deep seismic structure of the kaapvaal craton the kaapvaal craton is remarkable for the intensive jurassic cretaceous kimberlite magmatism there are indications that the hot spot heating of the upper mantle in southern africa continued in the cenozoic however surface wave data in spite of their sensitivity to temperature failed to provide any evidence of thermal agitation at depths less than 350 km beneath the kaapvaal craton bloch et al 1969 we investigated the s velocity structure in the depth range of the mantle transition zone with the aid of teleseismic p to s converted phases our analysis suggests that a layer of anomalously low s velocity is present in the 370 470 km depth interval beneath the kaapvaal craton the anomalous layer is most likely of thermal origin its upper boundary may separate the deep mantle root of the craton from the underlying mantle the anomaly could arise beneath the craton already in the mesozoic and melting within this layer could contribute to the kimberlite magmatism

phosphatic limestones from tuvalu ellice islands phosphatic limestones and associated soils occur on each of the nine islands of tuvalu central pacific deposits range from gram sized crusts to 500 000 metric tons carbonate hydroxylapatite dahllite forms multilaminar botryoidal crustose cement around calcareous bioclasts which it sometimes replaces consolidated rock occurs as hardpan with unconsolidated phosphatic layers above and below the genetic relationship of phosphatic limestone to unconsolidated phosphatic sediments and to phosphatic soil is unclear phosphatization has occurred either continuously or episodically within the vadose zone for at least 2 000 yrs while relative sea level has fluctuated the present deposits are exhumed accumulations of apatite formed at earlier higher holocene sea levels with phosphorous derived from possibly both terrestrial and marine organisms including birds plants and degradation of the bioclastic calcareous substrate humic complexes could enable phosphate to enter the lensoid fresh water table degradation of these humic complexes would liberate free hpo super 2 4 with subsequent precipitation of apatite under suitable conditions of alkalinity and carbonate and magnesium activity pisonia grandis should not be regarded as a geobotanical indicator of phosphate deposits in present day tuvalu

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

geochronology and geochemistry of the late triassic longtan pluton in south china termination of the crustal melting and indosinian orogenesis the indosinian orogeny is recorded by triassic angular unconformities in vietnam and south china and by widely occurring granitoids in the yunkai nanling and the xuefengshan belts of south china the longtan pluton in the northwestern part of the xuefengshan belt is a typical high k calc alkaline i type granitoid which can shed light on the relationship between the indosinian tectonic and magmatic activity in the region three precise zircon u pb ages yielded a mean of 218 0 8 ma which is taken as the age of crystallization the pluton consists of both granodiorite 64 59 68 01 sio2 and 3 25 4 22 k2o and granite 70 49 71 80 sio2 and 4 07 4 70 k2o the granodiorites are characterized by relatively high mg 54 57 low contents of na2o 3 2 4 3 wt low abundances of incompatible elements lile nb and p high initial 87sr 86sr 0 7175 0 7184 and negative end t 9 98 to 9 72 ree patterns show moderate fractionation la yb cn 8 07 18 80 with negative eu anomalies eu eu 0 62 0 86 compared with the granodiorite the granite has a wider range of mg 49 59 lower contents of na2o 2 8 4 2 wt higher initial 87sr 86sr 0 7232 0 7243 and more negative end t 12 07 to 11 24 values ree patterns are relatively flat la yb cn 14 73 29 37 with smaller negative eu anomalies eu eu 0 48 0 63 the granodiorite has lower k2o na2o and al2o3 mgo feotot values than the granite based on major and trace element geochemistry and sr nd isotopes we interpret the longtan granodioritic magma to have been derived by partial melting of interlayered proterozoic metabasaltic to metatonalitic source rocks whereas the granite was probably derived from a mixture of proterozoic metagraywackes and metaigneous rocks field petrographic and geochemical evidence indicate that partial melting and fractional crystallization were the dominant mechanism in the evolution of the pluton the longtan granodiorites and granites are petrologically and geochemically similar to typical indosinian varieties and are considered to have been produced in a similar manner the indosinian granitoids in the region show a magmatic peak age of 238 ma from the yunkai nanling belt in the southeast and a magmatic peak age of 218 ma of the xuefengshan belt to the northwest these early and late magmatic episodes of the indosinian granitoids also display slight variations of regular compositions end t values and t dm ages thus we propose a syncollisional extension model that indosinian granitoids were generated by decompressional partial melting of crustal materials triggered by two extensions during collision of the indochina and south china blocks the longtan pluton in the northwesternmost part of the orogenic belt marks the termination of the indosinian magmatism and orogenesis

receiver function analysis of the north american crust and upper mantle summary data from 27 permanent broad band stations spread over north america and sampling diverse tectonic regimes of varying ages from the archaean to the phanerozoic were analysed using the receiver function approach receiver functions a few hundred at most of the locations provide valuable constraints on the composition of the crust shaped by diverse processes characteristic of their time of formation the chemically inert and stable nature of the archaean domains of canada is well manifested in their low poisson ratio 0 26 felsic lower than global averages for precambrian age crust accompanied by a thicker than global average 35 km archaean crust data from the canadian cordillera and innuitian orogens of phanerozoic times indicate the possible operation of two distinctly different crustal processes resulting in thin crusts 35 km with dominantly felsic and more mafic compositions respectively these processes are restricted to the region above 410 km without disturbing the integrity of the mantle transition zone mtz which is evident from the remarkably homogeneous nature of the 410 and 660 km discontinuities beneath the canadian landmass contrastingly the thermally complex phanerozoic southern california region suggests the presence of a heterogeneous mantle transition zone possibly owing to localized 660 topography with a relatively uniform 410 overlain by a crust heterogeneous in composition and perhaps maturing to that of a continent

s velocity reversal in the mantle transition zone the transition zone differs from the rest of the mantle by high positive p and s wave velocity gradients however by applying s receiver function technique to recordings of about 50 globally distributed stations in 7 regions we obtain evidence of negative discontinuity at a depth around 500 km four of these regions correspond to the well known hotspots no other hotspots are sampled by our observations at this depth our modeling suggests that the negative discontinuity is hardly an effect of anisotropy of wadsleyite more likely it is caused by isotropic s velocity reduction of around 0 2 km s origin of the low velocity can be related to enhanced water content in wadsleyite

multiple seismic discontinuities near the base of the transition zone in the earth s mantle the seismologically defined boundary between the transition zone in the earth s mantle 410 660 km depth and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma spinel phase of olivine to magnesium perovskite and magnesiowustite laboratory measurements of these transformations of olivine have determined that the phase boundary has a negative clapeyron slope and does indeed occur near pressures corresponding to the base of the transition zone but a computational study has indicated that because of the presence of garnet minerals multiple seismic discontinuities might exist near a depth of 660 km ref 4 which would alter the simple negative correlation of changes in temperature with changes in the depth of the phase boundary in particular garnet minerals undergo exothermic transformations near this depth acting to complicate the phase relations and possibly effecting mantle convection processes in some regions here we present seismic evidence that supports the existence of such multiple transitions near a depth of 660 km beneath southern california the observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma spinel phase of olivine temperature anomalies calculated from the imaged discontinuity depths using clapeyron slopes determined for the various transformations generally match those predicted from an independent p wave velocity model of the region

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

mantle control of plate boundary deformation the seismic wavefield propagating along the recently instrumented pacific north american plate boundary california displays remarkable variation with regional shear waves arriving at coastal stations up to 20 seconds earlier than equidistant stations in eastern california broadband modeling of this data reveals that coastal paths sample fast upper mantle typical of miocene aged ocean plate 50 km thickness inland paths sample slower uppermost mantle with the seismic lithosphere or lid measuring less than 5 km thick characteristic of the basin and range extensional province the boundary in the uppermost mantle between these provinces is sharp expressing the juxtaposition of the stronger pacific plate with weaker continental north america the lid step coincides with regionally maximum dextral strain rates measured with gps suggesting the uppermost mantle modulates long term regional scale continental margin deformation and evolution

lithospheric and upper mantle stratifications beneath tibet new insights from sp conversions 1 we assess the upper mantle structure under the tibetan plateau from a s to p converted waves receiver functions study contrary to the ps receiver functions blurred by multiples from moho to 350 km depth the sp are better suited for imaging at these depths the moho is clearly recovered and often exhibits a complex and warped structure the upper crust is marked by a significant low velocity zone in the southern part of the plateau probably associated with partial melt which vanishes north of the bangong suture in the upper mantle between the moho and the 660 km discontinuity four stratification levels are identified the strongest converter at a depth ranging between 120 to 180 km corresponds to the bottom of a low shear wave velocity layer imaged by surface wave inversion

ubiquitous low velocity layer atop the 410 km discontinuity in the northern rocky mountains receiver functions from three 30 station iris passcal small aperture arrays 2 15 km station spacing operated for 10 months each in the northern rocky mountains show a ubiquitous negative polarity p to s conversion just preceding the 410 km discontinuity arrival data from the three arrays were sorted into nw se and sw back azimuth quadrants and stacked to form nine quadrant stacks remarkably the negative polarity arrival npa is apparent in 8 of the 9 quadrant stacks with 7 of the 8 having well correlated waveforms each quadrant stack also contains clear p to s conversions from the 410 and 660 km discontinuities moveout analysis shows that all the major phases display the correct moveout for forward scattered p s phases the waveshapes for the seven similar npa 410 km discontinuity arrivals are modeled with a five parameter double gradient slab model that is parameterized as follows a top gradient thickness and shear velocity decrease a constant velocity layer bottom gradient thickness and shear velocity increase model misfit is assessed via a grid search over the model space using a reflectivity code to calculate synthetic seismograms model likelihood is determined by calculating 1 and 2 d marginal probability density functions pdf for the five parameters the 1 d marginals display a range of peak values although significant overlap is observed for the top gradient thickness and its associated velocity decrement from the peak value of the summary pdf we find the top velocity gradient to be sharp 6 4 km and the shear velocity decrement to be large 8 9 vs defining an effective thickness of the low velocity layer as the mean layer thickness plus half the mean gradient thicknesses the 410 low velocity layer thickness is found to be 22 km a review of changes in the physical state required to match our new 410 lvl constraints suggests that the water filter model remains an operative hypothesis to test

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

global scale analysis of the mantle pds phases a global scale analysis of travel times of the mantle pds phases converted from p to s from the discontinuity at a depth d beneath the receiver is presented most of the data are related to p 410 s and p 660 s recorded by the broadband stations of geoscope incorporated research institutions for seismology iris and other networks in order to provide a good global scale coverage published measurements at 37 stations are complemented by our measurements at 45 stations lateral resolution of our data is many times higher than that of ss and its precursors previously used for similar purposes but we have fewer data in the oceans lateral variations of travel times of the major pds phases are strongly correlated with the lateral p and s velocity variations in the uppermost mantle as revealed by tomographic studies the scaling coefficient between the teleseismic s and p residuals as found from the variations of travel times of the major pds phases is around 3 7 for north america and around 3 3 for the rest of the world almost all the differential travel times t p 660 s t p 410 s are in the range of 1 s relative to the average value of 24 0 s if the lateral variations of the velocities within the mantle transition zone mtz can be neglected the corresponding variations of thickness of the mtz are in the range of 10 km relative to the standard depth of 250 km and do not show significant correlation with either velocities in the upper mantle above the mtz or with the variations of thickness of the mtz as found in some recent studies of precursors to ss there is a weak but marginally significant positive correlation between the variations of thickness of the mtz inferred from our data and the s velocity variations in the mtz as given by su et al 1994 this correlation suggests that both variables contain temperature dependent components lateral variability of the mtz is also manifested by extreme weakness of p 410 s at several locations this may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity at some stations there are arrivals that can be interpreted as p 520 s this interpretation implies that the strengh and or sharpness of 520 km discontinuity as well as its depth are laterally variable in a broad range

lithospheric structure of northern california from teleseismic images of the upper mantle teleseismic p wave travel time residuals from 120 earthquakes recorded across the u s geological survey california seismic network were used to determine the lithosphere p wave velocity structure beneath northern california a region characterized by complex interactions between the pacific north american and gorda plates lateral p wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals inversion results for the crust show strong correlations to volcanic features the active volcanic fields shasta medicine lake lassen and clear lake are characterized by crustal low velocity anomalies that average approximately 6 possibly identifying partially molten magma bodies cooled solidified magma bodies beneath the extinct volcanic fields sonoma southern clear lake and sutler buttes are denoted by relative velocity highs averaging 3 the largest upper mantle velocity variations occur in the depth range 30 110 km where velocities vary from 5 5 to 9 5 these velocity variations reflect changes in the thickness and geometry of the pacific north american and gorda plates where they interact at the mendocino triple junction north of the mendocino triple junction the steep 70 east dipping portion of the gorda plate is imaged as a 5 velocity high to depths near 270 km a presumed segment of the gorda plate observed beneath the northern great valley and south of the inferred edge of the plate is characterized by a 9 velocity high in the depth range 30 70 km beneath the northern coast ranges shallow asthenosphere is imaged in the depth range 30 100 km as a pronounced southward tapering 4 low velocity zone which we interpret as the slab window results from this study provide improved constraints on gorda plate subduction evolution of the san andreas fault system and development of the lithosphere beneath western north america

deep structure of the afro arabian hotspot by s receiver functions 1 we investigated deep structure of the afro arabian hotspot by using recordings from geoscope seismograph station atd the records are processed with the s receiver function technique which allows a detection of sp converted phases from the upper mantle discontinuities the seismic data reveal two unusual discontinuities the discontinuity at a depth of 160 km beneath the gulf of aden corresponds to the onset of melting if the water content in olivine is around 800 h 106si melting at this depth requires a temperature close to 1550 c about 120 c higher than the average another remarkable discontinuity is found at a depth of 480 km where s velocity drops with depth by about 0 2 km s this can be the head of another plume which is trapped in the mantle transition zone

regional three dimensional seismic velocity model of the crust and uppermost mantle of northern california 1 we present a three dimensional 3d tomographic model of the p wave velocity vp structure of northern california we employed a regional scale double difference tomography algorithm that incorporates a finite difference travel time calculator and spatial smoothing constraints arrival times from earthquakes and travel times from controlled source explosions recorded at network and or temporary stations were inverted for vp on a 3d grid with horizontal node spacing of 10 to 20 km and vertical node spacing of 3 to 8 km our model provides an unprecedented comprehensive view of the regional scale structure of northern california putting many previously identified features into a broader regional context and improving the resolution of a number of them and revealing a number of new features especially in the middle and lower crust that have never before been reported examples of the former include the complex subducting gorda slab a steep deeply penetrating fault beneath the sacramento river delta crustal low velocity zones beneath geysers clear lake and long valley and the high velocity ophiolite body underlying the great valley examples of the latter include mid crustal low velocity zones beneath mount shasta and north of lake tahoe

global de correlation of the topography of transition zone discontinuities analysis of long period shear wave underside reflections from upper mantle discontinuities shows the shape of the 410 km discontinuity to be neither globally nor locally correlated with that of the 660 km discontinuity the long wavelength pattern of 410 topography is dominated by degree 1 spherical harmonics and has a reasonable correlation with the seismic velocity perturbations in the mantle above it the global topography of 660 dominated by degree 2 spherical harmonics is strongly correlated with shear wave velocity variations in the transition zone the poor correlation between the observed topography of the 410 and 660 km discontinuities is inconsistent with a simple model of large scale whole mantle flow thus implying a more complex pattern which includes at least temporary accumulation of subducted material in the transition zone ringwood and irifune nature 331 1988 131 136 tackley et al nature 361 1993 699 704 the modest seismic discontinuity at 520 km is well observed when the reflections are under mid age oceans but is not observed when they are under continental shields interpretation of this observation in terms of olivine depletion in the deep continental mantle indicates the possible existence of continent ocean chemical heterogeneity at depths exceeding 500 km

mantle anomalies beneath southern africa evidence from seismic s and p receiver functions summary to understand deep structure and processes beneath southern africa we apply the doublestacking version of the s receiver function srf technique to the recordings of the south african seismic experiment in this technique the wavefields of s and sks are separated by space time filtration and the receiver functions are constructed separately for the s and sks seismic phases the results are consistent with those obtained from the srfs of the permanent stations bosa and lbtb evidence for a reduced s velocity atop the 410 km discontinuity is present in both srfs and p receiver functions the s velocity contrast at the 410 km discontinuity is 40 per cent larger than the norm and there are observations of s350p and p350s seismic phases from the negative 350 km discontinuity beneath the kaapvaal craton the s350p and p350s phases display a dependence on the azimuth which can be caused by anisotropy in the layer atop the 350 km discontinuity this dependence is consistent with observations of shear wave splitting in sks there are observations of the s450p phase from the 450 km negative discontinuity in the transition zone the most anomalous transition zone is found close to the region where the kalahari craton was located in the mesozoic lateral variations of the s velocity are found beneath the 660 km discontinuity teleseismic s and p traveltime residuals with respect to iasp91 model are evaluated from the traveltimes of the p410s and pp410s seismic phases in the uppermost mantle these residuals as well as s410p traveltimes require a reduced vp vs ratio around 1 75 0 01 versus 1 8 in iasp91 an effect of the depletion of the mantle lithosphere in basaltic material in the models obtained by a joint inversion of the receiver functions and the teleseismic traveltime residuals the low velocity zone lvz with the onset of low velocity at a depth of 140 20 km is present at most locations the minimum s velocity in the lvz is 4 5 km s 1 the lvz in our models is consistent with the s velocity vp vs velocity ratio and depth range of high temperature lerzholites in the mantle xenoliths from southern africa our most intriguing finding is a very low quality factor qs on the order of a few tens in the upper mantle we interpret high attenuation the lvz and the low s velocity atop the 410 km discontinuity as the effects of plume like phenomena in the upper mantle

thickness estimates of the upper mantle transition zone from bootstrapped velocity spectrum stacks of receiver functions summary we modify the receiver functions stacking technique known as velocity spectrum stacking vss so as to estimate combinations of velocity model v p and v s and depth that stack the ps conversion from upper mantle discontinuities most coherently we find that by estimating the diverences in the depths to the 660 and 410 km discontinuities using velocities that maximize the stacked amplitudes of p410s and p660s phases we can estimate the thickness of the transition zone more accurately than the depths to either of these discontinuities we present two examples indicating that the transition zone beneath obninsk russia is 252 6 km thick and that beneath pasadena california is only 220 6 km thick

observations of s410p and s350p phases at seismograph stations in california 1 we analyze a new set of seismic data from seismograph stations in california this data set consists of nearly 5000 s receiver functions for 47 seismograph stations as a rule the stacked srfs display a distinct s410p seismic phase s wave converted to p at the 410 km discontinuity the wave paths of s410p sample the upper mantle beneath california and the neighboring region of the pacific in northernmost california the s410p travel times are close to those of the iasp91 global model further south s410p usually arrives about 2 s earlier than predicted by the iasp91 model this early arrival can be explained either by an anomalously high vp vs velocity ratio 1 9 in a 125 km thick layer of the upper mantle versus 1 8 in iasp91 by a depression of the 410 km discontinuity of 15 km or by a combination of both effects with smaller amplitudes we observe systematically s350p phase which is converted from a negative discontinuity with a lower s velocity at the lower side near a depth of 350 km the observations of s350p are indicative of a low s velocity layer a few tens of kilometers thick atop the 410 km discontinuity beneath southern california and the neighboring oceanic region some receiver functions also display s480p phase which is interpreted as evidence of an intermittent low velocity layer in the transition zone

low velocity zone atop the 410 km seismic discontinuity in the northwestern united states the seismic discontinuity at 410 km depth in the earth s mantle is generally attributed to the phase transition of mg fe 2sio 4 from the olivine to wadsleyite structure variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations for example a cold anomaly would elevate the 410 km discontinuity because of its positive clapeyron slope whereas a warm anomaly would depress the discontinuity but trade offs between seismic wave speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities and structure often appears very complicated here we simultaneously model seismic refracted waves and scattered waves from the 410 km discontinuity in the western united states to constrain structure in the region we find a low velocity zone with a shear wave velocity drop of 5 on top of the 410 km discontinuity beneath the northwestern united states extending from southwestern oregon to the northern basin and range province this low velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic receiver function observations observed above the region we interpret the low velocity zone as a compositional anomaly possibly due to a dense partial melt layer which may be linked to prior subduction of the farallon plate and back arc extension the existence of such a layer could be indicative of high water content in the earth s transition zone

teleseismic travel time residuals in north america and anelasticity of the asthenosphere abstract the slope a of the relation t s a t p b for the teleseismic s and p travel time residuals t s and t p is practically the only robust observational value that is equally sensitive to the lateral s and p velocity variations in the upper mantle published estimates of a for north america are about twice higher than those at ultrasonic frequencies in olivine at comparable pressures and temperatures in our study lateral variations of the s residuals in north america are found not from the s wave readings which can be easily biased but from accurately determined delays relative to p of the ps phases converted from 410 km and 660 km discontinuities nevertheless the estimates of a thus derived are close to the values obtained with the standard technique the discrepancy between the ultrasonic and seismic estimates of a can be explained by physical velocity dispersion on the assumption of a single absorption band our value of a is consistent with the published values of q in the seismic frequency band and velocities in the ultrasonic range if the high frequency cutoff of the absorption band in the upper mantle beneath western north america is on the order of several hundred hertz however in the literature there are indications of a cutoff at around 1 hz then our data imply the presence of another absorption band at high frequencies outside the seismic frequency band this band may correspond to the viscous dissipation in the walsh walsh j b 1969 a new analysis of attenuation in partially melted rock j geophys res 74 4333 4337 model of penny shaped inclusions of melt

subcratonic low velocity layer and flood basalts 1 teleseismic recordings of several seismograph stations are processed with the aid of the s receiver function technique farra and vinnik 2000 the data provide evidence of a strong reduction of s velocity at a depth around 360 km station bosa and 280 300 km station lbtb in the east of southern africa a similar feature is found in the tunguska basin of the siberian platform from the recordings of station nril in both regions large volumes of flood basalts have been erupted 190 180 ma and 250 ma ago respectively the coincidence suggests a causal relation between the two phenomena this relation implies that the low velocity anomalies migrate with the plates

temperature and melting of a ridge centred plume with application to iceland part ii predictions for electromagnetic and seismic observables summary the dynamic and melting processes of a ridge centred plume have been investigated in a companion paper by ruedas et al hereafter referred to as paper i in a set of 3 d numerical fluid dynamic models with varying plume excess temperatures and melt extraction thresholds in paper i the modelled thickness of the generated crust has been compared to observations of the icelandic crust using the results of those plume models magnetotelluric mt transfer functions and seismic velocity anomalies are predicted in this paper together with paper i a dynamically consistent set of geophysical observables of a ridge centred plume is presented and applied to iceland temperature partial melting and the connectivity of the melt phase influence the electrical conductivity of crust and mantle rocks the temperature and melt fraction of our plume models are used to calculate 3 d conductivity models for mt modelling for the melt geometry ellipsoidal inclusions with appropriate aspect ratios were assumed to control melt connectivity the resulting transfer functions are compared to each other and to models not including a plume to separate signals from the ridge and the plume they may be applied to observed mt measurements if the plume head contains only 1 per cent of melt the plume signal cannot be distinguished from the ridge signal at least 3 per cent melt is needed for such distinction the other predicted observables calculated from the different numerical models are seismic velocity anomalies the temperature induced vp and vs anomalies were estimated including anharmonic and anelastic effects as well as the water induced increase of dislocation mobility that lowers seismic velocities realistic melt geometries as observed in laboratory experiments were used to calculate the effect of partial melts on the seismic velocities vs anomaly distributions are synthesized from the different plume models and compared to seismic observations to reconcile seismic anomalies of the plume head and plume stem a wet plume stem overlain by a partially molten dehydrated plume head is favoured the combined interpretation of available observations crustal thicknesses paper i and seismic results with our dynamic plume models paper i leads to a favoured plume model with 135 k excess temperature and a vertical velocity of approximately 13 cm yr 1 at 200 km depth with 1 per cent melt extraction threshold and a melting zone of approximately 500 km width and 100 km depth extent

three dimensional s wave velocity structure in southern california 1 the three dimensional s wave velocity structure was constructed from rayleigh wave phase velocities measured using teleseismic trinet data the s wave velocity maps show some features that are much more distinct than previous tomographic results in southern california there is a clear seismic velocity contrast across the san andreas fault the north american plate side is systematically slower than the pacific plate side under the eastern california shear zone there are distinct slow velocity anomalies in the crust suggesting a close connection to lower viscosity crust previously concluded from observation of the post seismic deformation after the landers earthquake major upper mantle features include a fast velocity root under the transverse ranges which is the dominant feature in the upper mantle and a slow velocity anomaly under the salton trough our results support that the velocity contrast across the san andreas fault extends below the moho

upper mantle stratification by p and s receiver functions summary seismic stratification of the upper mantle is investigated by applying two complementary techniques to the records of the graefenberg array in southern germany the anisotropic p receiver function technique kosarev et al 1984 vinnik montagner 1996 is modified by using summary seismic events instead of individual events and different weighting functions instead of the same function for the harmonic angular analysis of the sv and t components of the pds phases the summary events provide better separation of the second azimuthal harmonic than the individual events the parameters of the second harmonics of sv and t thus evaluated should be similar if they reflect the effects of azimuthal anisotropy this can be used as a criterion to identify the anisotropy to detect the sdp phases and their azimuthal variations caused by azimuthal anisotropy we have developed a stacking technique which can be termed the s receiver function technique it includes axis rotation to separate interfering p and s arrivals determination of the principal m component of the s wave motion deconvolution of the p components of many recordings by their respective m components and stacking of the deconvolved p components with weights depending on the level of noise and the angle between the m direction and the backazimuth of the event both techniques yield consistent results for the graefenberg array as indicated by the p receiver functions the upper layer of the mantle between the moho and 80 km depth is anisotropic with dvs vs around 0 03 and the fast direction close to 20 clockwise from north the fast direction of anisotropy below this layer is around 110 the boundary between the upper and the lower anisotropic layers is manifested by the detectable pds and sdp converted phases shear wave splitting in sks is strongly dominated by azimuthal anisotropy in the lower layer asthenosphere