joint inversions of seismic and geodynamic data for models of three dimensional mantle heterogeneity the seismic models of three dimensional 3 d mantle heterogeneity may be interpreted in terms of the density perturbations which drive mantle flow and thus provide important predictions of flow related observables such as the nonhydrostatic geoid the current models of global scale shear velocity heterogeneity provide reasonably good fits to the long wavelength nonhydrostatic geoid data 60 70 variance reductions but rather poor fits to the corresponding free air gravity anomalies 30 40 variance reductions this major difference is due to two factors 1 the very different amplitude spectrum of the gravity anomalies which is nearly flat for degrees l 3 the largest mismatch between the pattern of predicted and observed gravity anomalies is in the southern hemisphere this observation suggests that one reason for the poor overall match between the two fields may be that the global seismic data are not resolving sufficiently well the heterogeneity in the southern hemisphere portion of the deep mantle in contrast the gravity data provide accurate and geographically uniform constraints on the vertically integrated heterogeneity in the mantle we therefore perform a series of experiments in which we simultaneously invert a large set of seismic data which includes long period waveforms ss s and scs s differential travel times and normal mode structure coefficients and the long wavelength gravity anomaly data we thus determine whether it is possible to derive new 3 d heterogeneity models which satisfy both data sets the gravity anomaly data are interpreted in the context of spherically symmetric viscous flow models of the mantle in these inversion experiments we test several radial viscosity and ln ln profiles and thereby assess their plausibility the joint seismic geodynamic inversions reveal that it is indeed possible to greatly improve the fit to the free air gravity anomalies with variance reductions of 80 90 readily accessible while preserving the fit to the seismic data this improvement is achieved with some adjustment to the heterogeneity in the depth range 1500 2500 km where the seismic data constraints appear to be weakest the joint inversions also reveal new structures in the southern hemisphere portion of the lower mantle which apparently are not resolved by the seismic data alone

slab stagnation in the shallow lower mantle linked to an increase in mantle viscosity subducting slabs can stagnate in the lower mantle high pressure laboratory experiments show that the viscosity of a dominant mantle phase increases dramatically at shallow lower mantle depths which could cause the slabs to halt their descent

why cold slabs stagnate in the transition zone oceanic lithosphere sinks stagnates and is deflected sub horizontally beneath western pacific island arcs requiring buoyancy in the slab that is inconsistent with a thermal origin the transformation of pyroxene to majoritic garnet occurs by extremely slow diffusion and pyroxene is therefore unlikely to transform at equilibrium pressures and temperatures in the cold interior of slabs we present high resolution numerical simulations showing that when slow diffusion inhibits the dissolution of pyroxene into garnet the slab becomes buoyant relative to the ambient mantle and stagnates whereas when the phase transformations occur in equilibrium there is no effect on the slab we test the model by comparing slab temperature and geometry and find that sub horizontal slabs are more likely colder than average consistent with our numerical simulations

inferring the viscosity and the 3 d density structure of the mantle from geoid topography and plate velocities summary in a dynamic earth mantle mass heterogeneities induce gravity anomalies surface velocities and surface topography these lateral density heterogeneities can be estimated on the basis of seismic tomographic models recent papers have described a realistic circulation model that takes into account the observed plate geometry and is able to predict the rotation vectors of the present plates the relationship between the surface observables and the heterogeneities is sensitive to the viscosity stratification of the mantle here we use this model combined with a generalized least squares method in order to infer the viscosity profile of the earth from the surface observations and to get some new insight into the 3 d density structure of the mantle the computed radial viscosity profile presents a continuous increase of more than two orders of magnitude the asthenosphere has a viscosity close to 2 x lo2 pa s no sharp discontinuity is requested at the upper lower mantle interface the largest viscosity 7 x 10 pas is reached in the middle of the lower mantle at greater depth approaching the core mantle boundary the viscosity decreases by one order of magnitude the model suggests that the well known degree 2 and order 2 anomaly in the transition zone of the upper mantle is merely the signature of the slabs it also slightly increases the degree 2 and order 0 in the lower mantle and decreases it in the upper mantle in other words the inversion requests a hotter lower mantle beneath the equator and a colder upper mantle at the same latitudes

lateral variations in mantle rheology implications for convection related surface observables and inferred viscosity models summary over the past decade numerous analyses of convection related observables such as horizontal surface divergence geoid or gravity anomalies and dynamic surface topography have been carried out in the context of tomography based mantle flow models one of the major objectives of this modelling has been the inference of the rheological structure of the mantle with few exceptions these studies have been conducted in the framework of a viscous flow theory which assumes that the mantle rheology may be represented in terms of an effective viscosity which varies with depth only here we present a detailed assessment of the impact of lateral variations in viscosity on global convection related observables using forward modelling of buoyancy induced flow in a 3 d spherical shell we find that the resulting dynamic topography at the surface and the core mantle boundary as well as the gravitational response of the earth are affected relatively little by the inclusion of lateral viscosity variations lvv when compared with results for a purely 1 d radial viscosity model in particular we found that the effect of lvv on the global observables is significantly smaller than the variability due to uncertainties in the current seismic tomography models we also quantify the effect of lvv in the context of the viscosity inverse problem using two synthetic data sets generated with a 1 d viscosity profile and with a fully 3 d viscosity model in which the lvv span across three orders of magnitude we compared the 1 d viscosity profiles recovered from the inversions and found that lvv have virtually no effect on our inversion results the synthetic viscosity inversion further revealed that the effect of lvv is small in comparison to the uncertainties arising from the seismic tomography models the inversions also suggest that the 1 d viscosity profiles derived from actual surface data represent the depth variation of the horizontally averaged logarithm of the 3 d viscosity distribution in the mantle intr o duction constraining the rheological properties of the mantle using surface geophysical data has traditionally involved separate treatments of short seismic intermediate solid earth tides long postglacial rebound and very long mantle convection timescale processes in each case the initial focus historically speaking has been on the inversion of the data to determine a one dimensional 1 d purely radial variation of the relevant rheological parameter e g elastic moduli intrinsic attenuation and viscosity which provides the best fit to the data it has long been recognized particularly in the context of the seismic inverse problem that the search for a 1 d earth model is subject to possible bias due to the presence of lateral variations in structure which may be as significant as the radial variations especially at shallow depths this paper is concerned with the mantle viscosity field the earliest inferences of the radial i e horizontally averaged profile of viscosity based on glacial isostatic adjustment gia data e g mcconnell 1968 parsons 1972 were characterized by a pronounced increase of viscosity with depth in the top half of the mantle however these were followed by two decades of gia research that argued for an essentially isoviscous

radial models of mantle viscosity results from a genetic algorithm summary the long wavelength component of the geoid has been used to constrain radial variations in mantle viscosity using 3 d velocity anomaly models determined from seismic tomography as the buoyancy forces driving viscous flow using a genetic algorithm ga the robustness of mantle viscosity models obtained by calculusbased optimization methods is investigated gas are a relatively new class of optimization techniques which are generating a growing interest in the geophysics community these methods can be particularly useful for solving highly non linear optimization problems unlike traditional techniques gas do not require derivative information to form a jacobian matrix or sensitivity kernels models are constructed from pieces of successful models following a simple set of rules these simple rules are driven by stochastic rather than deterministic means because of the efficiency of the ga for the mantle viscosity problem it is possible to explore a greater range of potential solutions than with a traditional optimization method it is found that while both calculus based and ga optimization methods find viscosity models with a low viscosity transition zone the ga finds another class of models with a high viscosity in the transition zone that also satisfies the geoid data the geoid alone is unable to resolve low viscosity transition zone models from high viscosity transitionzone models the effect of the scaling between seismic velocity and density on the preferred viscosity model is also explored while a velocity to density scaling with a pronounced increase in the transition zone provides the best fit to the geoid the preferred vertical viscosity profile is insensitive to the velocity to density scaling

formation of a future supercontinent through plate motion driven flow coupled with mantle downwelling flow series of high resolution numerical simulations of three dimensional mantle convection were performed to examine the interaction between the drifting continental lithospheres and the underlying mantle structure for 250 m y from the present and to predict the configuration of the future supercontinent the density anomaly of the mantle interior was determined by the seismic velocity anomaly from global seismic tomography data sets which contain well resolved subducting slabs the present day plate motion was imposed for the first stage of the simulation as a velocity boundary condition at the top surface boundary instead of a shear stress free condition the switching time from the plate motion boundary to shear stress free conditions was taken as a free parameter the results revealed that australia eurasia north america and africa will merge together in the northern hemisphere to form a new supercontinent within 250 m y from the present the continental drift was assumed to be realized by plate scale mantle flow rather than large scale upwelling plumes that is continuously moving plates at the surface for the first stage of the simulation are mechanically coupled with the subducting slabs in the mantle this enhances the underlying mantle downwelling flow as a result persistent continental drift can be reproduced for long future time periods even though top surface boundary conditions may switch in response to shear stress free conditions the configuration of the numerically reproduced future supercontinent in this study is broadly consistent with the hypothetical model of amasia as indicated by previous findings from geological correlations and a paleogeographic reconstruction

on the amplitude of dynamic topography at spherical harmonic degree two abstract two large seismically slow regions in the lower mantle beneath africa and the pacific ocean are sometimes referred to as superplumes this name evokes images of large scale active upwellings however it remains unclear whether these features are real or represent collections of multiple regular mantle plumes here we investigate the implications of these upwellings for dynamic topography we combine detailed measurements of oceanic residual topography from hoggard et al 2016 with continental constraints derived from crust1 0 to produce a global model expanded in spherical harmonics observed dynamic topography is subsequently compared to predictions derived from mantle flow following steinberger 2016 using tomographic density models results yield relatively good overall agreement and amplitude spectra with similar slopes except for degree two i e 10 000 km wavelengths where predicted amplitude is more than two times as large and is dominated by contributions from the lower mantle predictive models suggest two large scale uplifted regions above the superplumes that are barely seen in the observed topography we suggest that this mismatch can only partly be reconciled by altering the seismic velocity to density conversion factor or by including the effects of lower mantle chemical heterogeneity in addition it may be important to consider more significant revisions to the lower mantle flow patterns such as those possibly induced by different radial viscosity profiles and laterally varying or anisotropic lower mantle viscosity

effect of lateral viscosity variations in the top 300 km on the geoid and dynamic topography summary we have investigated the impact of lateral viscosity variations lvv in the top 300 km of the mantle on the long wavelength gravitational response of the earth in contrast to the previous studies we demonstrate that the lvv may play a crucial role if a model with imposed plate velocities and partial layering is considered assuming lvv in the top mantle associated with the continental roots and the oceanic asthenosphere we are able to explain a significant portion of the geoid 85 per cent and of the free air gravity data 55 per cent without invoking a complex radial viscosity profile or assuming a lithosphere with an unrealistically low uniform viscosity the best fit to the data is found for models in which mass anomalies located at the boundary between upper and lower mantles reduce the mass flux across the 660 km discontinuity by a factor of 3 these models are characterized by a low average value of viscosity in the asthenosphere below the oceans 10 19 pa s and a large viscosity contrast 100 between the young suboceanic mantle and the deep continental roots the viscosity in the deep upper mantle is found to be around 3 10 20 pa s while a value higher by at least two orders of magnitude is typical for the lower mantle the predicted dynamic surface topography is influenced by the surface plate velocities and its amplitude amounts to a few hundred metres besides investigating the viscosity structures that reflect the different thicknesses of the lithosphere in continental and oceanic regions we have searched for a large scale pattern of lateral viscosity variation in the top 300 km that yields the best fit to the geoid the pattern obtained by this inversion shows a good correlation with the ocean continent distribution and 90 per cent of the geoid and 67 per cent of the free air gravity can be explained

the early differentiation history of mars from 182w 142nd isotope systematics in the snc meteorites we report here the results of an investigation of w and nd isotopes in the snc shergottite nakhlite chassignite martian meteorites we have determined that 182 w values in the nakhlites are uniform within analytical uncertainties and have an average value of 3 also while 182 w values in the shergottites have a limited range from 0 3 0 7 their 142 nd values vary considerably from 0 2 0 9 there appears to be no correlation between 182 w and 142 nd in the nakhlites and shergottites these results shed new light on early differentiation processes on mars particularly on the timing and nature of fraction ation in silicate reservoirs assuming a two stage model the metallic core is estimated to have formed at 12 myr after the beginning of the solar system major silicate differentiation established the nakhlite source reservoir before 4542 ma and the shergottite source reservoirs at 4525 19 ma these ages imply that within the uncertainties afforded by the 182 hf 182 w and 146 sm 142 nd chronometers the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contempo raneously possibly during crystallization of a global magma ocean the distinct 182 w 142 nd isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on mars two of which are depleted in incompatible lithophile elements relative to chondrites and the third is enriched the two depleted silicate reservoirs most likely reside in the martian mantle while the enriched reservoir could be either in the crust or the mantle therefore the 182 w 142 nd isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related a further implication is that the source reservoirs of the nakhlites and shergottites on mars have been isolated since their establishment before 4 5 ga therefore there has been no giant impact or efficient global mantle convection to thoroughly homogenize the martian mantle following the establish ment of the snc source reservoirs copyright 2005 elsevier ltd

effects of lateral viscosity variations on the geoid 1 we investigate the effects of lateral viscosity variations lvvs on mantle circulation model predictions of the global geoid the present study is motivated in part by earlier findings that lvvs due to stiff slabs in the lower mantle have a strong influence on the geoid and that slabs in the lower mantle are perhaps no stronger than the ambient mantle however more recently it has been argued based on global seismic tomography models that lvvs are a minor effect on the geoid in the light of these seemingly contradictory results we re visit the problem of slab strength in the lower mantle we substantiate that the geoid calculated from tomography is hardly affected by the presence of lvvs whereas the geoid computed from global slab models yields a poor fit to the observed geoid when lvvs are considered however this degradation of fit only occurs for very long wavelengths of flow indicating inherent differences between the slab and tomography models we also investigate the effects on the geoid due to weak plate boundaries strong cratonic keels and a low viscosity region in the d layer due to post perovskite in addition to the geoid we attempt to fit plate motions with a circulation model that has prescribed weak zones at plate boundaries motions are matched well and taking into account lvvs the geoid with appropriate surface velocity boundary conditions agrees with the observed geoid as well as for free slip cases

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 of solid solid phase transitions in mantle convection abstract with changing pressure and temperature conditions downwelling and upwelling crustal and mantle rocks experience several solid solid phase transitions that affect the mineral physical properties owing to structural changes in the crystal lattice and to the absorption or release of latent heat variations in density together with phase boundary deflections related to the non null reaction slope generate important buoyancy forces that add to those induced by thermal perturbations these buoyancy forces are proportional to the density contrast between reactant and product phases their volume fraction the slope and the sharpness of the reaction and affect the style of mantle convection depending on the system composition in a homogeneous pyrolitic mantle there is little tendency for layered convection with slabs that may stagnate in the transition zone because of the positive buoyancy caused by post spinel and post ilmenite reactions and hot plumes that are accelerated by phase transformations in the 600 800 km depth range by adding chemical and mineralogical heterogeneities as on earth phase transitions introduce bulk rock and volatiles filtering effects that generate a compositional gradient throughout the entire mantle with levels that are enriched or depleted in one or more of these components phase transitions often lead to mechanical softening or hardening that can be related to a different intrinsic mechanical behaviour and volatile solubility of the product phases the heating or cooling associated with latent heat and the transient grain size reduction in downwelling cold material strong variations in viscosity would enhance layered mantle convection causing slab stagnation and plume ponding at low temperatures and relatively dry conditions reactions are delayed due to the sluggish kinetics so that non equilibrium phase aggregates can persist metastably beyond the equilibrium phase boundary survival of low density metastable olivine ringwoodite pyroxene and pyrope garnet in the transition zone and uppermost lower mantle produces positive buoyancy forces that decrease the subduction velocity and may lead to slab stagnation in the transition zone the presence of deep metastable portions is still debated and should not be associated a priori with a completely dry slab as field observations suggest that heterogeneously hydrated oceanic plates could contain metastable dry portions surrounded by transformed wet rocks

evolution of a dune from crescentic to parabolic form in response to short term climatic changes rabjerg mile skagen odde denmark abstract rabjerg mile a complex parabolic dune reaches a height of 20 m above the deflation plain the width of the dune is 900 m and the length of the whole dune is 1500 m the dune is covered by 0 5 1 5 m high transverse dune ridges the dune has migrated approximately 1 km since 1887 a d accompanied by a change in the migration direction from ese 100 to ene 68 the morphology of rabjerg mile through the last 100 years is illustrated by a geographical information system arc info applied to a series of high quality topographical maps based on the digitized contour lines the evolution of rabjerg mile can be followed from an initial crescentic form in 1887 a d to a complex parabolic dune in this century dune dynamics and sites of erosion and deposition are evaluated from the digital terrain models changes in vegetation cover documented from the maps and air photos indicate that vegetation in the study area steadily increased with time calculations of resultant drift directions and gross bedform normal transport are based on climatic data from the skagen changed from an almost perfect transverse bedform 1887 1924 to an oblique imperfectly transverse bedform 1966 to present the bedform alignment followed the gross bedform normal transport rule in most years the evolution of a parabolic dune after 1924 is related to a period of reduced wind energy and increased vegetation cover this parabolic dune was maintained after 1960 when wind energy increased and the wind regime was more complex at the same time the dune became markedly asymmetric because of the frequent occurrence of strong southerly and easterly winds

the long wavelength geoid from three dimensional spherical models of thermal and thermochemical mantle convection the earth s long wavelength geoid anomalies have long been used to constrain the dynamics and viscosity structure of the mantle in an isochemical whole mantle convection model however there is strong evidence that the seismically observed large low shear velocity provinces llsvps in the lower mantle underneath the pacific and africa are chemically distinct and likely denser than the ambient mantle in this study we have formulated dynamically self consistent 3 d spherical mantle convection models to investigate how chemically distinct and dense piles above the core mantle boundary may influence the geoid our dynamic models with realistic mantle viscosity structure produce dominantly spherical harmonic degree 2 convection similar to that of the present day earth the models produce two broad geoid and topography highs over two major thermochemical piles in the lower mantle consistent with the positive geoid anomalies over the pacific and african llsvps our geoid analysis showed that the bottom layer with dense chemical piles contributes negatively to the total geoid while the layer immediately above the chemical piles contributes positively to the geoid canceling out the effect of the piles thus the bottom part of the mantle as a compensation layer has zero net contribution to the total geoid and the thickness of the compensation layer is 1000 km or 2 to 3 times as thick as the chemical piles our results help constrain and interpret the large scale thermochemical structure of the mantle using surface observations of the geoid and topography as well as seismic models of the mantle

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

the viscosity structure of the mantle during 1991 1994 the four year period covered by this report there has been a shift in emphasis away from one dimensional radial mantle viscosity models and toward an understanding of the three dimensional radial and lateral structure of mantle viscosity our understanding of mantle viscosity comes primarily from two sources studying the creep properties of mantle minerals under appropriate pressures and temperatures and using theoretical flow models to predict surface observables such as plate velocities gravity geoid and heat flow laboratory measurements of deformation indicate that the rheology of upper mantle minerals such as olivine mg fe 2sio4 is a strong function of temperatue grain size and stress cf karato and wu 1993 the deformation of minerals under mantle conditions generally follows a flow law of the form e a nd mexp q qrt 1 where is the deformation rate is the deviatoric stress is the shear modulus d is the grain size of the rock q is the activation energy for the deformation mechanism t is the temperature in kelvin r is the gas constant and a is a constant viscosity is defined as e 2 therefore deformation is directly related to viscosity substituting equation 1 into equation 2 an effective viscosity eff a n 1dmexp qrt 3 can be defined for temperature changes of 100 degrees k the viscosity changes by an order of magnitude at constant stress cf karato and wu 1993 changes of deviatoric stress by a factor of 2 change the viscosity by an order of magnitude cf karato and wu 1993 other factors such as partial pressure of oxygen and water content may also have important effects

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

strength of mg fe 2sio4 wadsleyite determined by relaxation of transformation stress abstract the growth of a reaction rim around a crystal undergoing a polymorphic phase transformation generates elastic strain energy as a consequence of the reaction volume change the strain energy inhibits the transformation by counteracting the chemical free energy driving force for growth stress relaxation enables growth to continue at a rate controlled primarily by the mechanical properties of the rim we studied this process in high pressure experiments in order to constrain the rheology of wadsleyite one of the high pressure polymorphs of olivine single crystal spheres of san carlos olivine surrounded by a quasi hydrostatic pressure medium of nacl or au were partially transformed to wadsleyite at 1100 c and 16 17 gpa to determine the rate of growth as a function of time microstructural observations indicate that the relict olivine remains largely undeformed while the wadsleyite rims deform by dislocation creep application of an elastoplastic model to fit the observed decrease in growth rates with time constrains the yield strength of wadsleyite at these conditions to between 4 and 6 gpa a decrease in growth rates was also observed in another study in which varying amounts of water were added to the samples kubo t ohtani e kato t shinmei t fujino k 1998a experimental investigation of the transformation of san carlos olivine single crystal phys chem miner 26 1 6 kubo t ohtani e shinmei t fujino k 1998b effects of water on the transformation kinetics in san carlos olivine science 281 85 87 modeling these results gives values for the yield strength of wadsleyite between 2 and 5 gpa under various conditions a strength difference of 1 gpa is indicated by experiments at 1030 c conducted under nominally dry 200 ppm h2o and water added 500 ppm h2o conditions supporting the hypothesis that hydrolytic weakening is an important process in wadsleyite the calculated yield strengths for wadsleyite indicate that it is three to six times stronger than olivine at the same conditions based on an extrapolation of the low temperature plasticity flow law for olivine these results imply a strong increase in the strength of subducting slabs in the mantle transition zone provided that the deformation mechanism is the same as in the experiments furthermore inhibition of growth by transformation stress may be an important factor in extending the depth range over which metastable olivine transforms to its high pressure polymorphs in subducting slabs

linked influences on slab stagnation interplay between lower mantle viscosity structure phase transitions and plate coupling abstract an endothermic phase transition in mantle material at 660 km depth constitutes a barrier that in most cases prevents the direct penetration of subducted slabs seismic tomography shows that subducted material is in many subduction zones trapped at the bottom of the transition zone just above the 660 km phase boundary recent tomographic models however also report subducted material that penetrates to the shallow lower mantle and there it is observed to flatten at about 1000 km depth models of slab dynamics that generally assume sharp rheological transition at 660 km depth however mostly predict slab stagnation at the bottom of the transition zone multiple lines of evidence including recent experiments indicate that viscosity may gradually increase in the uppermost 300 km of the lower mantle rather than simply changing abruptly at the upper lower mantle boundary here we present the results of a modeling study focused on the effects of rheological transition between upper and lower mantle material on slab deformation and stagnation we test the effects of smoothing the viscosity increase over 300 km and shifting it to a depth of 1000 km or even deeper we show that slab ability to penetrate to the lower mantle is mainly controlled by the trench migration rate which in turn is affected by crustal viscosity coupling between the subducting and overriding plates thus plays a key role in controlling slab penetration to the lower mantle and stagnation in the deep transition zone or shallow lower mantle models with strong crust and consequently negligible rollback display penetration to the lower mantle without much hindrance and no stagnation above or below the 660 km interface regardless of viscosity stratification in the shallow lower mantle models with weak crust are characterized by fast rollback and penetration is very limited as slabs buckle horizontally and flatten above the 660 km boundary most interesting from the point of view of shallow lower mantle stagnation are models with intermediate crustal viscosity here rollback is efficient though slower than in weak crust cases horizontally lying slab segments are trapped in the transition zone if the sharp viscosity increase occurs at 660 km but shifting the viscosity increase to 1000 km depth allows for efficient sinking of the flat lying part and results in temporary stagnation below the upper lower mantle boundary at about 1000 km depth

on dislocation glide in mgsio3 bridgmanite at high pressure and high temperature abstract dislocation glide in mgsio3 bridgmanite with pbnm perovskite structure is modeled at 30 and 60 gpa for the 100 010 and 010 100 slip systems the velocity of screw dislocations is calculated in the thermally activated regime based on the kink pair mechanism we show that the dislocation velocity determination can rely on the atomic scale calculations of a limited amount of parameters the peierls stress p and the formation enthalpy of a single kink h k from the dislocation velocities the evolution of stress as a function of temperature can be derived from the orowan equation at any strain rate calculations performed at laboratory strain rates of 10 5 s 1 reproduce well the high stress levels found experimentally this demonstrates the influence of lattice friction in the mechanical properties of bridgmanite the same calculations are performed at mantle strain rate 10 16 s 1 they demonstrate that in the lower mantle bridgmanite would always be in the thermally activated regime and that stresses close to 1 gpa are still necessary to move dislocations in bridgmanite in the uppermost lower mantle dislocation glide is inhibited and other deformation mechanisms involving diffusion are needed

viscosity jump in earth s mid mantle the viscosity structure of earth s deep mantle affects the thermal evolution of earth the ascent of mantle plumes settling of subducted oceanic lithosphere and the mixing of compositional heterogeneities in the mantle based on a reanalysis of the long wavelength nonhydrostatic geoid we infer viscous layering of the mantle using a method that allows us to avoid a priori assumptions about its variation with depth we detect an increase in viscosity at 800 to 1200 kilometers depth far greater than the depth of the mineral phase transformations that define the mantle transition zone the viscosity increase is coincident in depth with regions where seismic tomography has imaged slab stagnation plume deflection and changes in large scale structure and offers a simple explanation of these phenomena

an evolving view of transition zone and midmantle viscosity in a new study liu and zhong this issue investigate the impact of thermochemical anomalies in the lower mantle on the viscosity structure inferred from the geoid finding that both thermochemical and thermal only mantle models can be reconciled with the geoid with nearly the same viscosity structure they also find a preference for radial viscosity models where the lowest viscosity layer occurs in the transition zone as opposed to the upper mantle while their weak transition zone viscosity model is not unprecedented it differs from the most widely used viscosity models which assume the smallest viscosity occurs in the upper mantle liu and zhong s investigation does not support the midmantle viscosity increase recently suggested by rudolph et al 2015 who use a nearly identical methodology but different tomographic model

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle