three dimensional mantle dynamics with an endothermic phase transition three dimensional convection for the spinel to perovskite phase change has been simulated numerically results for rayleigh ra numbers of 0 106 show intermittent layering with a strong robust plume rising through the phase boundary many descending instabilities are deflected but merging cold sheets come together at a junction a pool of cold material accumulates underneath in the phase transition zone a strong gravitational instability results which precipitates a rapid and massive discharge of upper mantle material

the role of partial melting in the 15 ma geochemical evolution of gran canaria a blob model for the canary hotspot the subaerial portion of gran canada canary islands was built by three cycles of volcanism a miocene cycle 8 5 15 ma a pliocene cycle 1 8 60 ma and a quaternary cycle 1 8 0 ma only the pliocene cycle is completely exposed on gran canaria the early stages of the miocene cycle are submarine and the quaternary cycle is still in its initial stages during the miocene sio2 saturation of the mafic volcanics decreased systematically from tholeiite to nephelinite for the pliocene cycle sio2 saturation increased and then decreased with decreasing age from nephelinite to tholeiite to nephelinite sio2 saturation increased from nephelinite to basanite and alkali basalt during the quaternary in each of these cycles increasing melt production rates sio2 saturation and concentrations of compatible elements and decreasing concentrations of some incompatible elements are consistent with increasing degrees of partial melting in the sequence melilite nephelinite to tholeiite the mafic volcanics from all three cycles were derived from co2 rich garnet lherzolite sources phlogopite ilmenite sulfide and a phase with high partition coefficients for the light rare earth elements lree u th pb nb and zr possibly zircon were residual during melting to form the miocene nephelinites through tholeiites phlogopite ilmenite and sulfide were residual in the source of the pliocene quaternary nephelinites through alkali basalts highly incompatible element ratios e g nb u pb ce k u nb pb ba rb zr hf la nb ba th rb nb k nb zr nb th nb th la and ba la exhibit extreme variations in many cases larger than those reported for all other ocean island basalts but these ratios correlate well with degree of melting survival of residual phases at higher degrees of melting during the miocene cycle and differences between major and trace element concentrations and melt production rates between the miocene and pliocene tholeiites suggest that the miocene source was more fertile than the pliocene quaternary source s we propose a blob model to explain the multi cycle evolution of canary volcanoes and the temporal variations in chemistry and melt production within cycles each cycle of volcanism represents decompression melting of a discrete blob of plume material small degree nephelinitic and basanitic melts are derived from the cooler margins of the blobs whereas the larger degree tholeiitic and alkali basaltic melts are derived from the hotter centers of the blobs the symmetrical sequence of mafic volcanism for a cycle from highly undersaturated to saturated to highly undersaturated compositions reflects melting of the blob during its ascent beneath an island in the sequence upper margin corelower margin volcanic hiatuses between cycles and within cycles represent periods when residual blob or cooler entrained shallow mantle material fill the melting zone beneath an island

spectral analysis of volcanic glass chemistry along the east pacific rise 13 23 s an analysis is presented for the variation in chemistry of 162 normal mid ocean ridge basalt glass groups collected with an average spacing of about 10 km along a 1100 km section of the east pacific rise between 13 and 23 s long wavelength periodicities have been determined for oxides al2o3 cao cr2o3 feo k2o mgo mno na2o nio p2o5 sio2 tio2 for ratios feo mgo k p and k ti for feo na2o and sio2 corrected for fractionation to 8 0 wt mgo and for bathymetry the data were modeled as the sum of a long wavelength periodic component and a residual random component the power spectrum of the periodic component was determined using the maximum entropy method the spectral phase of significant spectral peaks was determined by cross correlation with a cosine wave with the wavelength of the respective peaks in most cases the characteristics of the random component were found to be consistent with a normal distribution spatial aliasing considerations and the variance of the random component place confidence limits upon the spectral features the results of the spectral analyses indicate substantial correlation in the power spectra of the oxides the oxide data have spectral peaks with wavelengths in the vicinity of 575 and 225 km and a local minimum in spectral power at wavelengths between 300 and 500 km in general oxide components compatible in the crystallizing minerals olivine and plagioclase are approximately 180 out of phase with the incompatible oxides indicating that fractionation is an important process in controlling compositional variations along this portion of the east pacific rise unlike the fractionation sensitive oxide data periodic variations are not pronounced for k ti indicating that the length scales of mantle composition being melted are uncorrelated with those of magmatic temperature variation the analysis of fractionation corrected values of na8 0 and fe8 0 indicates that degree and depth of partial melting show a strong spectral peak near a wavelength about 430 km there is also significant power in the spectrum of na8 0 near 260 km and of fe8 0 near 200 km bounding the average spectral peak for the oxides at 225 km there appears to be strong coupling between the degree and depth of melting and magmatic temperature at two length scales around 225 km and 400 600 km corresponding to wavelengths of geoid undulations observed in the vicinity of the east pacific rise and suggesting a relationship with convection

mid miocene propagation of the yellowstone mantle plume head beneath the columbia river basalt source region columbia river flood basalt volcanism is the product of mid miocene distortion of the yellowstone mantle plume head against the precambrian margin of north america the overall migration pattern of columbia river basalt vents reflects northward propagation of the plume head beneath the west southwest migrating north american plate thermal uplift mimics the vent migration over the 1 5 m y of grande ronde basalt extrusion the projected crest of the plume head migrated 105 km 7 cm yr from the chief joseph dike swarm east oortheast into the clearwater embayment where uplift occurred over the same period at a rate of 0 67 mm yr chemical variations reflect progressive melting of the plume and mantle lithosphere followed by the mixing of these components with an enriched source from the overriding craton rapid thickening of the plume head against the cratonic margin led to decompressional melting and magma rise beneath a thin oceanic lithosphere of accreted terranes

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

the possible reflection of mantle discontinuities in pacific geoid and bathymetry geoid anomalies over the pacific plate show lineated undulations approximately oriented in the direction of absolute plate motion conventional spectral analyses have revealed a broad range of dominant wavelengths but as filtering is direction dependent results are difficult to interpret here we present a new approach designed to quantify the correlation between the geoid undulations and pacific plate motion we calculate the spatial correlation between geoid lineations and arbitrarily oriented axisymmetric sinusoidal undulations of given wavelength the pole i e the orientation of the sinusoid which maximizes the correlation is determined the distance between this pole and the pacific hotspot pole is calculated and represents a measure of how well a geoid lineation of a given wavelength aligns with pacific plate motion this distance varies with undulation wavelength and has discrete minima for several wavebands these minima moreover occur at wavelengths whose values are remarkably close to the depths of seismically inferred mantle discontinuities a similar analysis of available bathymetry corroborates these findings if these correlations are significant they suggest that geoid and bathymetry undulations are influenced by mantle structure and thus are likely to reflect mantle dynamics

lateral heterogeneity and azimuthal anisotropy of the upper mantle love and rayleigh waves 100 250 s the lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring rayleigh and love wave phase velocities in the period range 100 250 s spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 m 10 slow regions are evident at the east pacific rise northeast africa tibet tasman sea southwestern north america and triple junctions in the northern atlantic and indian oceans fast regions occur in australia western pacific and the southern atlantic these features are also found by a completely different analysis based on the backus gilbert method the backus gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000 km radius and the errors are about 1 of the phase velocity in the zeroth order spherically symmetric earth inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors however resolution analysis by the backus gilbert method shows that the current data do not have enough resolution for everywhere on the globe only a few regions currently have adequate azimuthal coverage thus confirmation of the above correlation requires a more complete data set

an analysis of the altimetric geoid in various wavebands in the central pacific ocean constraints on the origin of intraplate features abstract seasat profiles were filtered in three different wavebands and interpolated to elaborate maps of the very short 25 110 km and short 80 320 km and 280 820 km wavelengths of the geoid anomalies for the south pacific for the very short wavelengths we mapped the geoid roughness i e the variations of amplitude of the anomalies two lines following present day plate motions and separated by 800 1000 km were identified the first is formed by the tuamotu archipelago the pitcairn crough alignment and the easter sala y gomez chain the second comprises the cook austral islands a seamount cluster mapped close to the east pacific rise south of 30 s and the chile rise alignments with a n150 e direction and corresponding to traces of old thermal activity were mapped on lithosphere older than 40 m y these features are also spaced of 800 1000 km the map for the 80 320 km waveband shows a pattern of 200 km wavelength undulations several trends are superimposed but the n95 e direction is dominant in the area we associate these geoid undulations to lithospheric phenomena such as fracturations and or small magmatic ridges the 280 820 km waveband reveals linear anomalies roughly following the direction of present day absolute plate motions the linear anomalies have a 400 500 km wavelength and are nearly continuous between the pacific and the nazca plates the topography is located on the highs but the anomalies generally precede the active ends of the volcanic chains this result favours the hot line hypothesis previously suggested by several authors we associate these bands to thermal heterogeneities probably due to a convective regime since the main topographic features shall be emplaced over the ascending limbs of two neighbouring cells the 800 1000 km spacing of the topography imposes a minimum size of 400 500 km for the cells so the convective cells cannot be limited to a thin layer beneath the lithosphere but will affect part of the upper asthenosphere the existence of hot lines aligned parallel to plate motions can explain the intermittent magmatic activity observed for the volcanic lineaments of the south pacific volcanic activity can be the result of the interaction between the hot lines and lithospheric zones of weakness and or of instabilities in the convective cells the regular spacing observed for topography formed before and after 40 m y suggests that the size of the cells is rather constant through time

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

the effect of post pangea subduction on global mantle tomography and convection there is an excellent correlation between tomographic patterns and global tectonics in the upper 200 km of the mantle below this depth there is little relationship with present tectonic provinces most of the power in the tomography is in the longest wavelengths l 1 3 with l 2 generally dominant this is quite different from expectations based on mantle convection models the question then arises what causes the long wavelength variations mapped by seismic techniques we investigate the hypotheses that past locations of supercontinents and subduction may control temperature variations in the mantle we look for correlations between pangea and post pangeatic subduction and seismic velocity variations at various depths the best correlations with time integrated slab locations occur near the base of the uppermantle suggesting that slabs bottom out in the mesosphere the pangea hemisphere has a colder than average uppermantle probably due to circum and intra pangea subduction hotspots and ridges avoid regions cooled by subduction over the past 180 ma the presence of large continents at the surface and large areas of cold slab at depth explain the dominance of long wavelengths in the mantle s thermal structure despite the high rayleigh number

mantle temperature anomalies along the past and paleoaxes of the galapagos spreading center as inferred from gravity analyses to better understand the effects of hot spots on mid ocean ridge thermal structure we investigate the subsurface density structure of the galapagos spreading center and nearby lithosphere using shipboard gravity and bathymetry data we obtain maps of mantle bouguer anomalies mba by removing from the free air gravity the attractions of seafloor topography and a 6 km thick model crust comparison of observed and theoretical mba profiles along isochrons for ages 0 0 7 7 ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle this result is consistent with the notion that seafloor topography along the galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs along the ridge axis the mba decreases from the east and west toward the galapagos hot spot by 90 mgal reaching a minimum nearest the hot spot at 91 w seafloor topography mirrors the mba along axis increasing by 1 1 km toward the hot spot these variations in mba and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50 25 c toward the hot spot the predicted crustal thickening of 2 4 km nearest the hot spot accounts for 70 75 of the along axis mba and bathymetry anomalies mantle density variations account for the rest of the anomalies from the crustal isochron of age 7 7 ma to the present day axis the along isochron amplitudes of mba decrease from 150 to 90 mgal the corresponding along isochron bathymetry anomalies decrease from 1 7 to 1 1 km these observations along the paleoaxes of the galapagos spreading center indicate that the axial temperature anomaly was 70 hotter in the past 86 25 c and has steadily decreased to 50 25 c as the ridge axis migrated away from the galapagos hot spot these along isochron temperature anomalies however have remained well below that estimated for the hot spot itself 200 c indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10 20 times greater than that along the ridge axis over the past 7 7 m y

generation of long wavelength heterogeneity in the mantle by the dynamic interaction between plates and convection lateral variations in seismic velocity through its dependence on temperature can easily be generated at the gravest harmonics including degrees one and two by the dynamic interaction between plates and convection models of thermal convection with a single non subducting plate have been formulated in a cylindrical geometry plates of width one to four times times the thickness of the convecting region strongly modulate the flow by being pushed over cold downwellings and inhibiting cooling of the fluid beneath during rapid motion off of hot regions a large scale pattern of shear is developed causing small uprising limbs to be swept into the largest upwellings both insulation and plume plume collisions pump energy into the lower wavenumber harmonics

crustal structure of the tuamotu plateau 15 s and implications for its origin we investigate the sedimentary and volcanic structure of the tuamotu plateau with multichannel seismic seismic refraction and gravity data along a ship track crossing the plateau near 15 s the volcanic basement of the central portion of the plateau is capped with a 1 to 2 km thick sediment layer composed of two compositional sequences the uppermost sequence with semblance derived p wave velocities of 1 6 1 9 km s and thicknesses of 0 2 0 9 km is composed of pelagic sediments the underlying sequence with velocities 2 5 3 5 km s and thicknesses of 0 5 1 5 km is composed of limestone and volcaniclastic sediments sonobuoy refraction data show the upper 1 km of the volcanic basement to have velocities 4 5 5 5 km s the gravity data indicate that the platform is compensated by an elastic lithosphere with effective thickness 5 5 km and that the volcanic thickness is 9 10 km thicker than normal oceanic crust with a volume of 2 0 2 6 l06 km3 the inferred eruption rates of 0 1 0 13 km3 yr are comparable to those of the hawaiian and marquesas island chains but substantially less than those of many oceanic plateaus radiometric and paleontological ages for the plateau and geomagnetic dates of the surrounding seafloor indicate that the northwestern portion of the plateau formed 600 km off the axis of the paleo pacific farallon spreading center on lithosphere of age 10 20 ma linear volcanic ridges and scarps bounding deep sediment filled basins however are similar to features of oceanic plateaus which formed at or near accretionary plate boundaries we suggest that these volcanic ridges and the gross plateau like morphology were formed by magma that was channelled along the lithospheric discontinuities left behind by a southward propagating rift segment of the nearby spreading center we attribute the formation of the northwestern portion of the tuamotu plateau to the passage of two hotspots during times 50 30 ma as they migrated beneath the pacific plate but remained west of the tuamotu propagator

present and past regional ridge segmentation evidence in geoid data global geoid data from the geosat altimeter satellite and topography data have been analysed in the pacific and atlantic oceans to study present along axis and past off axis regional mid ocean ridge segmentation along the mid atlantic ridge east pacific rise and pacific antarctic ridge axial geoid and depth variations are positively correlated at length scale 800 1000 km respective peak to peak amplitudes are in the range of 0 4 0 8 m and 200 400 m axial variations are correlated with the position of large offset transform faults which coincide with geoid and topography lows triple junctions are also located in geoid and topography lows the observed admittance between geoid and depth is estimated to 2 0 5 m km a value indicative of a dynamic convective origin for the along axis regional variability off axis analysis shows a regional pattern of geoid topography anomalies elongated in the direction of spreading this pattern has also a length scale of 800 1000 km and possibly represents past regional segmentation large offset fracture zones coincide with off axis geoid topography lows

temperature fluctuation and thermodynamic properties in earth s lower mantle an application of the complete travel time equation of state abstract the complete travel time equation of state ct eos enables us to obtain all the thermodynamic parameters from travel time or velocity measurements of elastic waves at simultaneously high pressure and temperature here we formulate the adiabatic ct eos and apply it to the lateral heterogeneity of seismic wave velocity in the lower mantle we obtained internally consistent results for temperature thermal expansivity and the gruneisen constant in the lower mantle at the depth 2741 km just above the d layer the most likely results are 2500 k for temperature 1 0 10 5 k 1 for thermal expansivity and 1 0 for the gruneisen constant we also discuss the geophysical implication of the small thermal expansivity suggested in the deep lower mantle

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

large scale motion between pacific and atlantic hotspots studies of true polar wander tpw the rotation of the solid earth with respect to the spin axis1 have suggested that there has been 10 15 of relative motion over the past 130 myr refs 2 4 in such studies the orientation of the spin axis is recovered from continental palaeomagnetic poles corrected for relative plate motions and compared with a deep mantle reference frame defined by hotspot locations but deducing relative plate motions becomes increasingly difficult for older mesozoic time periods hindering tests of tpw on timescales comparable to those of large scale mantle convection moreover the assumption of hotspot fixity is controversial5 6 we examine here a more direct approach7 8 using palaeolatitudes derived from pacific guyots contrary to predictions from tpw models these data suggest only minor latitudinal shifts of pacific hotspots during the cretaceous period instead of tpw relative motion between the atlantic and pacific hotspot groups9 is required at a velocity of approximately 30 mm yr 1 more than 50 larger than previously proposed5

depth extent of cratons as inferred from tomographic studies we address the question of the depth extent of mantle high velocity zones under ancient cratons by using seismic velocity maps from recent mantle tomographic studies we divided old continents into two age provinces 800 1700 ma middle proterozoic and older than 1700 ma archean and early proterozoic the areas included in these age provinces are cross correlated with the tomography at different depths to quantify the global occurrence of high velocity anomaly hva extensions beneath cratons statistically significant hvas underlie the oldest cratons to a depth of 250 km there is no significant consistent correlation between tomography and younger cratons the older cratons have correlated hvas extending to depths varying from 100 to 450 km there is no significant difference in the average velocity anomaly beneath the early proterozoic part and the archean part of the canadian shield whereas the middle proterozoic part does not contain any hva the mantle under early precambrian crust is distinctly different from mantle elsewhere we propose two different hypotheses to explain these results 1 cratons of early proterozoic age or older contain ancient and deep physically attached roots or 2 these cratons contain a small permanent physical root that induces cold downwellings beneath it a root thickness of 200 km can be explained by an feo poor olivine rich mineralogy whereas a requirement of high strength probably demands in addition a volatile poor composition different root depths for cratons of similar age may be related to their location in the various supercontinents in which they were involved and to their subsequent deformational history or they may represent different snapshots in time of transient roots such as thermal boundary layers

three dimensional mantle convection beneath a segmented spreading center implications for along axis variations in crustal thickness and gravity segmentation and along axis variations within individual segments indicate the inherently three dimensional nature of mantle up welling and melting beneath oceanic spreading centers numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center the experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher viscosity mantle half space a periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt the relative magnitudes of the buoyant and plate driven components of mantle flow are controlled by the spreading rate and mantle viscosity with buoyant flow more important at lower spreading rates and viscosities buoyant flow beneath the spreading axis amplifies along axis variations in upwelling near a ridge transform intersection and distributes the variations along the entire spreading axis buoyant flow may thus be responsible for the more three dimensional character of slow spreading centers away from the spreading axis thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along axis wavelength controlled by the prescribed thickness of the asthenosphere however the position and stability of rolls are influenced by the segmentation geometry in cases where the spreading center geometry does not allow a stable configuration of rolls the flow is time dependent along axis variations in upwelling cause variations in melt production which imply large variations in crustal thickness that dominate the surface gravity signal the crustal thickness distributions implied by these numerical experiments produce bulls eye shaped negative mantle bouguer anomalies centered over spreading segments as observed at several spreading centers the amplitude of the anomaly increases with decreasing spreading rate

gravity study of the pitcairn easter hotline shipboard free air gravity and bathymetric anomalies with an extension of 400 km were identified across the pitcairn easter hotline in the south pacific the anomalies are associated with one of the positive geoid undulations observed in the area from satellite data several smaller topographic features volcano tectonic ridges oriented n65 e are superimposed on the topographic high admittance computations and direct modeling show that the swell topography is compensated by a low density zone within the lithosphere 4 to 8 km below the crust the volcano tectonic ridges are locally compensated in a classical airy sense the swell and the associated ridges were probably created by the action of a thermal anomaly resulting from the interaction of the easter island hotspot and of the easter microplate accretion centers

mantle dynamics constrained by degree 6 surface topography seismic tomography and geoid inference on the origin of the south pacific superswell abstract the relationship between global geophysical observables i e seismic tomography geoid and earth surface topography is investigated this study focuses on the high correlation at degree 6 which in addition to degree 2 characterizes these global fields surface topography is found to be highly correlated at degree 6 to seismic velocity anomalies between the 150 km and 450 km depths in the upper mantle several highs of the degree 6 topography coincide with particular provinces of the earth surface among others the south pacific superswell and the afar region these provinces overlie low velocity hot upper mantle by developing a global circulation model in a compressible radially viscosity stratified mantle we show that the degree 6 global fields can be explained as the result of the mantle convective flow driven by density anomalies the viscosity structure that best fits the degree 6 observables also explains the low degree 2 and 3 components of the dynamical topography and geoid the best fit model has a viscosity increase of factor 30 at the upper mantle lower mantle discontinuity and a low viscosity layer extending from the 100 to the 400 km depth in the upper mantle with a viscosity drop of factor 30 with respect to the transition zone these values are in good agreement with most previously published results based on the low degree geoid alone from this study we infer that the south pacific superswell which lies over a broad zone of hot upper mantle extending down to the 450 km depth is generated by the degree 6 flow of the mantle circulation

pacific plate reconstructions and uncertainties abstract the pacific plate is positioned into the hotspot reference frame first directly relative to the pacific hotspots and then indirectly relative to the african hotspots by using seafloor spreading data the reconstructions are repeated for 10 15 37 48 and 61 ma the errors associated with each link for both approaches are expressed by covariance matrices which can be combined to yield the overall error in the case of the relative plate motion circuit assuming the hotspots are fixed and that the reconstructions record the true relative positions of the plates the reconstructions should be equivalent within their uncertainties in fact however there is agreement within the uncertainties only for 10 ma with the misfit increasing with age for the earlier reconstructions these misfits can he accounted for by three alternative explanations large hotspot location uncertainties a shift in time of the pacific group of hotspots relative to the african group or an additional plate boundary in the relative plate motion circuit which we assume to be in antarctica by examining each of these possibilities in turn we conclude that the uncertainties in individual hotspot locations required to bring the reconstructions into agreement are unreasonably large and that the data available at present do not allow us to distinguish which of the remaining two possibilities may be the primary source of reconstruction errors

evidence for diffuse extension of the pacific plate from pukapuka ridges and cross grain gravity lineations satellite altimeter measurements of marine gravity reveal 100 to 200 km wavelength lineations over a wide area of the pacific plate oriented roughly in the direction of absolute plate motion at least three mechanisms have been proposed for their origin small scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere diffuse n s extension of the lithosphere resulting in lineated zones of extension boudins and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism recently several chains of linear volcanic ridges have been found to be associated with the gravity lineations following ridgelike gravity signatures apparent in high resolution geosat gravity measurements we surveyed a series of volcanic ridges that extend northwest from the east pacific rise flank for 2600 km onto 40 ma seafloor our survey data as well as radiometric dates on samples we collected from the ridges provide tight constraints on their origin 1 individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion 2 the ridges formed along a band 50 to 70 km wide in the trough of one of the more prominent gravity lineations 3 radiometric dates of the largest ridges show no hotspot age progression moreover the directions predicted for minihotspot traces older than 24 ma do not match observed directions of either the gravity lineations or the ridges based on this last observation we reject the minihotspot model the occurrence of the ridges in the trough of the gravity lineation is incompatible with the small scale convection model which would predict increased volcanism above the convective upwelling we favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere however the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model perhaps the fault scarps are buried under more than 30 m of sediments and or covered by the elongated ridges finally we note that if ridge push force is much smaller than trench pull force then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

new observations on mid plate volcanism and the tectonic history of the pacific plate tahiti to easter microplate abstract we describe the geology and tectonics of a continuous swathe of seafloor between tahiti and the western edge of the easter microplate imaged by gloria and sea beam on two separate cruise transits in 1987 and 1988 the data reveal that mid plate volcanism is common in this region even on deep seafloor hundreds of kilometres from major lines of seamounts and islands this supports the idea of a thin weak lithosphere over the pacific superswell and the idea that the tops of major mantle plumes may spread out over diameters of the order of 1000 km the mid plate volcanism occurs in two distinct forms over most of our traverse it appears as fields of relatively young and acoustically strongly backscattering lava flows often accompanied by groups of numerous small circular volcanoes east of 122 w about chron 5a however we observed a distinct form major sharp crested constructional volcanic ridges many tens of kilometres long individually trending ene but lying en echelon along an e w regional trend these ridges appear morphologically identical to the cross grain ridges seen elsewhere in the pacific we attribute their formation to magma supplied from the regionally hot mantle leaking along tectonic lines of weakness however although these ridges are parallel to fracture zone trends seen farther west they are morphologically very different from any known fracture zone moreover individual ridges are somewhat oblique to the tectonic spreading fabric around them and so do not seem to follow actual fracture zone traces the whole line of en echelon ridges lies along part of the predicted trace of fracture zone 2 of okal and cazenave 15 and is probably its morphological expression however nowhere did we see a convincing conventional fracture zone trace in or following the predicted position or orientation we suggest instead that magma from an independent source has used lines of weakness along minor fracture zones to produce these en echelon features the austral fracture zone is the only major fracture zone crossed in our transit and here is characterised by four fossil transform strands its marked position on the aapg and gebco maps is found to be in error finally we found that the expected change from nnw to nne trending spreading fabric at chron 6c did not occur in a clear cut way as predicted by earlier tectonic histories of the pacific instead the post chron 6c fabric oscillates in a confused way between nne and nnw suggesting to us that this area has been characterised by an unstable plate boundary probably associated with a succession of propagating rifts or microplates from chron 6c to the present

numerical modeling of chemically buoyant mantle plumes at spreading ridges the geometry of spreading of plumes beneath midoceanic ridges is investigated by three dimensional numerical modeling with the goal of characterizing the width of the plume along the ridge or the waist width chemically buoyant plumes are modeled in order to compare to previously reported laboratory tank experiments the plume is generated near the bottom of the box and rises and forms a mushroom head with some entrainment of surrounding fluid the head flattens at a considerable depth beneath the ridge before rising to the surface once the plume reaches the surface the head is quickly divided by the diverging plates and the waist width is found to have reached a steady state value w the results show that w is proportional to the square root of the volumetric flux of the plume divided by the diverging plate speed which is consistent with previously reported experimental data our scaling law gives an independent method for estimating the volumetric flux of mantle plumes the calculated plume fluxes are two to four times larger than previous estimates if plume buoyancy is purely of thermal origin then excess temperatures can be estimated from the fluxes for iceland azores and the galapagos the calculated excess temperatures are 140 57 and 51 c respectively in agreement with recent independent estimates from modeling of gravity and bathymetry

seismicity of the gorda plate structure of the continental margin and an eastward jump of the mendocino triple junction analysis of 9 years of data from the humboldt bay seismic network sheds new light on the structure and evolution of the gorda plate and mendocino triple junction significant findings include the buttressing effect of the pacific plate which demonstrates that there is no underthrusting of the gorda plate along the mendocino fault and the pattern of left lateral northeast trending faults which demonstrates how the gorda plate accommodates n s shortening without such underthrusting focal mechanisms are consistent with northward compression of the gorda plate by the pacific plate until the gorda plate passes the triple junction beyond which the n s compressive stress is effectively removed and the focal mechanisms show a change from strike slip to normal faulting with downslab tension another significant feature is the shallow benioff zone 10 40 km which shows a double seismogenic layer unlike most other double seismic layers in deep subduction zones this one appears to be due to underplating with a new subduction zone being developed east of the former one a corner of continental margin material has been partially subducted we attribute this double seismic zone to reactivation of both the old and new subduction boundaries under n s compression produced by pacific gorda plate interaction at the triple junction because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the san andreas fault has migrated eastward we have constructed a model that accounts for both the eastward migration of the san andreas fault and the doubling of the seismic zone in the gorda plate this can be done by assuming that at the time of the last eastward jump of the fault the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the san andreas fault other scenarios for the evolution of the mendocino triple junction do not require that the subduction zone migrate eastward however the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place this is consistent with other evidence for an eastward jump of the san andreas fault

effects of multiple phase transitions in a three dimensional spherical model of convection in earth s mantle numerical models of mantle convection that incorporate the major mantle phase changes of the transition zone reveal an inherently three dimensional flow pattern with cylindrical features and linear features that behave differently in their ability to penetrate the 670 km discontinuity the dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670 km depth resulting in frequent avalanches of upper mantle material into the lower mantle the effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670 km phase change resulting in smaller and more frequent avalanches and a wider range of morphologies large quantities of avalanched cold material accumulate above the coremantle boundary cmb resulting in a region of strongly depressed mean temperature at the base of the mantle the 670 km phase change has a strong effect on the temperature field with three distinct regions being visible 1 the upper mantle containing linear downwellings and pools of cold material in the transition zone and characterized by a high amplitude long wavelength spectrum 2 the midmantle containing quasi cylindrical avalanche conduits and characterized by a low amplitude broad spectrum and 3 the deep mantle containing large pools of cold avalanched material and characterized by a high amplitude ultra red i e long wavelength spectrum the effect on the velocity field is very different flow penetration across the 670 km phase change is strongly wavelength dependent with easy penetration at long wavelengths but strong inhibition at short wavelengths thus when comparing numerical models with long wavelength seismic tomography diagnostics based on the density field such as the radial correlation function are much more sensitive to the effects of phase transitions than those based on the velocity field the amplitude of the geoid is not significantly affected by the partial layering because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanced by the contribution from deflection of the 670 km discontinuity avalanches are associated with geoid lows however a more complex viscosity structure is required to correctly match the sign of the geoid over slabs in earth

the scales of mantle convection seismic topographic and gravity data show that there are two important scales of mantle convection these are associated with spherical harmonic degrees l 2 and 6 the l 2 pattern corresponds to the pattern of subduction cooling since the breakup of pangea most hotspots and ridges occur in the half of the globe unaffected by this cooling and all large igneous provinces were generated over this part of the mantle the l 2 distribution of upwellings and downwellings is likely to be a long lived feature of the earth cold regions of the mantle repeatedly attract continents and subduction reinforces the coldness an l 1 pattern of convection is probably related to supercontinents and their breakup degree 6 convection shows up in the spectrum of hotspots and upper mantle tomography and in the correlation of tomography and topography with the geoid cratons with their deep cold keels control the l 6 pattern and may even cause it by their role in establishing lateral temperature gradients and relief at the top of the convecting mantle subduction zones reinforce the craton pattern downwellings preferentially occur under cratons upwellings and hotspots occur at complementary locations supercontinents and their associated subduction zones l 1 constantly assemble and reassemble in the african atlantic hemisphere and the continental fragments in the dispersed state periodically settle into the polar band of geoid lows l 2 and 6 that now includes the americas antarctica australia and india if cratons control the l 6 pattern of convection and many patterns of l 6 are possible e g sectoral zonal checker board then upper mantle convection may reorganize roughly every 30 ma as cratons move about the hotspot spectrum is probably related to lithospheric extension as well as to broad upwellings a third and smaller scale of convection order 400 1000 km in dimension is just below the resolution of global tomography but shows up in the gravity field geoid and topography this scale is controlled by the depth of an endothermic phase change which tends to stratify mantle convection and the thickness of the upper mantle low viscosity zone convective domains of this dimension are also implied by the scales of chemical homogeneity lengths of rifts ridges and seamount chains fracture zone spacing and mid ocean ridge segmentation the temperature geochemistry and fertility of these upper mantle scale domains is controlled by their previous history of subduction continental insulation or refrigeration and processing by ridges fertile or volatile rich or hot cells can be mistaken for plumes hotspot swells are typically of the dimension that we argue is a characteristic upper mantle scale rather than a deep mantle plume scale geochemical domains of various sizes exist in the upper mantle they are broad scale features rather than point sources as in plume theories lithospheric dynamics and geometric focusing not mantle dynamics control the dimensions of so called hotspot eruptives

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

mgsio3 fesio3 al2o3 in the earth s lower mantle perovskite and garnet at 1200 km depth natural pyroxene and garnet starting materials are used to study the effects of joint fe and al substitution into mgsio3 perovskite at 50 gpa garnet is found to coexist with perovskite in samples containing both fe and al to pressures occurring deep into the lower mantle 1200 km depth the volume of the perovskite unit cell is vo a3 162 59 5 95xfesio3 10 80xal2o3 with aluminum causing a significant increase in the distortion from the ideal cubic cell on the basis of a proposed extension of the mgsio3 al2o3 high pressure phase diagram toward fesio3 fe is shown to partition preferentially into the garnet phase the stability of garnet deep into the lower mantle may hinder the penetration of subducted slabs below the transition zone

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

periodic hotspot distribution and small scale convection in the upper mantle hotspots are commonly attributed to mantle plumes and to a first order the hotspot distribution is correlated with long wavelength geoid anomalies and with higher temperatures in the lower mantle suggesting that the plumes are generated in the lower mantle i show that the second order distribution is periodic with a typical spacing of 800 1000 km such a spacing would be too narrow for plumes to ascend directly from the lower mantle but is close to the thickness of the upper mantle the alignment of convection cells in the upper mantle seems to control the periodicity to reconcile the first a and second order distributions assuming limited mass fluxes across the 670 km boundary a lateral deflection of plumes as they breach the transition zone from below is required

glacial geologic confirmation of an intraplate boundary in the parana basin of brazil the predrift fit of south america and africa remains problematic given the inability to match all parts of the coastlines simultaneously early opening of the south atlantic ocean may have been facilitated by intraplate deformation in either south america or africa but the location of such deformation is controversial and poorly constrained broad scale remote sensing has suggested an intraplate boundary in south america stretching from the cochabamba bend in the andes to the rio grande walvis rise but the inferred boundary crosses the paranaa basin of brazil where any evidence of deformation is obscured by late jurassic flood basalts a subsurface study of the upper paleozoic glaciogenic infill of the paranaa basin the 1300 m thick itarare group ca 300 260 ma identifies repeated episodes of intracratonic rifting on either side of an accommodation zone crossing the basin along the trend of the inferred intraplate boundary renewed influence of this zone during the opening of the south atlantic is suggested by the asymmetric distribution of successive magma units within the serra geral lava pile and by the trend of dike swarms the intraplate boundary identified within the paranaa basin may extend westward along the northern margin of the chaco basin in bolivia

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

the upper truncated power law applied to earthquake cumulative frequency magnitude distributions evidence for a time independent scaling parameter earthquake cumulative frequency magnitude cfm distributions are described by the gutenberg richter power law where the exponent is the b value although it has been reported that the b value changes before large earthquakes we find that an upper truncated power law applied to earthquake cfm distributions yields a time independent scaling parameter herein called the value we analyze earthquakes associated with subduction of the nazca plate beneath south america and find a region of isolated seismic activity at depths greater than 500 km between 20 s and 30 s for the entire record 1973 2000 the earthquake cfm distribution in this isolated region is well described by a gutenberg richter power law with b 0 58 the data set includes several large seismic events with m b 6 8 a power law applied to the cfm distributions for short time intervals between the large events yields b values greater than the b value for the entire record cfm distributions for the short time intervals are better described by an upper truncated power law than by a power law the value determined by applying an upper truncated power law to the short time intervals is equal to the b value obtained by applying the gutenberg richter power law to the entire record temporal changes in b value are due to temporal fluctuations in maximum magnitude analysis of four flinn engdahl regions two in subduction zones and two along spreading ridges demonstrates wider applicability of the results the value is an unchanging characteristic of the system that may be determined from a short term record manuscript received 2 july 2001

intermediate wavelength 400 600 km south pacific geoidal undulations their relationship to linear volcanic chains abstract intermediate wavelength 400 600 km geoidal undulations observed in seasat data from the south pacific appear to be 1 much more pronounced and extensive than the short wavelengh lineations of haxby and weissel which start and are confined to within 1500 km of the epr 2 copolar aligned roughly parallel to the absolute motion of the pacific plate 3 continuous across transform faults or fracture zones and 4 surmounted at their crests in places by linear volcanic seamount chains formed since 43 ma whereas the short wavelength lineations could be caused by shallow gravitational instabilities forming in the uppermost mantle just below the base of the lithosphere we think that the intermediate wavelength lineations result from dynamic elastic deformation of the lithosphere caused by well developed secondary convection rolls extending much deeper into the upper mantle because individual hotspot volcanic chains invariably coincide with crestal highs of discrete positive geoid lineations the location of the hotspots is probably controlled by the rolls

temperature and chemistry of the core mantle boundary seismic anomaly structures at the bottom of the lower mantle may be interpreted in terms of large temperature variations based on new laboratory data at very high pressure 1 sound velocity measurements at mantle pressures show an increase in ln ln p with depth this and the strong decrease in the thermal expansion coefficient yield thermal anomalies t of 1000 k using the seismically measured lateral velocity variations 2 high melting gradients in fe and fe o compounds measured at core pressures yield a temperature increase across the core mantle boundary in excess of 1300 k 3 melting temperatures of mg fe sio3 perovskite measured to deep mantle conditions extrapolate to 7500 k at the bottom of the mantle and result in ttm values between 0 3 and 0 4 this may strongly decrease the temperature dependence of viscosity if the average temperature profile of the lower mantle is nearly adiabatic and if the viscosity systematics found at low pressure are applicable at lower mantle conditions and 4 for dry conditions mgsio3 in contrast to mgo does not react with molten iron at pressures of the core mantle boundary this suggests that chemical interactions between the mantle and the core play a minor role

geoid lineations of 1000 km wavelength over the central pacific altimeter profiles of the ers 1 and topex poseidon satellites have been used to compute a geoid surface from which we have extracted medium wavelength geoid anomalies over the central pacific in this region the geoid shows prominent elongated anomalies of 20 40 cm in amplitude with a spacing of 1000 km and oriented n60 w in the south central pacific the polynesian hotspot swells seem to be located on the linear geoid highs however the latter extend much farther eastward preceeding the active hotspots to the north other geoid lines are visible but they do not coincide with known tectonic features the wavelet transform applied to raw geoid data clearly detects a strong signal at the 1000 1200 km wavelength the amplitude of the lineations increases with age by a factor of 3 between 10 and 50 my analysis of seafloor topography corrected for age and sediments reveals topography anomalies correlated with the geoid lineations admittance and coherence calculations give high coherence 0 9 in the 1000 1200 km waveband and admittance values of 1 5 m km at 10 my and 3 m km at 60 my the mechanism producing the lineations is unclear their characteristics however are not incompatible with a convective origin

evidence for age and evolution of corner seamounts and great meteor seamount chain from multibeam bathymetry the comer seamounts in the western north atlantic and great meteor seamount chain in the eastern north atlantic are thought to progress in age from late cretaceous through late cenozoic they both presumably formed by volcanism above the new england hotspot when first the north american plate and then the mid atlantic ridge axis and african plate moved over the hotspot high resolution multibeam bathymetry of the seamounts shows geomorphic features such as guyots terraces and a base level plateau cruiser plateau that we interpret to have formed at sea level we have backtracked these features to sea level along the north atlantic crustal age depth curve in order to estimate their ages the derived age pattern of volcanism indicates formation of the comer seamounts at ca 80 ma to 76 ma with migration of the mid atlantic ridge plate boundary over the hotspot and formation of the cruiser plateau about 76 ma seamount ages suggest that subsequent volcanism on the african plate moved first northward in the late cretaceous to early cenozoic plato tyro and atlantis seamount groups then southward to great meteor seamount in the late cenozoic recurrent volcanism appears to have occurred at some seamounts up to 20 30 m y after their initial passage over the hotspot it would thus appear that intralithospheric conduits can link the hotspot to old seamounts several hundred kilometers away

westward migration of pacific ocean upper mantle into the southern ocean region between australia and antarctica the southern ocean south of the great australian bight hosts a globally anomalous region of mid ocean ridge the australian antarctic discordance in addition to its recognized morphological and geophysical anomalies the discordance represents the current on axis location of a proposed isotopic boundary between indian ocean and pacific ocean upper mantle convective regimes previously defined from analyses of 4 ma mid ocean ridge basalts morb pb sr and nd isotope analyses of 36 66 ma seafloor dredged from either side of the northward extrapolation of the australian antarctic discordance show that 36 ma seafloor to the east has an indian ocean morb isotopic signature this indicates that the indian pacific isotopic boundary does not extend directly north of the ridge toward the southern margin of australia progressive westward migration of an arcuate shaped front of pacific ocean upper mantle therefore appears to be a consequence of australian antarctic rifting and southern ocean opening suggesting that the current location of the isotopic boundary within the discordance may be entirely coincidental

long lived mid ocean ridge segmentation of the pacific antarctic ridge and the southeast indian ridge the pacific antarctic and the southeast indian ridges are examined for evidence of long lived tens of millions of years tectonic segmentation of the mid ocean ridge by examining a variety of geophysical parameters similar to those previously documented in the south atlantic kane and hayes 1992 these parameters include subsidence rate of the oceanic crust individual flanks and asymmetry zero age depth and geoid height decrease with age geoid rate the variability in these parameters along strike of the mid ocean ridge is systematic and serves to define a large scale ridge segmentation of the order of hundreds of kilometers the southeast indian ridge exhibits long lived segmentation in all parameters examined and is subdivided into five flow line corridors with boundaries between corridors occurring at or near major fracture zones while the pacific antarctic ridge exhibits conspicuous asymmetric crustal subsidence evidence of segmentation exists primarily in the along strike geoid rate the observations in these two areas are found to be internally inconsistent with simple thermal conduction models for oceanic crust strongly suggesting that factors other than cooling and isostasy are influencing the creation and subsequent modification of the oceanic crust and lithosphere

insights into spatial sensitivities of ice mass response to environmental change from the searise ice sheet modeling project i antarctica atmospheric oceanic and subglacial forcing scenarios from the sea level response to ice sheet evolution searise project are applied to six three dimensional thermomechanical ice sheet models to assess antarctic ice sheet sensitivity over a 500 year timescale and to inform future modeling and field studies results indicate i growth with warming except within low latitude basins where inland thickening is outpaced by marginal thinning ii mass loss with enhanced sliding with basins dominated by high driving stresses affected more than basins with low surface slope streaming ice and iii mass loss with enhanced ice shelf melting with changes in west antarctica dominating the signal due to its marine setting and extensive ice shelves cf minimal impact in the terre adelie george v oates and victoria land region of east antarctica ice loss due to dynamic changes associated with enhanced sliding and or sub shelf melting exceeds the gain due to increased precipitation furthermore differences in results between and within basins as well as the controlling impact of sub shelf melting on ice dynamics highlight the need for improved understanding of basal conditions grounding zone processes ocean ice interactions and the numerical representation of all three key points sensitivity study of antarctica to atmospheric oceanic and subglacial forcings different sectors of antarctica are vulnerable to the forcings atmospheric forcing lead to a growth but dynamic forcing lead to a mass loss 2013 american geophysical union all rights reserved

on the effect of continents on mantle convection at the earth s surface continents and oceans impose different thermal boundary conditions at the top of the mantle laboratory experiments are used to investigate the consequences of this for mantle convection the upper boundary of the experimental tank was made of copper plates enforcing a fixed temperature and had a conductive lid of finite width in the middle beneath this lid the thermal boundary condition was of the mixed type with a biot number depending on the dimensions and thermal conductivity of the lid experimental values of the biot number were scaled to earth values experiments were run for a large range of rayleigh numbers from 104 to 107 and for several lid widths the effects of temperature dependent viscosity and of the shape of the lid were investigated at steady state in all cases there is an upwelling beneath the conductive lid which feeds two symmetrical and elongated convective cells three different dynamic regimes were identified as a function of rayleigh number independently of the lid width at rayleigh numbers lower than 1 2 105 the upwelling is steady both in geometry and temperature structure at rayleigh numbers between 1 2 105 and 2 106 this central upwelling is intermittent at larger values of the rayleigh number there is no longer a simple upwelling structure but a set of small plumes rising together and distorted by a cellular circulation of large horizontal extent thus the conductive lid always imposes a large scale flow pattern the length of these convective cells is a function of lid width it is equal to the lid width at large values and decreases to the rayleigh benard value as the lid width decreases to zero a fluid loop model explains the most important features of this form of convection the cell length is such that the upwelling temperature is minimized for a given rayleigh number and lid width and is an increasing function of lid width and a decreasing function of rayleigh number transient experiments demonstrate that the large scale flow structure develops rapidly with even small horizontal temperature differences implications for the earth are that large scale convection cells exist in conditions which in the absence of continents would probably lead to a chaotic convection pattern dominated by plumes at high rayleigh number continental breakup is effected by a large scale line upwelling structure which includes a number of individual plumes

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

aspherical heterogeneity of the mantle from phase velocities of mantle waves long period surface waves are used to map the lateral heterogeneity of the upper 100 600 km of the mantle there is good correlation of velocity with surface tectonics and heat flow convergence regions are generally slow for love waves and fast for rayleigh waves back arc basins have slower than average shallow mantle some island arcs show evidence of fast material at greater depth deep seated slow anomalies underlie the red sea afar region of north east africa western north american northern east pacific rise indian ocean triple junction and the tasman sea campbell plateau regions the fastest regions are in the north central australia new guinea and the south atlantic

paleomagnetic tests of pacific plate reconstructions and implications for motion between hotspots the plate motion circuit through the south pacific and antarctica is shown to fail paleomagnetic tests of consistency these failures imply that reconstructions of pacific basin plates relative to surrounding plates inferred from this circuit are systematically in error and that estimates of motion between hotspots inferred from this circuit are probably too large therefore the motions between hotspots remain poorly known and may be much smaller than previously estimated

the northern nevada rift regional tectono magmatic relations and middle miocene stress direction as defined by the most recent aeromagnetic surveys the north northwest trending northern nevada rift zone extends for at least 500 km from southern nevada to the oregon nevada border at several places along the rift the magnetic anomaly is clearly related to north northwest trending dikes and flows that based on new radiometric dating erupted between 17 and 14 ma and probably during an even shorter time interval the tectonic significance of the rift is dramatized by its length its coincidence in time and space at its northern terminus with the oldest silicic caldera complex along the yellowstone hot spot trend and its parallelism with the subduction zone along the north american coast prior to the establishment of the san andreas fault the northern nevada rift is also equivalent in age trend and composition to feeder dikes that fed the main eruptive pulse 95 volumetrically off the columbia river flood basalts in northern oregon 15 5 16 5 ma because of these similarities both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south southeast and north northwest respectively from a central mantle plume the site of the initial breaching of the north america plate by this plume is probably the mcdermitt volcanic center at the north end off the rift near the oregon nevada border the present north northwest trend of the rift and its internal elements such as dikes and lava filled grabens record the orientation of the arc normal extensional stress in this back arc region at the time of emplacement paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift the present north northwest trend of the rift reflects the state of stress in the basin and range during middle miocene time and is consistent with stress indicators of similar age throughout the basin and range and rio grande rift provinces