structure of the hellenic subduction zone from gravity gradient functions and seismology the normalized full gradient nfg method has widespread applications in the analysis of potential fields especially the gravity and magnetic fields this method is used to identify the lateral and horizontal density variations in the crust and lithosphere in this study the nfg method was applied to the gravity data of the cretan arc and its surroundings because of the tectonic features of the eastern mediterranean the cretan arc and the neighboring areas are seismically very active especially the subduction zone and the complicated crustal features have been defined applying many different geophysical methods in this study first the nfg method is tested with synthetic prisms two cubes after that the nfg method was applied to the bouguer gravity data of the cretan arc and its subduction zone hellenic subduction zone and hellenic subduction zone was defined with the foci depth data usgs along the south north direction thus geometry of the focal depth distribution has been created to determine probable media depths and their localizations according to the nfg results vertical structural transitions were observed at a depth ranging between 10 and 180 km also these results were compared with the foci depth model and the other results of the related publications finally some considerations in vertical solution with the nfg method have been presented and locations of the different structures at horizontally have been defined with application of the nfg method

thermal hydrous and mechanical states of the mantle transition zone beneath southern africa abstract observations of p to s conversions from the seismic discontinuities near 410 and 660 km depth reveal the shoaling of the 410 km discontinuity and the deepening of the 660 km discontinuity beneath the archaean cratons in southern africa consequently the mantle transition zone is 20 km thicker than beneath post archaean regions and the global average the discontinuity structure combined with tomographically imaged seismic velocities suggests a relatively cold t 100 k and water enriched oh 20 35 saturation transition zone beneath the cratons the thicker than normal transition zone correlates with surface geology and does not follow the track of african plate motion as numerical simulations of small scale convection imply the observations suggest that cold thermal downwelling is not the primary cause of the anomalously thick transition zone beneath the cratons although it may exist in a confined area near the boundary between the kheis thrust belt and the namaqua natal belt if thermal downwelling exists its temperature anomaly at the 660 km discontinuity must be below the levels detectable by the method used in this study t 60 k the spatial correlation between surface geology high velocities beneath the cratons and the transition zone structure indicates that the cratonic keels extend to the base of the upper mantle substantially deeper than most estimates our results demonstrate the need to closely consider the effect of water content on seismic velocities beneath deep continental roots

fine structure of the 410 km discontinuity the april 14 1995 earthquake in western texas m w 5 7 produced a strong topside reflection off the 410 km discontinuity which was recorded on a multitude of seismic arrays throughout the southwestern united states data from 394 vertical short period and 24 broadband instruments provide dense coverage of this event from distances of 11 to 19 and provide a detailed look at the subcontinental 410 km structure the salient features of this data set are 1 the strong dependence on wavelength of the 410 km triplication range 2 the uniform amplitude ratio of the direct p and reflected p 410 phases on both short period and broadband recordings throughout the triplication and 3 the abrupt termination of the short period p 410 phase at 13 3 these features are best modeled by a composite discontinuity in which a sharp velocity jump of 3 is overlain by a linear velocity jump of 3 5 spread over 14 km the interference of energy turning in the diffuse and sharp portions of this discontinuity structure reproduces both the long and short period triplication range and the step like behavior of the p 410 short period amplitude which cannot be reproduced with either a simple linearly diffuse or a purely sharp discontinuity this composite structure produces a triplication range which depends on source frequency and has an apparent depth which depends on observation frequency additionally this is the structure expected from mineralogical arguments for the to olivine phase transition

upper mantle seismic structure beneath eastern mexico determined from p and s waveform inversion and its implications 1 we present compressional p and shear s wave seismic velocity models for the upper mantle beneath southeastern mexico derived from waveform inversion of triplicated seismic phases the seismic waveform data produced by an earthquake located near the mexico guatemala border were recorded by the la ristra passive seismic array the la ristra seismic array consists of 54 broadband seismometers arranged linearly from west texas to southeastern utah the orientation of the la ristra array is nearly along the great circle from the event and the distance 18 5 26 5 of the seismic array from southern mexico is such that the data are ideal for investigating localized seismic structure of the upper mantle previous tomography and receiver function studies provide a priori knowledge of receiver side crustal and upper mantle structure from which static adjustments were made to the seismic data the waveforms were inverted for mantle velocity from 40 to 1000 km depth using a conjugate gradient algorithm in the inversion we evaluated a suite of starting models with different depths of the 410 km and 660 km discontinuities and varying velocity gradients the best fitting models have velocity increases across the 410 km discontinuity of 6 2 and 7 3 for p and s wave velocities respectively the velocity jump across the 660 km discontinuity was found to be 3 3 for p waves and 6 3 for s waves the size of the upper mantle discontinuities that we find are more in agreement with a pyrolite composition than standard reference models imply a common feature of the best fitting models is a low velocity zone above the 410 km discontinuity that is more prominent in the shear velocity model than the compressional velocity model this feature may be due to partial melting induced by water release from the transition zone the overall jump in velocity at 410 km is also larger than in previously published models with a lower gradient below in addition the p wave data require a small discontinuity at 490 km depth that is not resolved in the s data finally the s wave data require an unusually high gradient beginning at about 600 km depth extending to the 660 km discontinuity this feature may be due to a thermal and or mineralogic anomaly due to a flat lying slab beneath eastern mexico

numerical simulation of scour and backfilling processes around a circular pile in waves abstract this study continues the investigation of flow and scour around a vertical pile reported by roulund et al 2005 flow and scour backfilling around a vertical pile exposed to waves are investigated by using a three dimensional numerical model based on incompressible reynolds averaged navier stokes equations the model incorporates 1 k turbulence closure 2 vortex shedding processes 3 sediment transport both bed and suspended load as well as 4 bed morphology the numerical simulations are carried out for a selected set of test conditions of the laboratory experiments of sumer et al 1997 2013a and the numerical results are compared with those of the latter experiments the simulations are carried out for two kinds of beds rigid bed and sediment bed the rigid bed simulations indicate that the vortex shedding for waves around the pile occurs in a one cell fashion with a uniform shedding frequency over the height of the cylinder unlike the case for steady current where a two cell structure prevails the rigid bed simulations further show that the horseshoe vortex flow also undergoes substantial changes in waves the amplification of the bed shear stress around the pile including the areas under the horseshoe vortex and the lee wake region is obtained for various values of the keulegan carpenter number the principal parameter governing the flow around the pile in waves the present model incorporated with the morphology component is applied to several scenarios of scour and backfilling around a pile exposed to waves in the backfilling simulations the initial scour hole is generated either by a steady current or by waves the present simulations indicate that the scour and backfilling in waves are solely governed by the lee wake flow in agreement with observations the numerical model has proven successful in predicting the backfilling of scour holes exposed to waves the results of the numerical tests indicate that the equilibrium depth of scour holes is the same for both the scour and the backfilling for a given keulegan carpenter number in full agreement with observations

crustal structure of the southeastern tibetan plateau from gravity data new evidence for clockwise movement of the chuan dian rhombic block abstract the crustal deformation beneath the chuan dian rhombic block cdb and surrounding regions has been studied in geological and geodetic methods and provide important insights into the kinematics and dynamics about the clockwise movement of this tectonic block in this work we present images of the normalized full gradient nfg of the bouguer gravity anomalies from five gravity profiles across the boundary faults of the cdb measured in recent years and investigate the distribution characteristics of the crustal anomalous bodies along the profiles firstly an anomalous body with eastward dipping exist beneath the xianshuihe fault suggesting that crustal mass move to east secondly near the xiaojiang fault two anomalous bodies dip westward with depth increasing the inferred movement direction of the north one is from west to east and the south one is from east to west thirdly anomalous bodies on the northeast and southwest sides of the red river fault suggest the directions of crustal movement is from northeast to southwest these results are also consistent with gps solutions and provide gravity evidence for crustal deformation of the cdb with clockwise rotation

new seismic model of the upper mantle beneath africa we present a seismic model of the upper 400 km of the mantle beneath africa and surrounding regions this model is constructed by inverse modeling of fundamental mode rayleigh wave phase velocities 40 200 s for about 8000 propagation paths among the most pronounced anomalies are high shear velocity structures as much as 6 higher than in the preliminary reference earth model beneath the west african congo and kalahari cratons that extend to about 250 km depth these structures have near vertical margins across which the shear velocity changes by as much as 6 over 500 km distance anomalous low shear velocities 3 4 lower than in the preliminary reference earth model structures are observed beneath the east african red sea and gulf of aden rifts and beneath the northwestern indian ocean these structures extend to a depth of at least 250 km our model cannot be reconciled with models that invoke a large number of plumes that have impinged on the base of the lithosphere nor does our seismic model indicate that high temperature low density material beneath the lithosphere is responsible for the uplift of southern africa

upper mantle seismic structure beneath southern africa constraints on the buoyancy supporting the african superswell we present new one dimensional sh wave velocity models of the upper mantle beneath the kalahari craton in southern africa obtained from waveform inversion of regional seismograms from an mw 5 9 earthquake located near lake tanganyika recorded on broadband seismic stations deployed during the 1997 1999 southern african seismic experiment the velocity in the lithosphere beneath the kalahari craton is similar to that of other shields and there is little evidence for a significant low velocity zone beneath the lithosphere the lower part of the lithosphere from 110 to 220 km depth is slightly slower than beneath other shields possibly due to higher temperatures or a decrease in mg number mg if the slower velocities are caused by a thermal anomaly then slightly less than half of the unusually high elevation of the kalahari craton can be explained by shallow buoyancy from a hot lithosphere however a decrease in the mg of the lower lithosphere would increase the density and counteract the buoyancy effect of the higher temperatures we obtain a thickness of 250 30 km for the mantle transition zone which is similar to the global average but the velocity gradient between the 410 and 660 km discontinuities is less steep than in global models such as prem and iasp91 we also obtain velocity jumps of between 0 16 0 1 and 0 21 0 1 km s across the 410 km discontinuity our results suggest that there may be a thermal or chemical anomaly in the mantle transition zone or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be refined overall our seismic models provide little support for an upper mantle source of buoyancy for the unusually high elevation of the kalahari craton and hence the southern african portion of the african superswell

long lasting epeirogenic uplift from mantle plumes and the origin of the southern african plateau 1 we investigate under what conditions the present day long wavelength anomalous elevation 500 m of the southern african plateau can be attributed to mesozoic plume events the anomalous long wavelength topography of the southern african plateau cannot be easily explained by recent heating of the upper mantle as opposed to other areas of the african superswell and geomorphological studies provide few hard constraints on the timing of the uplift a mesozoic origin for the plateau uplift is attractive in that southern africa experienced several large magmatic events in the mesozoic we formulate numerical and scaling models to investigate if plume material ponded beneath cratonic lithosphere in the mesozoic could produce 500 m of present day elevation we find that starting plume heads are ineffective at producing much long lasting uplift because the material spreads into a thin layer tens of km thick the ponded plume material also suppresses secondary convection by having a stable boundary at its base over time heat continues to flow out of the top of the lithosphere and a net imbalance of heat flow in the lithosphere develops resulting in subsidence even after the plume material has cooled to the temperature of the mantle adiabat secondary convection supplies less heat to the lithosphere than is lost upward by conduction the cratonic lithosphere returns to its original thermal structure on a timescale of less than 200 m y even two mantle starting plume heads one at the time of karoo volcanism ca 183 ma and the other at the time of kimberlite eruption ca 80 90 ma in our models cannot produce much 200 m of the present day anomalous elevation however significant uplift can be generated by plume tails provided they linger beneath the lithosphere for 25 30 m y and if the uplift effects of mesozoic plume heads and tails are considered together then it is possible to account for 500 m of present day elevation consequently a mesozoic plume model for plateau uplift in southern africa appears to be a viable model if plume tails delivered heat to the southern african lithosphere over a period of 25 m y or more

mantle structure and composition to 800 km depth beneath southern africa and surrounding oceans from broadband body waves abstract average one dimensional p and s wavespeed models from the surface to depths of 800 km were derived for the southern african region using travel times and waveforms from earthquakes recorded at stations of the kaapvaal and south african seismic networks the herglotz wiechert method combined with ray tracing was used to derive a preliminary p wavespeed model followed by refinements using phase weighted stacking and synthetic seismograms to yield the final model travel times combined with ray tracing were used to derive the s wavespeed model which was also refined using phase weighted stacking and synthetic seismograms the presence of a high wavespeed upper mantle lid in the s model overlying a low wavespeed zone lwz around 210 to 345 km depth that is not observed in the p wavespeed model was inferred the 410 km discontinuity shows similar characteristics to that in other continental regions but occurs slightly deeper at 420 km depletion of iron and or enrichment in aluminium relative to other regions are the preferred explanation since the p wavespeeds throughout the transition zone are slightly higher than average the average s wavespeed structure beneath southern africa within and below the transition zone is similar to that of the iasp91 model there is no evidence for discontinuity at 520 km depth the 660 km discontinuity also appears to be slightly deeper than average 668 km although the estimated thickness of the transition zone is 248 km similar to the global average of 241 km the small size of the 660 km discontinuity for p waves compared with many other regions suggests that interpretation of the discontinuity as the transformation of spinel to perovskite and magnesiowustite may require modification alternative explanations include the presence of garnetite rich material or ilmenite forming phase transformations above the 660 km discontinuity and the garnet perovskite transformation as the discontinuity

interpretation of gravity anomalies with the normalized full gradient nfg method and an example the normalized full gradient nfg method which was put forward about 50 years ago has been used for downward continuation of gravity potential data especially in the former union of soviet socialist republics this method nullifies perturbations due to the passage of mass depth during downward continuation the method depends on the downwards analytical continuation of normalized full gradient values of gravity data analytical continuation discriminates certain structural anomalies which cannot be distinguished in the observed gravity field this method has been used in various petroleum and tectonic studies the trapeze method was used for the determination of fourier coefficients during the application of this method no other techniques for calculating these coefficients have been used however the filon method was used for the determination of fourier coefficients during the application of the nfg method in this work this method rather than the trapeze method should be preferred for indicating abnormal mass resources at the lower harmonics in this study the nfg method using the filon method has been applied the first time to theoretical models of gravity profiles as example field at the hasankale horasan petroleum exploration province where successful results were achieved hydrocarbon presence was shown on the nfg sections by the application of nfg downward continuation operations on theoretical models important signs of hydrocarbon structure on the nfg section for field and model data at low harmonics are obtained more effectively using this method

the anisotropic seismic structure of the east european platform 1 we present models for the p and s velocity structure of the upper mantle beneath the east european platform including measurements of radial anisotropy the models were obtained by full waveform inversion of 3 component broadband seismograms from strike slip earthquakes located near the edge of the platform and recorded in russia and europe we used direct and multiply reflected body wave arrivals s ss p pp combined with fundamental mode love and rayleigh waves at source receiver distances between 15 and 50 to resolve mantle structure down to the 410 km discontinuity the platform is underlain by a radially anisotropic mantle lid extending to a depth of 200 km with a largely isotropic mantle below the model has a positive velocity gradient from 41 km to 100 km depth and a relatively uniform velocity structure from 100 km to 200 km depth with high sh and ph velocities 4 77 km s 8 45 km s shear anisotropy is uniform at 5 h v from 41 to 200 km depth drops to 2 from 200 to 250 km and is isotropic below that the average shear velocity from 100 to 250 km is also uniform at 4 65 km s and the drop in anisotropy is matched by a drop in h to 4 70 km s combined with an increase in v to 4 60 km s below 250 km there is a positive velocity gradient in both p and s velocity down to 410 km p anisotropy is not well resolved but p structure mimics the sh velocity structure suggesting that p is also anisotropic within the lid

constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveforms we examine stacks of several seismic phases having different sensitivities to mantle transition zone structure when analyzed separately underside p and s reflections pdp and sds are suggestive of very different structures despite similar raypaths and data coverage by stacking the radial component of pdp rather than the vertical pdp we show that this difference does not result from interference from other more steeply inclined phases such as pkp and ppdpdiff in general stacks of p to s converted phases pds appear to lack evidence of a 520 km discontinuity when examined without other phases when these phases and stacked topside p reflections ppdp are analyzed jointly using a nonlinear inversion method consistent but nonunique seismological models emerge these models show that a discontinuity at 653 km depth has smaller contrasts in density and velocity than found in most previous studies a sub 660 gradient can account for the majority of this difference a 1 6 0 5 p velocity contrast and a 2 2 0 3 density contrast at 518 km depth without a s velocity contrast can explain the lack of a p520s together with robust pp520p and s520s phases for models parameterized with a finite thickness for each discontinuity the 410 km discontinuity is consistently 3 times thicker than the 660 km discontinuity

upper mantle structure of the kaapvaal craton from surface wave analysis a second look 1 surface waves from earthquakes recorded during the two year southern africa seismic experiment have been used to estimate the average velocity structure beneath the kaapvaal craton by cross correlating the data with synthetics derived using mode summation techniques i measure frequency dependent phase delays for love and rayleigh waves as well as higher modes for 12 events ranging in epicentral distance between 12 and 91 i simultaneously invert the traveltime delays for structure beneath the array and outside of the array in this way heterogeneity along the path lying outside of the array is not mapped erroneously into structure beneath the array the differential dispersion between the love and rayleigh waves require some anisotropy in the upper mantle however models with a range of anisotropic depths as shallow as 150 km and as deep as 220 km can equally well fit the data the sensitivity of the fundamental mode surface waves used in this study begins to decline significantly below 200 km depth limiting interpretation of deeper structure nonetheless this range of anisotropic depths is significantly different than the 100 km depth proposed in a recent study by freybourger et al 2001 using a similar technique and different data from the same array i attribute the difference in their dispersion measurements and modeling to the improper use of a phaseless i e frequency dependent filter on the data which systematically biased their measurements at lower frequencies in addition i find no evidence of a low velocity layer as proposed by qui et al 1996 and priestley 1996

upper mantle sh and p velocity structures and compositional models beneath southern africa abstract we constrain sh and p wave velocity structures in the upper mantle beneath southern africa using triplicated phases recorded in the epicentral distance range of 11 28 for one shallow event we then explore thermal and compositional models appropriate for explaining the inferred seismic structures in the region both sh and p wave data suggest presence of a low velocity zone with velocity reductions of at least 5 for s waves and 2 for p waves beneath a 150 km thick high velocity lithospheric lid seismic observations also suggest that the p s ratio is larger 1 88 in the transition zone than in the lithospheric lid 1 70 the inferred p wave velocity jump across the 660 km discontinuity is small

average lithospheric structure of southern africa summary we present a new 1 d earth model for the crust and upper mantle beneath the archaean core of southern africa the crustal structure is constrained by published seismic refraction reflection data and by the modelling of teleseismic receiver function data the mantle structure is constrained by traveltime measurements for p and s waves by waveform inversion of multimode surface waves and by forward modelling of p waveforms for regional earthquakes the 42 km thick crust of southern africa consists of four layers a 2 km thick 5 05 km s surface layer a 5 km thick 6 08 km s upper crust a 20 km thick 6 30 km s l mid crust and a 15 km thick 6 73 km s lower crust below the crust there is an 80 km thick upper mantle lid v and v are 8 09 and 4 62 km s l respectively at the top of the lid and the compressional and shearvelocity gradients through the lid are 0 0008 s l and 0 0013 s respectively below the lid there is a substantial shear wave lvz with the shear wave velocity dropping from 4 72 km s l in the lower part of the lid to 4 32 km s at 250 km depth the shear wave lvz is required to fit both the v measurements and the regional surface waveforms forward modelling of p suggests that no upper mantle p wave lvz exists there is an increase in the p wave velocity gradient at 125 km depth to 0 0015 s and a second increase at 250 km depth to 0 0035 s velocities and densities in the seismic model of the lid agree closely with velocity and density estimates from the geochemical analysis of garnet periodotite nodules from kimberlites of the kaapvaal craton implying that the estimates from kirnberlites are representative of a wider region of the archaean core of southern africa not just of the restricted region beneath which the kimberlites are found a comparison of the southern african seismic model with seismic models for other shield regions where nodule data are not available shows that the nodule results may also be representative of the upper mantle in those regions

detection of high potential oil and gas fields using normalized full gradient of gravity anomalies a case study in the tabas basin eastern iran the normalized full gradient nfg represents the full gradient of the gravity anomaly at a point divided by the average of the full gradient at the same point the nfg minimum between two maxima in an nfg section or a closed minimum surrounded by closed maxima on an nfg map may indicate density deficient anomalies closely related to possible oil gas reservoirs on a cross section closed minima can be used to estimate the depth to centers of possible hydrocarbon reservoirs the nfg map can also be used to locate oil gas exploratory wells for estimation of the depth of possible reservoirs the objective of this paper is to use two and three dimensional 2d and 3d nfg on gravity data of the tabas basin in yazd province eastern iran a hypothetical model is first considered to explore the nfg characteristics and their relationship with the geometry of the model the physical properties of the model are then studied to simplify the interpretation of real data finally 2d and 3d nfg models are developed for real gravity data to predict the location of any possible high potential oil gas reservoirs the results obtained indicate two zones in the northern and central parts of the tabas basin suitable for hydrocarbon prospecting however the favorable zone located in the middle of the basin in which anticline e is detected at a depth of 5 7 km is more important for the purpose of hydrocarbon exploration

thermochemical structure and dynamics of the african superplume 1 we present a new three dimensional 3 d model of the thermochemical structure of the african superplume region obtained from simultaneous inversion of global seismic and convection related data convection related observations include the global free air gravity field tectonic plate motions dynamic surface topography and the excess ellipticity of the core mantle boundary a 3 d image of the chemically induced density perturbations provides direct evidence that intrinsically dense material is entrained within the superplume and concentrated into a rounded structure 1000 km above the core mantle boundary the thermally induced density perturbations are greater in magnitude than the chemically induced implying overall positive buoyancy throughout the superplume the observed morphology and density signatures are consistent with a thermochemical plume that has risen from a compositionally distinct pile at the base of the mantle and may be currently deforming under the influence of its intrinsic negative chemical buoyancy citation simmons n a a m forte and s p grand 2007 thermochemical structure and dynamics of the african superplume geophys res lett 34 l02301 doi 10 1029 2006gl028009

isostasy of the eastern anatolia turkey and discontinuities of its crust this paper presents a probable isostatic model of the east anatolian region which lies in a belt of significant plate movements probable locations of the horizontal and vertical discontinuities in the crust structure were determined using the normalized full gradient nfg method for the purpose of explaining the mechanism that supports topography corresponding to the crust thickness in the region calculations of effective elastic thickness t e were carried out initially by utilizing admittance and misfit functions according to these results the effective elastic thickness value obtained was less than the crust thickness even though the isostatic model does not conform with the airy model consequently it was assumed that there could be problems beneath the crust hence the nfg method was applied on the bouguer gravity data of the region in order to investigate probable discontinuities in the crust structure according to the nfg results vertical structural transitions were observed at a depth ranging between 10 and 30 km which begin immediately north of the bitlis zagros suture zone bzsz and continue in a northerly direction the relationship between the effective elastic thickness t e 13 km in average as determined in the last stage and the seismogenic zone in the region was investigated if the t e value happens to be less then the crustal thickness then one can say that there are problems in the crustal structure of the region similar to eastern anatolia indeed when nfg results of the study area are examined numerous vertical and horizontal discontinuities in the crust can be observed these discontinuities which correspond to low bouguer gravity anomalies and shallow focal depth earthquakes are probably the source of the factors which rule the tectonic mechanism of the region

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

ghosts of lithospheres past imaging an evolving lithospheric mantle in southern africa group ii 143 117 ma and group i kimberlites 108 74 ma intrude across the southwest boundary of the kaapvaal craton sampling the same volume of the subcontinental lithospheric mantle sclm in two time slices major and trace element analyses of 3699 peridotitic garnet xenocrysts were used to construct paleogeotherms for 17 kimberlite localities and to place each garnet at its depth of origin the ti contents of each garnet and the calculated x mg of its coexisting olivine were projected onto a southwest northeast section across the craton boundary and splines were used to interpolate between the virtual boreholes in each age group the sections show that the cratonic sclm extends at least 75 km southwest of the mapped craton boundary suggesting a dipping contact marked differences between the time slice sections show that between 117 and 108 ma the sclm on both sides of the craton boundary was heated and chemically refertilized by infiltrating asthenosphere derived melts thinning the depleted layer by 40 km the thermal and geochemical changes record a significant tectonothermal event that may be related to changes in the stress field associated with opening of the south atlantic or with mantle upwelling

evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath africa we present data which indicate that the broad low shear velocity anomaly beneath southern africa is stronger and more extensive than previously thought recordings of earthquakes in the southwestern atlantic ocean at an array of broadband seismic stations in eastern africa show anomalously large propagation time delays of the shear phases s scs and sks which vary rapidly with epicentral distance by forward modeling we estimate that the low velocity anomaly extends from the core mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3 lower than in standard models such as prem strong velocity contrasts exist at its margins 2 over about 300 km these seismic characteristic are consistent with recent numerical simulations of lower mantle mega plume formation

upper mantle velocity structure beneath southern africa from modeling regional seismic data the upper mantle seismic velocity structure beneath southern africa is investigated using travel time and waveform data which come from a large mine tremor in south africa mb 5 6 recorded by the tanzania broadband seismic experiment and by several stations in southern africa the waveform data show upper mantle triplications for both the 410 and 670 km discontinuities between distances of 2100 and 3000 km auxiliary travel time data along similar profiles obtained from other moderate events are also used p wave travel times are inverted for velocity structure down to 800 km depth using the wiechert herglotz technique and the resulting model is evaluated by perturbing it at three depth intervals and then testing the perturbed model against the travel time and waveform data the results indicate a typical upper mantle p wave velocity structure for a shield p wave velocities from the top of the mantle down to 300 km depth are as much as 3 higher than the global average and are slightly slower than the global average between 300 and 420 km depth little evidence is found for a pronounced low velocity zone in the upper mantle a high velocity gradient zone is required above the 410 km discontinuity but both sharp and smooth 410 km discontinuities are permitted by the data the 670 km discontinuity is characterized by high velocity gradients over a depth range of 80 km around 660 km depth limited s wave travel time data suggest fast s wave velocities above 150 km depth these results suggest that the bouyant support for the african superswell does not reside at shallow depths in the upper mantle