mantle discontinuity structure beneath china mantle reflectivity profiles obtained from long period recordings of multiple scs reverberations from intermediate and deep focus earthquakes surrounding china reveal remarkably sharp images of china deep structure the tibetan plateau and fold belts to the north are underlain by a midlithospheric low velocity zone lvz evidenced by coincident g lid lvz boundary and l base of the lvz reflectors and a thickened lithosphere consistent with uniform thickening as the dominant mode of north south deformation the mantle beneath northeastern china mongolia and southeastern russia has a typically cratonic reflectivity signature with h hales and deep l lehmann discontinuities the sino korean paraplatform of eastern china is a region of active extensional tectonics as expressed by an oceanic reflectivity pattern at shallow depths a g discontinuity at 80 km but a more continental structure below the southern sea of japan korean peninsula yellow sea and bordering regions of the china mainland are underlain by a 5 4 impedance decrease near 330 km depth that we interpret as the upper limit of dense silicate melt atop the 410 km discontinuity produced at the subduction zones to the east regional inversion of the reflection coefficient of the 410 km discontinuity r 410 reveals a pronounced high beneath mongolia and north to northwestern china which is best explained by increased upper mantle olivine content signifying extensive basalt depletion in a tectosphere that extends below the 410 km discontinuity a low value of r 660 associated with topographic depression of the 660 km discontinuity beneath eastern asia is consistent with a horizontally inclined slab atop the phase transition marking the seismic discontinuity reflection defocusing is an alternative but requires 38 km of intermediate scale topography within the depressed region exceeding the amplitude of the depression by a factor of 1 5 2 either mechanism requires strong heterogeneity far west of seismically active slabs two lower mantle reflectors one beneath southern honshu kyushu and the sea of japan the other below shantung province china and the yellow sea correlate with horizontally elongated velocity anomalies in western pacific seismic tomography models we speculate that they may represent slabs stagnating below the 660 km discontinuity

the effect of water on the 410 km discontinuity an experimental study 1 in order to evaluate the effect of h on the pressure and transition interval of the olivine wadsleyite transformation synthesis experiments were carried out in hydrous and anhydrous peridotite compositions at 1400 c under hydrous near saturated conditions the pressure of the transition is 1 0 0 5 gpa lower than under anhydrous conditions in addition the two phase interval appears to broaden from about 0 4 gpa 12 km in the anhydrous system to as much as 1 3 gpa 40 km in the hydrous system under hydrous conditions the spatial distribution of olivine and wadsleyite appears to be controlled by h diffusion extrapolating previously measured diffusion coefficients for h in olivine and in liquids it appears that h diffusion driven by gravitational equilibration may serve to narrow the transition interval between olivine and wadsleyite

strain partitioning in a pluton during emplacement in transpressional regime the example of the neouvielle granite pyrenees this microstructural and anisotropy of magnetic susceptibility study of the internal structures of the hercynian neouvielle granite pluton 100 km2 provides new data indicating that the pluton was emplaced during the main hercynian tectonic event recognized in the pyrenees it also provides new data about the later alpine deformation localized along narrow mylonitic bands these bands acted as reverse faults and have not rotated the hercynian structures that define the main part of the pluton the pluton is composed of two structural domains the northern half of the pluton displays a beak shape in map view with subhorizontal e w trending lineations of magmatic origin the southern half is semi circular and displays rather steeply northward plunging lineations corresponding to magmatic and high temperature ht solid state microstructures these features are associated with magma deformation during emplacement magma deformation corresponds in the northern half of the pluton to an e w strike slip deformation recognized in the enveloping pelitic metasediments of carboniferous age and in the southern half of the pluton to southward overthrusting recognized in the enveloping quartzites of devonian age juxtaposition in a single granite body of transcurrent and compressive domains is viewed as a strain partitioning in the magma this strain partitioning is linked to both the transpressive character of the main regional deformation event and the rheological contrast between the pelitic country rocks and quartzose country rocks

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

p v vp vs t measurements on wadsleyite to 7 gpa and 873 k implications for the 410 km seismic discontinuity the compressional p and shear wave s velocities for mg2sio4 wadsleyite have been measured to 7 gpa and 873 k using simultaneous ultrasonic interferometry and in situ x ray diffraction techniques from the velocity measurements we obtained the pressure and temperature derivatives for the elastic shear g and adiabatic bulk ks moduli g p t 1 5 1 g t p 0 017 1 gpa k ks 173 2 gpa ks p t 4 2 1 and ks t p 0 012 1 gpa k for the p and s waves we obtained vs p t 0 021 1 km s gpa vs t p 0 035 2 km s k vp p t 0 065 2 km s gpa and vp t p 0 038 2 km s k values in parentheses are standard deviations e g 1 5 1 1 5 1 independent equation of state analysis of p v t data provided an estimation of the temperature dependence for the isothermal bulk modulus of kt t p 0 022 12 gpa k and thermal expansion a bt coefficients of a 2 10 5 k 1 and b 2 5 10 8 k 2 using these data along with elastic properties for other mantle phases a velocity depth profile for a pyrolite model to 670 km depth is constructed using a finite strain method along a 1673 k adiabat in the transition zone the pyrolite model has a smaller gradient between 410 and 660 km than the body wave models from synthetic waveform analyses but converges with the seismic profiles at the bottom of the transition zone just above the 660 km discontinuity the pyrolite model has velocity jumps of 6 9 and 7 9 for p and s waves respectively over a thickness of 10 km for the phase transformation from olivine to wadsleyite which is in good agreement with short period p wave reflection data and a recent fine structure model c4 for both the velocity jumps and the thickness of the 410 km seismic discontinuity

seismic evidence for accumulated oceanic crust above the 660 km discontinuity beneath southern africa 1 high pressure assemblages of subducted oceanic crust are denser than the normal upper mantle but less dense than the uppermost lower mantle thus subducted oceanic crust may accumulate at the base of the upper mantle direct observational evidence for this hypothesis however remains elusive we present an analysis of a negative polarity shear wave converted from compressional wave at a seismic discontinuity near 570 600 km depth beneath southern africa the negative polarity of the converted phase indicates a 2 2 0 2 s velocity decrease with depth at the seismic discontinuity this velocity reduction is associated however with a low velocity contrast at the 660 km discontinuity the exsolution of ca perovskite in former oceanic crust at depth greater than 600 km and the associated small volume fraction of ringwoodite are plausible explanations for the apparent paradox between the negative velocity discontinuity and the low velocity contrast at the 660 km discontinuity

transition zone thickness beneath indonesia as inferred using the receiver function method for data from the jisnet regional broadband seismic network 1 we analyzed broadband waveforms recorded by the newly deployed japan indonesia seismic network to determine the depths of the 410 km and 660 km discontinuities and thickness of the mantle transition zone beneath indonesia by the receiver function method the discontinuities have an undulation with a maximum difference of 40 km with respect to the iasp91 model the thickness of the transition zone is well correlated with seismicity and seismic velocity anomalies suggesting that the topography is caused by temperature anomalies associated with the subducted slabs beneath indonesia

slab dehydration in the earth s mantle transition zone abstract because of the high water solubility in wadsleyite and ringwoodite the mantle transition zone is possibly a large water reservoir the potentially high water content of the earth s mantle transition zone is a key element of several global mantle dynamic models nevertheless to keep the transition zone relatively wet the tendency for convection to distribute water over the entire mantle has to be offset by other mechanisms one such mechanism could be linked to the dehydration of the slab studies of slab dehydration mainly focus on hydrous minerals phase diagrams that provide the depth of water exsolution in this study we investigate the water diffusion process that occurs prior to such exsolution the diffusion process is controlled by both chemical gradients and thermal gradients via the temperature dependence of water solubility we have addressed how this diffusion phenomenon influences slab water transport into the transition zone by developing a theoretical model of coupled thermal and chemical diffusion in a flat lying slab stalled in the transition zone model solutions demonstrate that even if intrinsic water solubility is decreasing with increasing temperature which normally would act to impede a chemical diffusion from the slab the water concentration profile adjusts to reduce the adverse effects of temperature dependent solubility the self adjustment entails the concentration of water reaching a local maximum below the top edge of the stagnant slab under specific conditions this concentration maximum can reach the solubility limit and thus exsolve fluid i e create of hydrous melt or aqueous silicate fluid consequently model water flux computations are relatively independent of solubility variation and indicate that much of the water flux introduced by relatively well hydrated slabs is taken up into the transition zone by diffusion

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

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

seismic waves converted from velocity gradient anomalies in the earth s upper mantle summary modelling of elastic wave propagation in 1 d structures is frequently performed using reflectivity techniques in which the earth s velocity profile is approximated by stacks of homogeneous layers the complete reflection transmission r t response of a zone with arbitrary 1 d depth variation including both gradients and discontinuities in material properties can however be calculated using invariant embedding techniques results from earlier studies are here extended to derive exact expressions for r t matrices in arbitrary 1 d anisotropic media using a form of born approxi mation valid for thin scatterers and which does not assume small perturbations in material properties the r t matrices are solutions to a system of non linear ordinary differential equations of ricatti type and may be manipulated using standard r t matrix algebra in an equivalent description the wavefield within the heterogeneous zone is considered in terms of depth dependent contributions from up and downgoing waves propagating within the embedding reference medium this leads to efficient calculation of the internal wavefield using r t matrices of the heterogeneous stratification and portions thereof at minor additional expense mode conversion of teleseismic p and s phases from velocity gradients is examined by way of examples and comparison with three component data from broad band stations of the yellowknife seismic array the frequency dependence of such wave interactions depends on the differences in vertical slowness between incident and scattered modes it is shown that significant energy is converted from transition zones with extent l p 2 a broader interval than will generally produce intramode reflections a layer structure identified from ps conversions near 75 km depth below the slave craton is shown to be compatible with a 10 km thick gradient zone in which anisotropy increases from ambient levels to vp 5 per cent vs 2 5 per cent at a discontinuous upper boundary this characterization supports a previous interpretation as the upper strata of a former oceanic plate juxtaposed against overriding lithosphere during an ancient episode of shallow subduction

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

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

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

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

super deep low velocity layer beneath the arabian plate 1 s and p receiver functions reveal indications of a low s velocity layer at 350 410 km depths beneath the arabian plate a similar layer was previously found beneath the kaapvaal craton in southern africa and tunguska basin of the siberian platform we hypothesize that the boundary at 350 km depth may separate dry mantle root of the arabian plate from the underlying wet mantle layer this boundary is not found beneath the gulf of aden where the root is destroyed by sea floor spreading

initial intensification of east asian winter monsoon at about 2 75 ma as seen in the chinese eolian loess red clay deposit 1 the loess paleosol red clay sequence in the chinese loess plateau provides a record of the evolution of atmospheric circulation and monsoon climate over east asia during the last 7 8 ma detailed analyses of the baishui and jingchuan sections show that the grain size of eolian loess increased abruptly at about 2 75 ma which is accompanied with a reversal of depositional pattern from the red clay to loess paleosol deposits suggesting that the dust transporting circulation was reorganized this is the earliest clear expression of the east asian winter monsoon intensification at the pliocene pleistocene climate transition coincident with the initiation of major northern hemisphere glaciation over the eurasian arctic and northeast asia implying that at the early stage of the pliocene pleistocene global cooling the east asian winter monsoon circulation had been closely linked with the build up of ice sheet and climate changes over the high latitudes of northern hemisphere the intensified winter monsoon winds would have in turn acted as an important positive feedback in strengthening global cooling by increasing albedo and dust loading

variability of p660s phases as a consequence of topography of the 660 km discontinuity abstract records from the broad band nars nl array deployed in the netherlands western germany and belgium have been analyzed for p to s converted phases from the 660 km discontinuity p660s no individual seismogram from this array shows a clear p660s phase stacks of up to 113 crosscorrelograms show p660s energy that is weak in comparison with similar stacks elsewhere very high amplitude observations of p660s in single seismograms from the same region have been made in a previous study paulssen 1985 geophys res lett 12 709 712 paulssen 1988 j geophys res 93 10489 10500 such variations may be due to relief on the 660 km discontinuity in this paper we quantify the effects of different topographies on the time delays amplitudes and waveforms of the direct p wave as well as on the p660s converted wave synthetic p and p660s waveforms are calculated by applying a kirchhoff helmholtz integration over the wave fields at the 660 km discontinuity interface for topography on scales of a few hundreds of kilometers the effects on the p wave are very small compared with those on the p660s phases time delays focused and defocused amplitudes and waveform distortion of p660s phases influenced by such topography diminish the coherence of p660s in synthetic stacks our results show that topography of the 660 km discontinuity with dominant scale lengths of a few hundreds of kilometers and variations in depth of 15 km to 25 km is consistent with the pattern of p660s observations in single seismograms and stacks

can sharp seismic discontinuities be caused by non equilibrium phase transformations a long posed problem of the seismic discontinuities at the depths of 410 and 660 km in earth s mantle is whether these discontinuities are caused by phase transitions alone or together with chemical layering among several tests the sharpness of the boundary is one of the most crucial recent data suggest that the transition region thickness is less than 4 km for both the 410 and the 660 km discontinuities this may be smaller than predicted by phase equilibria we suggest that if there is a sufficiently large nucleation barrier for phase transformation and if the transformation front cannot move faster than the convective flow the transformation in rising or descending material does not take place until the metastable overshoot becomes equal to the nucleation barrier an avalanche like transformation following this overshoot occurs in a very narrow region and can be the cause of a sharp seismic discontinuity in this case the topography of the phase boundary is also substantially modified

global variability of transition zone thickness 1 we analyze recordings from more than 3000 shallow earthquakes and measure s410s s660s differential travel times on a global scale these differential times are obtained using a novel cross correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone our measurements show absolute perturbations up to 8 s from the global average 242 km which imply transition zone thickness variations of 20 km on a length scale of 2000 km the transition zone under major subduction zones e g the western pacific and south america is significantly thicker than the global average this could be associated with large scale depressions of the 660 km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle a narrow transition zone is observed under the central pacific and northern atlantic oceans we also observe a modest anticorrelation between s410s s660s times and the delay times of shear waves predicted using s12 wm13 su et al 1994 in the transition zone which suggests considerable thermal influence on the large scale topography of the 410 and 660 km discontinuities the lack of a stronger anticorrelation suggests that 1 compositional heterogeneities may be present and 2 a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410 and the 660 km discontinuities a regional analysis shows that the transition zone is on average 6 8 km thicker under continents than under oceans this implies a temperature difference of 50 70 c however we do not observe a clear correlation between thickness and the age of the crust

imaging the mountainless root of the 1 8 ga cheyenne belt suture and clues to its tectonic stability receiver function analysis across the cheyenne belt an archean proterozoic suture images a 100 km wide zone of 50 60 km thick crust this thick crust is thought to be a remnant from the original 1 8 ga suturing event the thick and presumably buoyant crust is remarkable in that it is not associated with a large topographic or gravitational anomaly suggesting isostatic balance and a high density mass at depth to compensate the crustal root these features could account for the lithosphere s long term stability the presence of eclogite below the crustal root is a likely source for the high density mass as it would not produce a seismically unusual moho yet it would provide the required excess mass