latent heat effects of the major mantle phase transitions on low angle subduction abstract very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate we have investigated the effects of latent heat for this situation where mantle material is pushed through the major mantle phase transitions during shallow low angle subduction below the overriding plate the significance of the buoyancy forces arising from the latent heat effects on the dynamics of the shallowly subducting slab is examined by numerical modeling when a 32 ma old slab is overridden with 2 5 cm yr by a continent flat subduction occurs with a 4 5 cm yr convergence rate when latent heat is included in the model forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces under these circumstances the flat slab segment subducts horizontally about 350 km further and for about 11 ma longer than in the case without latent heat before it breaks through the 400 km phase transition the style of subduction strongly depends on the mantle rheology increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow similarly an overriding velocity of less than 1 cm yr leads to steep subduction which gradually changes to flat subduction when increasing the overriding velocity however these model parameters do not change the aforementioned effect of the latent heat provided that low angle subduction occurs in all models latent heat resulted in a substantial increase of the flat slab length by 300 400 km varying the olivine spinel transition clapeyron slope from 1 to 6 mpa k reveals a roughly linear relation between and the horizontal length of the slab based on these results we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low angle subduction below an overriding plate

tracking slabs beneath northwestern pacific subduction zones article i nfo article history this study uses the amplitudes of bottom side reflected shear waves to constrain the morphology and dynam ics of subducted oceanic lithosphere beneath northwestern pacific subduction zones across honshu arc the 410 and 660 km seismic discontinuities are detected at the respective depths of 395 5 and 685 5 km within the wadati benioff zone their topographies are negatively correlated along slab dip showing the dominant effect of temperature on the olivine phase changes within the upper mantle transition zone the base of the upper mantle shows broad depressions as well as localized zones of shallow average depths be neath korea and northeast china the 15 km peak to peak topography west of the wadati benioff zones suggests that the stagnant part of the subducted pacific plate is not as flat as previously suggested eastward slab pile up is also possible at the base of the upper mantle across southern kuril arc the shear wave reflec tion coefficients of major olivine phase boundaries fall below 5 within the wadati benioff zone the appar ent reflection gaps and the spatial connection between a strong reflector at 900 km depth may imply 1 possible compositional variations at the top and bottom of the transition zone and 2 substantial mass heat flux across the 660 km seismic discontinuity we also identify strong reflectors within the subducted oceanic lithosphere at mid transition zone depths the depths and strengths of these reflectors are highly variable be tween honshu and southern kuril islands

two dimensional three dimensional waveform modeling of subducting slab and transition zone beneath northeast asia subduction plays a fundamental role in dynamics of the mantle convection and in material circulations of the earth s interior slabs have been imaged to subduct near horizontally in the transition zone tz beneath the northeast asia in seismic tomography triplication waveform modeling is an effective tool to study the detailed seismic structure in the tz however tz triplication modeling has traditionally relied on 1 d models in this study we use the spectral element method to explore influences of 2 d 3 d slab structure on tz triplication waveforms and to model for the first time the slab and tz structures beneath the northeast asia synthetic tests suggest that for a subduction zone earthquake slab structure can have important influences on tz triplication waveforms and that even in a narrow azimuth range the effects from 2 d 3 d slab structure on the wave propagation can lead to erroneous conclusions with 1 d modeling our data are high quality triplicated sh waveforms at distances of 10 32 from a deep event below 410 km discontinuity in the pacific subducting slab our 2 d 3 d waveform modeling results suggest that a simple model of the subducting slab 5 high velocity anomaly and 100 km thick down to 560 km but normal below 560 km can match most of the observed waveforms remarkably well the bottom of the tz of the sampling region north of the yellow sea contains a patch of very slow anomaly the results indicate that subhorizontal slab above the 660 discontinuity is not everywhere beneath northeast asia and the subducting slab is not everywhere continuous to the bottom of the tz compared with the traditional 1 d modeling our new 2 d 3 d approach provides better fits to the data and allows us to constrain the slab geometry and to separate tz structure from slab structure

morphology of the distorted subducted pacific slab beneath the hokkaido corner japan abstract the intersection of the japan and kurile arcs is expressed as a cuspate feature at the trench a bend in the japanese islands and a complex lithospheric structure and is known as the hokkaido corner the pacific plate is subducting beneath the two arcs in the northwest pacific at different velocities which has resulted in an arc arc collision and distortion of the subducting oceanic lithosphere using p and s wave tomographic images and seismicity the distorted shape of the subducted pacific plate beneath the hokkaido corner can be interpreted in three dimensions the pacific plate is imaged as continuous along the japan kurile arc with dip angle increasing from south to north but at the arc arc junction the geometry is complex and appears crumpled beneath the hokkaido corner there are two features in the tomography that are clearly imaged a region of low velocity at approximately 50 150 km depth between 142 5 145 e and 41 43 n and a distorted buckled lower slab boundary these two unusual characteristics in the slab morphology are likely to be related to deformation of the subducted pacific plate at the arc arc junction

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

accretion and exhumation structures formed by deeply subducted seamounts in the kamuikotan high pressure temperature zone hokkaido japan 1 the lower cretaceous iwashimizu complex in the southern kamuikotan zone japan was formed by accretion of seamounts which subducted to the lawsonite albite to blueschist facies the complex is a pile of four nappe units each unit comprises a duplex with horses of seamount volcanics sedimentary rocks and ramp shear zones suggesting a thin skinned peeling of subducted seamounts each unit bounded fault is regarded as the floor thrust of an overlying duplex and the roof thrust of an underlying duplex reflecting a downward progression of underplating accretion the structurally upper pirashuke unit was metamorphosed at higher pressure than the structurally lower units this inverted arrangement of metamorphic grade can be explained by shallowing of progressively younging subducted slab structurally upper units were exhumed synchronously with growth of the structurally lower units this synaccretionary exhumation can be explained by repetition of undercritical duplex forming compression and overcritical extensional unroofing during seamounts subduction

three dimensional forward and backward numerical modeling of mantle plume evolution effects of thermal diffusion we investigate the effects of thermal diffusion on the evolution of mantle plumes by means of three dimensional numerical modeling forward and backward in time mantle plumes are fed by a hot low viscous material from the thermal boundary layer the material of the plumes is mainly advected toward the earth s surface with some effects of thermal diffusion however the feeding can become weaker with time and then thermal diffusion can take over and control the evolution of the plumes numerical experiments forward in time show that a week feeding of mantle plumes by the hot material from the boundary layer results in the diffusive disappearance of plume tails first and plume heads later this is the most likely explanation for the seismically detected low velocity mantle structures mantle plumes with prominent heads and almost invisible tails at midmantle depths we develop restoration models backward in time to recover strong features of mantle plumes in the geological past after they have dissipated due to thermal diffusion and analyze effects of thermal diffusion and temperature dependent viscosity on the reconstruction of the mantle plumes we investigate the impact of thermal diffusion on the performance of our restoration variational data assimilation algorithm for a given range of rayleigh number ra and two values of the viscosity ratio r between the upper and lower boundaries of the model domain we show that 1 the residuals between the temperature predicted by the forward model and that reconstructed by the backward modeling become larger and 2 the restoration process becomes poorer as ra decreases and r increases we assimilate temperature obtained from high resolution seismic tomography data for the southeastern carpathians and show that present diffused mantle structures can be restored to their prominent state in the miocene times we discuss the problems of smoothness of model input and output data errors associated with the modeling and some other challenges in the data assimilation for thermoconvective flow in the mantle

deep recycling of oceanic asthenosphere material during subduction uncertainties in the origin and composition of oceanic asthenosphere lead to different views on its temporal evolution upon subduction we investigate the evolution of asthenosphere material during subduction using high resolution geodynamic models in contrast to some earlier models suggesting that limited amount of asthenosphere material can be entrained during subduction we find that much of the subslab mantle 100 km thick could recycle into the deep mantle following the slab even if this mantle layer remains buoyant and less viscous during entrainment our results support the hypothesis that observed trench parallel subslab seismic anisotropy is a downward continuation of the anisotropic asthenosphere the entrainment of sometimes buoyant asthenosphere material provides a new mechanism for reducing slab dip angle which is consistent the shallower pacific slab underneath japan than that farther south episodic release of entrained buoyant materials during subduction can also explain enigmatic intraplate volcanism such as the changbaishan volcano in northeast china

plumes in central panthalassa deductions from accreted oceanic fragments in japan accretion of a large number of seamounts or fragments of huge oceanic plateaus is recorded in the geology of japan major accretion occurred twice a late early carboniferous seamount chain accreted in latest permian to middle jurassic time and a late jurassic oceanic plateau accreted in the early cretaceous the late jurassic plateau now in northern japan and sakhalin is here named the sorachi plateau and a tithonian age for it is well documented on the basis of microfossils from pelagic cherts and limestones petrochemical characteristics of basalts suggest two kinds of sources depleted mantle under oceanic lithosphere and enriched mantle related to plume upwelling all the accreted fragments in japan can be traced back to the central part of the panthalassa by using paleomagnetic data and plate trajectory analysis the timing of generation of the accreted seamount chain and the plateau is consistent with a time of global change their place of origin in the mid pan thalassa retains a positive residual geoid anomaly and slow vp in the lower mantle which suggest a past upwelling of hot material from the core mantle boundary

sinking hot anomaly trapped at the 410 km discontinuity near the honshu subduction zone japan abstract a fate of hot anomaly dragged down by subducting slab is studied by simple 2d and 3d fluid dynamical models in which the slab movement is kinematically imposed the 410 km discontinuity treated as an olivine wadsleyite phase change acts as a barrier to the downward movement of hot anomaly as a result the hottest part is located near the 410 km discontinuity the shape of high temperature anomaly near the 410 km discontinuity is similar to that of the low velocity anomaly under the ocean ward side of the subduction zone near the honshu arc japan which was recently convincingly confirmed and it was interpreted as a mainly thermal anomaly the time for the hot anomaly to stay near the 410 km discontinuity depends on many factors such as the intensity of thermal anomalies and the viscosity we set up a simple model in which such a hot anomaly was produced in the area of the pacific superplume region and carried toward the japanese islands by the horizontal flow associated with the plate movement the hot anomaly whose size is equivalent to 2 3000 km horizontal 200 km vertical may stay there longer than 100 million yr we will discuss the implication of this result in terms of the relation between the present high temperature anomaly near the honshu subduction zone and the past activity of the pacific superplume

slab dehydration and fluid migration at the base of the upper mantle implications for deep earthquake mechanisms summary water enters the earth s mantle via subduction of oceanic lithosphere and sediments a lot of this water immediately returns to the atmosphere through arc volcanism but part retained in dense hydrous magnesium silicates dhmss and nominally anhydrous minerals nams like olivine is expected to be subducted as deep as the bottom of the upper mantle 660 km depth then due to its low solubility in lower mantle minerals water is likely to be released as a hydrated fluid during the spinel post spinel phase change the dynamics of this fluid phase is investigated through a 1 d model of compaction in which a source term has been introduced to take the fluid precipitation into account the competition between the advective transport by the descending slab and the buoyant rise of the fluid results in three distinct situations depending on the properties of the solid and the fluid phases low matrix permeability and high fluid viscosity inhibit compaction and favour the entrainment of fluid towards the deep mantle in this case the entire slab water content would enter the lower mantle and would be mixed at large scale however realistic values of the fluid viscosity and matrix permeability make this possibility unlikely when effective compaction results in an accumulation of fluid at and below the phase boundary then depending on the value of the matrix viscosity the situation evolves differently above 1020 pa s accumulation of fluid extends below the phase boundary and the pressure difference between the fluid and the matrix increases continuously exceeding the yield strength of rocks as a result cracks would form and evolve towards the formation of dykes in case of very low mantle viscosity possibly due to strong grain size reduction during phase change compaction becomes very efficient and the fluid remains confined within the phase change horizon without increasing pressure in the long term this last situation appears unstable and would also evolve towards the formation of dykes thus it is expected that water returns to the upper mantle by dykes propagating in the direction of the maximum compressive stress since maximum compressive stress appears to follow the dip of the slab below 410 km depth we predict the formation of dykes extending from the 660 km phase change to 410 km depth the possible existence of such dykes in slabs offers the necessary conditions for strong double couple component earthquake in the deepest part of the upper mantle

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

genesis of flood basalts from eclogite bearing mantle plumes on the basis of the geochemistry of flood basalts we investigate the hypothesis that the source material for these magmas includes a significant eclogite component derived from ancient subducted oceanic crust entrained into mantle plumes at the core mantle boundary we present finite element models of the melting in starting plumes plume heads in both continental and oceanic regions starting plume models in which the plume heads are composed solely of pyrolite mantle are incapable of generating sufficient volumes of melt or any melt at all for reasonable plume head temperatures 1400 1600 c or lithosphere ages 6 25 100 m y this result is in contrast to the results of farnetani and richards 1994 whose models require higher plume head temperatures 1600 1700 c and a weak mobile lithosphere in order to generate volumes of melt similar to flood basalts plumes composed of 15 wt eclogite yield volumes of melt comparable to flood basalts in both oceanic and continental settings the maximum extent of melting in these models is about 10 30 this agrees with the model of hofmann and white 1982 who argued that ocean island basalts are derived from subducted oceanic crust if the embedded eclogite thermally reequilibrates with the surrounding plume material then melt volumes and total extents of melting increase by a additional factor of 2 3 a simple two dimensional rifting model for a slow spreading ocean basin underlain by a lens of hot plume material shows that plume related melting occurs over 50 60 m y a much longer period than for flood basalts

numerical simulations of mantle flow around slab edges abstract a simple kinematic dynamic model of mantle flow around the slab edge is constructed in order to understand the flow complexity there the flow velocity on the top and the small boundary region around the shallow plate boundary is kinematically imposed in order to achieve a subduction like feature and the flow in other part is dynamically calculated the geometry of the plate mimics the region around the junction of aleutian islands and kamchatka that are examples of the convergent transform fault plate boundaries in a simple model in which the overlying plate is almost stationary the lateral flow from the mantle under the subducting slab to the mantle under the neighboring plate is of minor importance once the slab penetrates into the high viscosity layer where the downward flow encounters the resistance similar situation was found when the trench is advancing that is the trench moves toward the overlying plate for the case with retreating trench that is the trench moves toward the subducting plate a lateral flow exists even after the slab penetrates into the high viscosity layer although its magnitude is significantly smaller than that of the plate velocity the presence of a low viscosity layer just beneath the subducting plate may promote the emergence of lateral flow a significant lateral flow is observed when the high temperature anomaly that is buoyant and low viscosity block carried by the movement of subducting plate approaches the slab these results may have important implications for the possible existence of trench parallel flow in the sub slab mantle

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

subducted lithosphere beneath the kuriles from migration of pp precursors abstract we seismically image both thermal and chemical heterogeneity of the mantle beneath the kurile subduction zone using p wave energy reflected from the underside of discontinuities arriving as precursors to the seismic phase pp we take advantage of new broadband seismic data provided by the high lava plains seismic experiment and earthscope s usarray collecting a dataset of 31 high quality sumatran earthquakes sampling beneath the kuriles we employ high resolution array analysis techniques including migration and vespagrams to identify precursory arrivals and study lateral variations in discontinuity depth sharpness and impedance of the mantle transition zone we find the 410 km boundary is at 395 km near the subducting kurile slab though the boundary is 410 425 km deep elsewhere in regions away from subduction we do not detect a laterally continuous underside reflection of p waves from the 660 km discontinuity however in the vicinity of the subducting kurile slab we detect robust p660p reflections from interfaces near 620 670 km depth signifying an increase in the impedance contrast at 660 km depth we also detect deeper reflectors down to 720 km depth beneath the kurile slab in a localized area cold aluminum depleted harzburgitic lithosphere residing at the base of the transition zone best explains the local enhancement of the 660 km discontinuity p wave impedance contrast our new discontinuity measurements support the hypothesis of cold depleted lithosphere stagnating at the 660 km discontinuity beneath the kuriles subduction zone and imply the 660 km boundary can locally impede mantle flow and produce chemical heterogeneity within the transition zone

thermal evolution and geometry of the descending lithosphere beneath the se carpathians an insight from the past abstract mantle heterogeneities imaged by seismic tomography in the se carpathians contain information on the present thermal state of the mantle based on p wave seismic velocity anomalies we develop a model of the present mantle temperature beneath the region and combine the model with a model of crustal temperature constrained by heat flow data the modeled temperatures are assimilated into the geological past using the information on the regional movement in the early and middle miocene prominent thermal states of the lithospheric slab descending in the region are restored from its diffuse present state in miocene times the slab geometry clearly shows two portions of the sinking body the northwest southeast oriented portion of the body is located in the vicinity of the boundary between the east european and scythian platforms and this portion of the sinking body may be a relic of cold lithosphere that has traveled eastward another portion has a northeast southwest orientation and is related to the present descending slab above a depth of 60 km the slab had a concave thermal shape confirming the curvature of the carpathian arc and a convex surface below that depth the slab maintained its convex shape until it split into two parts at a depth of about 220 km we propose that this change in the slab geometry which is likely to be preserved until the present can cause stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region also we hypothesize that either the processes of dehydration and partial melting of the descending lithospheric slab or dragging down of hotter rocks from the adjacent uppermost mantle by the slab are possible causes of the reduction in seismic velocities beneath the transylvanian basin

the melt content of the low velocity layer in the eastern south china implications for the subduction process of the western pacific plate abstract the low velocity layer lvl atop the mantle transition zone involves diapiric segregating melt upwelling mantle mixing magmatic eruption and the remnants the lateral variations of the lvl could record the process of the subducting plates the lvl beneath the eastern south china intensively affected by the subduction of western pacific plate since mesozoic is imaged with observed p wave triplications recorded by the dense chinese digital seismic network the thicknesses of the lvl are 57 km for sub region a 40 km for sub region b and 20 km for sub region c the melt fractions related to the different velocity structures of the lvl beneath three north to south adjacent sub regions a b and c of the eastern south china are estimated with the bulk solid compositions reference potential temperatures compositions and dihedral angles the spatial characteristics of the melt fractions and the lvl thicknesses are in accord with the recession directions of the subduction fronts of the western pacific plate this consistency provides a powerful information for understanding the plate subduction process

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

waveform inversion of ss precursors an investigation of the northwestern pacific subduction zones and intraplate volcanoes in china abstract the arrival time and amplitude of underside reflections from mantle seismic discontinuities ss precursors have made major contributions to the understanding of mantle composition and dynamics in this study we introduce a nonlinear waveform inversion technique to simultaneously constrain shear velocities and discontinuity depths beneath the northwestern pacific subduction system based exclusively on a large ss precursor waveform dataset we are able to clearly delineate the morphology of the descending pacific plate which flattens at the base of the upper mantle and extends westward by 1500 km toward northern central china our grid search yields the maximum correlation between shear velocity and transition zone thickness at an angle of 30 consistent with the reported average slab dip beneath the study region the strongly positive correlation suggests predominantly thermal rather than compositional variations along the descending pacific plate the joint depth velocity solution also shows a 5 10 km depression of the 410 km discontinuity and an average decrease of 1 2 in upper mantle shear velocity beneath the intraplate volcanic fields in northeastern china this anomaly which reaches the middle of the upper mantle transition zone beneath the changbai hotspot initiates at a significantly shallower 320 km depth beneath the wudalianchi region high amplitude reflections at depths greater than 410 km suggest a water poor melt layer in possible association with 1 decompression melting from passive upwelling and or 2 active upwelling through a slab window

early tertiary rupture of the pacific plate 1700 km of dextral offset along the emperor trough line islands lineament abstract between 67 and 40 ma ago a northwest southeast trending fracture system over 8000 km long split the pacific plate and accumulated at least 1700 km of dextral offset between the east and west portions this system here named the emperor fracture zone efz system consisted of several segments one along the present trace of the emperor trough and another along the line islands joined by short spreading ridges the efz terminated at its northern end against the kula pacific ridge and at its southern end in a ridge transform system called the emperor spreading system which extended to the west north of australia the finite angular velocity vector describing the relative motion between the east and west pacific plates is 0 6 ma about a pole at 36 n 70 w this vector added to the known early tertiary motion of the pacific plate with respect to the global hotspot reference frame accounts in large part for the nnw trend of the emperor seamount chain relative to the wnw hawaiian trend without violation of the integrity of the antarctic plate the meiji emperor and emperor hawaiian bends date respectively the initiation 67 ma ago and cessation 40 ma ago of seafloor spreading on the emperor spreading system the postulated early tertiary relative motion along the efz between the east and west pacific plates explains 1 the present misalignment of the two sets of magnetic bights of the pacific 2 the abrupt truncation of eastern pacific bathymetric lineaments against the emperor trough and line islands 3 the contrast in paleolatitude between the eastern and western pacific as indicated by paleomagnetic and sedimentologic studies and 4 the anomalous gravity signature of the central hawaiian ridge that indicates that the ridge loaded thin hot lithosphere

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

changbaishan volcanism in northeast china linked to subduction induced mantle upwelling the changbaishan volcanic complex in china cannot be easily explained as the consequence of a mantle plume seismic images from the region identify buoyant mantle material that may have been entrained and dragged downwards by the subducting pacific plate but is now escaping upwards through a gap in the plate and producing the intraplate volcanism

geochemical evidence for a mid cretaceous superplume basalt lavas with a high nb y ratio for a given nb zr ratio occur in the polynesian superswell region of the south pacific which probably formed by upwelling of a deep mantle superplume the distinctive geochemical characteristics of the polynesian basalts may be attributed to melting of a mantle source that is more enriched in a basaltic ancient mid oceanic ridge basalt component basalts displaying such chemical signatures have been found on shatsky rise the ontong java plateau and greenstones from subduction zone complexes of sakhalin island the occurrence of polynesian type basalts together with an estimate of their ages suggests that the south pacific superplume was active as long ago as 90 150 ma the superplume activity preceded the onset of the superchron supporting an idea that the superplume acted as a trigger for such a global event

distinct velocity variations around the base of the upper mantle beneath northeast asia abstract both the global and regional p wave tomographic studies have revealed significant deep structural heterogeneities in subduction zone regions in particular low velocity anomalies have been observed beneath the descending high velocity slabs in a number of subduction zones the limited resolution at large depths and possible trade off between the high and low velocities however make it difficult to substantiate this feature and evaluate the vertical extent of the low velocity structure from broadband waveform modeling of triplicated phases near the 660 km discontinuity for three deep events we constrained both the p and sh wave velocity structures around the base of the upper mantle in northeast asia for the two events beneath the southern kurile the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending pacific slab our preferred models consistently suggest normal to lower p and significantly low sh velocities above and below the 660 km discontinuity extending to about 760 km depth compared with the global iasp91 model corroborating previous observations for a slow structure underneath the slab in contrast both high p and sh velocity anomalies are shown in our preferred models for the japan subduction zone region likely reflecting the structural feature of a slab stagnant above the 660 km discontinuity the velocity jumps across the 660 km discontinuity were found to be on average 4 5 and 7 for p and s waves under the south kurile and 3 and 6 under the japan subduction zone the respective velocity contrasts in the two regions are consistent with mineralogical models for colder slab interior and hotter under slab areas based on mineral physics data the depth averaged 1 5 p and 2 5 sh velocity differences in the depth range of 560 760 km between the two regions could be primarily explained by a 350 450 k temperature variation although the presence of about 0 5 1 wt water might also contribute to the subtle velocity variations near the base of the transition zone in the southern kurile from our modeling results we speculate that the slow structure in the southern kurile may be correlated to the low velocity zone observed previously around the 410 km discontinuity under northern honshu if this is the case both may be associated with a thermal anomaly rooted in the lower mantle beneath the subduction zone in northeast asia

depth extent of the lau back arc spreading center and its relation to subduction processes seismic tomography and wave form inversion revealed that very slow velocity anomalies 5 to 7 percent beneath the active lau spreading center extend to 100 kilometer depth and are connected to moderately slow anomalies 2 to 4 percent in the mantle wedge to 400 kilometer depth these results indicate that geodynamic systems associated with back arc spreading are related to deep processes such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab the slow regions associated with the tonga arc and the lau back arc are separated at shallow levels but merge at depths greater than 100 kilometers suggesting that slab components of back arc magmas occur through mixing at these depths

effects of strongly temperature dependent viscosity on time dependent three dimensional models of mantle convection numerical simulations of thermal convection in a wide 8 8 1 cartesian box heated from below with temperature dependent viscosity contrasts of 1000 and rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern with rigid upper and lower boundaries spoke pattern flow with small diameter 1 5 cells is obtained consistent with laboratory experiments and previous numerical results however with the arguably more realistic stress free boundaries the flow chooses the largest possible wavelength forming a single square cell of aspect ratio 8 with one huge cylindrical downwelling surrounded by upwelling sheets the addition of stress dependence to the rheology weakens the stiff upper boundary layer resulting in smaller cells though still with upwelling sheets and downwelling plumes

glacial lake outburst floods as drivers of fluvial erosion in the himalaya himalayan rivers are frequently hit by catastrophic floods that are caused by the failure of glacial lake and landslide dams however the dynamics and long term impacts of such floods remain poorly understood we present a comprehensive set of observations that capture the july 2016 glacial lake outburst flood glof in the bhotekoshi sunkoshi river of nepal seismic records of the flood provide new insights into glof mechanics and their ability to mobilize large boulders that otherwise prevent channel erosion because of this boulder mobilization glof impacts far exceed those of the annual summer monsoon and glofs may dominate fluvial erosion and channel hillslope coupling many tens of kilometers downstream of glaciated areas long term valley evolution in these regions may therefore be driven by glof frequency and magnitude rather than by precipitation

surface area control of organic carbon accumulation in continental shelf sediments the relationship between organic carbon oc and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments cores from many north american shelf environments show downcore decreases in oc to similar refractory background concentrations if expressed relative to the surface area of the sediments this consistent concentration is 0 86 mg oc m 2 which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces a more global collection of sediment water interface samples show that this relationship is even more extensive with exceptions occurring in areas of very high riverine sediment input organic pollution or low oxygen water columns density separations indicate that organic matter is largely adsorbed to mineral grains the microtopography of surfaces was examined with n2 sorption and most surface area was found to be inside pores of 10 nm width these data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay such protection would likely work in concert with other protection mechanisms such as humification this consistent surface area correlation with oc concentration may explain control of spatial and temporal variations in oc burial rates by sedimentation rates the pore protection hypothesis provides a causal mechanism for this observed control

high temperature anomalies oceanward of subducting slabs at the 410 km discontinuity abstract our p wave whole mantle tomography revealed a low velocity region oceanward of the northern honshu slab of the pacific plate at depths around the 410 km seismic discontinuity resolution tests and scrutiny of the traveltime residuals for the ray paths passing through the low velocity region indicate that this anomaly is a resolvable feature and not an artifact due to the strong slab anomalies the existence of the slow anomalies is also supported by the analysis of the p wave records from the j array a large aperture seismic array in japan for a bonin earthquake the p arrivals to northern honshu at epicentral distances of 13 20 are strongly triplicated because of the 410 km discontinuity the later arrivals along the retrograde branch where ray paths pass through the low velocity region are anomalously slow comparison of the observed and synthetic waveforms indicates not only slow anomalies but also depression of the 410 km discontinuity this depression represents the direct evidence for the low velocity zone of primarily thermal origin an excess temperature of 200 k and the associated fractional melt of less than 1 can explain both the results of the tomographic and waveform analyses