveals the very fine structure of the marine geoid up until now unknown at a global scale this new data set will be most useful for marine geophysical and tectonic investigations

thermal isostasy response of a moving lithosphere to a distributed heat source spreading ridges and hot spot swells are identified by their high surface heat flow shallow seafloor and high geopotential to understand these and other thermotectonic features the oceanic lithosphere is modeled as a thermomechanical boundary layer moving through a three dimensional time independent heat source the heat source mimics the heat advection associated with a spreading ridge or hot spot without introducing the nonlinearities of these flow processes the fourier transforms of three green s functions response functions which relate the three observable fields to their common heat source are determined analytically each of these reponse functions is highly anisotropic because the lithosphere is moving with respect to the source however the ratio of the gravity response function to the topography response function i e gravity topography transfer function is nearly isotropic and has a maximum lying between the flexural wavelength and 2 times the thickness of the thermal boundary layer the response functions are most useful for determining the surface heat flow seafloor topography and geopotential for complex lithospheric thermal structures in practice these three observables are calculated by multiplying the fourier transform of the heat source by the appropriate response function and inverse transforming the products almost any time independent thermotectonic feature can be modeled using this technique included in this report are examples of spreading ridges and thermal swells although more complex geometries such as ridges offset by transform faults and rrr type triple junctions can also be modeled because forward modeling is both linear and computationally simple the inverse of this technique could be used to infer some basic characteristics of the heat source directly from the observed fields

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

gravity anomalies and sea surface heights derived from a combined geos 3 seasat altimeter data set gravity anomalies and sea surface heights have been computed on a 0 125 grid in the ocean areas from a combined geos 3 seasat altimeter data set the basic estimation procedure used least squares collocation where model covariance functions are tailored to individual areas through altimeter residual variance scaling preliminary tests led to production prediction procedures using a reference model defined by a set of potential coefficients complete to degree 180 comparisons of the predicted anomalies with ship derived values showed agreement varying from 9 to 30 mgal no correction to the altimeter implied sea surface heights was made for sea surface topography effects the maximum anomaly predicted is 396 mgal near hawaii and the most negative anomaly is 361 mgal over the puerto rican trench the actual resolution of the predicted quantities was estimated to be about 0 19 based on a power spectrum analysis the resolution is limited by our data selection process which uses 300 data points from a thinned altimeter data set for the prediction of the 0 125 in a 3 3 block with one data selection and matrix inversion the predicted data are used to compute 104 potential coefficient spectrums using flat earth approximations developed by forsberg the spectra were classified in terms of smooth mild or rough areas the great majority 78 of the spectra were in the smooth classification good agreement was found from spectra computed from topographically reduced land data and the mean ocean data up to degree 800 we found that the spectra decayed as l 3 6 as compared to the decay of l 3 implied by the kaula model the gridded data was used to compute 1 1 and 0 5 0 5 mean values the 1 mean anomalies were compared to terrestrial data where an rms difference of 7 mgal was found in comparing 10 139 values these new values have allowed us to identify 1 mean anomalies based on terrestrial estimates that are in substantial error

a late miocene plate boundary reorganization along the westernmost pacific antarctic ridge abstract magnetic and bathymetric measurements carried out in the southwest pacific between new zealand and antarctica south of 60 s combined with the satellite derived gravity map add new information on the structural fabric and tectonic development of the westernmost portion of the pacific antarctic plate boundary the plate boundary includes in its northern part a series of short spreading centres offset by nnw sse trending fracture zones towards the south it becomes structurally more complicated and appears to be formed by extension along an ancient strike slip lineation as a consequence of a change in the pacific antarctic relative motions that resulted in a geometric readjustment of the plate boundary the identification of marine magnetic lineations crossing this portion of the boundary testifies that this readjustment is relatively recent the newly created crust is not older than late miocene and is presently surrounded by oceanic regions where the magnetic anomalies range from 19 to 24 41 53 ma northeastward and from 7 to 9 24 5 28 ma southwestward this segment of the plate boundary formed along an older fracture zone that was pulled apart by the new plate motions and the segmented ridge transform system then evolved

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

comparison of along track resolution of stacked geosat ers 1 and topex satellite altimeters cross spectral analysis of repeat satellite altimeter profiles was performed to compare the along track resolution capabilities of geosat ers 1 and topex data geophysical data records were edited differentiated low pass filtered and resampled at 5 hz all available data were then loaded into three dimensional files where repeat cycles were aligned along track 62 cycles of geosat exact repeat mission 16 cycles of ers 1 35 day orbit 73 cycles of topex the coherence versus wave number between pairs of repeat profiles was used to estimate along track resolution for individual cycles eight cycle average profiles and 31 cycle average profiles geosat and topex only coherence which depends on signal to noise ratio reflects factors such as seafloor gravity amplitude regional seafloor depth instrument noise oceanographic noise and the number of cycles available for stacking averaging detailed resolution analyses are presented for two areas the equatorial atlantic a region with high tectonic signal and low oceanographic noise and the south pacific a region with low tectonic signal and high oceanographic variability for all three altimeters along track resolution is better in the equatorial atlantic than in the south pacific global maps of along track resolution show considerable geographic variation on average globally the along track resolution 0 5 coherence of eight cycle stacks are approximately the same 28 29 and 30 km for topex geosat and ers 1 respectively topex 31 cycle stacks 22 km resolve slightly shorter wavelengths than geosat 31 cycle stacks 24 km the stacked data which are publicly available will be used in future global gravity grids and for detailed studies of mid ocean ridge axes fracture zones sea mounts and seafloor roughness

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

terrestrial and planetary gravity fields the determination of the gravity field together with knowledge of the surface topography provides one of the primary means of inferring the density structure and dynamics of planetary interiors phillips and lambeck 1980 hager 1985 after removing the topographic gravity signal the distribution of internal density anomalies caused by thermal or compositional differences can be studied gravity field models can also be used to study the compensation of surface topography which can provide information on the mechanical properties and state of stress of the lithosphere the determination of these models has been accomplished using a wide variety of different measurement types and solution techniques the near earth measurement types can be divided into satellite tracking measurements surface gravity measurements and satellite altimeter measurements in both near earth and interplanetary contexts tracking data are used to measure gravitational perturbations affecting satellites with the accuracy and spatial temporal distribution of the data being the most important factors in the resulting field accuracy and resolution surface gravity data provide a more direct measurement of the gravity field but acquiring data uniformly over the earth has always been difficult satellite altimetry provides precise measurements of the marine gravity field provided that satellite orbit errors and non geoidal sea surface height variations can be adequately modeled comprehensive gravity field solutions must incorporate these disparate data types in order to estimate mathematical parameters describing the gravity field such as spherical harmonic coefficients the proper combination of these data along with the desire to acquire an accurate representation of the model errors require that complex procedures be developed for computing these solutions

structure and tectonic setting of the 77 e and 75 e grabens kerguelen plateau south indian ocean the central kerguelen plateau south indian ocean is characterized by abundant north south striking normal faults which comprise two prominent north south rifts known as the 77 e and 75 e grabens the 77 e graben is a well defined structure which extends over some 800 km from the eastern margin of the kerguelen plateau to about 58 5 s over most of its length it is associated with a 10 30 km wide axial rift and with a 100 150 km wide uplift the 75 e graben is less well documented but the available data suggest that its dimensions and internal structure resemble that of the 77 e graben in the better documented 77 e graben six rift segments 50 100 km long are identified faulting is more developed at the northern and southern ends of the 77 e graben possibly resulting from the interaction with other rifts to the north the 77 e graben abuts the highly faulted eastern margin of the kerguelen plateau and the northern part of an even larger rift zone the plate boundary rift zone which extends along the boundary with the australian antarctic basin to the south the 77 e graben adjoins the northwestern end of the southern kerguelen plateau rift zone the 77 e and 75 e grabens and the other rift zones on the kerguelen plateau appear to have been formed at approximately the same time between 72 and 60 ma they are all part of an important extensional phase which occurred in the region and mark the beginning of the process which led to the development of the plate boundary rift zone into the southeast indian ridge between 46 and 43 ma the north south trend of the 77 e and 75 e grabens is different from that of the other rift zones which are oriented northwest southeast this geometry suggests that some strike slip motion may have occurred along the north south trending grabens as a result of extension on the northwest southeast trending rifts particularly the southern kerguelen plateau rift zone however since near surface extension estimates for the southern kerguelen plateau rift zone are small the strike slip motion along the 77 e graben must be equivalently small also the available seismic data from this graben do not show typical seismic characteristics of a strike slip environment such as zones of compression or flower structures thus the results of this work are inconsistent with any model for the development of the south indian ocean which requires significant amount of transform motion on the 77 e or 75 e grabens finally the seismic data from this area provide a unique opportunity to compare rifting on an oceanic plateau environment with continental rifting we find great similarities between the two processes in the segmentation of the rifts the asymmetric cross section and the associated shoulder uplifts

seafloor spreading in the weddell sea and southwest atlantic since the late cretaceous geosat radar altimetry defines a pattern of closely spaced curvilinear gravity anomalies in the northern weddell sea and adjacent parts of the southwest atlantic these anomalies are caused by a fracture zone pattern resulting from seafloor spreading mainly on the southern flank of a ridge system which now exists only to the east of the south sandwich trench flowlines derived from these anomalies are combined with isochrons resulting from an interpretation of all available magnetic anomaly profiles in the region to produce a new tectonic summary chart this chart reveals major changes in spreading direction during i the cretaceous magnetic superchron ii the latest cretaceous paleocene 65 ma and iii the mid eocene 50 ma the latter two changes enclose a period of very slow spreading and define a kink in the flowlines which is discernable throughout most of the weddell sea preliminary modelling indicates that trends from the western weddell sea to near the south atlantic triple junction can be explained by the separation of just two plates there is some disagreement between the observed flowlines and those predicted by rotations published for the south atlantic and southwest indian ridge but abrupt changes in spreading direction and a slowing of rates are observed on all three branches of the south america africa antarctica plate system between the times of anomalies c31 and c21 this suggests that the lack of closure results from errors in the calculated rotations and that the seafloor of the weddell sea and southwest atlantic was created as a direct consequence of south america antarctica plate motion the data do not support the existence of a late cretaceous malvinas plate within the weddell sea although microplates in the northernmost weddell sea do appear to have formed briefly as a result of interactions between the former spreading ridge and a subduction zone located at the south scotia ridge

oceanwide gravity anomalies from geos 3 seasat and geosat altimeter data three kinds of satellite altimeter data have been combined along with 5 5 bathymetric data to calculate a 0 125 ocean wide gridded set of 2 3 106 free air gravity anomalies the procedure used was least squares collocation that yields the predicted anomaly and standard deviation the value of including the bathymetric data was shown in a test around the dowd seamount where the root mean square rms difference between ship gravity measurements decreased from 40 mgal to 20 mgal when the bathymetry was included comparisons between the predicted anomalies and ship gravity data is described in three cases in the banda sea the rms differences were 20 mgal for two lines in the south atlantic rms differences over lines of 2000 km in length were 7 mgal for cruise data in the antarctica region the discrepancies were 12 mgal comparisons of anomalies derived from the geosat geodetic mission data by marks and mcadoo 1992 with ship data gave differences of 6 mgal showing the value of the much denser geosat geodetic mission altimeter data

evolution of the lofoten vesteralen margin inferred from gravity and crustal modeling 1 the lofoten vesteralen continental margin in norway shows pronounced elongated positive and negative gravity anomalies the source of these anomalies cannot be explained by the flat bathymetry but can be explained by the crustal density anomalies that are observed we propose a boudinage mechanism to explain the generation of these density anomalies and respective crustal layer undulations free air gravity anomalies have been derived using ers 1 and geosat altimetry data and have been validated by comparison with shipborne data structural models of the margin are available from extensive seismic surveys which have been used to carry out a gravity inversion to cross validate the observed gravity anomalies with seismic data the crustal structure data reveal two wavelengths of crustal undulations the first is related to crustal blocks separated by listric faults and the second wavelength is related to an undulated moho the undulations were likely generated during the opening of the north atlantic 56 myr ago and were particularly formed during the transition phase between rifting and spreading the dominating wavelengths of the undulations are 46 and 109 km respectively to explain the evolution of crustal undulations in an extensional stress regime we use a two dimensional finite element model a viscoelastoplastic power law rheology is used to study the growth of instabilities developing in a three layer lithosphere prescribed random vertical undulations show wavelength dependent growth under extension and the dominating wavelengths are 18 65 and 129 km respectively these results are in good agreement with the gravity observations and provide a strong indication that the proposed boudinage mechanism influences the evolution of the passive margin segment

deep crustal structure of the area of intersection between the shackleton fracture zone and the west scotia ridge drake passage antarctica abstract the shackleton fracture zone which forms the boundary between the antarctic and scotia plates in the drake passage is characterized by a present day left lateral motion the west scotia ridge an extinct spreading centre formed the oceanic crust of the western scotia plate and intersects the shackleton fracture zone in a complex deformed area multichannel seismic gravity magnetic and multibeam swath bathymetry data were acquired during the antpac 97 98 cruise with the spanish vessel b o hesperides in the area of intersection of these two tectonic features the new data reveal its asymmetrical deep crustal structure which developed as a consequence of the overprinting of extensional and contractional deformation events the main seismic features of the crust of the scotia and antarctic plates are deep dipping braided reflectors which may be a consequence of an initial stretching deformation related to the shackleton fracture zone at present this fracture zone is characterized by thickened oceanic crust deformed by reverse and transcurrent faults and locally bounded by areas of crustal thinning the present morphology of the west scotia ridge has the characteristics of slow spreading centres with a central valley bounded by two elongated highs however its structure reveals that after spreading ended there was a nw se contractional deformation event with a thrust of about 40 km of slip that resulted in asymmetrical crustal thickening incipient subduction initiates subparallel to the strike of the spreading centre this tectonic event may be related to an episode of closure of south america and the antarctic peninsula probably of pliocene age we conclude that in oceanic domains areas with a weak crust fracture zones and spreading centres constitute the most sensitive regions for analysing the regional tectonic evolution since deformation events are better recorded there than in normal oceanic crust

statistical characterization of geosat altimetry noise dependence on environmental parameters 1 sea surface height ssh measurements from the geosat satellite altimetry mission are analyzed to statistically characterize their noise process which is defined here as the small scale 50 km time varying component of the altimetry data this analysis is one component of a larger effort to extract meaningful information about seafloor roughness from the small scale fabric of the altimetric gravity field both small scale seafloor topography e g abyssal hills and ssh noise significantly influence the measured gravity fabric hence a global quantitative understanding of ssh noise is required to infer what portion of the gravity fabric is associated with the true geoid it is also anticipated that the ssh noise analysis will prove useful in designing improvements to altimetry processing algorithms the ssh data are first processed by differencing nearest neighbor tracks to minimize the static component of variability and then by high pass filtering to remove variability associated with mesoscale oceanic circulation autocovariance analysis reveals that ssh noise can be decomposed into uncorrelated white noise and correlated semiperiodic components three parameters are measured which characterize the variance of both components and the correlation length of the correlated component the global distribution of uncorrelated noise variance is closely correlated to significant wave heights directly linking the uncertainty of the altimeter return time to the roughness of the sea surface the variance and correlation length of the correlated component of noise can be associated with three separate environmental parameters sea ice precipitation rates and upper atmospheric variability associated with the subtropical jet stream as widely differing as these environmental processes are it is hypothesized that features like sea ice rain storms and atmospheric turbulence can cause sudden perturbations in the return time of the altimetry signal creating decaying oscillations associated with the onboard tracking algorithms