detection of mantle plumes in the lower mantle by diffraction tomography hawaii abstract a new method is proposed to detect mantle plumes in the lower mantle the method is akin to diffraction tomography and relies on the scattering of long period seismic body waves by nearly vertical heterogeneities the theory is described in a companion paper ying ji h c nataf earth planet sci lett this issue here we apply this method to an actual set of long period digital seismograms recorded on the world wide network of seismic stations between 1980 and 1994 we select seismograms that illuminate the lower mantle in a 20 20 region around hawaii we construct an image of plume like heterogeneities using a cell size of 1 1 by a lsqr inversion of the scattered waves this image shows a strong slow anomaly about 200 km northwest of hawaii although the image is somewhat noisy resolution tests indicate that this feature is fairly robust the amplitude of the anomaly is between 30 and 60 times larger than what we predict from a simple thermal plume model built with a 600 k maximum temperature excess and a gaussian horizontal cross section with an 1 e diameter of 250 km it is the first time that such a method is applied to the mantle while we think that the existence of this anomaly is real we do not know how to explain such a large amplitude if real this observation has a number of important geodynamical consequences it indicates that a mantle plume is indeed responsible for the hawaii hotspot as speculated by morgan w j morgan nature 230 1971 42 43 the plume is nearly vertical it originates from the d region at the base of the lower mantle the amplitude of the anomaly suggests that partial melt or and a chemical anomaly must be present the plume source is to the northwest of its surface expression as predicted by some models of plume advection in the mantle wind

low velocity structure beneath africa from forward modeling seismic waveforms observed in south africa containing the first arrival crossover of s to sks 70 to 110 are analyzed the data consist of analog records from the world wide seismographic station network wwssn of deep events beneath south america the s waves arrive 2 to 3 s early relative to prem at ranges from 70 to 95 and then become increasingly delayed becoming 5 to 6 s late at 110 the sks phase is late by 3 to 5 s over the entire range this pushes crossover between s and sks normally observed at about 81 out about 2 to 3 which is the most anomalous shift ever reported to model such features we modified grand s tomography model grand et al gsa today 7 1997 1 7 and generated 2d synthetics to match the data the overall shape and position of the lower mantle low velocity anomaly proposed by grand predicts good results if lower mantle anomalies are enhanced to a level of about 4 this results in a complex tabular structure extending upward from the core mantle boundary about 1500 km into the mantle these features appear to be consistent with a large young plume which is erupting off the cmb

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

strong seismic scatterers near the core mantle boundary west of mexico pkpphasesobservedinjapan showstrongpre cursors with clear onsets for earthquakes that occurred in south america the onset times of the pkp precursors can allbeexplainedbyexistenceofseismicscatterersinthelow ermost 100 kmof themantlewest of mexico withinan area of 200 km300 km forward modeling of pkp precursor amplitudes indicates that a p wave velocity variation of at least 6 is required to explain the large amplitudes of the observed pkp precursors suggesting that the core mantle boundary region beneath west of mexico is a highly anoma lous region characterized by strong seismic scatterers

seismic evidence for a mantle plume oceanwards of the kamchatka aleutian trench junction summary a non linear iterative p wave traveltime tomography has revealed a mantle plume originating at a depth of nearly 1000 km rising across the 600 km discontinuity and deflecting subhorizontally in the uppermost mantle presumably by shear flow due to the overlying moving plate data from the geophysical survey of russia 1955 1997 were inverted jointly with the catalogues of international seismological centre and usgs national earthquake information centre 1964 1998 the result shows a 300 500 kmwide cylindrical low velocity anomaly yx2 per cent that extends from a depth of greater than 900 km to shallower than 200 km the anomaly is almost vertical at depths up to y400 km and rises obliquely to the north up to y200 km under the ocean floor near the northern end of emperor seamounts above y300 km depth a subsidiary anomaly extends subhorizontally to the nw in fair agreement with the direction of movement of the pacific plate the overlying seafloor is characterized by anomalously high heat flow which may be attributed to the thermal effect of the mantle plume

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

lateral variations in compressional shear velocities at the base of the mantle observations of core diffracted p p diff and sh sh diff waves recorded by the missouri to massachusetts moma seismic array show that the ratio of compressional p seismic velocities to horizontal shear sh velocities at the base of the mantle changes abruptly from beneath the mid pacific vp vs 1 88 also the value predicted by reference earth models to beneath alaska vp vs 1 83 this change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin a textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern pacific paleotrench

plumes in a convecting mantle models and observations for individual hotspots the motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented a new list of 44 possible hotspots with associated tracks has been compiled for all of them calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow plume source depth has usually been assumed to be the top of d but an alternative source depth at the 670 km discontinuity has also been considered using models of relative plate motions and boundaries hotspot tracks on plates have been calculated and compared with age data ocean floor topography and distribution of volcanics on continents absolute plate motions have been redetermined under consideration of hotspot motion using a new least squares method for the hawaiian and yellowstone hotspots source locations and hotspot motion have been computed for a total of up to 23 different models the results show plume conduits being tilted with source regions at the d moving in the lowermost mantle flow generally toward large scale upwellings under southern africa and the south central pacific hotspot surface motion often represents the horizontal component of midmantle flow which is frequently opposite to plate motion toward ridges and away from subduction zones in particular almost all models tested predict southward motion of the hawaii and kerguelen hotspots and westward motion of the iceland hotspot for models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as or better than for the fixed hotspot model but sometimes fixed hotspots give the best fit in some cases where the track ends at a subduction zone e g for the bowie hotspot results can give indications about the otherwise unknown age of the hotspot in other cases especially for the tahiti hotspot results suggest an origin shallower than d and in yet other cases particularly east africa the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes

geographic correlation between hot spots and deep mantle lateral shear wave velocity gradients abstract hot spot volcanism may originate from the deep mantle in regions exhibiting the earth s most pronounced lateral s wave velocity gradients these strong gradient regions display an improved geographic correlation over s wave velocities to surface hot spot locations for the lowest velocities or strongest gradients occupying 10 of the surface area of the core mantle boundary cmb hot spots are nearly twice as likely to overlie the anomalous gradients if plumes arise in an isochemical lower mantle plume initiation should occur in the hottest thus lowest velocity regions or in the regions of strongest temperature gradients however if plume initiation occurs in the lowest velocity regions of the cmb lateral deflection of plumes or plume roots are required the average lateral deflections of hot spot root locations from the vertical of the presumed current hot spot location ranges from 300 to 900 km at the cmb for the 10 30 of the cmb covered by the most anomalous low s wave velocities the deep mantle may however contain strong temperature gradients or be compositionally heterogeneous with plume initiation in regions of strong lateral s wave velocity gradients as well as low s wave velocity regions if mantle plumes arise from strong gradient regions only half of the lateral deflection from plume root to hot spot surface location is required for the 10 30 of the cmb covered by the most anomalous strong lateral s wave velocities we find that strong gradient regions typically surround the large lower velocity regions in the base of the mantle which may indicate a possible chemical in addition to thermal component to these regions

three dimensional structure at the base of the mantle beneath the central pacific forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models a broad zone of low s velocity 4 percent with respect to standard models which may correspond to the base of a thermal plume narrows and is deflected as it extends to about 1000 kilometers above the core mantle boundary to the east of this zone a localized region of fast s velocity 5 percent suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages which may or may not involve core material its presence could also explain the observation of precursors to core reflected phases in this region

tomographic inversion of s sks times for shear velocity heterogeneity in d degree 12 and hybrid models differential travel times between s or diffracted s and sks were measured to study the global distribution of shear wave velocity heterogeneity in the lowermost 250 km of the mantle the d region commencing with 3000 s sks times with variable qualities we minimize uneven path coverage by thinning redundantly sampled regions e g the fiji tonga to north america corridor and collect additional data for sparsely sampled areas especially in the southern hemisphere about 1500 paths were retained distributed to reconcile both high density and homogeneous sampling we compare 1 spherical harmonic and 2 equal area block parameterizations of d shear velocity heterogeneity for identical minimum resolving lengths and mean model errors we show that the two parameterizations result in indistinguishable patterns of heterogeneity and power distribution for resolution down to a block size of 4 5 4 5 and maximum spherical harmonic degree of 40 we demonstrate synthetically that a high degree l inversion followed by a lower degree ls spherical harmonic synthesis effectively circumvents model contamination from expansion truncation effects tomographic inversion of this data set yields a global distribution of d heterogeneity robust up to degree 12 in our preferred model l 40 ls 12 rms 1 surface hotspots and estimated lower mantle plume roots are located in or at edges of low velocity regions vs 2 for both the atlantic indian low velocity corridor as well as the low velocities beneath the central pacific although the above parameterizations entail no intrinsic difference in resolvability higher resolution is regionally achieved by removing the derived degree 12 model from the data then inverting the corrected residuals using 4 5 4 5 blocks this hybrid method prevents power loss at intermediate to long wavelengths from otherwise severe damping and recovers smaller scale structure where constraints are better than the global average such as beneath the pacific and eurasia a synthetic recovery test yields resolvability maps that reflect both the path geometry and the quality of data which help to identify robust features in both the degree 12 and the hybrid d models

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

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

the structure of the core mantle boundary from diffracted waves diffracted p and s waves pd sd traveling around the core mantle boundary cmb of the earth give us information about the velocity structure and therefore the thermochemistry of d the base of the earth s mantle by examining pd and sdarrivals we determined the apparent ray parameter for different regions at the base of the mantle by comparing the data slownesses to those found from reflectivity synthetic seismograms we were able to quantify d average velocities using these averaged velocities with a thermochemical modeling of lower mantle minerals using a birch mumaghan equation of state we have been able to make chemical and physical inferences as to the causes of lateral variations at the cmb examinations found significant lateral heterogeneity at the base of the mantle amounting to 4 for both p and s velocities these velocities did not always vary in parallel and the poisson ratio varied regionally by almost 6 the most unusual region of the cmb found was under indonesia where velocities 3 slower than the preliminary reference earth models were found adjacent to a region of faster than average velocities these regions currently correspond to areas of core up welling and down welling respectively found by voorhies 1986 which if mostly held in place by core mantle coupling might cause a flux of heat and iron into the mantle making the anomaly both thermally and chemically derived at the cmb under the northern pacific rim the fastest shear velocities were found but the same region yielded slower than average p velocities while the presence of fast shear velocities here would support the idea that we are seeing the cold dregs of mantle convection perhaps continuing down from the north pacific subduction zones the presence of slow p velocities suggests additional complications our thermochemical modeling suggests that the d poisson ratio is very sensitive to variations in the silicate oxide ratio and that a decrease in the amount of perovskite relative to magnesiowustite may play an important role in this region

geochronology of pediment surfaces in southern peru implications for quaternary deformation of the andean forearc abstract the geomorphology of the andean forearc has historically been viewed as an old remnant of a late miocene planar landscape with no significant active structures accommodating quaternary deformation however the well preserved sequence of planation surfaces and strath terraces developed within the forearc of southern peru provide evidence of recent uplift along the western margin previously the abandonment of these surfaces was attributed to uplift of late miocene age however abrupt changes in topography and drainage incision within the pediment surfaces indicate recent deformation we use in situ produced 10be to determine the exposure ages of these abandoned surfaces in order to derive the spatial and temporal extent of this tectonic activity our new results in contrast to previous work yield pediment surface ages of 119 ka 203 ka 278 ka 549 ka and 1003 ka indicating a youthful morphology additionally these surfaces are affected by steeply dipping active faults producing localized deformation which can be quantified incision rates based on the exposure ages of abandoned strath surfaces are on the order of 0 04 0 3 mm yr the data set presented here not only suggests significant active deformation within the forearc but also highlights a sharp contrast between the style of deformation observed on the eastern and western margins of the andes while the general notion is that active deformation is localized in the subandean fold and thrust belt our data support an emerging view where active deformation is occurring in the western margin as well

the origin of large scale structure in mantle convection effects of plate motions and viscosity stratification convection in earth close quote s mantle is dominated by long wavelength structure as evidenced by the very open quote open quote red close quote close quote spectra of both seismic velocity heterogeneity in the deep mantle and the non hydrostatic gravity field or geoid here we show that this large scale structure may be a consequence of two factors that influence the scale of mantle convection first the existence of surface plates which tend to organize the flow second a substantial increase in lower mantle viscosity for which there is considerable independent geophysical evidence combining these two factors in 3 d spherical mantle convection models explains rather well the observed seismic spectrum of mantle heterogeneity copyright american geophysical union 1996

the excess temperature of plumes rising from the core mantle boundary we study the rise of conduit type thermal plumes from the core mantle boundary cmb in a simplified numerical model viscosity is a strong function of temperature and pressure assuming that all plumes start with the same excess temperature at the cmb we find that their temperature in the upper mantle is controlled by the plume flux and is insensitive to variations in viscosity or other material parameters or to the kind of flow law newtonian or non linear the reduction of excess temperature with height is stronger for weaker plumes plumes with a buoyancy flux of less than 1000 kg s should have cooled so much that they would not melt beneath old lithosphere the existence of such hot spots possibly indicates that not all plumes rise from the cmb

intense seismic scattering near the earth s core mantle boundary beneath the comoros hotspot i report strong seismic pkp precursors recorded in the seismic stations in tanzania for earthquakes occurred in the fiji subduction zone the observed pkp precursors show clear onsets and rapid variations of amplitude for different events and stations i develop a technique to use the precursor onset times to determine the location of seismic scatterers and apply forward modeling of precursor amplitudes to place constraints on the magnitude of seismic anomalies the travel time analysis indicates that these recorded pkp precursors are caused by the seismic scattering in the lowermost 360 km of the mantle beneath the comoros hotspot the amplitudes of the observed pkp precursors suggest a rather complex seismic structure with a p velocity variation of at least 8 likely associated with partial melt

shear wave speeds at the base of the mantle we inverted 4864 scs s and 1671 s diff sks residual travel times for shear wave speed anomalies at the base of the earth s mantle we applied ellipticity corrections accounted for mantle structure outside of the basal layer using mantle tomography models and employed finite size sensitivity kernels the basal layer thickness was set to 290 km however the data allow thicknesses between 200 and 500 km the residuals were inverted using a spherical harmonic basis set of degree 30 for a model that both is smooth and has a small euclidean norm which limited spectral leakage of higher order structures into low order wavelengths hotspots dominantly overlay slow wave speed regions nonsightings of ultralow velocity zones ulvzs most frequently appear in fast regions suggesting that slow regions at the base of the mantle are associated with ulvzs however ulvz sightings appear in both slow and fast regions recently active subduction zones do not correlate with velocity anomalies however the locations of subduction zones active prior to 90 ma correlate extremely well with fast anomalies implying that slabs descend as fast as 2 cm yr through the lower mantle the correlation continues through the historical subduction record to 180 ma suggesting that slabs remain in the deep mantle at least 90 myr fast anomalies reach 2 percent while slow anomalies extend to 5 percent if we assume that the anomalies are thermal and anharmonic in origin and apply a wave speed thermal anomaly conversion the temperature deviations would be over 500 k cold in fastest regions and over 1000 k hot in the slowest regions which would initiate plumes much hotter than those observed at the surface author

on the applicability of thermal conduction models to mid plate volcanism comments on a paper by gass et al the proposition that mid plate hot spot magmatism is strongly discouraged by a combination of thick lithosphere and rapid plate motion is examined empirically on a global basis and found to be inadequate although hot spots perhaps tend to avoid cratonic areas the nonrandom worldwide distribution of hot spots appears primarily to reflect the state of the sublithospheric mantle plate thickness or speed is of lesser importance the high level of african mid plate volcanism since 25 m y b p appears to be part of a global pattern controlled by events in the mantle rather than a result of the africa plate coming to rest over sublithospheric thermal sources about 40 m y b p

the fate of slabs inferred from seismic tomography and 130 million years of subduction the volume and location of subducted plate since 130 ma are reconstructed from magnetic anomalies and updated finite rotation parameters describing relative motions in the ocean basins and between plates and hotspots the area of subducted plate is calculated from the relative motions between overriding and overridden plates along the length of convergence in the hotspot reference frame in 5 ma intervals ridge locations are reconstructed by rotating magnetic anomalies from their present to their former positions the distances between trenches and ridges at a certain point of time in the spreading direction are thus estimated and are used to determine the ages of trenches according to the spreading history recorded in present day oceans the thickness of the oceanic plate is obtained from the age based on the half space cooling model about 3 45 10 10 km 3 of oceanic plate has been subducted during the past 130 ma with maximum accumulation beneath southeast eurasia at spherical harmonic degree l 2 where subduction flux peaks excellent correlation is found between four seismic tomographic models at the top of the lower mantle about 800 1100 km deep and predicted slab locations for some seismic models excellent correlation is also found at other degrees in that depth range some models also have good correlations with predicted slab locations in the deep mantle at l 2 direct comparison between subduction and seismic tomographic patterns at 800 1100 km shows that subduction history correlates with tomography very well in terms of location and amplitude of anomalies cold downwellings which may be related to slabs appear to be trapped in the mesosphere or middle mantle correlations between seismic tomography and subduction over different time periods support this conclusion there are also some correlations in the upper mantle and in the deep lower mantle although they are generally not as significant as those in the 800 1100 km depth range there may be a significant boundary in the mantle near the 800 1100 km depth from a geodynamic and chemical point of view the lower mantle may start at a depth closer to 1000 km than to 670 km

horizontal transition from fast to slow structures at the core mantle boundary south atlantic recent tomographic studies of the deep mantle have revealed large scale low velocity zones beneath africa and relatively fast velocity zones beneath south america here we conduct a concentrated core mantle boundary cmb study across this transition zone from fast to slow beneath the south atlantic deep south american earthquakes recorded in africa provide s and scs waveform data at core grazing distances of 85 105 the waveform interference pattern produced at these ranges provides an excellent sample of the cmb structure in the vicinity of the scs bounce point which can be used to map out regions of low velocity zones 10 reductions the strongest anomalies occur at the western edge roughly beneath the tristat plateau with weaker structures to the east beneath the so called great africa plume these results agree with dynamic models where strong local upwellings develop at the edges of d fast structures

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

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

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

p velocity structure of the core mantle boundary region inferred from pkp ab pkp bc differential travel times pkp ab pkp bc differential travel time residuals from the bulletins of the international seismological centre are inverted to retrieve the p velocity anomalies in the lowermost 300 km of the mantle we show that a realistic core mantle boundary cmb topography may have only a marginal contribution to the residuals the block parametrization used allows us to discard poorly sampled areas prior to a least squares inversion the main features are negative anomalies beneath eurasia and north america and a positive anomaly beneath the southwest pacific the inferred map is not consistent with other maps resulting from previous p and s velocity studies which are correlated at long wavelengths with each other the preferred explanation for this challenging inconsistency is the presence of a thin heterogeneous layer just above the cmb

the pacific plume as seen by s scs and sks the forward modeling of s sks and scs s differential travel time residuals sensitive to structure beneath a localized region of the central pacific suggests the presence of two anomalous domains first a broad zone which becomes thinner as it extends into the mid mantle where the s velocity is reduced by 2 to 4 and which is deflected to the south east as it rises towards the surface adjacent to it a smaller domain where the s velocity is about 4 higher these strong velocity anomalies are likely the results of chemical heterogeneity that may involve subducted material and partial melting strong heterogeneity appears to extend at least locally well above d but the morphology of the anomalous domains is more reminiscent of localized rising or sinking currents rather than of a relatively uniform and thick geochemical reservoir

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

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

zircon u pb ages in myanmar magmatic metamorphic events and the closure of a neo tethys ocean abstract previous isotopic age studies have concluded that the main metamorphism in the mogok metamorphic belt mmb in myanmar was tertiary and caused by collision with or underthrusting by india we present here results of zircon u pb age determinations on 18 samples from myanmar largely on intrusive rocks from the north trending 550 km long shan scarps segment of the sickle shaped mmb the oldest determination is 491 ma on orthogneiss north of mandalay interpreted as a protolith age and similar to the stratigraphic age of the bawdwin volcanics augen gneiss at kyaukse has a 114 ma protolith age our zircon u pb ages on undeformed intrusive rocks in the mmb range from cretaceous to miocene the oldest 128 ma on a diorite near yebokson implies an earliest cretaceous minimum age for regional metamorphism here younger ages on undeformed intrusions include 91 ma on diorite at mokpalin 72 ma on the weakly foliated garnet bearing nattaung granite 44 and 48 ma respectively on the mec and sedo granites and 17 20 ma on granite dykes for the 128 ma diorite sr and nd isotopes indicative of a continental crust component and mapped continuity of metamorphic host rocks suggest correlation with mid early cretaceous plutons in the eastern transhimalayas zircon u pb ages for the mec sedo and other granites within the mmb and slate belt help to define a late cretaceous to early eocene age range for mostly ilmenite series peraluminous granites within the western granite belt and more restricted western tin belt of myanmar and southern thailand in the wuntho popa arc west of the dextral sagaing fault our new u pb zircon age of 105 ma on diorite at salingyi supports previous k ar ages 106 93 ma for pre tertiary intrusions in the cretaceous this arc may have occupied the andaman sea and continued northwards through the mokpalin diorites and west of the mmb to the 103 80 ma i type plutons in the gandise batholith of southern tibet we interpret the mmb and slate belt as part of a western myanmar block separated in the jurassic from the shan plateau to the east by the southwestward continuation of the bangong nujiang luxi neo tethys i ocean we relate most intrusive and metamorphic events in and near the mmb to westward subduction of this ocean and end jurassic collision of the plateau with the overriding western myanmar slate belt block to orogenic polarity reversal and early cretaceous westward translation of the plateau sequence over the suture zone and to eastward subduction of the ancestral indian ocean or neo tethys ii beneath myanmar with generation of the wuntho popa arc beginning before the late cretaceous the mmb includes meta plateau rocks near mandalay but meta slate belt rocks to the north and south within the mmb previous monazite xenotime thorite ages of eocene oligocene and oligocene early miocene cooling ages on micas with no deformation in eocene granites suggest a high temperature thermal event related to intrusion of these and other granites during a previously proposed regional extension our data imply that the main fabric forming metamorphic event in the mmb pre dated the india asia collision

comparing p and s wave heterogeneity in the mantle from the reprocessed data set of engdahl and co workers we have carefully selected matching p and s data for tomographic imaging we assess data and model error and conclude that our s model uncertainty is twice that of the p model we account for this in our comparison of the perturbations in p and s wavespeed in accord with previous studies we find that p and s perturbations are positively correlated at all depths however in the deep mantle systematic differences occur between regions that have undergone subduction in the last 120 million years and those that have not in particular below 1500 km depth ln vs ln vp is significantly larger in mantle regions away from subduction than in mantle beneath convergent margins this inference is substantiated by wavespeed analyses with random realizations of the slab non slab distribution through much of the mantle there is no significant correlation between bulk sound and s wave perturbations but they appear to be negatively correlated between 1700 and 2100 km depth which is also where the largest differences in ln vs ln vp occur this finding supports convection models with compositional heterogeneity in the lowermost mantle