small scale heterogeneity at the top of the lower mantle around the mariana slab abstract short period waveform data recorded at the western us seismic array for 14 mariana deep earthquakes near 19 n show two anomalous wave packets within about 30 s after the arrival of the direct p wave simply called p or p wave one at about 13 s called x1 phase and the other around 29 s called x2 phase we perform array analyses to locate the sources of these phases in the first step we measure arrival time slowness back azimuth and amplitude of these phases relative to p the amplitudes of the x1 and x2 phases correlate with each other for the individual events and vary more than an order of magnitude among the events which cluster in a region spanning less than 50 km the amplitude correlation suggests that the two waves have a similar origin and the large amplitude variation eliminates receiver side reverberation as the origin of the phases the x1 phase has a slowness and an arrival azimuth which are not distinguished from those of p waves across the entire array and has nearly constant delay times regardless of the focal depths from 567 to 605 km the x2 phase arrives as a more emergent wave packet with slowness and azimuth different from those of p in the second step we compute composite semblance coefficients for the cases of p to p and s to p single scattering near the foci the x1 and x2 phases are best interpreted as s to p scattered waves generated in the uppermost lower mantle north of the focal region based on three observations 1 highest composite semblance values 2 scatterer locations mutually consistent between the two event groups near the depths of 500 and 600 km and 3 a reasonable amount of elastic property anomalies required the x1 phase scatterer is determined at 19 8 0 3 n 145 7 0 3 e 710 30 km on the other hand the scatterer of the x2 phase appears to split into two objects one at 20 4 0 4 n 145 4 1 0 e 900 80 km and the other at 20 6 0 4 n 147 4 0 5 e 860 40 km although the geometries of the scatterers of x1 and x2 phases are not constrained horizontal or subhorizontal discontinuities are unlikely the changes in elastic properties associated with these heterogeneous objects probably occur within several kilometers according to their high efficiency at converting short period waves they are thus likely to represent sharp chemical variations in major element composition these objects are located within a thickened high velocity anomaly at the top of the lower mantle which has been determined by previous seismic studies a plausible tectonic interpretation of these objects is that they are fragments of former oceanic crust which are entrained in the pacific slab impinging on the more viscous lower mantle

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

new inferences of mantle viscosity from joint inversion of long wavelength mantle convection and post glacial rebound data we perform joint inversions for mantle viscosity of geophysical observables associated with both mantle convection and glacial isostatic adjustment gia our data include coefficients of the non hydrostatic geoid up to degree 8 and decay times associated with post glacial relative sea level rsl variations in hudson bay and fennoscandia we find that both data sets may be reconciled using a single profile of mantle viscosity which is characterized by a significant increase with depth across this region this result weakens previous arguments that the mantle rheology has significant transient effects the viscosity profiles we obtain are found to reconcile the observed free air gravity anomaly over hudson bay

the 3d numerical modeling of subduction dynamics plate stagnation and segmentation and crustal advection in the wet mantle transition zone 1 water content in the mantle transition zone mtz has been broadly debated in the earth science community as a key issue for plate dynamics in this study a systematic series of three dimensional 3d numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the mtz under wet conditions this hypothesis is based on the seismic observations of structural variations associated with large scale flattened high velocity anomalies i e stagnant slabs in the mtz in our model weak low viscosity fault zones wfzs which presumably correspond to the fault boundaries of large subduction earthquakes are imposed on the top part of subducting plates the phase transitions of olivine to wadsleyite and ringwoodite to perovskite magnesiowustite under both dry and wet conditions are considered based on recent high pressure experiments the effect of viscosity reduction of wet garnet is also considered for oceanic crust in the mtz results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km the weaker and denser crustal material garnet rich is fed into the wfzs and is advected faster than the plate main body of peridotite olivine rich thus the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet mtz condition

global mantle heterogeneity and its influence on teleseismic regional tomography abstract a new global p wave tomography model is determined using a flexible grid parameterization this new model better reveals the mantle structure under the polar regions than the previous tomographic models the subducting slabs are generally imaged clearly as high velocity high v zones the young slabs are still subducting in the upper mantle and the mantle transition zone mtz whereas the old and ancient slabs are either stagnant in the mtz or have subducted down to the lower mantle even reaching the core mantle boundary low velocity low v anomalies are generally revealed in the mantle under the hotspot regions it seems that a variety of mantle upwelling plumes exist some strong plumes are visible in the whole mantle under the long living hotspots such as those in south central pacific africa hawaii and iceland whereas weak plumes are visible in only some depth range under the minor hotspots under the intraplate volcanoes in east eurasia bering sea and west alaska significant low v anomalies are revealed in the upper mantle which may reflect hot and wet upwelling associated with corner flows in the big mantle wedge bmw above the stagnant pacific slab in the mtz and perhaps deep slab dehydration as well the subduction triggered magmatism in the bmw may be a new class of mantle plumes we also used the new global model to investigate the influence of whole mantle heterogeneity on the determination of upper mantle tomography under japan with a teleseismic inversion method the results show that the mantle heterogeneities outside the target volume of regional tomography can cause significant changes 0 2 0 4 s to the observed relative travel time residuals of a distant earthquake the pattern of regional tomography remains the same even after correcting for the whole mantle heterogeneity but there are some changes in the amplitude of velocity anomalies in the regional tomography hence it is necessary to correct for the mantle heterogeneities outside the target volume so as to obtain a better regional tomography

first principles prediction of fast migration channels of potassium ions in kalsi3o8 hollandite implications for high conductivity anomalies in subduction zones materials sharing the hollandite structure were widely reported as fast ionic conductors however the ionic conductivity of kalsi3o8 hollandite k hollandite which can be formed during the subduction process has not been investigated so far here first principles calculations are used to investigate the potassium ion k transport properties in k hollandite the calculated k migration barrier energy is 0 44 ev at a pressure of 10 gpa an energy quite small to block the k migration in k hollandite channels the calculated ionic conductivity of k hollandite is highly anisotropic and depends on its concentration of k vacancies about 6 k vacancies in k hollandite can lead to a higher conductivity compared to the conductivity of hydrated wadsleyite and ringwoodite in the mantle k vacancies being commonly found in many k hollandite samples with maximum vacancies over 30 the formation of k hollandite during subduction of continental or alkali rich oceanic crust can contribute to the high conductivity anomalies observed in subduction zones

stress distribution within subducting slabs and their deformation in the transition zone abstract the mechanical behavior of slabs in the mantle transition zone is a poorly understood phenomenon to obtain a better understanding of the stress and deformation pattern inside subducting slabs we performed 2d numerical simulations of the subduction process in a purely viscous non linear rheological model including diffusion creep dislocation creep and a stress limiting mechanism the model calculations are used to investigate the effects of yield stress plate age and surface boundary condition free slip versus given plate velocity on the stress development in and around a subducting slab and on the deformation of the subducting plate in the transition zone the deformation and morphology of subducted slabs may differ between free slip and kinematic boundary condition models in some cases especially for young and relatively weak slabs even though kinematically driven subduction morphology may look realistic in most cases our results show that stress distributions can differ significantly if the discrepancy between free slip and kinematic subduction velocity is large and in some model cases even influence the ability of the plate to penetrate into the lower mantle stress distributions such as the ones presented in this study can be a valuable tool in understanding the complex and enigmatic interaction between subducting slabs and the transition zone

seismic evidence for a chemical heterogeneity in the midmantle a strong and slightly dipping seismic reflector beneath the mariana subduction zone 1 a clear later phase 80 s after the direct p wave is observed in most of individual seismograms recorded by a short period seismometer network in japan j array from a cluster of deep earthquakes that occurred at the northern mariana subduction zone this phase 1 shows a p wave particle motion 2 arrives later from earthquakes with shallower focal depths 3 has a steeper incident angle than that of p wave and 4 shows a deviation of a few degrees in the arrival azimuth from that of p wave we interpret it as an s to p converted wave which takes off downward from the source and is reflected at a velocity discontinuity reflector below the earthquakes travel time inversion shows that the seismic reflector dips southwest by 20 at 24 25 n 144 75 e and at a depth of 1115 km with a lateral extension at least 100 100 km the location corresponds to the lower edge of a high velocity anomaly in global tomographic models amplitude and waveform analyses suggest a decrease of s wave velocity by 2 6 and an increase of density by 2 9 within the reflector there is almost no difference in p wave velocity 1 between the reflector and the surrounding mantle the estimated thickness of the reflector is 12 km these observations indicate that the observed seismic structure is more likely to be a chemical reservoir rather than a purely thermal anomaly the seismic reflector might be a piece of subducted oceanic crust as suggested by a previous study it also could be related to the break down of the d phase of dense hydrous magnesium silicates at midmantle pressure condition reported by recent mineral physics studies both scenarios imply that mechanical chemical segregation might occur within the subducted slab at midmantle condition

seismic scatterers at the shallowest lower mantle beneath subducted slabs abstract data from a western united states short period seismic network are analyzed in order to investigate anomalous later phases within a time window from about 10 s to 120 s after direct p waves for deep and intermediate depth earthquakes in circum pacific subduction zones the anomalous phases are best interpreted as s to p scattered waves wavelengths of 10 km from heterogeneities in the shallow lower mantle depths 950 km several s to p scatterers where elastic properties of the rocks must substantially change within several kilometers are detected in the shallowest 300 km of the lower mantle beneath four circum pacific regions kuril bonin fiji and peru around most of the observed scatterers the seismic tomography models have delineated high seismic velocity anomalies which are associated with recently subducted pacific slab or nazca slab the most likely origin of the scatterers would be basalt which used to form the oceanic crust interestingly enough the majority of the scatterers are located near the bottom boundaries of the slabs given rather moderate degrees of folding and or buckling of the pacific and nazca slabs in the shallowest lower mantle at the study areas the existence of the oceanic crust associated with the most recent slab subduction at the scattering sites is unlikely alternative and more likely explanations include the presence of ancient basaltic rocks localized dehydration of hydrous minerals or a sharp boundary between different rock fabrics such as isotropic and anisotropic lower mantle rocks associated with mantle flow dragged by the slab subduction

physical properties and rock physics models of sediment containing natural and laboratory formed methane gas hydrate this paper presents results of shear strength and acoustic velocity p wave measurements performed on 1 samples containing natural gas hydrate from the mallik 2l 38 well mackenzie delta northwest territories 2 reconstituted ottawa sand samples containing methane gas hydrate formed in the laboratory and 3 ice bearing sands these measurements show that hydrate increases shear strength and p wave velocity in natural and reconstituted samples the proportion of this increase depends on 1 the amount and distribution of hydrate present 2 differences in sediment properties and 3 differences in test conditions stress strain curves from the mallik samples suggest that natural gas hydrate does not cement sediment grains however stress strain curves from the ottawa sand containing laboratory formed gas hydrate do imply cementation is present acoustically rock physics modeling shows that gas hydrate does not cement grains of natural mackenzie delta sediment natural gas hydrates are best modeled as part of the sediment frame this finding is in contrast with direct observations and results of ottawa sand containing laboratory formed hydrate which was found to cement grains waite et al 2004 it therefore appears that the microscopic distribution of gas hydrates in sediment and hence the effect of gas hydrate on sediment physical properties differs between natural deposits and laboratory formed samples this difference may possibly be caused by the location of water molecules that are available to form hydrate models that use laboratory derived properties to predict behavior of natural gas hydrate must account for these differences

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

mantle convection with longest wavelength thermal heterogeneity in a 3 d spherical model degree one or two 1 the formation of longest wavelength mantle convection in the sluggish lid regime is investigated using a three dimensional spherical model the bottom rayleigh number is fixed at 107 considering temperature dependent rheology degree one dominant thermal convection occurs for both purely basal heating and mixed i e basal and internal heating modes for the purely basal heating mode degree one convection occurs when the viscosity contrast due to temperature dependent rheology is 103 104 in both boussinesq and extended boussinesq fluids however with extended boussinesq fluid degree one convection may only occur in the basal heating mode in the mixed heating mode degree one convection shifts to one with high degree modes presumably because of enhanced viscous dissipation in the highly viscous lid over up downwelling plumes the geophysically relevant degree two convection with sheet like downwellings is not observed in this study the inclusion of visco plastic rheology in the top part of the mantle breaks down degree one convection

physical conditions producing slab stagnation constraints of the clapeyron slope mantle viscosity trench retreat and dip angles abstract recent seismic tomography has revealed various morphologies in the subducted lithosphere in particular significant flattening and stagnation of slabs around the 660 km boundary are seen in some areas beneath the northwestern pacific subduction zones we examined the cause of slab stagnation in terms of the clapeyron slope of the phase transformation from ringwoodite to perovskite magnesiowustite trench retreat velocity dip angles and high viscosity of the lower mantle based on two dimensional 2 d numerical simulations of thermal convection in particular we examined the conditions necessary for slab stagnation assuming a very small absolute value of the clapeyron slope which were proposed based on recent high pressure high temperature high p t experiments our calculations show that slabs tend to stagnate above the 660 km boundary with an increasing absolute value of the clapeyron slope viscosity jump at the boundary and trench retreat velocity and a decreasing initial dip angle stagnant slabs could be obtained numerically for a realistic range of parameters obtained from high p t experiments and other geophysical observations combining buoyancy high lower mantle viscosity and trench retreat we found that a low dip angle of a descending slab at the bottom of the upper mantle plays an important role in slab stagnation two main regimes underlie slab stagnation buoyancy dominated and viscosity dominated regimes in the viscosity dominated regime it is possible for slabs to stagnate above the 660 km boundary even when the value of the clapeyron slope is 0 mpa k

can long wavelength dynamical signatures be compatible with layered mantle convection analyses of the long wavelength geoid with seismic tomographic models have been providing for a long time important estimates of mantle viscosity these estimates have nearly been derived under the assumption of whole mantle flow it has been commonly held that a fully impermeable boundary at 660 km depth is incompatible with the long wavelength gravity signal on the other hand models with whole mantle circulation which can explain a large portion of the geoid signal usually produce excessive amplitudes of the dynamical topography especially for long wavelengths using recent tomographic models together with genetic algorithm we have successfully demonstrated that the layered convection model can also produce a reasonable fit to the geoid which is comparable in quality with that obtained for the whole mantle model the layered model can simultaneously yield realistic amplitudes of the dynamical topographies of the surface and the 660 km discontinuity

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

taliks in relict submarine permafrost and methane hydrate deposits pathways for gas escape under present and future conditions we investigate the response of relict arctic submarine permafrost and gas hydrate deposits to warming and make predictions of methane gas flux to the water column using a 2 d multiphase fluid flow model exposure of the arctic shelf during the last glacial cycle formed a thick layer of permafrost protecting hydrate deposits below however talik formation below paleo river channels creates permeable pathways for gas migration from depth an estimate of the maximum gas flux at the present time for conditions at the east siberian arctic seas is 0 2047 kg yr 1 m 2 which produces a methane concentration of 142 nm in the overlying water column consistent with several field observations for conditions at the north american beaufort sea the maximum gas flux at the present time is 0 1885 kg yr 1 m 2 which produces a methane concentration of 78 nm in the overlying water column shallow sediments are charged with residual methane gas after venting events sustained submergence into the future should increase gas venting rate roughly exponentially as sediments continue to warm studying permafrost associated gas hydrate reservoirs will allow us to better understand the arctic s contribution to the global methane budget and global warming

joint seismic geodynamic mineral physical modelling of african geodynamics a reconciliation of deep mantle convection with surface geophysical constraints recent progress in seismic tomography provides the first complete 3 d images of the combined thermal and chemical anomalies that characterise the unique deep mantle structure below the african continent we present a tomography based model of mantle convection that provides an excellent match to fundamental surface geodynamic constraints on internal density heterogeneity that includes both compositional and thermal contributions where the latter are constrained by mineral physics the application of this thermochemical convection model to the problem of african mantle dynamics yields a reconciliation of both surface gravity and topography anomalies to deep seated mantle flow under the african plate over a wider range of wavelengths than has been possible to date on the basis of these results we predict flow in the african asthenosphere characterised by a clear pattern of focussed upwellings below the major centres of late cenozoic volcanism including the kenya domes hoggar massif cameroon volcanicline cape verde and canary islands the flow predictions also reveal a deep seated large scale active hot upwelling below the western margin of africa under the cape verde islands that extends down to the core mantle boundary the scale and dynamical intensity of this west african superplume is comparable to the south african superplume that has long been assumed to dominate the large scale flow dynamics in the deep mantle under africa we evaluate the plausibility of the predicted asthenospheric flow patterns through a comparison with seismic azimuthal anisotropy derived from independent analyses of african shear wave splitting data

methane oxidation following submarine permafrost degradation measurements from a central laptev sea shelf borehole submarine permafrost degradation has been invoked as a cause for recent observations of methane emissions from the seabed to the water column and atmosphere of the east siberian shelf sediment drilled 52 m down from the sea ice in buor khaya bay central laptev sea revealed unfrozen sediment overlying ice bonded permafrost methane concentrations in the overlying unfrozen sediment were low mean 20 m but higher in the underlying ice bonded submarine permafrost mean 380 m in contrast sulfate concentrations were substantially higher in the unfrozen sediment mean 2 5 mm than in the underlying submarine permafrost mean 0 1 mm using deduced permafrost degradation rates we calculate potential mean methane efflux from degrading permafrost of 120 mg m 2 yr 1 at this site however a drop of methane concentrations from 190 m to 19 m and a concomitant increase of methane 13c from 63 to 35 directly above the ice bonded permafrost suggest that methane is effectively oxidized within the overlying unfrozen sediment before it reaches the water column high rates of methane ebullition into the water column observed elsewhere are thus unlikely to have ice bonded permafrost as their source

effects of trench migration on fall of stagnant slabs into the lower mantle abstract global seismic tomography has recently revealed horizontally lying slabs near the upper and lower mantle boundary beneath the northwestern pacific region although physical mechanisms that could produce such slab stagnation have been proposed based on numerical simulations there has been little research into what occurs after slab stagnation we proposed trench advance and trench jumps as effective mechanisms related to the fall of stagnant slabs into the lower mantle and our numerical simulations of temperature and fluid flow associated with slab subduction in a 2 d box model confirmed these mechanisms our results indicate that a supply of slab material associated with further slab subduction after slab stagnation plays an important role in differentiating further slab stagnation from the falling of slabs into the lower mantle a shortage of material supply would produce extended slab stagnation near the 660 km boundary for ringwoodite to perovskite magnesiowustite phase transformation whereas downward force due to further slab subduction on a stagnant slab would enhance its fall into the lower mantle the behaviors of falling stagnant slabs were not affected by clapeyron slope values associated with phase equilibrium transformation within the range from 3 0 to 0 0 mpa k compared with models of normal mantle viscosity a high viscosity lower mantle played a role in hindering the fall of slabs into the lower mantle resulting in complicated shapes and slow falling velocities lower mantle viscosity structure also affected slab behavior slabs tended to stagnate when a low viscosity zone lvz existed just below a depth of 660 km because friction between the slab and the lvz was weak there slab stagnation around a depth of 660 km also occurred when a high viscosity zone existed below a depth of 1200 km and acted as a resistive force against a slab even if the slab existed in the lower part of the upper mantle

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

trench dynamics effects of dynamically migrating trench on subducting slab morphology and characteristics of subduction zones systems abstract understanding the mechanisms of trench migration retreat or advance is crucial to characterizing the driving forces of earth s tectonics plates the origins of subducting slab morphologies in the deep mantle and identifying the characteristics of subduction zones systems which are among the fundamental issues of solid earth science a series of numerical simulations of mantle convection focusing on plate subduction in a three dimensional 3 d regional spherical shell coordinate system was performed to examine subduction zone characteristics including geodynamic relationships among trench migration back arc stress and slab morphology the results show that a subducting slab tends to deflect around the base of the mantle transition zone and form a sub horizontal slab because its front edge its toe is subject to resistance from the highly viscous lower mantle as the sub horizontal slab starts to penetrate into the lower mantle from its heel the toe of the slab is drawn into the lower mantle the results for models with dynamically migrating trenches suggest that trench retreat is the dynamically self consistent phenomenon in trench migration the reason for this is that the strong lateral mantle flow that is generated as a sequence of events leading from corner flow at the subduction initiation to return flow of the formation of a sub horizontal slab in the shallower part of mantle wedge produces the retreat of the subducting slab in fact a mantle suction force which is generated in the mantle wedge to fill space left by the retreating subducting plate is enhanced by the subsequent trench retreat even when upwelling flow with significant positive buoyancy originates just above a mantle phase boundary at a depth of 410 km as inferred from independent seismic tomographic geodynamic geochemical and mineral physics reaches the base of the overriding plate and the overriding plate is slightly thinned lithospheric stress tends to be compressed above the upwelling flow the reason for this is that the strong lateral mantle flow originating from the upwelling flow generates resistance drag force at the base of the overriding plates this situation may apply to a case of east asia under which the typical morphology of sub horizontal slabs can be seen by seismic tomography the strong lateral velocity observed in the shallower mantle wedge in the present numerical simulation may account for both the compressional subduction tectonics and back arc compression in the japan kuril kamchatka aleutian and south chile trenches as well as for weak plate slab coupling strong seismic coupling and the possibility of great earthquakes along these trenches

the role played by a low viscosity zone under a 660 km discontinuity in regional mantle layering abstract our previous work from regional geoid inversion over oceans has shown that there exists a low viscosity zone lvz under the 660 km boundary nonlinear inversion has been applied for fitting to the intermediate wavelength oceanic geoid without invoking a priori any impermeable boundary in this study we have conducted a systematic search of the dependence of the depth of this regional boundary from 660 to 1000 km we found that the use of this boundary at the depth of the lvz exerts little influence on the inversion the lvz can act regionally as an impermeable boundary and enhances very fast lateral flow at the uppermost lower mantle on the other hand impermeable boundary alone is not caused by a lvz as shown also by analysis of the 2d flow patterns driven by internal density anomalies inferred from tomography from the intermediate wavelength geoid we can not resolve the existence of layering when the lvz exists beneath the 660 km boundary however a lvz is needed to fit the geoid this lvz can have profound influence on the geodynamical behavior of mantle circulation and the dispersal of geochemical anomalies and the generation of seismic anisotropy

effect of the mid mantle viscosity and phase transition structure on 3d mantle convection abstract recent geophysical evidence shows some sort of layering in the lower part of the mantle transition zone down to a depth of 1000 km seismic observations have revealed a sharp reflector surface at around 900 or 1000 km depth for which a possible explanation can be given in terms of a new phase transition of the lower mantle constituent minerals furthermore new results from the inversion of the oceanic geoid show the existence of a second low viscosity zone lvz somewhere between 660 and 1000 km depth the existence of the second lvz may be linked to the mid mantle phase transitions the phase and viscosity stratification of the transition zone have been included in a series of 3d convection simulations in a 4 4 1 rectangular box with a surface rayleigh number of 2 10 7 beneath the well known 400 and 660 km phase changes we assumed a hypothetical weak endothermic transition at 1000 km in some of our models the principal controlling factor of the style of mantle convection is still the 660 km endothermic transition which sets up a partial or full barrier to flow causing stratified circulation we used various viscosity profiles with emphasis on the model containing the second lvz the main consequence of this zone is to enhance flow layering many plumes can emanate from the transition zone and small scale instabilities develop in the second lvz when the 1000 km endothermic phase transition is included these instabilities can grow only at a few places but then they form strong downwellings two distinct types of penetrative deep downwellings can be present at the same time one which crosses the whole transition zone and another one which crosses only the 660 km discontinuity and stops at 1000 km at least temporarily this can explain seismological observations which suggest that subducted slabs can be retarded not only by the 660 km boundary but also by some deeper obstacle near 1000 km depth

the aeromagnetic method as a tool to identify cenozoic magmatism in the west antarctic rift system beneath the west antarctic ice sheet a review thiel subglacial volcano as possible source of the ash layer in the waiscore abstract the west antarctic ice sheet wais flows through the volcanically active west antarctic rift system wars the aeromagnetic method has been the most useful geophysical tool for identification of subglacial volcanic rocks since 1959 64 surveys particularly combined with 1978 radar ice sounding the unique 1991 97 central west antarctica cwa aerogeophysical survey covering 354 000 km 2 over the wais 5 km line spaced orthogonal lines of aeromagnetic radar ice sounding and aerogravity measurements still provides invaluable information on subglacial volcanic rocks particularly combined with the older aeromagnetic profiles these data indicate numerous 100 1000 nt 5 50 km width shallow source magnetic anomalies over an area greater than 1 2 10 6 km 2 mostly from subglacial volcanic sources i interpreted the cwa anomalies as defining about 1000 volcanic centers requiring high remanent normal magnetizations in the present field direction about 400 anomaly sources correlate with bed topography at least 80 of these sources have less than 200 m relief at the wais bed they appear modified by moving ice requiring a younger age than the wais about 25 ma exposed volcanoes in the wars are the present rapid changes resulting from global warming could be accelerated by subglacial volcanism

wet subduction versus cold subduction 1 in situ x ray diffraction study of post spinel transformation in hydrous peridotite 2 wt h2o indicates that the phase boundary is shifted to higher pressures by 0 6 gpa relative to anhydrous peridotite at 1473 k whereas it shows no obvious shift at high temperature around 1873 k a linear equation for the boundary is p gpa 0 002 t k 26 3 which is applicable for temperatures below 1800 k the present data shows that a significant part of the depressions seen in the 660 km seismic discontinuity may be affected by existence of water it is also well resolved that the olivine wadsleyite phase transformation corresponding to the 410 km seismic discontinuity is shifted to lower pressures by 1 2 gpa by the addition of water thus the topography of seismic discontinuities in the transition zone associated with slabs can be attributed not only to cold subduction but also wet subduction

using linear discriminant analysis lda of bulk lake sediment geochemical data to reconstruct lateglacial climate changes in the south carpathian mountains abstract a multi proxy approach is widely used for reconstructing climatic change in alpine lake sediments this study applied bulk sediment geochemistry to reconstruct lateglacial and early holocene climatic change in a glacial lake lake brazi 1740 m a s l in the retezat mts south carpathians romania the lowermost 1 m part of a 4 9 m long sediment core covering the period between 9950 and 15 750 cal bp was used for high resolution bulk analysis of major elements al 2 o 3 sio 2 tio 2 cao mgo k 2 o na 2 o fe 2 o 3 mno so 3 linear discriminant analysis lda was used to compare a priori classified main chemical groups subsamples from the core were priory ordered to warm and cold groups respectively according to their age and evidence of cold and warm events in the record as suggested by proxy correlation with the lateglacial event stratigraphy of north greenland ice core project ngrip the discriminant function was calculated using concentrations of al 2 o 3 tio 2 cao mgo k 2 o na 2 o fe 2 o 3 and mno after log ratio transformation loss on ignition silicon and sulphur concentrations were not used for the discriminant analysis but regarded as comparison proxies for checking up the validity of outputs sediments deposited during cold and warm events were separated significantly by the lda function 85 1 of the originally grouped cases were correctly classified under the lda analysis the calculated discriminant scores indicated four cold and four warm events sediments ordered into the warm group contained larger amounts of organic matter and sulphur while sediment samples of the cold group were characterized by the overall predominance of major oxides bound into inorganic silicates the discriminant scores showed strong correlation with the ngrip 18 o data r 0 8135 and with the pollen percentage sum of trees and shrubs r 0 9460 loss on ignition showed a somehow weaker but still conclusive linear relationship with the discriminant scores r 0 7505 discriminant analyses of bulk sediment major oxide chemical data may be a useful tool to identify the impact of climatic events upon the nature and composition of materials delivered to a lake basin

plastic deformation of garnets systematics and implications for the rheology of the mantle transition zone abstract plastic properties of materials with garnet structure have been studied under wide temperature conditions ranging from room temperature to 95 of the melting temperatures using uniaxial compression and hot microhardness tests garnets studied include single crystals of oxide garnets y 3 al 5 o 12 gd 3 ga 5 o 12 and y 3 fe 5 o 12 and silicate garnets various solid solutions including grossular almandine andradite pyrope spessartine and uvarovite both uniaxial compression and hot hardness tests indicate that there is a general trend in the plasticity of garnets when the data are compared at normalized conditions t t m and and that the resistance to plastic deformation in garnets is significantly higher than most of the other minerals in the earth s mantle based on both stress dip tests and microstructural observations it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by recovery processes this conclusion is consistent with the high peierls stress inferred from the hot hardness tests the high peierls stress in garnets is presumably due to the large unit cell i e long burgers vectors and or the bcc packing which are common to all garnets we postulate therefore that the present results can be applied to the strength of high pressure garnet majorite and suggest that garnet rich layers in the earth such as subducted oceanic crust in the transition zone or a possibly garnet rich bottom part of the transition zone will be considerably stronger than surrounding regions

dipping low velocity layer in the mid lower mantle evidence for geochemical heterogeneity data from western united states short period seismic networks reveal a conversion from an s to a p wave within a low seismic velocity layer greater than or equal to the 4 percent velocity difference compared to the surrounding mantle in the mid lower mantle 1400 to 1600 kilometers deep east of the mariana and izu bonin subduction zones the low velocity layer about 8 kilometers thick dips 30 to 40 southward and is at least 500 kilometers by 300 kilometers its steep dip large velocity contrast and sharpness imply a chemical rather than a thermal origin ancient oceanic crust subducted into the lower mantle is a plausible candidate for the low velocity layer because of its broad thin extent

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

minimum distribution of subsea ice bearing permafrost on the u s beaufort sea continental shelf 1 starting in late pleistocene time 19 ka sea level rise inundated coastal zones worldwide on some parts of the present day circum arctic continental shelf this led to flooding and thawing of formerly subaerial permafrost and probable dissociation of associated gas hydrates relict permafrost has never been systematically mapped along the 700 km long u s beaufort sea continental shelf and is often assumed to extend to 120 m water depth the approximate amount of sea level rise since the late pleistocene here 5 000 km of multichannel seismic mcs data acquired between 1977 and 1992 were examined for high velocity 2 3 km s 1 refractions consistent with ice bearing coarse grained sediments permafrost refractions were identified along 5 of the tracklines at depths of 5 to 470 m below the seafloor the resulting map reveals the minimum extent of subsea ice bearing permafrost which does not extend seaward of 30 km offshore or beyond the 20 m isobath

water transport into the deep mantle and formation of a hydrous transition zone abstract the phase relations in the peridotite water system to the top of the lower mantle reveal that a certain amount of water can be transported and stored in the transition zone by various hydrous minerals resulting in formation of a hydrous transition zone results of preliminary experiments to clarify the effect of water on kinetics of the olivine wadsleyite transformation are given and show that water enhances the transformation kinetics significantly there are potential dehydration sites in the deep mantle four sites along the descending slabs depending on their temperature profiles i e the mantle wedge of the subduction zone mantle transition zone top of the lower mantle where hydrous ringwoodite and superhydrous phase b decompose and deep in the lower mantle around 1200 1500 km depth where decomposition of phase d g occurs we also expect one site of dehydration melting at the base of the upper mantle where the hot mantle plume hydrated in the transition zone can be dehydrated the transition zone can serve as a water absorber for the mantle scale water circulation system some of the recent seismic works including the p wave tomography images reveal the existence of low velocity anomalies in the transition zone suggesting possibly a wet transition zone elevation of the 410 km discontinuity and lack of clear evidence for a metastable olivine wedge in cold slabs may be consistent with the equilibrium phase transformation from olivine to wadsleyite in cold and wet slabs on the basis of our kinetic study of the transformation under wet conditions

rheological control of oceanic crust separation in the transition zone mineral physics observations suggest that distinct density and rheological differences exist between the crustal component of oceanic lithosphere and the underlying mantle we have conducted numerical experiments to investigate the influence of both density and viscosity on the effectiveness of recycling of oceanic crust into the lower mantle confirming previous results the density inversion at 670 km depth alone is not sufficient to prevent crustal recycling however a soft layer may exist between the strong garnet crust and cold slab interior models employing a simplified newtonian sandwich model show that this thin weak layer can effectively decouple the crust and slab once entrained into the lower mantle the then lighter crust can rise sufficiently fast as a rayleigh taylor instability to avoid further entrainment these results suggest that the crustal component of slabs may be trapped at 670 km depth leading to a garnet enriched transition zone

effects of water and iron content on the rheological contrast between garnet and olivine abstract the effects of water and iron content on the relative creep strengths of garnet and olivine were investigated by shear deformation experiments garnet and olivine samples were sandwiched together between alumina pistons in a simple shear geometry and were deformed at p 1 2 gpa t 1473 k and strain rates ranging from 10 5 to 10 3 s 1 using a griggs type solid medium apparatus the stress and strain rate relation as well as the deformation microstructures including lattice preferred orientation and dynamic recrystallization indicates that the deformation by dislocation creep the creep tests show that the fe rich garnet alm 67 prp 29 grs 3 was slightly weaker than olivine fo90 whereas the mg rich garnet alm 19 prp 68 grs 12 was significantly stronger than olivine under dry conditions the wet experiments show that the creep rate of the mg rich garnet is more sensitive to water than olivine the water fugacity exponent on strain rate was estimated to be 2 4 for garnet and 1 2 for olivine and the mg rich garnet becomes weaker than olivine in a water rich environment the experimental results show that the rheological contrast between garnet and olivine depends strongly on water content and to a lesser degree on fe content consequently the geodynamic behavior of geochemical reservoirs can be sensitive to their chemical environments in the upper mantle

on the separation of relatively buoyant components from subducted lithosphere subducted oceanic lithosphere consists of a sandwich of component layers morb underlain by depleted parent mantle these layers give rise to intrinsic density contrasts within the slab which have been hypothesized to cause component separation after subduction we explore this process through numerical models of simple layered slabs and through convection models which incorporate large local chemical density contrasts we show that the components of positive and negative buoyancy density contrast component volume must be almost equal in order to cause significant component separation or to selectively block deep penetration of buoyant slab components in the transition zone published petrological constraints on component densities and relative volumes indicate that such buoyancy neutralization is not achieved in subducted slabs therefore component separation should not occur on the timescale associated with the sinking of slabs through the upper mantle and transition zone

petroleum systems modeling on gas hydrate of the first experimental exploitation region in the shenhu area northern south china sea abstract two dimensional gas hydrate petroleum system modeling was conducted to understand gas hydrate accumulation mechanisms and spatial distribution related to geologic and geochemical processes in the first experimental gas hydrate exploitation region in the shenhu area of the south china sea based on the gas production in core samples a new biogenic methane generation model was created and incorporated into a kinetic model with the results of a rock pyrolysis experiment a thermogenic gas kinetic model for deep formations in the study area was also developed the entire dynamic evolution process from gas generation to migration and gas hydrate formation and accumulation in the perspective area was simulated with the petroleum system modeling method the results showed that the gas hydrate stability zone ghsz began to form after 3 88 ma and its thickness was greatest when the paleowater depth peaked at 3 66 ma the mass of thermogenic gas generation in deep formations is large and that of biogenic gas in shallow formations is relatively small formations that overlap the gas hydrate bearing layers act as cap rocks that prevent the gas from migrating vertically to shallow layers there was a geological time gap between thermogenic gas generation migration and ghsz formation however the timing of biogenic gas generation which is typically of recent generation in the shallow layer matched well with the formation of the ghsz finally coupling of the age and space of gas migration and ghsz formation has allowed gas hydrate accumulation at structural highs in the shenhu area the evaluation technique of petroleum system modeling is of great significance for the improvement of gas hydrate exploration in frontier areas in the future particularly where survey data are limited

origin and stability of a permafrost methane hydrate occurrence in the canadian shield abstract relatively little attention has been given to the stability of methane hydrates formed during periods of past climate change currently in areas of continuous permafrost although a large portion of the canadian arctic is underlain by crystalline rocks the occurrence phase and origin of alkanes in crystalline rocks under thick permafrost conditions 500 m have not been reported for the first time composition and isotopic data for gases from a crystalline shield environment currently under permafrost conditions are presented gas and water samples were collected from exploration boreholes and seeps between 890 and 1130 m depths in the lupin gold mine nunavut canada gases were methane dominate 64 87 with nitrogen 10 37 the next largest component and smaller amounts of ethane propane and carbon dioxide pressure and temperature measurements indicated gas hydrates were stable at the site prior to mining operations a conclusion supported by noble gas and salinity determinations gas hydrate stability over the last 120 kyr glacial cycle was demonstrated by calculating transient subsurface pressure and temperature conditions utilizing the memorial university university of toronto glacial systems model mun uoft gsm and the hydrogeosphere groundwater flow model model results also indicated glacial loading increased subsurface pressures resulting in increased hydrate stability fields during glacial periods subglacial groundwater recharge would be limited by any significant formation of gas hydrates gas composition combined with carbon and hydrogen isotopic determinations on methane 56 to 42 vpdb and 349 to 181 vsmow carbon dioxide 55 to 15 vpdb ethane 37 to 27 vpdb and 330 to 228 vsmow and propane 34 to 27 vpdb and 196 to 172 vsmow indicated formation of natural gases by thermogenic processes mixed with bacteriogenic gas reasonable given site geologic history methane hydrate formation affected gas composition and gas 2 h values complicating the interpretation gas production is not a modern process at this location and the overall contribution to the global carbon budget is small glacial groundwater recharge models need to account for methane hydrate formation and dissipation due to changes in subsurface stability fields during glaciation and the effect of methane hydrate on hydraulic conductivity

inferences of viscosity from the oceanic geoid indication of a low viscosity zone below the 660 km discontinuity abstract we have attempted to infer details of the viscosity structure in the top 1000 km of the mantle from the geoid and tomographic structure beneath the oceans in order to eliminate the gravity signal from problematic masses located below the subduction zones and the continents we have considered only the intermediate degrees of the oceanic geoid l 12 25 a genetic algorithm has been used to determine the family of viscosity models which give the best correlation with the observed geoid our inversion clearly identifies the asthenosphere just below the lithosphere and also confirms the viscosity increase in the lower mantle predicted by previous inferences but suggests that the main viscosity jump occurs at a depth of about 1000 km and not at the usually stated 660 km boundary somewhere in the depth range of 400 1000 km a low viscosity zone may exist where the viscosity decreases to a value comparable with the asthenosphere existence of such a low viscosity zone is supported by recent analysis of deep mantle anisotropy which favours a flow pattern with a strong horizontal component in the top part of the lower mantle unfortunately the resolution of the inversion as well as the quality of recent seismic tomographic models are not sufficient to localize the depth and to come up with a higher accuracy for the viscosity of this low viscosity channel

plate tectonics on the early earth limitations imposed by strength and buoyancy of subducted lithosphere the tectonic style and viability of modern plate tectonics in the early earth is still debated field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the archean continue to accumulate here we present the first numerical modeling results that address for a hotter earth the viability of subduction one of the main requirements for plate tectonics a hotter mantle has mainly two effects 1 viscosity is lower and 2 more melt is produced which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation our modeling results show that eclogitization significantly relaxes this limitation for a developed ongoing subduction process furthermore the lower viscosity leads to more frequent slab breakoff and sometimes to crustal separation from the mantle lithosphere unlike earlier propositions not compositional buoyancy considerations but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter earth these results suggest a new explanation for the absence of ultrahigh pressure metamorphism uhpm and blueschists in most of the precambrian early slabs were not too buoyant but too weak to provide a mechanism for uhpm and exhumation

intrapermafrost gas hydrates from a deep core hole in the mackenzie delta northwest territories canada visible gas hydrate and possible pore space hydrate samples have been recovered for the first time in north america from within ice bonded permafrost in a 451 m deep core hole in the mackenzie delta northwest territories canada the visible hydrate was found as thin icelike layers that released methane gas initially upon retrieval but stabilized for up to 4 h at atmospheric pressure conditions and subfreezing temperatures the temporary stabilization of hydrate samples is attributed to the self preservation phenomenon described by others on the basis of laboratory studies high methane concentrations in well ice bonded sediments and gas releases suggest that pore space hydrate may be found at depths as shallow as 119 m geochemical and isotopic determinations suggest that the methane hydrate observed in the core hole is biogenic microbial in origin

testing the accuracy of the geomagnetic polarity time scale gpts at 2 5 ma utilizing40ar 39ar incremental heating data on whole rock basalts abstract recent40ar 39ar dating and astrochronologic calculations indicate that the k ar derived gpts underestimates the ages of geomagnetic field reversals in the interval 2 0 ma we report tests to check the accuracy of k ar data utilized to date the reunion event 2 1 ma and part of the early pliocene portion 4 6 ma of the gpts 40ar 39ar incremental heating studies on whole rock basalts from the island of reunion yield an age of2 14 0 03 ma for the reunion event somewhat older than the earlier k ar value of2 07 0 02 ma on the same set of lava flows study of a single specimen from the newer volcanics australia indicates that the earlier k ar date was accurate but the new40ar 39ar results yield a plateau age 4 56 ma with a standard error of 0 6 as compared to the k ar value 3 the improved precision and accuracy resulting from such40ar 39ar incremental heating studies will play a key role in calibrating more precisely the gpts for the interval 10 0 ma

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

variable seismic structure near the 660 km discontinuity associated with stagnant slabs and geochemical implications abstract this study reports a compilation of reflectivity synthetic modeling for the structure of the upper mantle transition zone with high velocity anomalies hva s associated with the northwestern pacific subduction zones here the employed method of reflectivity synthetics effectively determines the structure of flattened hva i e stagnant slab as triplicated regional body waves are very sensitive to the velocity discontinuities in the transition zone results show a distribution of hva s with or without a depression of the 660 km discontinuity depth which indicates a possible variation of geochemical properties at the bottom of the upper mantle a hypothesis is proposed for this implication i e that the structural variation may represent the contrast between a hydrous garnet rich layer subducted crust and bulk peridotite associated with a stagnant slab which is supported by the results of recent laboratory experiments the garnet rich layer can flow and descend faster than bulk peridotite as hydrous garnet is weaker and denser than peridotite in the transition zone given that the clapeyron slope for hydrous garnet perovskite is positive at 660 km two zones of hva with and without a depression in the discontinuity depth may exist next to each other at the bottom of the transition zone this variation of the discontinuity depths coincides with the segmentation identified at deeper depths 629 km in a p wave travel time tomography model although the resolution of the tomography inversion is limited to elaborate the discontinuity structure

late pleistocene and holocene sea level change in the australian region and mantle rheology summary spatial and temporal variations in sea level are produced by the melting of the late pleistocene ice and by the earth s response to the redistribution in surface loads by examining different parts of the sea level curves of the past 20 000 yr from geographically widely distributed regions it becomes possible to constrain models of the melting history of the ice sheets and of the earth s rheology observations from sites away from the former arctic ice sheets such as the australian and south pacific region are particularly important for constraining the total meltwater volumes added into the oceans in the past 20 000 yr and the rates at which this occurred these observations indicate that the antarctic ice sheets provided a significant contribution to the sea level rise at a rate that was approximately synchronous with the melting of the laurentide ice sheet except for the interval 9000 6000 yr ago when it may have lagged behind minor melting of the antarctic ice sheet appears to have continued throughout the late holocene differential observations of the late holocene sea level change recorded at sites in the same region are particularly useful for estimating parameters describing the earth s non elastic response to surface loading the effective parameters used here are a lithospheric thickness an upper mantle viscosity and a lower mantle viscosity describing the response below 670 km depth with the observations used here it is not possible to separate the lithospheric thickness h from the upper mantle viscosity and the viscosity results are based on the assumption that 50 h 100 km neither do these observations provide a resolution of the depth dependence of viscosity in the upper mantle and the resulting estimates are effective parameters only differential sea levels along continental margins and along the shores of large gulfs and bays constrain the effective upper mantle viscosity to be about 1 2 1020 pa s 1 while differential values from islands of different sizes are suggestive of a somewhat lower value the lower mantle taken to be below 670 km depth viscosity is about two orders of magnitude greater than this the estimated ice and rheological models explain many of the holocene sea level observations throughout the australian and southern pacific region the tilting of continental margins as exemplified by observations of variable amplitudes of the holocene high stands and the variable times at which sea levels first reached their present level along the north queensland coast and great barrier reef is well represented by these models differential holocene sea levels observed along the narrow spencers gulf of south australia are also well explained by the models and in neither case is it necessary to invoke tectonic motions predicted late holocene sea levels at small to medium sized islands are characterized by small amplitude high stands that reached their maximum values about 4000 2000 yr ago consistent with observations from the society cook and tuamotu islands

origin of pingo like features on the beaufort sea shelf and their possible relationship to decomposing methane gas hydrates 1 the arctic shelf is currently undergoing dramatic thermal changes caused by the continued warming associated with holocene sea level rise during this transgression comparatively warm waters have flooded over cold permafrost areas of the arctic shelf a thermal pulse of more than 10 c is still propagating down into the submerged sediment and may be decomposing gas hydrate as well as permafrost a search for gas venting on the arctic seafloor focused on pingo like features plfs on the beaufort sea shelf because they may be a direct consequence of gas hydrate decomposition at depth vibracores collected from eight plfs had systematically elevated methane concentrations rov observations revealed streams of methane rich gas bubbles coming from the crests of plfs we offer a scenario of how plfs may be growing offshore as a result of gas pressure associated with gas hydrate decomposition

core mantle boundary topography estimated from numerical simulations of instantaneous mantle flow core mantle boundary cmb topography is estimated from numerical simulations of instantaneous mantle flow in a three dimensional spherical shell geometry density anomaly models of the earth s mantle are derived from geodynamic models constrained principally by seismic tomography results to account for the small 1 5 km cmb topographic relief inferred from recent seismological results lateral viscosity variations in the mantle compositionally dense piles in the deep mantle and a low viscosity d layer are all required for the numerical models low viscosity subducting slabs in the lower mantle serve to reduce the topographic amplitude below high velocity regions i e the circum pacific belt the existence of dense piles may cause the local topographic depressions even below low velocity regions i e the south pacific and atlantic africa regions

dynamical consequences of depth dependent thermal expansivity and viscosity on mantle circulations and thermal structure the effects of both depth dependent thermal expansivity and depth dependent viscosity on mantle convection have been examined with two dimensional finite element simulations in aspect ratio ten boxes surface rayleigh numbers between 107 and 6 107 have been considered the effects of depth dependent properties acting singly or in concert are to produce large scale circulations with a few major upwellings the interior of the mantle is cooled by the many cold instabilities which are slowed down and eventually swept about by the large scale circulation the interior temperature of the mantle can be influenced by the trade off between depth dependent properties and internal heating for chondritic abundance of internal heating and depth dependent thermal expansivity the viscosity increase across the mantle can be no greater than a factor of around ten in order to keep the lower mantle adiabatic the thermal contrasts between the cold blobs and the surrounding mantle are strongly reduced by depth dependent properties whereas the lateral differences between the hot upwelling and the ambient lower mantle can be significant over several hundred degrees depth dependent properties also encourage the formation of a stronger mean flow in the upper mantle which may be important for promoting long term polar motions

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

on the possibility of a folded crustal layer stored in the hydrous mantle transition zone abstract previous numerical studies of mantle convection focusing on subduction dynamics have indicated that viscosity rheological heterogeneity in the subducting plates and the surrounding mantle may have a primary effect on the behavior of subducting plates meanwhile the existence of a low viscosity layer lvl beneath the mantle transition zone mtz has been suggested by several geodynamic studies we investigated the effects of a rheologically weaker crustal layer in a hydrous wet mtz with an underlying lvl on the behavior of subducting plates and determined the trace of crustal material using a numerical simulation model of subduction dynamics in three dimensional regional spherical geometry the combined effect of a rheologically weaker crustal layer and an lvl beneath the mtz produces characteristic transient behavior of the crustal layer whose trace shows a folded loop like feature in the wet mtz along with a folded subducting slab in the lvl this phenomenon is caused by mechanical interaction of the crustal layer with an upward positive bending force due to the main body of the subducting slab folding in the lvl and an enhanced downward negative buoyancy force due to an increase in density from the garnet to the post garnet phase of crustal material the transiently folded crustal layer behind the root of the stagnant slab is also observed this result is consistent with seismological evidence suggesting a piece of subducted oceanic crust in the uppermost lower mantle beneath the subducting slab in the mariana trench over half of the subducted crust is temporarily stored in the mtz however this stored crustal material begins to penetrate into the lower mantle within 10 myr a relatively short timeframe relative to the overturn time of mantle convection eventually most of the crustal material is entrained into the lower mantle but a non negligible portion remains stored in the mtz for 50 myr after the beginning of the accumulation of subducted crust in the mtz

how valid are dynamic models of subduction and convection when plate motions are prescribed a detailed comparison between fully dynamic and kinematic plate formulations has been made in models of mantle convection plate velocity is computed self consistently from fully dynamic plate models with temperature and stress dependent viscosity and preexisting mobile faults in fully dynamic models the flow is driven solely by internal buoyancy while in kinematic models the flow is driven by a combination of the prescribed surface velocity and internal buoyancy only a temperature dependent viscosity close to the effective viscosity determined from the fully dynamic models is used in the kinematic models the two types of models give very similar temperature structures and slab evolutionary histories when the effective viscosity and surface velocity are nearly identical in kinematic plate models the additional work introduced by the prescribed velocity boundary condition is apparently dissipated within the lithosphere and has little influence on the convection under the lithosphere in models with periodic lateral boundary conditions slabs sink into the lower mantle at an oblique angle and this contrasts with the vertical sinking which occurs with reflecting boundary conditions models show that we can simulate fully dynamic models with kinematic models under either periodic boundary conditions or reflecting boundary conditions

glacial rebound of the british isles iii constraints on mantle viscosity summary observations of sea level change since the time of the last glacial maximum provide important constraints on the response of the earth to changes in surface loading on time scales of 103 104 years this response is conveniently described by an effective elastic lithospheric thickness and effective viscosities for one or more mantle layers considerable trade off between the parameters describing these layers can occur and different combinations can give rise to comparable predictions of sea level change in particular the trade off between lithospheric thickness and upper mantle viscosity can be important and for any reasonable value for the lithospheric thickness a corresponding mantle viscosity structure can be found that gives a plausible comparison of sealevel predictions with observations in particular thin lithosphere models will lead to low estimates for the upper mantle viscosity while thick lithosphere models lead to high viscosity values however either solution may represent only a local minimum in the model parameter space and may not correspond to the optimum solution it becomes important therefore that in the inversion of observational data a comprehensive search is conducted throughout the entire model parameter space to ensure that the solution identified does indeed correspond to the optimum solution the sea level data for the british isles lend themselves well to such an inversion because of the relatively high quality of the data the good geographic distribution of the data relative to the former ice sheet and reasonable observational constraints on the dimensions of the former ice sheet and on its retreat furthermore because of the contribution to the sea level signal from the distant ice sheets as well as from the melt water load the observational data base for the region also has some resolving power for the viscosity of the deeper mantle the parameter space explored is defined by up to five mantle layers the lithosphere of effective elastic thickness d and a series of upper mantle layers i 2 4 extending down to depths of 200 400 and 670 km respectively each of viscosity qi and a lower mantle layer of viscosity qlm extending down to the coremantle boundary the range of parameters explored is 30 d i 120 km 3 x loi9 iqi

dynamic buckling of subducting slabs reconciles geological and geophysical observations abstract ever since the early days of the development of plate tectonic theory subduction zones have been engrained in geological thinking as the place where steady linear slabs descend into the mantle at a constant uniform dip angle beneath volcanic arcs however growing evidence from geological and geophysical observations as well as analog and numerical modeling indicates that subducting slabs buckle in a time dependent manner in contrast to the steady state linear cartoons that dominate the literature to evaluate the implication of time dependent slab buckling of geological events we conduct a series of 2 d numerical dynamic kinematic subduction experiments by varying the viscosity increase across the 660 km discontinuity and the strength of the subducting slab our results show that slab buckling is a universal figure in all the experiments when rate of the trench migration vtrench is relatively slow vtrench the slab buckling behavior in our experiments explains a variety of puzzling geological and geophysical observations first the period of slab buckling is consistent with short periodic variations 25 myr in the motions of the oceanic plates that are anchored by subduction zones second the scattered distributions of slab dips from 20 to 90 in the upper mantle are snapshots of time dependent slab dip third the current compressional and extensional stress environments in back arcs are well correlated with shallowing and steeping slab dip resulting from slab buckling fourth the temporal evolution of stress environments in the andes is well correlated with alternate slab dip these correlations indicate that time dependent slab buckling is a major factor controlling subduction zone dynamics

subducting slabs stagnant in the mantle transition zone the p wave velocity structure beneath the western pacific is found from the international seismological center first arrival data special attention was paid to the deep structure beneath the wadati benioff zone we discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks this way of discretization minimizes a problem with tomographic studies of regional scale difficulty in making corrections for the effects outside the region of interest our solution is iterative with the alternate step of the relocation of earthquakes using the whole mantle model of inoue et al 1990 as a starting model a first order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model the essential result depends little on whether the reference spherical model is smooth or discontinuous near 400 and 670 km depths we examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere the resolution is depth dependent but in general good enough to see the slab configuration beneath the southern kurile japan izu bonin arcs and the java arc it is relatively poor beneath the northern kurile and mariana arcs the seismic image of subducting slab beneath the southern kurile to bonin arcs bends to subhorizontal near the leading edge of the wadati benioff zone and extends continentward over a distance of more than 1000 km the subhorizontal portion of the slab connects a high velocity blob to the bottom that reaches a depth of at least 800 km across the 670 km discontinuity under the japan arc although the image of the java slab directly penetrates the 670 km discontinuity it then bends to a shallow dip with a considerable spread reaching a depth of about 1200 km these results suggest that descending slabs of lithosphere in the western pacific tend to be stagnant in the transition zone under a subtle control of the 670 km discontinuity although stagnant slab materials eventually descend into the lower mantle they no longer maintain their original configuration below the 670 km discontinuity

compositional vs thermal buoyancy and the evolution of subducted lithosphere we formulate 2 d cartesian finite element models that explore the fate of compositionally defined lithosphere as it encounters a viscosity increase at the boundary between the upper and lower mantle subducted lithosphere is represented as a cold stiff layered composite of denser eclogite underlain by more buoyant harzburgite slabs impinging on a lower mantle 30 and 100 times more viscous than the upper mantle thicken and fold strongly as they penetrate the lower mantle approximately a factor of two thickening occurs via pure shear just above the discontinuity with additional enhancement due to folding by over a factor of two no separation of the individual slab components occurs at the discontinuity and direct comparison with models in which compositional buoyancy is explicitly ignored indicates that slab evolution is largely controlled by the thermal buoyancy these results are at odds with hypotheses about slab evolution in which the compositional buoyancy contributions lead to component separation and the formation of slab megaliths or a compositionally layered upper mantle

dynamical consequences of mid mantle viscosity stratification on mantle flows with an endothermic phase transition recent geophysical evidences from seismology and geoid inversions have pointed out the existence of some layering between 900 and 1000 km depth and the possibilities of a low viscosity zone between 660 and 1000 km depth using a finite difference and spectral code we have conducted 3 d calculations of thermal convection with the endothermic phase transition at 670 km depth and a family of viscosity profiles with emphasis on the presence of two low viscosity zones one in the upper mantle between 100 and 250 km depth and the second one between 670 and 1000 km depth below 1000 km the viscosity increases with depth the surface rayleigh number used was in the range of 10 7 the dominating effect is that due to the second low viscosity zone below the phase change layered convection is induced by the presence of this second asthenosphere which causes horizontal flow to develop in there alternatively a viscosity increase at 670 km depth would shift convection to the single layer regime the potential sharp variations of the viscosity structure between 670 and 1000 km depth can greatly influence the global flow dynamics of the mantle

effects of submarine groundwater discharge on the present day extent of relict submarine permafrost and gas hydrate stability on the beaufort sea continental shelf we investigate the role of submarine groundwater discharge on the offshore temperature and salinity field and its effect on the present day extent of submarine permafrost and gas hydrate stability on the north american beaufort shelf with a two dimensional numerical model based on the finite volume method this study finds that submarine groundwater discharge can play a large role in submarine permafrost evolution and gas hydrate stability suggesting that local hydrology may control the evolution of submarine permafrost as strongly as does sea level or paleoclimatic conditions submarine permafrost evolution shows transient behavior over potentially long time scales e g several glacial cycles before a balance of density and pressure driven flows is established with the permeability variations imposed by the overlying permafrost layer the detectable offshore permafrost extent is related to the quasi stationary location of the saltwater freshwater transition larger values of submarine groundwater discharge allow permafrost to extend farther offshore because fresh pore water preserves relict ice therefore differences in the permafrost extent at locations that share similar paleoclimatic history may be explained in part by differences in the local hydrology gas hydrate stability on the north american beaufort shelf may be more widespread than currently thought because low ice saturation highly degraded submarine permafrost likely exists beyond the boundary detectable by common geophysical methods

the influence of rheological weakening and yield stress on the interaction of slabs with the 670 km discontinuity results of high resolution seismic tomography showing subducting slabs deflected in the transition zone and thickened in the lower mantle seem to call for slab material weaker than inferred from mineral physics deformation mechanisms a possible mechanism suggested by several authors could be the weakening due to grain size reduction which should occur in the cold portion of fast slabs after an exothermic phase transition at a depth of 400 km since the amount of weakening as well as the rate of subsequent strengthening due to the grain growth are not precisely known we present here a parametric study of slab behavior in the transition zone and upper part of the lower mantle we simulate a subducting slab in a two dimensional 2 d cartesian box in the numerical model with composite rheology including diffusion creep dislocation creep and a general stress limiting rheology approximating peierl s creep we concentrate on two rheologic effects the dynamic effect of slab weakening due to grain size reduction at the phase boundary and the effect of yield stress of stress limiting rheology the effect of trench migration on slab deformation is also included in our study results show that the slab ability to penetrate into the lower mantle is not significantly affected by a trench retreat in the absence of grain size weakening however in case of a 4 cm yr trench retreat grain size weakening provides a viable mechanism to deflect the slab in the transition zone provided that stress limiting deformation mechanism would limit the effective viscosity outside the areas of grain size weakening to about 10 24 pa s 2002 elsevier science b v all rights reserved

defining the updip extent of the gas hydrate stability zone on continental margins with low geothermal gradients 1 the distribution of gas hydrate on a continental slope is often characterized as a wedge that pinches out on the seafloor this part of the hydrate stability zone is particularly relevant for studies of the dynamics of hydrate accumulations such as processes related to slope stability or hydrate dissociation leading to methane release into the overlying ocean for regions with very low geothermal gradients we have produced a series of thermobaric models of the shallow hydrate stability zone that contain an unexpected geometrical distribution of hydrated sediments in these models the shallowest part of the stability field thickens and bulges landward such a feature is more likely to happen in regions where low geothermal gradients are further lowered by high sedimentation rates also the effect is greater beneath colder oceans although a hydrate stability zone bulge would be difficult to image with conventional seismic methods there are numerous locations around the world where such a system could develop