improving global shear wave traveltime tomography usingthree dimensional ray tracing and iterative inversion a fully non linear approach to global tomography using s wave arrival times has been implemented using 3 d ray tracing with an iterative linearized inversion scheme the starting model for the 3 d inversion was the long wavelength saw12d model derived from inversion of global waveforms the traveltime tomography leads to the introduction of smaller scale structure and a final model in which small scale detail exists and can be resolved with a smooth interconnection through long wavelength structure the two point ray tracing was implemented using a pseudo bending approach for a full spherical 3 d model of the mantle generally the ray paths in the full 3 d model and 1 d reference model are quite close but the inclusion of a more accurate treatment of the rays improves the resolution of wave speed gradients and the positioning of heterogeneity particularly near strong variations in wave speed for example in subduction zones a further advantage of the use of 3 d ray tracing is that it is possible to undertake resolution tests with fewer approximations with the aid of the non linear inversion a number of global s models have been constructed using different assumptions about the character of the model for example solutions can be produced that are designed to introduce minimum differences from a 1 d reference model a variance reduction of 48 per cent was achieved in the inversions with considerable benefit from the inclusion of iterative inversion with 3 d ray tracing and the improved quality of the data set used in this study resolution in the lower part of the mantle has been improved by supplementing the s arrival time data with sks times for the distance range from 84 to 118 the new global s models retain the general features of models derived by one pass linearized inversion with 1 d ray tracing but provide more focused images with a higher perturbation level for the same damping parameters the new models are able to provide a good definition of features revealed by regional tomography using arrival time and waveform data for example the complex slab morphology beneath the tonga and kermadec regions and the sharp boundary between slow and fast uppermost mantle regions beneath western and eastern europe

low seismic resolution cannot explain s p decorrelation in the lower mantle 1 inverted models of the deep mantle show a decorrelation between maps of shear vs and compressional vp wave velocities an anti correlation between the bulk sound velocity v and vs and a much larger variability of vs with respect to vp expressed by large values of the ratio of their relative lateral variations we carried out synthetic tests to verify if these features could be artifacts explained by limits in tomographic resolution synthetic data are calculated for an input model and linearly inverted as in tomography to find an output model comparing the values of the aforementioned parameters for two different chemically homogeneous input models with the associated reconstructed output ones we found that artifacts caused by realistic data noise and the nonuniform distribution of seismic sources and stations over the globe are not sufficient to introduce the features previously described we confirm that compositional effects are required to explain them

correlated shear and bulk moduli to 1400 km beneath the mediterranean region we present a new model for the shear and compressional wave velocity structure of the mediterranean plate boundary region the model is obtained by a combined inversion of s and p data where we solve in one single step for shear wave and bulk sound velocity variations the ratios between the types of seismic velocities inferred from the combined inversion constrain the origin of heterogeneity our model is dominated by fast anomalies from subducted oceanic lithosphere we found that only a slight amount of bulk sound velocity heterogeneity is required to explain the combined set of p and s data thus compressional wave velocity anomalies are dominated by shear modulus heterogeneity and the ratio between bulk sound and shear wave velocity heterogeneity suggests a predominantly thermal origin 2006 elsevier b v all rights reserved

joint seismic tomography for bulk sound and shear wave speed in the earth s mantle high quality p and s travel times are now available from careful reprocessing of data reported to international agencies a restricted data set has been extracted for which comparable ray coverage is achieved for p and s and used for a joint inversion to produce a three dimensional model for shear and bulk sound velocities represented in terms of 2 2 cells and 18 layers in depth through the mantle about 106 times for each of p and s are combined to produce 312 549 summary rays for each wave type linearizing about the ak135 reference model 583 200 coupled tomographic equations are solved using an iterative partitioned scheme clear high resolution images are obtained for both bulk sound speed and shear wavespeed the bulk and shear moduli have differing sensitivity to temperature and mineral composition and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations different heterogeneity regimes can be recognised in the upper mantle the transition zone most of the lower mantle and the lowermost mantle in the upper mantle many features can be explained by thermal effects but in some orogenic zones e g western north america the opposite sense of the bulk sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects in the lower mantle pronounced narrow structures which may represent remnant subduction are most marked in shear the level of large scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km below this depth the variability of both wave speeds increases near the core mantle boundary the variations of the two wave speeds show little concordance suggesting the presence of widespread chemical heterogeneity

geochemistry of intrusive rocks associated with molybdenite mineralization kigom complex northern nigeria abstract the kigom igneous complex of northern nigeria is a small intrusive complex made up of peralkaline arfvedsonite granite riebeckite biotite granite and riebeckite aegirine granite the granites show coherent major element chemistry typical of that of the nigerian younger granites and are enriched in rb li zr nb th y f and ree and depleted in ba and sr compared to the world average for rocks of similar composition the riebeckite aegirine granite exhibits a two to three fold increase in the concentration of zr nb li f th zn and ree over comagmatic granites concentrations of the highly charged cations in these granites are believed to be the result of halogen alkali rich fluid activity during the fractionation of the magma corresponding trace element increases in the granites in areas of alkali metasomatism albitization support this argument and reflect the partial confinement of this alkali volatile phase within the high level magma chambers it is important to recognize that the chemical signatures of this liquid state fractionation process may be explanatory mechanisms in the formation of some kinds of lithophile ore deposits the degree of peralkalinity maintained miscibility in the residual fluid between silicate and aqueous phase to low temperatures so that mineralizing components continuously accumulated together the interaction between this phase and early formed minerals led to mo mineralization the well known classification of granites into sedimentary protolith s type igneous protolith i type and a type indicates that the granites studied here have typical a type features an origin by partial melting of lower crustal material probably granulite facies under a high flux of mantle derived volatiles is suggested for these rocks as a result of mixing of crustal and mantle derived fluids a syenitic magma was produced which then experienced selective contamination as a result of further reaction melting during fractionation and ultimately produced a peralkaline differentiate

using patchy pulsating aurora to remote sense magnetospheric convection five patchy pulsating aurora ppa patches have been identified in data obtained from the gillam time history of events and macroscale interactions during substorms themis all sky imager asi we found that azimuthal velocities of five patches derived from themis asi data were close to the same value as the local convection velocities as obtained from analysis of super dual auroral radar network data consistent with the idea that the patch motion is primarily due to e b convection we argue that this means that we can infer 2 d maps of the time evolving convection from time sequences of ppa further given that the e b convection is understood to be a projection of magnetospheric convection this means that provided auroral and viewing conditions cooperate patch motion can be used for remote sensing magnetospheric convection over across extended regions with fairly high time resolution this is the first detailed demonstration of the equivalence of patch velocities and e b convection

slab pull mantle convection and pangaean assembly and dispersal two global scale mantle convection cells presently exist on earth centred on upwelling zones in the south pacific ocean and northeast africa one cell panthalassan contains only oceanic plates the other pangaean contains all the continental plates they have remained fixed relative to one another for 400 ma a transverse rheic tethyian subduction system splits the pangaean cell poloidal plate motion in the oceanic cell reflects circumferential pull of panthalassan slabs but toroidal flow in the pangaean cell reflected by vortex type motion of continents toward the altaids of central east asia throughout the phanerozoic has resulted from the competing slab pull forces of both cells the combined slab pull effects from both cells also controlled pangaean assembly and dispersal assembly occurred during palaeozoic clockwise toroidal motion in the pangaean cell when gondwana was pulled into pangaea by the ne trending rheic subduction zone forming the appalachian variscide altaid chain pangaean dispersal occurred when the rheic trench re aligned in the jurassic to form the nw trending tethyside subduction system which pulled east gondwanan fragments in the opposite direction to form the cimmerian himalayan alpine chain this re alignment also generated a new set of indian mid ocean ridge systems which dissected east gondwana and facilitated breakup 100 200 myr long phanerozoic wilson cycles reflect rifting and northerly migration of gondwanan fragments across the pangaean cell into the rheic tethyian trench pangaean dispersal was amplified by retreat of the panthalassan slab away from europe and africa which generated mantle counterflow currents capable of pulling the americas westward to create the atlantic ocean thermal blanketing beneath pangaea and related hotspot activity were part of a complex feedback mechanism that established the breakup pattern but slab retreat is considered to have been the main driving force the size and longevity of the two cells organised and maintained by long lived slab pull forces favours deep mantle convection as the dominant circulation process during the phanerozoic

a geochemical study of the concentration process of tungsten and tin in the taourirts granites of central ahaggar algeria abstract the here presented geochemical study of the granite of three intrusions in the central ahaggar algeria was carried out in an effort to discriminate between w and sn bearing intrusions in the nahada intrusion w is not correlated with the other elements as this intrusion is linked to a large w deposit the correlation of w should be made with the elements of the fluid phase however this has not been taken into account in the present study although the tamazaror intrusion was also affected by a strong fluid phase only a minor w deposit was observed due to the fact that the w remained partially absorbed in a ferruginous phase caused by an extensive crustal contamination in the el bema intrusion accompanied by intrabatholitic sn veins w is correlated with a lithophile factor characterizing a fractional crystallization process which is recognized as an indicator of granite specialization for sn the chemical features of the intrusive granites allow a lithogeochemical approach to the prospection problems for w and sn in the central ahaggar

geochemical studies in the drainage basin of the rio vouga portugal abstract the hydrothermal waters in the vouzela region portugal have a temperature of 68 c and are characterized by a high content of f li b bicarbonate sulphate and silica ninety three water samples including two hydrothermal waters were analyzed for major and trace elements in order to investigate the genesis of the hydrothermal water in the vouzela region two older hercynian granites are exposed the abas granite and the sao pedro do sul granite the abas granite is intersected by the ribama fault albitization is ubiquitous other alteration effects are tourmalinization k feldspathization and muscovitization these processes and the associated sn w deposits were induced by the intrusion of the younger viseu granite in the present situation rainwater penetrates into the abas granite especially via the ribama fault and the greisenization zone it becomes part of a hydrothermal system leaches li f b and silica and rises via the termas fault in the hydrothermal springs of termas de sao pedro do sul geothermal equilibrium calculations yield a temperature of 205 10 c for the thermal reservoir the percentage of mixing with cold groundwater near the surface in the circulation system is estimated at 73 the circulation system is possibly maintained by the heat generated by the decay of u th k and their daughter products elements which are encountered in extremely high concentrations in the sao pedro do sul granite

application of a three dimensional ray tracing technique to global p pp and pdiff traveltime tomography summary a 3 d ray path tracing algorithm was successfully applied to global p wave traveltime tomography the inversion was conducted iteratively using the resultant p wave velocity model as the initial model for the subsequent iteration the lsqr method was adopted to solve a large and sparse system of equations this iteratively linearized inversion with 3 d ray tracing increased wave speed anomalies located heterogeneities better and reduced smearing as compared to those derived from a conventional one step inversion using 1 d ray tracing although the general pattern of velocity anomalies was similar a major difference was found in the lowermost mantle where the departure of a ray path from the great circle path tends to be in general greatest in particular a pronounced high velocity anomaly develops beneath the indian ocean a feature not obvious in the result of 1 d inversion the final p wave velocity model was obtained by including reported pp and pdiff traveltime data the addition of the pp data sharpened the images and enhanced velocity anomalies in the upper mantle especially at latitudes above 45 of the northern hemisphere the addition of the pdiff data sharpened and amplified velocity anomalies in the lowermost mantle in general

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

simultaneous inversion for 3 d variations in shear and bulk velocity in the mantle until recently most of the seismic tomographic modeling has been addressing the question of lateral heterogeneity either in p or s wave velocities the s wave velocity variations are larger and hence provide stronger signal on long period waveforms the direct p travel times being the first arrivals on the other hand are most frequently reported in the international seismological centre isc bulletins in mineral physics experiments the variation in bulk velocity is more often measured to better understand the differences between vp and vp patterns and better link the results from mineral physics to those of seismic tomography we formulate the inverse problem in terms of relative perturbations in the shear velocity vs 12 and bulk sound velocity v k 12 we use a large data set which consists of waveforms waveform derived travel times and travel times from the isc bulletins the earthquakes are relocated using corrections for lateral heterogeneity the events which cannot be reliably determined are discarded the model is defined as spherical harmonics to degree 12 horizontally and as chebyshev polynomials to order 13 radially for both shear and bulk sound velocity the inversion is performed under smoothness constraint the resolution tests and bootstrapping analysis indicate that the model is well recovered particularly at long wavelength the results indicate a much larger variability of shear than bulk sound velocity the model explains observations well the most intriguing result obtained in this study is that the variations in shear velocity and bulk sound velocity are negatively correlated in the lowermost mantle the explanation is not very clear from the mineral physics point of view it is not unlikely that this could be explained in terms of thermal variation even though we are unwilling to rule out the possibility of large wavelength compositional variations

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

palaeozoic granites of the umberatana region south australia the role of volatiles in the crystallization of some alkaline peralkaline granites palaeozoic alkaline to peralkaline sodic granites of the umberatana area of south australia have high mno p5o5 nb ta be f and b abundances as is typical of a type or anorogenic granites abundant f and b in the magma permitted primary muscovite to crystallize at pressures that may have been as low as 1 kbar because of the lowered solidus temperatures these volatile rich magmas precipitated both albite and two generations of k rich alkali feldspar the hydrothermal fluids released during crystallization of these magmas resulted in considerable element redistribution and recrystallization both in the plutons and the adjacent country rocks

joint bulk sound and shear tomography for western pacific subduction zones abstract detailed regional body wave tomographic inversion of the western pacific region has been performed using p and s travel times from common sources and receivers with a joint inversion in terms of bulk sound and shear wave speed variations in the mantle this technique allows the separation of the influence of bulk and shear moduli and hence a more direct comparison with mineral physics information the study region is parameterized with cells of side 0 5 to 2 and 19 layers to a depth of 1500 km while the rest of the mantle was parameterized with 5 5 cells with 16 layers between the surface and the core mantle boundary a simultaneous inversion is made for regional and global structures to minimize the influence of surrounding structures on the regional image a nested iterative inversion scheme is employed with local linearization and three dimensional ray tracing through the successive model updates the results of the regional tomographic inversion reveal the penetration of a subducted slab below the 660 km discontinuity at the kurile kamchatka trench while flattening of slabs above this depth is observed in the japan and izu bonin subduction zones on both the bulk sound and shear wave speed images the penetration of a subducted slab down to a depth of at least 1200 km is seen below the southern part of the bonin trench mariana philippine and java subduction zones fast shear wave speed perturbations associated with the subducted slabs down to the 410 km transition zone are larger than the comparable bulk sound perturbations for all these subduction zones except the philippines the bulk sound signature for the subducted slab is more pronounced than for shear in the philippines talaud new guinea solomon and tonga subduction zones where penetration of the slab into the middle mantle is observed variation in the amplitude ratio between bulk sound and shear wave speed anomalies correlates well with the subduction parameters of the descending slab slabs younger than 90 ma at the trench show bulk sound dominance in the upper mantle while older slabs have a stronger shear wave speed signature spreading of the fast shear wave speed zone between 800 and 1000 km is observed in the areas of deep subducted slab penetration but has no comparable expression in the bulk sound images this high velocity feature may reflect physical or chemical disequilibria introduced to the lower mantle by subducted slabs

fast p and s wave velocities associated with the cold stagnant slab beneath the northern philippine sea abstract previous tomographic studies using short period p times have typically shown that the fast pacific slab is stagnant in the mantle transition zone beneath the northern philippine sea and eastern asia however there has been no consensus about the s wave velocity anomalies of the stagnant slab so that it has remained difficult to interpret the stagnant slab in terms of thermal and or compositional state here we analyze both p and s wave travel time anomalies associated with the stagnant slab in the izu bonin subduction zone based on broadband records from japan korea and china we found that p and s waves traveling through the izu bonin stagnant slab exhibit systematically travel time residuals of 3 to 4 s for p waves and 6 to 8 s for s waves with respect to the iasp91 model the observed p residuals are consistent well with an existing three dimensional p velocity model indicating that the stagnant slab beneath the studied region has fast p wave velocity anomalies of 1 4 the observed s residuals are consistent with an s wave velocity model constructed from the p wave velocity model with a constant scaling factor ln v s ln v p r of 1 4 agreement of the experimentally estimated r 1 38 and the seismologically obtained r 1 4 values suggests that both p and s velocity anomalies can be explained by a stagnant slab colder than the surrounding mantle by about 250 300 k

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

seismic tomography with p and s data reveals lateral variations in the rigidity of deep slabs abstract regional seismic tomography of the northwest pacific island arcs using p and s wave arrival time data with similar path coverage reveals an oceanic lithospheric slab deflected in the mantle transition zone beneath the izu bonin region in good agreement with the results of earlier tomographic and other seismic studies in the region the s wave images however do not show clearly the deflection of the slab this implies that the stagnant lithospheric slab in the transition zone is more likely a bulk sound structure which is strongly supported by our results from joint inversions for bulk sound and shear wavespeeds for the izu bonin region where trench migration has been reported the properties of the deflected slab lying on top of the 660 km discontinuity show strong bulk sound and weak shear signatures in contrast to the descending slab itself there is an indication in the shear images that a component of the former oceanic lithosphere is penetrating into the top of the lower mantle suggesting that a slab split has occurred

geochemical aspects of the evolution and mineralization of the amo younger granite complex northern nigeria abstract the amo complex forms one of the prominent ring intrusions in the jos plateau and it is lithologically composed of granite porphyry riebeckite biotite granite hornblende biotite granite and later intrusives of biotite granite there are also small intrusions of albite riebeckite granite and albite biotite granite major element compositions of the principal rock units do not show significant differences comparison of the variations found in the granites with results of laboratory studies suggest either that water vapor and volatile transfer were important in the local magma series or at least they accompanied other systematic variations trace element associations vary anomalous enrichments of rb li f u th zr nb and hree occur over mildly peralkaline riebeckite biotite granite peralkaline albite riebeckite granite and albite biotite granite with peralkaline tendency in contrast to their peraluminous equivalents these cannot be explained by crystal liquid fractionation processes and require the evolution of a na enriched fluid it is suggested that in the albite riebeckite granite and the albite biotite granite the combined effect of f li and rb along with other volatiles may have led to a lower crystallization temperature such that two separate alkali feldspars albite and microcline crystallized individually cassiterite and columbite mineralization occur mainly as magmatic disseminations within the terminal phases of the biotite granites and albite biotite granite diffused greisenization in association with quartz veins also carry cassiterite mineralization in the tega and timber creek biotite granite phases although the magma may have supplied the ore elements and f for complexing actual mineralization appears to be a product of postmagmatic processes

on the resolution of density anomalies in the earth s mantle using spectral fitting of normal mode data summary the resolution of the 3 d density structure in the earth s deep interior has long eluded geoscientists high quality data from digital seismic instruments emplaced this decade have renewed interest in the measurement of low frequency earth normal modes with the goal of extracting heterogeneous density structure here we perform a series of synthetic experiments aimed at investigating the resolution of lateral variations in the mantle from normal mode spectral data contamination effects between seismic velocities and density are examined in two ways 1 by using resolution matrices computed from data kernels and 2 by inverting synthetic spectra computed from realistic input earth models the first type of experiment assumes that the non linearity of the inverse problem is weak no such assumptions are necessary in the second type of tests which nevertheless concur in their results with the former these synthetic tests indicate that density structure retrieved from presently available normal mode data is not reliable contamination of seismic velocity structure into the density model space produces patterns that resemble those obtained from inversions of real data the resulting density models appear to be dependent on a combination of the starting velocity models and the model parametrization

whole earth structure estimated from seismic arrival times the arrival times of 10 compressional and shear phases p s pp pcp ss scs pk pab pk pbc pk pdf sk sac and two sets of differential times ss s410s and ss s660s are used to infer the earth s three dimensional structure with the exception of ss sds the raw data are composed of arrival times from the international seismological centre isc catalogue the isc events have been relocated using p and s phases and the ak135 radial earth model a total of 818 655 summary arrival times constrain lateral velocity variations in the crust mantle and core we also determine relocation parameters for all events stations corrections for each phase boundary topography 410 km 660 km core mantle boundary inner core boundary and inner core anisotropy variations our model of p and s velocity variations in the crust and mantle contains the general features observed in other body wave inversions a low velocity ring around the pacific ocean basin in the depth range 35 400 km high velocity anomalies associated with subducting slabs most prominent in the depth range 660 1070 km and high velocity anomalies surrounding the pacific basin just above the core mantle boundary in the outer core there is evidence of a latitudinal variation in velocity approaching 0 6 in magnitude with lower p velocities at the poles and higher velocities near the equator this pattern is observed throughout the outer core but is most prominent in the depth range 3671 4071 km and near the inner core boundary synthetic tests indicate that outer core features are moderately resolvable and inversions of random arrival times indicates that the anomalies are a factor of 2 above the estimated parameter noise level lateral heterogeneity in the inner core trades off with anisotropy resulting in poor resolution of isotropic velocity variations anisotropy in the inner core is predominantly subparallel to the earth s axis and of the order of 3 but this value can decrease if more inner core isotropic lateral heterogeneity is allowed a comparison of structure above and below the core mantle boundary cmb indicates that velocity heterogeneity is uncorrelated across the cmb but there is a moderate correlation between cmb topograpy and outer core heterogeneity an inversion in which the pk pdf residuals were absent indicates that the latitudinal pattern of outer core velocity variations is not due to the influence of inner core velocity structure

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

seismic heterogeneity in the mantle strong shear wave signature of slabs from joint tomography abstract the primary source of information on heterogeneity within the earth comes from seismic tomography a powerful tool for examining the character of heterogeneity comes from the comparison of images of bulk sound and shear wavespeed extracted in a single inversion since this isolates the dependencies on the elastic moduli however particularly in such multi parameter inversions there are many hidden facets which can have a strong influence on the results such as the weightings between parameters and in the misfit functions joint inversion with restricted data sets giving comparable cover for p and s waves provides useful checks on more inclusive studies and can provide relatively high resolution in some areas the relative behaviour of bulk sound and shear wavespeed can provide a useful guide to the definition of heterogeneity regimes for subduction zones a large part of the tomographic signal comes from s wavespeed variations in the upper mantle and transition there can be significant bulk sound speed contributions for younger slabs and in stagnant slabs associated with slab roll back for subducted oceanic lithosphere older than about 90 ma shear wavespeed variations nearly always are dominant and so the p wave images are controlled by shear modulus variations the narrow segments of fast wavespeeds in the depth range 900 1500 km in the lower mantle are dominated by s variations with very little bulk sound contribution deep in the mantle there are many fast features without obvious association with subduction in the last 100 ma which suggests long lived preservation of components of the geodynamic cycle

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

spatial and temporal evolution of the subducting pacific plate structure along the western pacific margin 1 tomographic images of the subducting pacific plate beneath the izu bonin mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western pacific island arcs but reasons for the variation in geometry and morphology are still poorly understood by comparing high resolution tomographic images of the western pacific and updated paleogeographic reconstructions we are able to link the spatiotemporal evolution of the subducting pacific plate back to the mid miocene we have reconstructed tectonic motions along the kurile japan izu bonin mariana arc system and the ryukyu arc to provide an independent additional interpretation of the subducting pacific plate using the most current plate motion data we then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images the new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the ogasawara plateau with the trench in the mid miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary the combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes

statistical patterns of high latitude convection obtained from goose bay hf radar observations we have derived patterns that describe the statistical interplanetary magnetic field imf dependencies of ionospheric convection in the high latitude region of the northern hemisphere the observations of plasma motion were made with the hf coherent backscatter radar located at goose bay labrador over the period september 1987 to june 1993 the area covered by the measurements extended poleward of 65 to a working limit of about 85 distributions of electrostatic potential have been derived and expressed as series expansions in spherical harmonics the patterns are the first derived from direct ground based observations of ionospheric convection that approach in completeness and level of detail the patterns derived in recent satellite studies rich and hairston 1994 weimer 1995 we show the dependence of the convection on imf angle in the gsm y z plane for three intervals of imf magnitude in this plane except for predominantly northward imf the convection is primarily two cell the dusk cell is larger in terms of both spatial extent and potential variation the effect of imf by is apparent in the global shaping of the cells and the orientation of the overall pattern in mlt for by by the dusk dawn cell is more round crescent shaped and the pattern more rotated toward earlier mlts the by effect on the nightside convection is pronounced and is hemispherically antisymmetric like the well known day side by effect for imf increasingly northward the convection trajectories on the dayside become increasingly distorted evolving through a three cell to a four cell circulation the additional cells appear on either side of the noon meridian and result in sunward flow the overall agreement with the results of the satellite studies is good and extends to quite fine detail in the case of the comparison with weimer 1995 there are significant differences with the statistical patterns derived from magnetometer measurements which tend to show domination by the dawn rather than the dusk cell

lateral variations in mantle velocity structure and discontinuities determined from p pp s ss and ss sds travel time residuals on the basis of p pp s ss arrival times and ss s410s ss s660s differential times we construct models of mantle p and s velocity structure and boundary topography of the 410 km and 660 km discontinuities events from the catalog of the international seismological centre isc are relocated relative to the international association of seismology and physics of the earth s interior 1991 iasp91 velocity model using both p and s arrival times the arrival times are corrected for ellipticity and the pp and ss residuals are corrected for the topography at the bounce point the cap averaged pp p and ss s differential time residuals plotted at the pp and ss surface reflection points form broad coherent patterns the geographic distribution of the cap averaged residuals agrees quite well with pp p and ss s differential time residuals derived from long period global digital seismograph network gdsn data a robust lp inversion scheme is used to infer global mantle structure synthetic tests indicate that for regions well sampled by ss s410s and ss s660s differential times the velocity estimates are not seriously contaminated by the topography of the 410 and 660 km discontinuities however estimates of boundary deflections may be influenced by extensive p and s velocity variations of 3 or greater we find the 410 km discontinuity to be depressed by as much as 24 km beneath north america conversely the discontinuity is deflected upward underneath eurasia in some regions the topography of the 660 km discontinuity is quite distinct from that of the 410 km discontinuity but the two appear to be positively correlated a series of depressions are found at several intersections of the 660 km discontinuity with known subduction zones the elevated topography in the 410 km discontinuity beneath europe is underlain by a trough in the 660 km discontinuity a number of subduction zones are characterized by a thinning of the transition zone negative p and s velocity anomalies underlying back arc basins and tectonically active continental regions encircle the pacific where they are resolved the stable continental cratons are systematically positive velocity features that extend below 200 km with the inclusion of pp and ss travel time residuals we are better able to constrain midmantle structure most notably in the depth range 35 660 km beneath the northwest pacific we observe high p velocity where they are resolved mid ocean ridges are most clearly imaged as low velocity features in the s model the northern portion of the mid atlantic ridge is underlain by negative s velocity anomalies in the pacific the east pacific rise is an extensive low s velocity anomaly

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

improvement of superdarn velocity measurements by estimating the index of refraction in the scattering region using interferometry 1 in past calculations of convective velocities from super dual auroral radar network superdarn hf radar observations the refractive index in the scattering region has not been taken into account and therefore the inferred ionospheric velocities may be underestimated in light of the significant contribution by superdarn to ionospheric and magnetospheric research it is important to refine the velocity determination the refractive index in the ionosphere at superdarn observation f region altitudes has typical values between 0 8 and close to unity in the scattering region where conditions are more extreme the index of refraction may be much lower a simple application of snell s law in spherical coordinates bouguer s law suggests that a proxy for the index of refraction at the scattering location can be determined by measuring the elevation angle of the returned ionospheric radar signal using this approximation for refractive index the doppler velocity calculation can be refined for each superdarn ionospheric echo using the elevation angles obtained from the superdarn interferometer data a velocity comparison of dmsp and superdarn observations has revealed that the superdarn speeds were systematically lower than the dmsp speeds a linear regression analysis of the velocity comparisons found a best fit slope of 0 74 when the elevation angle data were used to estimate refractive index the best fit slope rose 12 to 0 83 as most superdarn radars employ an interferometer antenna array for elevation angle measurements the improvement in velocity estimates can be done routinely using the method outlined in this paper

j362d28 a new joint model of compressional and shear velocity in the earth s mantle summary the discrimination between chemical and thermal heterogeneity in the earth s mantle remains one of the most important and challenging questions to be answered by observational and theoretical geophysics to answer this question requires a thorough knowledge of the ratio between compressional and shear velocity anomalies we describe results of a joint inversion for compressional and shear velocity in the mantle using a large and diverse data set consisting of traveltimes complete waveforms and surface wave dispersion measurements a horizontal tessellation consisting of 362 spherical splines is used to parametrize the model which is approximately equivalent to a spherical harmonic of degree 18 in resolution the model contains peak variations from prem of up to 7 per cent in s velocity and 2 5 per cent in p velocity in the upper mantle these variations decrease to 1 5 and 0 6 per cent respectively at 1000 km depth and reach 2 5 and 1 0 per cent respectively in the d region the rms ratio of s to p velocity perturbations is fairly constant between 2 0 and 2 5 in the lower mantle but a local minimum in this ratio occurs at a depth of approximately 1700 km resolution tests show that the recovery of p and s velocity is not geographically uniform but also show that this amplitude ratio is well resolved between 670 and 2700 km depth we also observe a persistent negative correlation between bulk sound and shear velocity throughout the same region of the mantle in addition the model contains a minimum correlation between p and s velocity between 670 and 1100 km this feature is supported by both the favourable outcome of resolution experiments and the poor fit provided by the starting model from the inversion in which the p and s velocities are perfectly correlated to our data set of p wave traveltimes the power spectra of both p and s velocity heterogeneity are similar although we note a slightly larger dominance of degree two in the spectra for p velocity in the mid mantle where resolution is highest

tectonic magmatic stages of shield evolution the pan african belt in northeastern egypt abstract the arabian nubian shield illustrates an example of plate tectonics during the pan african orogenic event ca 950 550 ma it is considered one of the most remarkable proterozoic shield areas yet known although there is now a general agreement that the shield has evolved by a series of magmatic arcs and terrain accretion the evolution of its northernmost part exposed in northeastern egypt remains controversial two contrasting tectonic models have been proposed for the evolution of this region regional crustal extension and magmatic arc regimes there is no convincing evidence of older archean sialic materials within the region although the region is flanked by archean crust the neoproterozoic crustal components exposed in northeastern egypt preserve a record for its evolution a large gabbro diorite tonalite complex gdt 881 ma emplaced during the early stage of the pan african orogeny is geochemically primitive exhibits a low initial 87 sr 86 sr ratio 0 704 and shows trace element characteristics of oceanic arc related lavas the gdt complex was produced by fractionation from a mantle derived tholeiitic magma formed within an island arc tectonic environment a synorogenic stage characterized by extensive volcanism that produced the dokhan volcanic suite dvs 620 ma forms a continuous basalt to rhyolitic ignimbrite calc alkaline magma series exhibiting features of arc related volcanism with ree profiles similar overall to andean andesites and ignimbrites the dokhan suite was produced in a continental arc setting a granodiorite adamellite leucogranite composite batholith 552 ma emplaced during the late stage of the pan african orogeny exhibits typical features of i type complexes trace element traits of volcanic arc granites and is also interpreted to have been formed in an andean type setting a trondhjemite pluton was also formed at this late orogenic stage by partial melting of gdt host rocks at depth cooling and relaxation of the newly formed pan african crust caused extensive fracturing which was followed by intrusion of ne sw and nnw sse trending dyke swarms at 493 ma this episode marks a fundamental inversion from orogenic compressional to anorogenic extensional processes around the neoproterozoic paleozoic boundary anorogenic magmatism was locally associated with such extension induced structures the mount gharib peralkaline granite 476 ma for example exhibits trace element traits of a type within plate granites and a high initial 87 sr 86 sr ratio 0 711 and was formed in a rift environment thus the region is characterized by lengthy episodes of discontinuous subduction 880 550 ma during which it evolved to form a primitive crust by early pan african island arcs and late pan african continental arcs this was followed by an even longer period of crustal extension 550 90 ma that produced localized anorogenic magmatism

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

evolution of mantle structure beneath the northwest pacific evidence from seismic tomography and paleogeographic reconstructions 1 plate motions and subducting slab morphology are intricately connected and through the integration of seismicity tomographic images and relative plate motions the evolution of mantle structure can be interpreted tomographic images of p wave shear wave speed and bulk sound speed perturbations of the northwest pacific region have been interpreted to define the extent and geometry of the subducting pacific plate we have found that the subducted pacific plate beneath the japan and kurile arcs is coherent but is very complex at the junction of the two arcs near the hokkaido corner as the slab subduction angle decreases from north to south while the slab thickens the pacific slab beneath the japan arc is slightly thinner and subducting through the upper mantle at a less steep angle than the slab beneath the kurile arc as the slab reaches the 660 km discontinuity it becomes stagnant and lies in a horizontal position on top of the transition zone both beneath the izu and japan arcs in contrast the slab beneath the kurile arc is subducting at a steeper angle and penetrates through the transition zone into the lower mantle the difference in slab morphology appears to be due to a combination of change in dip of the subducting plate and the convergence velocity of the pacific plate along the margin a new paleogeographic reconstruction of northeast asia was assembled to evaluate the plate motions that could explain the foundation of the current slab morphology the plate reconstruction was based on euler pole motions of the pacific philippine amurian okhotsk and north american plates relative to stable eurasia plate the new tectonic model illustrates the collision of the japan and kurile arcs the opening of the kurile basin disparity in pacific plate velocities along the arc and different rates of trench retreat along the izu japan and kurile segments of the western pacific margin the plate motion model was interpreted in conjunction with the physical properties of the mantle imaged with the p wave and joint tomography to assess the evolution of the pacific plate morphology since the mid miocene and to provide constraints on the plausible plate motions in the region

global mantle shear velocity model developed using nonlinear asymptotic coupling theory we present a three dimensional shear velocity model of the whole mantle developed using s h waveform data the model is expressed horizontally in terms of spherical harmonics up to degree 12 and vertically in terms of legendre polynomials up to degrees 5 and 7 in the upper and lower mantle respectively what distinguishes this model from other tomographic models published to date is 1 the theoretical normal mode based wave propagation approach where we include across branch mode coupling terms in order to model the body wave sensitivity to structure along the path more accurately 2 the wave packet weighting scheme which allows to balance contributions from high amplitude and low amplitude phases increasing the resolution in some parts of the mantle we also relax the constraints on the moho depth which is allowed to vary in the inversion thus absorbing some uncertainties in crustal structure the resulting model is generally in good agreement with other recent global mantle s velocity models and with some regional models the rms profile with depth has more power than other models in the upper mantle lower mantle transition region and the zone of increased power and low degree structure near the base of the mantle is confined to the last 500 km in depth this model provides a particularly good fit to the non hydrostatic geoid through harmonic degree 12 79 variance reduction as well as good fits to observed splitting functions of s velocity sensitive mantle modes indicating that both large scale and small scale features are really well constrained