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

formation and growth of deep mantle plumes summary if whole mantle convection occurs in the earth s mantle then the core mantle boundary constitutes the lower boundary layer for mantle convection this boundary layer appears to be unstable on a small scale and thus may be a source of plumes of hot matter which penetrate the mantle and occasionally even the lithosphere producing hot spots a finite amplitude numerical code is used to study the formation of such plumes and their growth through the mantle the plumes are restricted to being two dimensional sheets rather than cylinders the initial conditions consist of a steadily convecting mantle and plumes are produced by introducing a perturbation in the form of either a pulse or a steady stream of heat into the bottom of the mantle two main results are obtained 1 a critical perturbation size has been found for a mantle with a rayleigh number of lo small perturbations produce plumes which fail to penetrate the mantle and instead are swept up by the pre existing convective pattern while large perturbations succeed in penetrating the mantle and reaching the lithosphere the critical perturbation size is shown empirically to be proportional to the effective bouyancy and to a factor related to the shape of the perturbation a perturbed region 150km wide and 60 km deep should produce a successful plume when the temperature perturbation is 200k or more 2 deep mantle plumes appear to require on the order of 50 100myr to penetrate the mantle episodic plumes on shorter time scales appear unlikely a similar time is required for plumes forming in an initially static uniform temperature mantle

slabs hotspots cratons and mantle convection revealed from residual seismic tomography in the upper mantle to further investigate the fate of slabs the structure of hotspots and the style of mantle convection we calculate residual tomographic maps in the upper mantle by excluding from the tomography the first order effects of conductive cooling of oceanic plates deep cratonic roots and cooling or partial melting associated with subducted lithosphere no correlation is found between residual tomography in the upper mantle and the 30 130 ma subduction the good correlations between residual tomography in the transition zone 400 650 km and 0 30 ma subduction at spherical harmonic degree l 2 can be explained either by slab accumulation in the transition zone beneath some subduction zones or by the poor lateral resolution of seismic tomography hotspots correlate with the residual tomography in the shallow 100 400 km and the lowermost mantle at degree l 2 however correlations of hotspots and seismic velocities in the middle mantle and correlations between residual tomography at shallow depths and lowermost mantle tomography are poor therefore the connection between residual tomography at shallow depths and seismic tomography in the lowermost mantle is not clear craton distribution correlates with distribution of hotspots and residual topography very well at degree l 6 this might indicate that cratonic roots may affect the locations of upwellings and thus modulate upper mantle convection or that the degree l 6 could be intrinsic in upper mantle convection with cratons and and other near surface features establishing the pattern

fe si oxyhydroxide deposits at a slow spreading centre with thickened oceanic crust the lilliput hydrothermal field 9 33 s mid atlantic ridge diffuse and focused low temperature fluids emanate at 9 33 s mid atlantic ridge and precipitate fe sioxyhydroxides that form chimneys mounds and flat lying deposits this extensive vent field named lilliput lies at the axial zone of a spreading segment with a significantly thickened crust 11 km theoretically much more heat needs to be removed from a thick crust spreading center compared to a spreading center with typical thickness of 6 km therefore settings with thickened crust should be favourable for supporting very powerful hydrothermal systems capable of producing large mineral deposits this is the first report on the composition of seafloor hydrothermal deposits at abnormally thickened oceanic crust due to hotspot ridge interaction our studies revealed that generally the lilliput hydrothermal deposits are very similar in morphology structure composition and lateral extent to other low temperature hydrothermal deposits of mid ocean ridges and intraplate volcanoes deposits at the lilliput vent field are composed of si containing goethite and ferrihydrite have very low contents of a number of transition and rare earth elements and show ree distribution patterns with negative ce and eu anomalies the speciation and precipitation of the main deposit forming elements fe and si at the hydrothermal field appear to be partially controlled by live microbes and exuded organic compounds the 18o values of the precipitated silica containing feoxyhydroxides point to low temperature formation and sr nd pb isotope variations suggest that the hydrothermal precipitates scavenged metals predominantly from the ambient seawater these findings are in agreement with the biogeochemical scenario for their precipitation

the empirical age depth relation and depth anomalies in the pacific ocean basin this paper presents a new age depth relation and a comprehensive depth anomaly map for the pacific ocean age depth and sediment thickness were averaged for each 1 1 area bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep hot spots were treated as thermal anomalies and data within 800 km of them were eliminated from further analysis an age depth equation was determined by fitting a least squares line to the remaining data the resulting depth versus age distribution confirms the linear dependence of depth on age up to 80 ma all older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted this flattening of the age depth distribution requires that a separate function be fit to data older than 80 ma as a result data representing the deepest seafloor older than 80 ma were selected for curve fitting and the age depth distribution was approximated by an exponential function it is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age depth data from the pacific ocean a depth anomaly map prepared by comparing observed depth with the depth predicted by the new age depth formula shows several newly recognized features including swells over the louisville ridge in the south pacific and in the gulf of alaska seamounts

protracted continental collisions argue for continental plates driven by basal traction abstract in plate tectonic theory collision between two continents should quickly terminate because of continental buoyancy if convergence is to continue it should do so at a new subduction zone where oceanic crust can be consumed the protracted continental collisions in the alps zagros and himalayas which have continued to deform continental crust since the early or middle cenozoic are therefore anomalies in standard plate tectonic theory it is commonly held that plates are driven by slab pull but this does not account for the continuing tethyan collisions where the descending slab has detached from the subducting continent these protracted continental collisions are better explained by horizontal traction of the mantle on the base of deep continental roots dragging the northern and southern continents together along a tethyan axis of mantle convergence continental undertow thus resolves the collision anomaly in plate tectonics

the origin of hotspot traces evidence from eastern australia interpretation of available isotopic ages and of published geologic maps of igneous rocks in eastern australia indicates a north south time transgressive pattern of cessation of igneous activity along a curvilinear trace beginning about 35 ma at 20 sup 0 s and extending to the present near 37 sup 0 s however volcanic activity began 70 60 ma over the length of the highlands and persisted through the early cenozoic until progressive cessation began the latitudinal rate of termination is compatible with that predicted by plate reconstructions relative to the hawaiian hotspot but the trend of the trace is incompatible with that predicted unless the hawaiian emperor bend is somewhat greater than 50 m y in age relative to the labrecque et al 1977 time scale the volcanic pattern appears to be incompatible with a hotspot or plume hypothesis paleostress and contemporary stress indicators as well as the volcanic pattern support a model in which the trace forms due to intraplate extension normal to the trend of the trace resulting in pressure release and melting at the base of the lithosphere beginning in the late cretaceous this stress field persisted until 35 ma when progressive reorientation of the stress field occurred cessation of igneous activity reflectedmore onset of compression normal to the trend of the trace as a consequence a migrating stress node is recorded in the progressive pattern of extinction of volcanic activity along the trace evidence from other hotspot traces suggests that they too represent intraplate stress controls it is not necessary to postulate deep sources mantle plumes for the origin of hotspot traces intraplate stress due to drag plate margin forces thermal contraction and nonsphericity of the earth can also reflect absolute motion less

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

implications of volcano and swell heights for thinning of the lithosphere by hotspots the heights of hotspot volcanoes are modeled by assuming isostatic equilibrium between the magma column and the adjacent lithosphere the depth to the level of compensation is assumed to be related to the thickness of the lithosphere after it has been reheated and thinned by a hotspot there is a square root relationship between volcano height and lithospheric age relationships between volcano height and lithospheric thickness and between reheated thickness and age of the lithosphere can be constrained within the limits imposed by uncertainties in lithospheric and magma densities if the reheated thickness determined from the isostatic model can be compared with lithospheric thickness determined from thermal models of the lithosphere then a relationship between the thickness of the lithosphere before and after reheating can be derived combining the upper limit on the reheated thickness age relationship with a theoretical expression derived by s t crough which relates swell height to lithospheric age reset thickness and physical constants yields a swell height age relationship that is in good agreement with empirical swell height data this agreement supports the assumption that the lithospheric thickness defined isostatically by volcano heights is closely related to the thermal thickness of the lithosphere a result that in turn supports the thermal origin of hotspot swells

effects of lateral viscosity variations on long wavelength geoid anomalies and topography models for the earth s geoid and dynamically supported topography usually account only for radial vertical variations in mantle rheology and flow structure however expected lateral variations in viscosity due to temperature or stress dependent rheology can degrade our ability to infer mantle structure from geoid models based on seismically determined density contrasts in the mantel theoretically the main difficulty is that the harmonic components of the flow stress deformation field are no longer independent and exact analytical methods cannot account for this mode coupling for large thermal density contrasts of small lateral dimension e g subducted slabs it is difficult to form any general theory for these effects however we believe that the largest geoid anomalies which occur at the lowest harmonic degrees l 2 3 are due to a global pattern of relatively mild temperature variations for convection with mainly broadscale thermal and rheological changes some relatively straightforward ideas can be developed combining results from simple perturbation theory and numerical models for convective flow we find the following 1 if we are correct in inferring viscosity variations of less than one order of magnitude globally the longest wavelength geoid anomalies l 2 3 are not seriously contaminated by lateral viscosity variations 2 contamination of the higher degrees l 4 is expected and in particular self coupling between l 2 heterogeneity and viscosity variations may be observable at degree 4 the most severe contamination occurs at the doubled harmonic l 4 of the dominant heterogeneity harmonic l 2 3 perturbation techniques are adequate to model global scale lateral viscosity variations of an order of magnitude or less 4 the steady state surface deformation problem at the heart of our geoid models may be more robust with respect to broadscale lateral viscosity variations than are models for transient deformations e g postglacial rebound 5 for loading by broadscale density heterogeneity the effects of stress dependent rheology are probably secondary to the effects of radial viscosity stratification

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

viscosity of the lower mantle as inferred from rotational data until recently most estimates of the deep mantle viscosity were based on analyses of postglacial uplifts and there were no serious estimates of the sensitivity of the derived viscosity solutions to variations of the input parameters such as those associated with the surface loading in this paper the viscosity of the lower mantle is arrived at by making comparisons of the observed secular motions of the earth s rotation axis with theoretical results from a layered viscoelastic rotating earth which has been subjected to glacial forcing our model consisting of an elastic lithosphere a two layer adiabatically stratified viscoelastic mantle and an inviscid core is essentially analytical and this makes it economically feasible to use as an aid in conducting an extensive sensitivity analysis of the lower mantle viscosity from changes in the parameters connected with the deglaciation phenomenon both sets of rotational data polar wander and nontidal deceleration of the length of the day have been employed to constrain the viscosity structure on the basis of this type of investigation the viscosity of the lower mantle is found invariably to be larger than that of the upper mantle and lies between 1 and 4 1022 p our results which just rely on the second degree harmonic of the strain field also lead to a determination of an upper bound to the thickness of the globally averaged lithosphere this value ranging between 130 and 190 km suggests that strong lateral heterogeneities between oceanic and continental plates may extend a few hundred kilometers into the upper mantle

observational hints for a plume fed suboceanic asthenosphere and its role in mantle convection an asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection in this study we discuss geochemical seismic and geoid depth evidence for lateral flow within this type of asthenosphere in particular we suggest that there are large scale layered horizontal flow structures that connect upward plume input beneath hotspots to near ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere lateral asthenosphere flow is also shaped by oceanic subduction zones with a partial return flow from trenches and by deep continental roots that are migrating barriers to asthenosphere flow this alternative paradigm offers relatively simple explanations for several puzzles about mantle convection for example the low mantle heat flow beneath continents it also offers an explanation for why mid ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large scale mantle flow while in contrast trenches appear to be strongly coupled to mantle thick regions of fast colder seismic velocity anomalies we also discuss several implications of this paradigm that should be testable in future studies such as the prediction of cogenetic off axis seamount volcanism that is created between an off axis hotspot and its neighboring ridge axis

aspherical earth structure from fundamental spheroidal mode data free oscillation data reveal heterogeneity in the earth s mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid the heterogeneity can be modelled as localized in the transition zone 420 670 km depth and may be related to a large scale component of mantle convection

lithospheric heating by mantle plumes summary a broad topographic swell is associated with the hawaiian hotspot a kinematic model is developed assuming that downstream of the hotspot this swell is produced by lithospheric heating by a mantle plume material from the model plume flows radially in the asthenosphere and eventually heats the lower lithosphere upstream from the hotspot the model swell owes its existence mainly to hot material at asthenospheric depths the model predicts extensive lithospheric heating downstream from the hotspot interpreted data to tell if the lower lithosphere behaves in this way are lacking

formation of seafloor swells by mantle plumes the prominent oceanic hot spots are located at the crest of broad topographic swells on the seafloor available evidence indicates that swells are supported by anomalously thin or altered lithosphere as might result from the intrusion of a mantle plume into the lithosphere a set of experiments have been made to measure the free surface uplift and subsidence produced by intrusion of a low viscosity buoyant plume into a rheological boundary layer a deep layer of strongly thermoviscous sugar solution with a free surface was cooled from above to simulate the lithosphere asthenosphere system low viscosity low density plumes were released at depth the free surface displacement generated by the ascending plume was recorded as a function of time using a proximity measurement system the plumes ascend through the asthenosphere as nearly spherical diapirs and collapse flatten upon intrusion into the base of the lithosphere evolution of the surface topography can be divided into three phases 1 a low axisymmetric rise supported by the plume deep beneath the lithosphere 2 rapid uplift and steepening as the plume reaches the lithosphere base and 3 slow axial subsidence as the plume stagnates within the lithosphere the measured uplifts are compared with the uplift calculated for buoyant plumes in a uniform viscous half space using morgan s image method the agreement is good for thin lithosphere for thick lithosphere the observed is less than the calculated uplift scaling laws have been derived which relate the measured maximum uplift and maximum uplift rate to diapir parameters extrapolation of these to the scale of ocean floor swells gives uplift rates of approximately 1 km in 5 m y by this mechanism the most significant qualitative results of the experiments are that swells formed by interaction of low viscosity mantle plumes and oceanic lithosphere can be characterized by axial symmetry rapid uplift and shallow compensation

q tomography of the upper mantle using three component long period waveforms summary we present a degree 8 3 d q model qrlw8 of the upper mantle derived from three component surface waveform data in the period range 60 400 s the inversion procedure involves two steps in the first step 3 d whole mantle velocity models are derived separately for elastic sh transverse component and sv vertical and longitudinal component velocity models using both surface and body waveforms and the non linear asymptotic coupling theory nact approach in the second step the surface waveforms thus aligned in phase are inverted to obtain a 3 d q model in the depth range 80 670 km various stability tests are performed to assess the quality of the resulting q model and in particular to assess possible contamination from focusing effects we find that the 3 d patterns obtained are stable but the amplitude of the lateral variations in q is not well constrained because large damping is necessary to extract the weak q signal from data the model obtained agrees with previous results in that a strong correlation of q with tectonics is observed in the first 250 km of the upper mantle with high attenuation under oceanic regions and low attenuation under continental shields it is gradually replaced by a simpler pattern at larger depth at the depths below 400 km the q distribution is generally dominated by two strong minima one under the southern pacific and one under africa yielding a strong degree 2 pattern most hotspots are located above regions of low q at this depth ridges are shallow features in both velocity and q models

anisotropy and shear velocity heterogeneities in the upper mantle long period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle the dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure the data are corrected for the effect of surface layers and both love and rayleigh waves are used shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km the shear wave velocity distribution to 200 km depth correlates with surface tectonics except in a few anomalous regions below that depth the correlation vanishes cold subducted material shows up weakly at 350 km as fast s wave anomalies in the transition region a large scale pattern appears with fast mantle in the south atlantic s anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones the pacific shows a nw se fabric

mantle transition zone structure beneath the south pacific superswell and evidence for a mantle plume underlying the society hotspot abstract underside reflections of shear waves from the discontinuities at 410 and 660 km depth are used to map lateral variations in the thickness of the mantle transition zone beneath the south pacific superswell and surrounding regions differential travel times of reflected waves indicate that the transition zone is about 25 km thinner than normal and thus hotter than normal over an area 500 km or less in diameter beneath the society hotspot there is no general difference however in transition zone thickness between the superswell area and its surroundings our observations support the inference that the thermal flux from lower to upper mantle beneath the south pacific superswell occur on the lateral scale of a mantle plume several hundred kilometers rather than that of a superplume several thousand kilometers

free surface formulation of mantle convection i basic theory and application to plumes in mantle convection models the top surface is traditionally approximated as a free slip boundary and the dynamic topography is obtained by assuming that the normal stress on the free slip boundary is compensated instantaneously through surface deformation it has already been shown that this approximation is valid for long wavelength topography based on both viscous and viscoelastic models with a free surface we have found that the characteristic time for topographic growth is comparable to the timescales of mantle convection 10 6 year for short and intermediate wavelengths 10 3 km or less and or a high effective lithospheric viscosity 10 24 pa s this suggests that the topography is history dependent under these conditions and that a free surface formulation is required to study the topography at these wavelengths we have developed a new eulerian finite element technique to model a free surface since the technique is based upon an undeformable eulerian grid this enables us to study long term free surface dynamics in the presence of evolving buoyancy we have compared numerical with analytic solutions of viscous relaxation for fixed buoyancy problems as long as the magnitude of topography is much smaller than the wavelength we find that the finite element method is very accurate with relative errors of less than 1 per cent this numerical technique can be applied to a variety of geophysical problems with free surfaces in applying this technique to dynamic models of mantle plumes we find that surface relaxation retards the topography at intermediate and short wavelengths and produces a smoother topography compared with topography from free slip calculations this reduced topography has a significant influence on the geoid at the corresponding wavelengths moreover free surface models by allowing vertical motion on the free surface yield a hotter lithosphere over ascending plumes than models with free slip boundaries

seismic evidence for a thinner mantle transition zone beneath the south pacific superswell broadband seismograms recorded by the seismic stations deployed on oceanic islands in the south pacific for two deep earthquakes in 1998 are used to investigate the mantle transition zone structure beneath the south pacific where a large scale hot plume might ascend from the core mantle boundary cmb by stacking s waves reflected at the mantle transition zone discontinuities we find clear signals associated with the 410 km and 660 km discontinuities the transition zone thickness which is constrained from the travel time difference between the reflected waves from the 410 km and 660 km discontinuities is observed to be approximately 15 km thinner beneath the south pacific superswell than that beneath other regions the result suggests that the transition zone temperature beneath french polynesia is approximately 100 200 k higher than the surrounding mantle

significant groundwater contribution to antarctic ice streams hydrologic budget satellite observations have revealed active hydrologic systems beneath antarctic ice streams but sources and sinks of water within these systems are uncertain here we use numerical simulations of ice streams to estimate the generation flux and budget of water beneath five ice streams on the siple coast we estimate that 47 of the total hydrologic input 0 98 km3 yr 1 to whillans wis mercer mis and kamb kis ice streams comes from the ice sheet interior and that only 8 forms by local basal melting the remaining 45 comes from a groundwater reservoir an overlooked source in which depletion significantly exceeds recharge of the total input to bindschadler bis and macayeal macis ice streams 0 56 km3 yr 1 72 comes from the interior 19 from groundwater and 9 from local melting this contrasting hydrologic setting modulates the ice streams flow and has important implications for the search for life in subglacial lakes

the modern geoid and ancient plate boundaries abstract the non hydrostatic geoid is dominated by three large anomalies an area of high gravity potential in the equatorial pacific another stretching from greenland through africa to the southwest indian ocean and a semi continuous low region passing from hudson s bay through siberia to india and on to antarctica none of these three high amplitude greater than 60 m and long wavelength anomalies corresponds to present day plate boundaries however if the modern geoid is plotted over the positions of continents and plate boundaries at 125 ma b p reconstructed relative to hotspots a strong correlation emerges the modern geoidal low corresponds in position to the areas of subduction surrounding the pacific 125 ma ago the geoidal high now centered on africa is entirely contained within ancient pangaea and the equatorial pacific high overlies the location of the spreading centers preserved in the magnetic anomalies of the central pacific the most plausible cause of the large geoidal undulations is lower mantle convection only weakly coupled to plate motions the correspondence between modern geoid and ancient plate boundaries implies either that the coupling was much more intimate in the past or that there is a lag of at least 100 ma in response of the lower mantle to upper mantle conditions

an analytic model for a mantle plume fed by a boundary layer summary an approximate analytical solution for flow in a mantle plume of constant radius viscosity and density contrast is obtained in cylindrical coordinates the differential equations for vertical velocity of the mantle surrounding the plume and for topography are homologous to the equation for flexure of an elastic plate although the model is too simple to be fully applicable to the earth one can conclude that the vertical velocity in the mantle changes significantly away from plumes that the viscosity of the plume is important for controlling flow rate and that the long wavelength geoid anomalies are sensitive to the viscosity of the surrounding mantle the first induced upwelling away from a plume is quite weak and unlikely to control the spacing of plumes

global shear velocity heterogeneities in the d layer inversion from sd sks differential travel times a global map of shear velocity in the d layer results from the inversion of 340 differential travel times of diffracted s sh minus sks sv sd sks from long period records of global seismic networks the two phase design reduces contamination from upper mantle heterogeneities and errors in location and origin time of the events additional corrections are made for 1 azimuthal anisotropy at stations where shear wave splitting parameters are available and for 2 travel time perturbations due to lower mantle asphericity although both effects are minor compared with the observed residuals with respect to the preliminary reference earth model prem dziewonski and anderson 1981 the corrected residuals ranging from 16 to 18 s are attributed to anomalies in d sampled by both phases taking these residuals as data and assuming a constant 250 km thick d layer we invert for a lateral velocity variation model of d using spherical harmonics in parameterizing d velocities a high degree expansion l 14 avoids aliasing but only the reliably determined low degree components li l produce optimal models the optimal model li 6 achieves a variance reduction of 56 and mean error 0 019 km s and the velocity is insensitive to the thickness of the d layer assumed a baseline shift of 0 022 km s from prem is resolved the magnitude of variation 0 2 km s or 3 is nearly twice as large as that of tanimoto 1990 and su et al 1994 while comparable to that of liu and dziewonski 1994 which includes s sks data and li and romanowicz 1996 degree 2 variation dominates the d with positive anomalies beneath the asian australian and american continents and southernmost pacific and negative anomalies beneath the pacific western africa atlantic and the southern indian ocean this pattern is similar to that found in previous tomograpic models significant discrepancies in nondegree 2 features are discussed in terms of the constraints from sd the fast and slow regions of d correlate with the inferred paleoslab positions and the major hotspot distribution respectively both at the 99 confidence level implications of these correlations for mantle dynamics are suggested the global model agrees reasonably well with the velocities determined regionally from the sd slowness measurement and the scs s residuals in other studies the latest radial models of d constrained from s triplications at distances of 75 90 provide a completely independent benchmark models for eurasia alaska and the caribbean predict sd travel times corresponding to 0 03 0 08 km s anomalies consistent with our lateral variation model in respective regions these vertical structures are also consistent with this study in scs times indicating that they agree in a vertically averaged sense major inconsistency exists for the india model syll young and lay 1987b which predicts sd and scs arrivals much slower than those expected from the 0 1 km s fast anomaly we found suggesting either the presence of strong lateral heterogeneities or the need for further mapping in this area

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

thermal haloes around plume tails summary the inward flow of ambient mantle material can cause thermal haloes around plume tail conduits to be narrow the plume is hard to detect tomographically within depth ranges where such flow occurs this may occur in the transition zone beneath iceland the influx rate per length of conduit needs to be of the order of 25 10 6 m 2 s 1 to have noticeable effects such flow rates would quench weak plumes but not greatly perturb strong ones a pressure deficit of the plume conduit of several mpa would drive this flow if the mantle viscosity did not exceed 10 22 pa s

a simple model of whole mantle convection isotopic evidence of the chemical variability of basalts implies the existence of distinct long lived chemical reservoirs in the mantle this has commonly been interpreted as evidence that mantle convection is layered since known models of whole mantle convection have too much shearing to allow preservation of these reservoirs however the geochemical evidence requires at least three reservoirs and models of layered convection have dynamical difficulties casting serious doubt on the viability of these models this difficulty has been resolved in the present paper by the development of a simple model of whole mantle convection which allows the preservation of chemically distinct reservoirs in this model heating of the mantle from below produces axisymmetric plumes which carry heat upward through the mantle without causing overturning hence the lower mantle is much less sheared than in the usual two dimensional model mantle convection is driven by the penetration of cold lithospheric slabs from above it is argued that the pattern of convection is governed primarily by the dependence of viscosity on temperature and this dependence drives the mantle toward an isoviscous state these ideas are quantified in a simple one dimensional model of mantle convection in which the slab mass flux as a function of radius is governed by the requirement that the temperature profile remains isoviscous the resulting convection cycles material through the upper mantle more rapidly than through the lower mantle this provides a mechanism for an initially homogeneous mantle to become chemically stratified with a depleted upper mantle and primitive lower mantle

the bahia seamounts test of a hotspot model and a preliminary south american late cretaceous to tertiary apparent polar wander path abstract detailed bathymetric and magnetic data collected over the bahia seamounts in the western south atlantic brazil basin are analyzed six paleomagnetic poles are determined one from three seamounts sharing a ridge and five from isolated seamounts two poles fall near a continental late cretaceous south american pole and are assumed to be of late cretaceous age another seamount has a preliminary 40 ar 39 ar age of 62 4 ma we check this age and estimate the ages of other seamounts by rotating their poles into north american coordinates and comparing them with north american paleomagnetic reference poles these ages and the characteristic magnetic polarities of all bahia seamounts are used to test hotspot models of formation of the seamount group the northern and central bahia seamount chains could have formed from a single hotspot the southern bahia chain may require a second hotspot the two postulated hotspots were probably concurrent and separated by 150 to 200 km the paleomagnetic poles trace out a smooth curve from the late cretaceous poles to a pole that is indistinguishable from the spin axis which we estimate as having an early eocene age the pole ages remain preliminary until better age dates are found and rocks are recovered from more seamounts

anelastic tomography a new perspective on upper mantle thermal structure abstract anelastic tomography provides new insights into the thermal structure of the earth s upper mantle lateral variations in q are largest in the upper 300 km but remain significant down to at least 500 600 km they correlate with the distribution of plates in the upper 300 km in agreement with cooling plate models and with the distribution of hotspots at greater depths two major upwellings are revealed under hawaii and northeastern africa which may originate at the bottom of the transition zone or deeper low q structure beneath the main ridge systems on the other hand does not appear to extend vertically below about 300 km

monteregian hotspot track a long lived mantle plume the monteregian hills the younger white mountains and the new england seamounts constitute a well defined hotspot track the evolution of the plume through time is obtained from the marine track the buoyancy flux of the plume when it made the new england seamounts was about 2 mg s 1 which would make it the seventh or eighth most active plume today the hotspot declined in activity after it made nashville seamount the buoyancy flux when it made corner seamounts and today is less than 0 5 mg s 1 the changes in the flux of the marine hotspot limit how far the marine flux can be extrapolated to the west onto the continental track and what can be learned in general about vulnerability of the continental lithosphere to hotspots the lack of an obvious track west of montreal may be due either to failure of the plume to penetrate the canadian shield to the lack of recognizable intrusions or to strengthening of the plume when it approached montreal

superplumes plates and mantle magmatism in two dimensional numerical models 1 two dimensional numerical models are systematically presented for magmatism in a convecting mantle internally heated by incompatible radioactive elements plate tectonics is self consistently modeled as a part of convection of newtonian fluid with temperature and stress history dependent viscosity driven by thermal and compositional buoyancy while magmatism is modeled as an upward migration of basaltic magma generated by decompression melting ridge magmatism actively takes place to chemically differentiate the mantle and a superplume is well developed in the lowermost mantle as a broad hot region enriched in basaltic component provided that the lithosphere is mechanically strong enough to inhibit spontaneous initiation of subduction by its own weight and that the internal heating rate is sufficiently high hot narrow plumes frequently rise up from the superplume to induce deep magma generation and hot spot magmatism which in turn frequently induces new plate margins the frequent plate margin formation keeps the activity of plate tectonics rather steady the thermal and chemical state of the mantle remains rather steady too as the internal heating rate becomes lower however the narrow plumes from superplume become sporadic and significantly colder deep magma generation becomes rare and new plate margin formation becomes less frequent as a consequence the activity of plate tectonics and the thermal and chemical state of the mantle become fluctuating much with time the mantle structure and plume behavior modeled here match with many observations for those of the earth from the archean to the present

thermal origin of the hawaiian swell heat flow evidence and thermal models the lithospheric reheating hypothesis for the origin of the hawaiian swell has been tested by heat flow measurements along the swell between hawaii and midway ninety five measurements at eight carefully selected sites show that the heat flux on the swell increases systematically from near normal values 52 2 mw m 2 near hawaii to values near midway 59 2 mw m 2 about 20 25 higher than normal for lithosphere of that age 110 m y this heat flow pattern is qualitatively consistent with a deep seated thermal anomaly created in a lithospheric plate moving across a fixed heat source modeling of the heat flow and bathymetry data on the hawaiian swell strongly suggest that the swell is caused by a rapid heating within 40 to 50 km depth of normal 90 m y old lithosphere over a relatively broad 500 1000 km region near hawaii this model is also consistent with gravity and geoid anomalies associated with the swell which require a low density region in the lower lithosphere and with flexure studies of seamounts in the hawaiian chain which indicate that the rigidity of the lithosphere is not significantly reduced by the reheating appendices a c are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 005 1 00 payment must accompany order

geoid anomalies and dynamic topography from convection in cylindrical geometry applications to mantle plumes on earth and venus summary a variety of evidence suggests that at least some hotspots are formed by quasi cylindrical mantle plumes upwelling from deep in the mantle we model such plumes in cylindrical axisymmetric geometry with depth dependent newtonian viscosity cylindrical and sheet like cartesian upwellings have significantly different geoid and topography signatures however rayleigh number nusselt number systematics in the two geometries are quite similar the geoid anomaly and topographic uplift over a plume are insensitive to the viscosity of the surface layer provided that it is at least 1000 times the interior viscosity increasing the rayleigh number or including a low viscosity asthenosphere decreases the geoid anomaly and the topographic uplift associated with an upwelling plume increasing the aspect ratio increases both the geoid anomaly and the topographic uplift of a plume the nusselt number is a weak function of the aspect ratio with its maximum value occurring at an aspect ratio of slightly less than 1

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

variations in magma supply rate at kilauea volcano hawaii when an eruption of kilauea lasts more than 4 months so that a well defined conduit has time to develop magma moves freely through the volcano from a deep source to the eruptive site at a constant rate of 0 09 km3 yr at other times the magma supply rate to kilauea estimated from geodetic measurements of surface displacements may be different for example after a large withdrawal of magma from the summit reservoir such as during a rift zone eruption the magma supply rate is high initially but then lessens and exponentially decays as the reservoir refills different episodes of refilling may have different average rates of magma supply during four year long episodes in the 1960s the annual rate of refilling varied from 0 02 to 0 18 km3 yr bracketing the sustained eruptive rate of 0 09 km3 yr for decade long or longer periods our estimate of magma supply rate is based on long term changes in eruptive rate we use eruptive rate because after a few dozen eruptions the volume of magma that passes through the summit reservoir is much larger than the net change of volume of magma stored within kilauea the low eruptive rate of 0 009 km3 yr between 1840 and 1950 compared to an average eruptive rate of 0 05 km3 yr since 1950 suggests that the magma supply rate was lower between 1840 and 1950 than it has been since 1950 an obvious difference in activity before and since 1950 was the frequency of rift zone eruptions eight rift zone eruptions occurred between 1840 and 1950 but more than 20 rift zone eruptions have occurred since 1950 the frequency of rift zone eruptions influences magma supply rate by suddenly lowering pressure of the summit magma reservoir which feeds magma to rift zone eruptions a temporary drop of reservoir pressure means a larger than normal pressure difference between the reservoir and a deeper source so magma is forced to move upward into kilauea at a faster rate

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

the dynamical and thermal structure of deep mantle plumes abstract if the interpretation of the d layer at the base of the mantle as a thermal boundary layer with a temperature increment in the order of 800 k is correct then the formation of deep mantle plumes to vent material from it appears inevitable we demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long lived chimneys that are 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material the speed of flow up the axis of the plume is correspondingly fast 1 6 m y 1 at the base and 4 8 m y 1 at 670 km depth thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there this convergence which contributes only a small increment to the flux of material up the plume also serves to throttle the flow in the chimney the global plume mass flux necessary to transport 1 6 10 12 w of core heat upward through the mantle is 1 8 10 6 kg s 1 at its base plume material is probably still significantly below its solidus or eutectic temperature but substantial partial melting is very likely as it rises we speculate that a small fraction of this fluid component eventually emerges at the surface in hot spots with the fate of the remainder being unknown the behaviour and properties of d and of plumes are closely coupled not only are plumes a necessary consequence of a thermal boundary layer but their existence is impossible without that layer

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

possible perturbations to hotspot traces and implications for the origin and structure of the line islands perturbations to hotspot signals recorded on the surface of the earth will produce hotspot traces with characteristics more complex than those of simple time progressive volcanic chains some of these perturbations are a consequence of normal plate tectonics some are observed interactions of hotspot magmatism with lithospheric structure and processes and others are hypothetical the line islands volcanic chain has a complex structure and some characteristics that cannot be reconciled with those expected of a simple time progressive hotspot trace two models for the development of the line islands are presented model 1 assumes that the line islands were formed between 130 and 67 ma and model 2 assumes an age span of 90 to about 15 ma and incorporates the tuamotu islands as an extension of the line islands in model 1 most of the complex structure and subsidence of the chain can be explained in terms of perturbations to the hotspot signal model 2 gives a good geometric fit to the volcanic chain but fails to explain the anomalous subsidence of the line islands

effect of a global plume distribution on earth normal modes we show that a global distribution of small scale heterogeneities in the lower mantle induces both frequency splitting and amplitude scatter of low angular order modes these effects can be used in order to map the global distribution of small scale structures in the mantle such as plumes two independent datasets consisting respectively of lower mantle modes interaction coefficients and of observed amplitudes of core modes can then be used to constrain the low angular degrees of the spatial distribution and the mean size of heterogeneities allowing us to constrain the mantle structure at different scales if these heterogeneities are assumed to be plumes associated with hotspot activity the mean size of plumes obtained using the two datasets is then in good agreement with that obtained from geodetical or numerical convection studies

seismic stratigraphy of detroit seamount hawaiian emperor seamount chain post hot spot shield building volcanism and deposition of the meiji drift 1 detroit seamount one of the northernmost seamounts of the hawaiian emperor seamount chain was formed at ca 76 ma new seismic data suggest renewed volcanism as late as 25 m y after initial seamount formation we use high resolution single channel seismic scs data acquired over the summit of detroit seamount in 2001 on ocean drilling program odp leg 197 supplemented by older scs data acquired as part of the gloria mapping program of the u s geological survey to characterize the seismic stratigraphy of detroit seamount volcanic edifices occur on the summit of the seamount and are older than the oldest beds of the meiji drift early oligocene ca 34 ma on the basis of ash layers in odp drill holes we suggest the edifices were active throughout much of the eocene ca 52 34 ma with activity possibly extending into the early oligocene 34 ma hence the age difference between the shield building lavas and the postshield cones on detroit is far greater than the shield postshield age differences observed on the hawaiian islands suggesting that renewed volcanic activity and tectonic collapse may be possible on any of the hawaiian islands we confirm earlier assertions that the thick sediment cap oligocene and younger in age was deposited by an ocean bottom current with a southeastward flow direction along the northeast facing flank of the emperor seamount chain this sediment cap the meiji drift was deposited by a lower velocity current than many other sediment drifts a low angle normal fault dipping 19 suggests topographic collapse of detroit seamount sometime during the eocene or late cretaceous

a dynamic model of venus s gravity field unlike earth long wavelength gravity anomalies and topography correlate well on venus venus s admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either airy or pratt isostasy but is consistent with dynamic support from mantle convection a model using whole mantle flow and a high viscosity near surface layer overlying a constant viscosity mantle reproduces this admittance curve on earth the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle these viscosity estimates may be biased by the neglect of lateral variations in mantle viscosity associated with hot plumes and cold subducted slabs the different effective viscosity profiles for earth and venus may reflect their convective styles with tectonism and mantle heat transport dominated by hot plumes on venus and by subducted slabs on earth convection at degree 2 appears much stronger on earth than on venus a degree 2 convective structure may be unstable on venus but may have been stabilized on earth by the insulating effects of the pangean supercontinental assemblage

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy 3 inversion lateral heterogeneity in the earth s upper mantle is investigated by inverting dispersion curves of long period surface waves 100 330 s models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies we also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities the data are from the observation of about 200 paths for love waves and 250 paths for rayleigh waves for both the regionalized and the spherical harmonic inversions corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters these corrections are found to be important especially at low spherical harmonic order the trench region and fast velocities down to 250 km under shields below 200 km under the oceans both s velocity and s anisotropy support a model of small scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough the spherical harmonic models clearly demonstrate a posteriori the relation between surface tectonics and s velocity heterogeneities in the first 250 km all shields are fast most ridges are slow below 300 km a belt of fast mantle follows the pacific subduction zones however at greater depths large scale heterogeneities that seem to bear no relationship to surface tectonics are observed the most prominent feature at 450 km is a fast velocity region under the south atlantic ocean smaller scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths an anomalously slow region centered in the south central pacific is possibly linked to intense hot spot activity a very fast region southeast of south america may be related to subduction of old pacific plate between 200 and 400 km a belt of sv sh anisotropy follows part of the ridge and subduction systems indicating vertical mantle flow in these regions the spherical harmonic results open new horizons for the understanding of convection in the mantle perspectives for the improvement of the models presented are discussed

global isostatic geoid anomalies for plate and boundary layer models of the lithosphere abstract isostatic geoid anomalies are usually interpreted using a flat earth one dimensional idealization isostatic anomalies on the spherical self gravitating earth differ from this idealization because 1 degree one terms in the spherical harmonic expansion vanish 2 each term in the spherical harmonic expansion is multiplied by l 2 l 0 5 relative to the flat earth case 3 mass in cones rather than straight sided columns is constant and 4 further deformation of the earth is induced by the gravitational attraction of the deformation caused by the isostatic potential anomaly when the effect of each of these is quantified the second third and fourth nearly cancel leaving the degree one over the horizon effect providing the most important difference calculations of model isostatic geoid anomalies for the spherical analogues developed here of the plate and boundary layer thermal models show that this effect can bias estimates of geoid slopes by over 20 although the effect is usually less than 5 the geoid anomalies for these two models are quite different over old ocean basins but they are unlikely to be distinguishable on the basis of global geoid observations owing to the presence of other larger perturbations in the geoid stripping the effects of plate aging and a hypothetical uniform 35 km thick isostatically compensated continental crust from the observed geoid emphasizes that the largest amplitude geoid anomaly is the geoid low of almost 120 m over west antarctica this anomaly is a factor of two greater in amplitude than the low of 60 m over sri lanka

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

rifting of africa and pattern of mantle convection beneath the african plate abstract the mesozoic and cenozoic tectonic setting of the african continent is characterized by an extensional stress regime as is evident from the evolution of the west central and east african rift systems the opening of the indian and atlantic oceans and the development of tethys domain as well as from the results of geophysical investigations the most prominent feature of the bouguer gravity anomaly map of africa is a great belt of negative bouguer anomalies which extends across the entire african continent from ethiopia in the east to benguela in the west in conjunction with the red sea spreading zone this belt appears to encircle a centre in cameroon it probably corresponds to a distinct zone of lithospheric thinning and extension the entire african continent is characterized by higher than normal elevations it is located inside the african residual geoid high earthquake fault plane solutions confirm the extensional character of the present tectonic regime of eastern and southern africa where normal faulting predominates the average orientation of t axes is n s in south africa nw se in zambia mozambique and madagascar e w in kenya and ethiopia and ne sw in the red sea region from north to south the orientation of t axes rotates systematically in a clockwise sense by 130 and describes a radial pattern centred on equatorial west africa this indicates that the african lithosphere has been subject to continent wide radial extension around a hypothetical west african spreading centre referred to as the african spreading centre a asc a which coincides with the geographical centre of the african plate shear traction exerted by the convecting mantle on the base of the lithosphere is considered to be a major driving force of plate movements in the case of africa the continent wide regularities in the pattern of lithospheric extension the elevated position of the continent as well as the evolution of the mid oceanic ridge system surrounding africa provide distinct boundary conditions for the pattern of large scale mantle flow beneath the african plate it is proposed that relatively warm and less dense mantle material rises below africa forming a single mega plume or a number of plumes at the base of the african lithosphere the ascending material diverges and flows radially away from the centre of ascent corresponding to the asc a this ascending flow forms part of a very large mantle convection cell which occupies the eurafrican hemisphere the descending branch of this convection cell is located at a distance of 80 90 from the asc a the mid oceanic ridges surrounding the african continent are located between the upwelling and the downwelling branches of this convection cell at a distance of 50 60 from the asc a upper mantle flow below the sea floor spreading axes is uni directional and horizontal and is directed away from the asc a

geology and geochronology of the line islands geological and geophysical studies along the entire length of the line islands were undertaken in order to test the hot spot model for the origin of this major linear island chain volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls volcanic rocks from the line islands are alkalic basalts and hawaiites in addition a tholeiitic basalt and a phonolite have been recovered from the central part of the line chain microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high tio2 1 0 4 0 wt and al2o3 3 4 9 1 wt and are of alkaline to peralkaline affinities major element compositions of the line islands volcanic rocks are very comparable to hawaiian volcanic rocks trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas the line islands lavas are very much unlike typical mid ocean ridge or fracture zone basalts dating of these rocks by 40ar 39ar k ar and paleontological methods combined with deep sea drilling project data from sites 165 315 and 316 and previously dated dredged rocks provide ages of volcanic events at 20 localities along the chain from 18 n to 9 s a distance of almost 4000 km all of these dates define mid cretaceous to late eocene edifice or ridge building volcanic events eocene volcanic events took place from 15 n to 9 s and late cretaceous events took place from 18 n to 9 s in the southern line islands both cretaceous and eocene events took place on the same edifice or ridge indicating recurrent volcanism at a single locality the irregular distribution of atolls in the chain the fact that late cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern line islands and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t1 2 related subsidence path magnetic surveys of 11 seamounts show that four of the seamounts from the central line chain give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of cretaceous seamounts these four poles agree with other paleomagnetic data of middle late eocene early oligocene age from the pacific one of these four seamounts yielded a 40ar 39ar total fusion age of 39 ma because the poles of all four seamounts fall into a tight group we infer that they are probably of middle late eocene age to early oligocene age the other three seamounts as well as one seamont from the line islands analyzed previously all give virtual geomagnetic poles which agree with late cretaceous paleomagnetic data from the pacific of these four seamounts three give late cretaceous 40ar 39ar ages ranging from 71 to 85 ma another in the southern line islands is interpreted on paleontological evidence to be of late cretaceous age the origin of the line islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots the single hot spot model cannot account for all of the volcanic edifices in the line islands although it does explain a general age progression of 9 6 0 4 cm yr from north to south along the chain derived from a number of dated edifices the four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data the petrologic data argue against a mid ocean ridge or transform fault origin proposed by earlier workers the complex volcanic province represented by the line islands remains a challenge to existing models for the origin of midplate volcanism in the pacific an atoll drilling program which could determine edifice building histories paleolatitudes of seamount formation and subsidence rates and patterns in the line islands is needed

plate dynamics caribbean map corrections and hotspot push summary in 1986 the author s previous paper describing a simple mathematical model for the forces driving the plates had an rms error of 1 62 per cent in its best case the model included slab pull and its global and local reactions proportional to speed times age 3 2 viscous drag due to shear under moving plates and in the flow towards mantle sinks at midocean ridges and sources at subduction zones ridge push proportional to plate age and friction at continental collision zones strengths of the various forces were found by least squares fitting three different improvements now reduce the error to 1 14 per cent they are 1 revision of the n s american plate boundary east of the caribbean sea to run along the 15 20 fracture zone ii removal of a transform segment from the caribbean subduction zone and iii allowing for hotspot push hotspots cause upward flow in the mantle which spreads outwards under the lithosphere above and pushes two plates apart if their boundary runs nearby in a way which is calculated the results and their statistical significance are estimated by a method given by backus park garbasz 1981 modified slightly because three of the equations in the model are linearly dependent on the remainder all the physical mechanisms envisaged in this work as driving plates turn out to be significant except local reaction to slab pull the effect of which is very small anyway

the interaction of plume heads with compositional discontinuities in the earth s mantle the effects of compositional discontinuities of density and viscosity in the earth s mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique three specific problems are considered 1 a plume head rising away from a deformable interface 2 a plume head passing through an interface and 3 a plume head approaching the surface of the earth for the case of a plume attached to a free surface the calculated time dependent plume shapes are compared with experimental results two principal modes of plume head deformation are observed plume head elongation or the formation of a cavity inside the plume head the inferred structure of mantle plumes namely a large plume head with a long tail is characteristic of plumes attached to their source region and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high to low viscosity region as a rising plume head approaches the upper mantle most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle if the plume head moves from a high to low viscosity region the plume head becomes significantly elongated and for the viscosity contrasts thought to exist in the earth could extend from the 670 km discontinuity to the surface plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry the dynamic surface topography induced by plume heads is bell shaped when the top of the plume head is at depths greater than about 0 1 plume head radii as the plume head approaches the surface and spreads the dynamic topography becomes plateau shaped the largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical and the surface expression of these stresses is likely to be dominated by radial extension as the plume spreads compressional stresses on the surface are produced beyond the edges of the plume consequently extensional features will be produced above the plume head and may be surrounded by a ring of compressional features

the geoid effect of compensated topography and uncompensated oceanic trenches the geoid is becoming increasingly important in interpretation of global tectonics most of the topography of the earth is isostatically compensated so removal of its effect from the geoid is appropriate before tectonic modeling the oceanic trenches however are dynamically depressed features and cannot be isostatically compensated in the classical way continental topography compensated at 35 km gives intracontinental geoidal undulations of up to 15 m over mountain ranges in a spherical harmonic expansion to order and degree 22 oceanic topography compensated at 40 km reasonable for the thermally supported long wavelengths matches the 10 m difference between old continents and old oceans in a detailed nasa gsfc geoid removing the assumed compensation for the oceanic trenches leaves negative anomalies of up to 9 m amplitude caused by their uncompensated mass deficit this mass deficit acts as a partial regional compensation for the excess mass of the subducting slabs and partly explains why geoidal and gravity anomalies over the cold slabs are less than thermal models predict

upper mantle origin of the yellowstone hotspot fundamental features of the geology and tectonic setting of the northeast propagating yellowstone hotspot are not explained by a simple deep mantle plume hypothesis and within that framework must be attributed to coincidence or be explained by auxiliary hypotheses these features include the persistence of basaltic magmatism along the hotspot track the origin of the hotspot during a regional middle miocene tectonic reorganization a similar and coeval zone of northwestward magmatic propagation the occurrence of both zones of magmatic propagation along a first order tectonic boundary and control of the hotspot track by preexisting structures seismic imaging provides no evidence for and several contraindications of a vertically extensive plume like structure beneath yellowstone or a broad trailing plume head beneath the eastern snake river plain the high helium isotope ratios observed at yellowstone and other hotspots are commonly assumed to arise from the lower mantle but upper mantle processes can explain the observations the available evidence thus renders an upper mantle origin for the yellowstone system the preferred model there is no evidence that the system extends deeper than 200 km and some evidence that it does not a model whereby the yellowstone system reflects feedback between upper mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep mantle plume hypothesis

antipodal hotspots and bipolar catastrophes were oceanic large body impacts the cause abstract one aspect of the hotspot distribution that has received little attention is its antipodal character of 45 primary hotspots found in most hotspot compilations 22 49 form antipodal pairs within observed hotspot drift limits 20 mm yr in addition the available ages or possible age ranges for both hotspots of an antipodal pair tend to be similar 10 myr difference or overlap monte carlo simulations indicate that the antipodal primary hotspots locations and ages are not due to chance at the 99 confidence level p

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

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