topographic expressions of mantle dynamics in the mediterranean abstract the surface of the earth is the ever changing expression of the dynamic processes occurring at depth and at or above its surface however our ability to read landscapes in terms of their underlying tectonic or climatic forcing remains rudimentary during the last decade particular attention has been drawn to the deep dynamic contributions to topography related to the stresses that are produced at the base of the lithosphere by mantle convection and their relevance compared to the iso static component of topography here we use examples from the mediterranean and estimate residual and dynamic topography we then compare those with surface uplift from geology for specific regions to disentangle the dynamic from the static components considering the different topographic signatures of tectonic processes e g actual residual and dynamic topography as well as uplift rates jointly provides a powerful way to distinguish between the contributions of mantle crustal and surface processes such an approach might bring us closer to reading topographic expressions in terms of their geological cause

contribution of gravitational potential energy differences to the global stress field summary modelling the lithospheric stress field has proved to be an efficient means of determining the role of lithospheric versus sublithospheric buoyancies and also of constraining the driving forces behind plate tectonics both these sources of buoyancies are important in generating the lithospheric stress field however these sources and the contribution that they make are dependent on a number of variables such as the role of lateral strength variation in the lithosphere the reference level for computing the gravitational potential energy per unit area gpe of the lithosphere and even the definition of deviatoric stress for the mantle contribution much depends on the mantle convection model including the role of lateral and radial viscosity variations the spatial distribution of density buoyancies and the resolution of the convection model gpe differences are influenced by both lithosphere density buoyancies and by radial basal tractions that produce dynamic topography the global lithospheric stress field can thus be divided into 1 stresses associated with gpe differences including the contribution from radial basal tractions and 2 stresses associated with the contribution of horizontal basal tractions in this paper we investigate only the contribution of gpe differences both with and without the inferred contribution of radial basal tractions we use the crust 2 0 model to compute gpe values and show that these gpe differences are not sufficient alone to match all the directions and relative magnitudes of principal strain rate axes as inferred from the comparison of our depth integrated deviatoric stress tensor field with the velocity gradient tensor field within the earth s plate boundary zones we argue that gpe differences calibrate the absolute magnitudes of depth integrated deviatoric stresses within the lithosphere shortcomings of this contribution in matching the stress indicators within the plate boundary zones can be corrected by considering the contribution from horizontal tractions associated with density buoyancy driven mantle convection deviatoric stress magnitudes arising from gpe differences are in the range of 1 4 tn m 1 a part of which is contributed by dynamic topography the egm96 geoid data set is also used as a rough proxy for gpe values in the lithosphere however gpe differences from the geoid fail to yield depth integrated deviatoric stresses that can provide a good match to the deformation indicators gpe values inferred from the geoid have significant shortcomings when used on a global scale due to the role of dynamically support of topography another important factor in estimating the depth integrated deviatoric stresses is the use of the correct level of reference in calculating gpe we also elucidate the importance of understanding the reference pressure for calculating deviatoric stress and show that overestimates of deviatoric stress may result from either simplified 2 d approximations of the thin sheet equations or the assumption that the mean stress is equal to the vertical stress

assimilating lithosphere and slab history in 4 d earth models we develop methods to incorporate paleogeographical constraints into numerical models of mantle convection through the solution of the convection equations the models honor geophysical and geological data near the surface while predicting mantle flow and structure at depth and associated surface deformation the methods consist of four constraints determined a priori from a plate history model 1 plate velocities 2 thermal structure of the lithosphere 3 thermal structure of slabs in the upper mantle and 4 velocity of slabs in the upper mantle these constraints are implemented as temporally and spatially dependent conditions that are blended with the solution of the convection equations at each time step we construct earth like regional models with oceanic and continental lithosphere trench migration oblique subduction and asymmetric subduction to test the robustness of the methods by computing the temperature velocity and buoyancy flux of the lithosphere and slab full sphere convection models demonstrate how the methods can determine the flow associated with specific tectonic environments e g back arc basins intraoceanic subduction zones to address geological questions and compare with independent data both at present day and in the geological past e g seismology residual topography stratigraphy using global models with paleogeographical constraints we demonstrate 1 subduction initiation at the izu bonin mariana convergent margin and flat slab subduction beneath north america 2 enhanced correlation of model slabs and fast anomalies in seismic tomography beneath north and south america and 3 comparable amplitude of dynamic and residual topography in addition to improved spatial correlation of dynamic and residual topography lows

erosion driven uplift in the gamburtsev subglacial mountains of east antarctica the relative roles of climate and tectonics in mountain building have been widely debated central to this debate is the process of flexural uplift in response to valley incision here we quantify this process in the gamburtsev subglacial mountains a paradoxical tectonic feature in cratonic east antarctica previous studies indicate that rifting and strike slip tectonics may have provided a key trigger for the initial uplift of the gamburtsevs but the contribution of more recent valley incision remains to be quantified inverse spectral free air admittance and bouguer coherence methods indicate that unusually for continents the coherence between free air gravity anomalies and bedrock topography is high 0 5 and that the elastic thickness of the lithosphere is anomalously low 15 km in contrast to previously reported values of up to 70 km the isostatic effects of two different styles of erosion are quantified dendritic fluvial incision overprinted by alpine style glacial erosion in the gamburtsevs and outlet glacier type selective linear erosion in the lambert rift part of the east antarctic rift system 3d flexural models indicate that valley incision has contributed ca 500 m of peak uplift in the gamburtsevs and up to 1 2 km in the lambert rift which is consistent with the present day elevation of oligocene miocene glaciomarine sediments overall we find that 17 25 of gamburtsev peak uplift can be explained by erosional unloading these relatively low values are typical of temperate mountain ranges suggesting that most of the valley incision in the gamburtsevs occurred prior to widespread glaciation at 34 ma the pre incision topography of the gamburtsevs lies at 2 2 5 km above sea level confirming that they were a key inception point for the development of the east antarctic ice sheet tectonic and or dynamic processes were therefore responsible for ca 80 of the elevation of the modern gamburtsev subglacial mountains

effects of latent heat release at phase boundaries on flow in the earth s mantle phase boundary topography and dynamic topography at the earth s surface abstract mantle flow models that do not consider the effects of latent heat on phase boundaries typically predict dynamic surface topography too large to be compatible with observations here these effects were implemented in a mantle flow model and resulting changes in dynamic topography and topography of phase boundaries were computed inclusion of these effects was found to reduce the rms amplitude of dynamic topography by about 50 60 m still leaving a substantial misfit from observations in most cases considered generally it also leads to a slight improvement of correlation with dynamic surface and phase boundary topography constrained by observations results thus indicate that the model implemented here is applicable to the earth but that other effects need to be considered as well in order to fully explain phase boundary and dynamic surface topographies

upper mantle structure below the european continent constraints from surface wave tomography and grace satellite gravity data 1 we here exploit fundamental mode rayleigh and love seismic wave information and the high resolution satellite global gravity model ggm02c to obtain a 1 1 3 d image of a upper mantle isotropic shear wave speeds b densities and c density vscoupling below the european plate 20 n 90 n 40 w 70 e the 3 d image of the density vscoupling provides unprecedented detail of information on the compositional and thermal contributions to density structures the accurate and high resolution crustal model allows us to compute a reliable residual topography to understand the dynamic implications of our models the correlation between residual topography and mantle residual gravity anomalies defines three large scale regions where upper mantle dynamics produce surface expression the east european craton the eastern side of the arabian plate and the mediterranean basin the effects of mantle convection are also clearly visible at 1 the eastern sirt embayment 2 the west african craton northern margins 3 the volcanically active region of the canarian archipelago 4 the northern edge of the central european volcanic province and 5 the northeastern part of the atlantic ocean between greenland and iceland strong connections are observed among areas of weak radial anisotropy and areas where the mantle dynamics show surface expression although both thermal and additional dependencies have been incorporated into the density model convective down welling in the mantle below the east european craton is required to explain the strong correlation between the estimated negative mantle residual anomalies and the negative residual topography

role of the lithosphere in mantle convection plate geometry and kinematics generally reflect the mechanical properties of the solid lithosphere rather than those of the fluid mantle underneath and plate formation and subduction account for most of the heat transport from the earth s interior correspondingly mantle convection models must incorporate a stiff but mobile boundary layer like the lithosphere before they can reproduce the main features of mantle convection a relatively easy way to accomplish this in numerical models is to combine a temperature dependent viscosity with an imposed piecewise constant surface velocity boundary condition it is shown how surface heat flux topography gravity and geoid but not plate velocities or stresses can then be derived numerical models confirm that a lithosphere has a first order effect on the underlying flow structure for internally heated models approx imating the case of whole mantle convection the calculated surface signatures are in first order agreement with observations a level of empirical success which hitherto has not been approached by models of mantle convection companion papers exploit the observations more fully to constrain the main features of mantle convection

slab avalanche induced tectonics in self consistent dynamic models subducted slabs often temporarily stagnate within the transition zone and then periodically penetrate into the lower mantle in the form of a slab avalanche we investigate the influence of such slab avalanches on tectonics with dynamically consistent numerical models we suggest that slab avalanches can induce substantial changes in surface tectonics including synchronous broad scale basin inversion and dynamic subsidence in regions above the avalanched slab and reducing or even reversing trench retreat however basins distal from the trench and avalanched slab continue extension following avalanche onset we suggest that a slab avalanche event beneath the sundaland continental promontory began in the early miocene to trigger a wide range of tectonic responses within southeast asia including 1 a reduction of the sunda java trench retreat velocity that may have transitioned to advance regionally since the early miocene 2 synchronous basin compression and subsidence within southern sundaland since the early miocene and 3 continuous basin extension with some developing to ultra deep rifted basins in the northern part of sundaland we also discuss the implication of the slab avalanche model in the context of the history of the taranaki basin offshore new zealand

topography the geoid and compensation mechanisms for the southern rocky mountains the southern rockies of colorado are anomalously high elevations greater than 2800 m topographically rough implying active uplift and underlain by significant low velocity anomalies in the upper mantle that suggest an intimate relationship between mantle geodynamic processes and the surface topography the region is in isostatic equilibrium i e near zero free air gravity anomaly however the poor correlation between the high topography and crustal thickness makes the application of simple compensation models e g pure heiskanen or pratt hayford problematic knowledge of how the current topography of the rockies is isostatically compensated could provide constraints on the relative role of sublithospheric buoyancy versus lithospheric support here we evaluate the geoid and its relationship to the topography using the geoid to elevation ratio gtr in the spatial domain and the admittance in the frequency domain to constrain the mechanism of compensation we separate the upper mantle geoid anomalies from those with deeper sources through the use of spherical harmonic filtering of the egm2008 geoid we exploit the fact that at wavelengths greater than the flexural wavelength where features are isostatically compensated the geoid topography ratio can be used to estimate the depth of compensation and the elastic thickness of the lithosphere the results presented below indicate that the main tectonic provinces of the western united states have moderate geoid topography ratios between 3 5 and 5 5 m km 3 9 for the southern rockies 4 25 for the colorado plateau and 5 2 for the northern basin and range suggesting shallow levels of isostatic compensation in terms of the elastic thickness of the lithosphere our results indicate an elastic thickness of less than 20 km these value support the notion that a major portion of the buoyancy that has driven uplift resides at depths less than 100 km and that upper mantle processes such as small scale convection may play a significant role in the buoyant uplift of the southern rockies as well as other actively uplifting areas of the western united states further support for this hypothesis is provided by high coherence for the geoid topography relationship for nearly all wavelengths between 50 and 1000 km

isostasy dynamic topography and the elevation of the apennines of italy abstract the elevation of an orogenic belt is commonly related to crustal lithosphere thickening here we discuss the apennines as an example to show that topography at a plate margin may be controlled not only by isostatic adjustment but also by dynamic mantle driven processes using recent structural constraints for the crust and mantle we find that the expected crustal isostatic component explains only a fraction of the topography of the belt indicating positive residual topography in the central apennines and negative residual topography in the northern apennines and calabria the trend of the residual topography matches the mantle flow induced dynamic topography estimated from regional tomography models we infer that a large fraction of the apennines topography is related to mantle dynamics producing relative upwellings in the central apennines and downwellings in the northern apennines and calabria where subduction is still ongoing comparison between geodetic and geological data on vertical motions indicates that this dynamic process started in the early pleistocene and the resulting uplift appears related to the formation and enlargement of a slab window below the central apennines the case of the apennines shows that at convergent margins the elevation of a mountain belt may be significantly different from that predicted solely by crustal isostasy and that a large fraction of the elevation and its rate of change are dynamically controlled by mantle convection

the late eocene early miocene unconformities of the nw indian intraplate basins and himalayan foreland a record of tectonics or mantle dynamics a well developed late eocene to miocene unconformity termed the base miocene unconformity bmu is found throughout the intraplate basins of north western india and has previously been ascribed to himalayan tectonics this hypothesis is investigated by first describing the nature and age of the bmu in the northwest indian intraplate basins and then reconstructing the location of the bmu relative to the himalayan deformation front at the time it formed we suggest that formation of the bmu in western india cannot be related to himalayan tectonic processes associated with plate loading and flexure unless the indian plate had an elastic thickness of 125 km which is highly unlikely furthermore the resumption of deposition post unconformity rules out inversion due to compression associated with india asia convergence as a cause as these compressive forces are still present we note the coeval nature of the unconformity in the nw indian plate intraplate basins and the himalayan peripheral foreland basin if the unconformities of the himalayan peripheral foreland basin and the nw indian intraplate basins formed by a common process uplift due to circulation in the mantle is the only possible regional scale mechanism such circulation could be the result of the intrinsically time dependent high rayleigh number convection in the mantle which has resulted in well documented unconformities elsewhere or be the result of subducting slab break off beneath the himalaya

the tilting continent a new constraint on the dynamic topographic field from australia a pronounced latitudinal asymmetry in the present day morphology of the australian continental shelf is reflected in neogene stratigraphic relationships the northern australian margin has a broad shelf typically 200 500 km wide and a neogene record of stratal onlap relative to the continent sea levels are currently as high as at any stage during the neogene in contrast the southern shelf is typically less than 100 km wide and shows a record of progressive offlap with neogene palaeo shorelines commonly many hundreds of kilometres inland at elevations up to 250 m above present day sea level this continental scale reciprocal stratigraphy implies 250 300 m n down ssw up apparent vertical motion with respect to sea level since the mid miocene the apparent vertical motion can be attributed to variations in dynamic topography and the geoid along australia s nne plate circuit specifically the movement of the southern margin off the dynamic topography low geoid low presently centred on the australian antarctic discordance and the northern margin towards a dynamic topography low geoid high associated with the subduction realm to the north variations in the geoid appear to account for about 10 of the total apparent motion depending on assumptions about how the geoid field has evolved during australia s northward motion this inferred neogene continental scale dynamic n s tilting rate of 15 20 m myr provides a compelling new constraint on the nature of the earth s dynamic topographic field 2007 elsevier b v all rights reserved

slab flattening and the rise of the eastern cordillera colombia abstract the topographic growth of a mountain belt is commonly attributed to isostatic balance in response to crustal and lithospheric thickening however deeper mantle processes may also influence the topography of the earth here we discuss the role of these processes in the eastern cordillera ec of colombia the ec is an active double vergent fold and thrust belt that formed during the cenozoic by the inversion of a mesozoic rift and topography there has risen up to 5 000 m cocuy sierra the belt is located 500 km away from the trench where two separate portions of the nazca plate subduct below the south american plate north of 5 n the ec rises above a flat slab subduction region volcanic arc migration implies slab shallowing by 10 ma and flattening up to the present day configuration at 6 ma the occurrence of a high vp vs anomaly and clustered seismicity below the belt at 160 km depth delineates the slab geometry and has been related to dehydration of the slab suggesting the presence of a hydrated mantle wedge we compiled thermochronologic data and inverted for the exhumation history of the chain over the last 20 ma using the age elevation relationship and the different closure temperatures of multiple thermochronologic systems results indicate that exhumation rates increased during the plio pleistocene at different wavelengths and amplitudes the small wavelength and large amplitude signals could be related to shallow crustal deformation whereas the source of the long wavelength and moderate amplitude signal has yet to be identified pulses of fast exhumation are found to be concomitant with the uplift that occurred from 7 ma to the present day previous studies suggested that the high topography of the chain cannot be achieved solely through isostatic adjustment the highest residual topography is centered on the highest elevations of the ec whereas the lowest residual topography corresponds to the magdalena valley following the regional slab geometry we propose that the recent uplift and exhumation events were triggered by the transition from regular to flat slab subduction along with the hydration of the mantle wedge above the slab we test the dynamic feasibility of our hypothesis with a series of numerical models for the present day state predicting the correct trends in elevation requires a flat slab geometry and a weak and buoyant mantle wedge

constraining mantle viscosity structure for a thermochemical mantle using the geoid observation long wavelength geoid anomalies provide important constraints on mantle dynamics and viscosity structure previous studies have successfully reproduced the observed geoid using seismically inferred buoyancy in whole mantle convection models however it has been suggested that large low shear velocity provinces llsvps underneath pacific and africa in the lower mantle are chemically distinct and are likely denser than the ambient mantle we formulate instantaneous flow models based on seismic tomographic models to compute the geoid and constrain mantle viscosity by assuming both thermochemical and whole mantle convection geoid modelling for the thermochemical model is performed by considering the compensation effect liu zhong 2015 of dense thermochemical piles and removing buoyancy structure of the compensation layer in the lower mantle thermochemical models well reproduce the observed geoid thus reconciling the geoid with the interpretation of llsvps as dense thermochemical piles the viscosity structure inverted for thermochemical models is nearly identical to that of whole mantle models in the preferred model the lower mantle viscosity is 10 times higher than the upper mantle viscosity that is 10 times higher than the transition zone viscosity the weak transition zone is consistent with the proposed high water content there the geoid in thermochemical mantle models is sensitive to seismic structure at mid mantle depths suggesting a need to improve seismic imaging resolution there the geoid modelling constrains the vertical extent of dense and stable chemical piles to be within 500 km above cmb our results have implications for mineral physics seismic tomographic studies and mantle convection modelling this article is protected by copyright all rights reserved

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

dynamic topography gravity and the role of lateral viscosity variations from inversion of global mantle flow lateral viscosity variations lvvs in the mantle influence geodynamic processes and their surface expressions with the observed long wavelength geoid free air anomaly gravity gradient in three directions and discrete high accuracy residual topography we invert for depth and temperature dependent and tectonically regionalized mantle viscosity with a mantle flow model the inversions suggest that long wavelength gravitational and topographic signals are mainly controlled by the radial viscosity profile the pre cambrian lithosphere viscosity is slightly one order of magnitude higher than that of oceanic and phanerozoic lithosphere plate margins are substantially weaker than plate interiors and viscosity has only a weak apparent dependence on temperature suggesting either a balancing between factors or a smoothing of actual higher amplitude but short wavelength lvvs the predicted large scale lithospheric stress regime compression or extension is consistent with the world stress map thrust or normal faulting both recent compiled high accuracy residual topography and the predicted dynamic topography yield 1 km amplitude long wavelength dynamic topography inconsistent with recent studies suggesting amplitudes of 100 to 500 m such studies use a constant positive admittance transfer function between topography and gravity in contrast to the evidence which shows that the earth has a spatially and wavelength dependent admittance with large negative admittances between 4000 and 10 4 km wavelengths

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

a model of transformational superplasticity in the upper mantle we develop a model of transformational superplasticity of the mantle as it undergoes a solid solid phase change by considering various scenarios of the evolution of grain geometry in a polycrystalline material composed of two phases of different densities we estimate the strain rate associated with the reshaping of the grains required to accommodate the volume change we relate the deviatoric strain rate of the reshaping grains to the macroscopic dilatation rate of the entire composite where the latter is evaluated both by applying a kinetic theory of the transformation and by implementing the seismically observed sharpness of the phase transformation we estimate that depending on the grain geometry and the kinetics the deviatoric strain rates can exceed the dilatational strain rates by an order of magnitude we calculate the degree of softening of the mantle that would occur at the beginning of the phase transformations at 400 and 670 km depths for a power law rheology with stress exponent n 3 mantle viscosity decreases by up to one to two orders of magnitude within the first 1 5 km of the upper transition and by two to three orders of magnitude within the first 1 km for the transition at 670 km depth to account for uncertainties in strain rate or stress and grain size we construct a deformation mechanism map for a three component mantle and a variety of grain sizes tectonic stresses and strain rates in the dislocation creep regime the high transformational stresses place an upper bound on the effective viscosity of the composite we calculate the transformational superplasticity ts field for a particular mantle flow model and show that variations of effective viscosity of the order of one order of magnitude occur at half of the dominant flow wavelength we describe the effects of a phase transformation on mantle dynamics as jump conditions on the vertical and lateral velocities across the thin two phase layer an abrupt change in the azimuthal velocity would facilitate mixing across the phase change region and cause refraction of currents passing through this depth the largest deviation of the flow velocities occurs within the major up and downwellings we also show that when ts is included the change in the long wavelength geoid is comparable to that caused by a 50 per cent increase in the viscosity of the lower mantle and the change in the short wavelength geoid is similar to an extension of an upper mantle low viscosity zone down to 450 km depth

sulfide melts graphite interaction at hpht conditions implications for diamond genesis abstract the interaction of graphite with fe ni 9s8 and fes sulfide melts has been studied at p 6 3 7 5 gpa and t 1450 2200 c in experiments with a duration from 2 to 65 h it has been found that both pentlandite and pyrrhotite melts interacting with graphite show similar behavior at

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

renormalisation of global mantle dynamic topography predictions using residual topography measurements for normal oceanic crust abstract we compare mantle dynamic topography predicted from mantle convection modelling with residual topography measurements for oceanic regions where crustal basement thickness is 10 0 km or less measurements of residual topography calculated by removing the isostatic effects of crustal thickness variation bathymetry sediments ice and lithosphere thermal anomalies from the observed topography are inaccurate for continents and oceanic plateaus due to uncertainties in determining their crustal thickness and density as a consequence residual topography measurements for these regions are unsuitable for testing mantle dynamic topography predictions residual topography is more accurately determined for oceanic crust we use global mapping of crustal basement thickness using gravity anomaly inversion to identify oceanic crust of 10 0 km thickness or less to select measured residual topography for comparison with predicted mantle dynamic topography for these oceanic regions we compare mantle dynamic topography and residual topography and using amplitude histogram matching and grid searches compute the amplitude rescaling and shift which needs to be applied to predicted mantle dynamic topography to fit the observed residual topography we examine three global compilations which use different approaches to determine mantle dynamic topography i steinberger 2007 which uses seismic topography deeper than 220 km to determine mantle density ii flament et al 2013 which uses plate velocity and subduction history and iii steinberger et al 2017 which uses seismic tomography including that above 220 km to determine shallow upper mantle densities our analysis shows that for the steinberger 2007 and flament et al 2013 compilations the predicted mantle dynamic topography for oceanic regions requires a rescaling of approximately 0 5 and a negative shift of approximately 500 m to match the observed residual topography in contrast steinberger et al 2017 which includes shallow upper mantle densities above 220 km requires only a small shift 50 m but a greater scaling of 0 375 maps of renormalised rescaled and shifted mantle dynamic topography for steinberger et al 2017 show a close resemblance to measured residual topography

predicting and testing continental vertical motion histories since the paleozoic abstract dynamic topography at the earth s surface caused by mantle convection can affect a range of geophysical and geological observations including bathymetry sea level change continental flooding sedimentation and erosion these observations provide important constraints on and test of mantle dynamic models based on global mantle convection models coupled with the surface plate motion history we compute dynamic topography and its history for the last 400 ma associated with pangea assembly and breakup with particular focus on cratonic regions we propose that burial unroofing histories of cratons inferred from thermochronology data can be used as a new diagnostic to test dynamic topography and mantle dynamic models our models show that there are currently two broad dynamic topography highs in the pacific and africa for the present day earth that are associated with the broad warm structures i e superplumes in the deep mantle consistent with previous proposals of dynamical support for the pacific and african superswells our models reveal that pangea assembly and breakup by affecting subduction and mantle upwelling processes have significant effects on continental vertical motions our models predict that the slave craton in north america subsides before pangea assembly at 330 ma but uplifts significantly from 330 ma to 240 ma in response to pre pangea subduction and post assembly mantle warming the kaapvaal craton of africa is predicted to undergo uplift from 180 ma to 90 ma after pangea breakup but its dynamic topography remains stable for the last 90 ma the predicted histories of elevation change for the slave and kaapvaal cratons compare well with the burial unroofing histories inferred from thermochronology studies thus supporting our dynamic models including the development of the african superplume mantle structure the vertical motion histories for other cratons can provide further tests of and constraints on our mantle dynamic models

a rapid method to map the crustal and lithospheric thickness using elevation geoid anomaly and thermal analysis application to the gibraltar arc system atlas mountains and adjacent zones we present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness the main assumptions are local isostasy and a four layered model composed of crust lithospheric mantle sea water and the asthenosphere we consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle we perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of rms error of elevation and geoid anomaly databases the application of this method to the gibraltar arc system atlas mountains and adjacent zones reveals the presence of a lithospheric thinning zone sw ne oriented this zone affects the high and middle atlas and extends from the canary islands to the eastern alboran basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western betics eastern rif rharb basin and gulf of cadiz a number of different even mutually opposite geodynamic models have been proposed to explain the origin and evolution of the study area our results suggest that a plausible slab retreating model should incorporate tear and asymmetric roll back of the subducting slab to fit the present day observed lithosphere geometry in this context the lithospheric thinning would be caused by lateral asthenospheric flow an alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the canary island and extending as far as central europe

exhumation of a collisional orogen a perspective from the north american grenville province combined structural and geochronologic research in the southernmost portion of the contiguous grenville province of north america ontario and new york state show protracted periods of extension after the last episode of contraction the grenville province in this area is characterized by synorogenic extension at ca 1040 ma supported by u pb data on titanites and 40 ar 39 ar data on hornblendes followed by regional extension occurring along crustal scale shear zones between 945 and 780 ma as recorded by 40 ar 39 ar analysis of hornblende biotite and k feldspar by ca 780 ma the southern portion of the grenville province from ontario to the adirondack highlands underwent uplift as a uniform block tectonic hypotheses have invoked various driving mechanisms to explain the transition from compression to extension however such explanations are thus far geodynamically unconstrained numerical models indicate that mechanisms such as gravitational collapse and mantle delamination act over timescales that cannot explain a protracted 300 m y extensional history that is contemporaneous with ongoing uplift of the grenville province rather the presence of a plume upwelling underneath the laurentian margin combined with changes in regional stress directions permitted the observed uplift and extension in the grenville province during this time the uplift history while on a slightly different timescale from those of most plume models is similar to that seen in models of uplift and extension caused by the interaction of a plume with the base of the lithosphere some of the protracted extension likely reflects the contribution of farfield effects possibly caused by tectonic activity in other cratons within the rodinian supercontinent effectively changing the stress distributions in the grenville province of northeastern north america

the dynamics of western north america stress magnitudes and the relative role of gravitational potential energy plate interaction at the boundary and basal tractions summary we investigate the forces involved in driving long term large scale continental deformation in western north america and quantify the vertically averaged deviatoric stress field arising from internal buoyancy forces and the accommodation of relative plate motions in addition we investigate the ability of regional models to resolve the level of tractions acting at the base of the lithosphere we directly solve force balance equations for vertically averaged deviatoric stresses associated with differences in values of 1 lithospheric thickness times the gravitational potential energy per unit area gpe the gpe values are inferred using both the etopo5 topographic data set and the crust2 0 crustal thickness model deviatoric stresses associated with basal tractions are calculated globally with inputs determined from an isoviscous upper mantle 10 21 pa s 3 d large scale convection model in which mantle density variations were inferred from tomographic data and the history of subduction in a 211parameter iterative inversion we then solve for a stress field boundary condition by fitting stress field indicators i e the directions and relative magnitudes of the principal axes of kinematic strain rates magnitudes of the total vertically averaged deviatoric stress field sum of gpe solution with the boundary condition solution range from 5 to 10 mpa within a 100 km thick lithosphere these magnitudes are calibrated by the gpe differences along with the spatial variation in deformation style there is a trade off between the scaling of the basal traction deviatoric stress field and the boundary condition solution however the combined boundary conditions plus basal traction solution is robust in both magnitude and style and when added to the contribution from gpe differences provides a global minimum of misfit between the total deviatoric stress solution and the stress field indicators gpe variations account for 50 per cent of the deviatoric stress magnitudes driving deformation while boundary condition stresses account for the remaining 50 per cent of deviatoric stress magnitude by comparing possible end member strength profiles with our vertically averaged deviatoric stresses we infer that the bulk of the strength within the lithosphere in western north america lies within the brittle seismogenic layer

geodynamic inversion to constrain the non linear rheology of the lithosphere a common method to determine the strength of the lithosphere is through estimating its effective elastic thickness from the coherence between gravity and topography this method assumes a priori that the lithosphere is a thin elastic plate floating on a viscous mantle whereas this seems to work well with oceanic plates it has given controversial results in continental collision zones usually continental collisions zones are well studied areas for which additional geophysical datasets such as receiver functions and seismic tomography exist that constrain the geometry of the lithosphere and often show that it is rather complex yet lithospheric geometry by itself is insufficient to understand the dynamics of the lithosphere as this also requires knowledge of the rheology of the lithosphere experimental results show significant variability between various rock types and there are large uncertainties in extrapolating laboratory values to nature which leaves room for speculation

inversion for mantle viscosity profiles constrained by dynamic topography and the geoid and their estimated errors summary we perform a joint inversion of earth s geoid and dynamic topography for radial mantle viscosity structure using a number of models of interior density heterogeneities including an assessment of the error budget we identify three classes of errors those related to the density perturbations used as input those due to insufficiently constrained observables and those due to the limitations of our analytical model we estimate the amplitudes of these errors in the spectral domain our minimization function weights the squared deviations of the compared quantities with the corresponding errors so that the components with more reliability contribute to the solution more strongly than less certain ones we develop a quasi analytical solution for mantle flow in a compressible spherical shell with newtonian rheology allowing for continuous radial variations of viscosity together with a possible reduction of viscosity within the phase change regions due to the effects of transformational superplasticity the inversion reveals three distinct families of viscosity profiles all of which have an order of magnitude stiffening within the lower mantle with a soft d layer below the main distinction among the families is the location of the lowest viscosity region directly beneath the lithosphere just above 400 km depth or just above 670 km depth all profiles have a reduction of viscosity within one or more of the major phase transformations leading to reduced dynamic topography so that whole mantle convection is consistent with small surface topography

models of isostatic and dynamic topography geoid anomalies and their uncertainties knowledge of the dynamic topography at earth s surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics we calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust the oceanic lithosphere and the continental tectosphere we also calculate the isostatic geoid anomaly that would result from these compensated near surface density anomalies because there is controversy about the thermal structure of old oceanic lithosphere we investigate two models of the cooling of the oceanic lithosphere half space and plate and calculate the effect of each on the inferred dynamic topography peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately 1 km we estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors to obtain the spectral coefficients of the fields we use weighted least squares inversions the calculated dynamic topography arising from sublithospheric sources residual geoid and errors associated with these fields can be used in studies of mantle dynamics

quantifying the relationship between short wavelength dynamic topography and thermomechanical structure of the upper mantle using calibrated parameterization of anelasticity oceanic residual depth varies on 5000 km wavelengths with amplitudes of 1 km a component of this short wavelength signal is dynamic topography caused by convective flow in the upper 300 km of the mantle it exerts a significant influence on landscape evolution and sea level change but its contribution is often excluded in geodynamic models of whole mantle flow using seismic tomography to resolve buoyancy anomalies in the oceanic upper mantle is complicated by the dominant influence of lithospheric cooling on velocity structure here we remove this cooling signal from global surface wave tomographic models revealing a correlation between positive residual depth and slow residual velocity anomalies at depths 300 km to investigate whether these anomalies are of sufficient amplitude to account for short wavelength residual depth variations we calibrate an experimentally derived parameterization of anelastic deformation at seismic frequencies to convert shear wave velocity into temperature density and diffusion creep viscosity asthenospheric temperature anomalies reach 150 c in the vicinity of major magmatic hotspots and correlate with geochemical and geophysical proxies for potential temperature along mid ocean ridges locally we find evidence for a 150 km thick low viscosity asthenospheric channel incorporating our revised density structure into models of whole mantle flow yields reasonable agreement with residual depth observations and suggests that 30 km deviations in local lithospheric thickness account for a quarter of total amplitudes these predictions remain compatible with geoid constraints and substantially improve the fit between power spectra of observed and predicted dynamic topography this improvement should enable more accurate reconstruction of the spatio temporal evolution of cenozoic dynamic topography

sublithospheric small scale convection and its implications for the residual topography at old ocean basins and the plate model 1 the seafloor topography and heat flux differ significantly from the predictions of the half space cooling hsc model at old ocean basins understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large scale mantle dynamics a widely used model that explains significant fraction of the deviations is the plate model but the dynamical feasibility of the plate model has never been demonstrated in this study we investigated the effects of sublithospheric small scale convection ssc and of internal heating on seafloor heat flux and topography and mantle thermal structure and we examined the dynamic feasibility of the plate model by formulating high resolution two dimensional numerical models of mantle convection with strongly temperature and depth dependent rheology we found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere especially when significant internal heating is present this trapped heat may exist in the middle mantle when the ssc is absent and it may also be redistributed by the ssc to shallow depths to reheat the lithosphere and to homogenize mantle temperature when internal heating accounts for 60 of the total heat output the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via ssc while maintaining uniform mantle temperature we suggest that the trapped heat and the ssc are responsible for the residual heat flux and topography at old ocean basins relative to the hsc model predictions our models also show that for the plate model to be dynamically viable both the ssc and significant internal heating 60 are necessary this is because only the ssc in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere which is the basic assumption of the plate model with the viscosity structure and internal heating rate for the present day mantle we think that the plate model is dynamically viable

lithosphere mantle coupling and the dynamics of the eurasian plate mechanical equilibrium of tectonic plates implies that lithospheric edge and body forces are balanced by forces arising from interaction with the underlying mantle we use this quantitative physical relation to integrate existing modelling approaches of lithosphere dynamics and mantle flow into a new combined approach applied to the eurasian plate by combining a thorough analysis of lithospheric forces with the requirement of torque balance we constrain the orientation of the torque on eurasia arising from mantle tractions we use this constraint to evaluate convective mantle flow models driven by tomographic or subduction history model anomalies and observed plate motion mantle forcing is considered through both shear at the bottom of the plate and induced dynamic topography we find that instantaneous semi analytic flow models without lateral viscosity variations generate tractions that meet the constraint from eurasian lithosphere dynamics but only for specific ranges of mantle flow parameters of the explored set of mantle anomaly models only mantle flow models based on s wave tomography anomalies can balance eurasia for realistic viscosity profiles and velocity density scaling choices in mantle density forcing and viscosity are crucial in that they govern the relative magnitude of tractions due to convective mantle flow active tractions and resistive tractions due to plate motion passive tractions we find mechanical balance is only achieved for similar torque magnitudes of active and passive shear the two shear contributions do however in no case balance each other and a considerable dominant net torque from edge forces is required to balance total mantle tractions and lithospheric body forces lbfs our analysis provides a range of mechanically consistent total force sets acting on the eurasian plate using this result we find that mantle buoyancy forces and lbfs acting on eurasia itself are important driving forces but do not drive eurasia in the observed direction continental collision at eurasia s southern boundary significantly deviates eurasia northwards our combined torque balance approach in which mantle tractions from convective mantle flow modelling are combined with explicitly applied edge forces thus emphasizes the role of plate interactions to the dynamics of tectonic plates

topography and geoid due to lithospheric mass anomalies summary a model of lithospheric thickness and a recent compilation of moho depths are used to compute the earth s isostatic surface topography and associated gravity anomalies the results are strongly influenced by the uncertainties in lithospheric depth and crustal density profiles the preferred models explain most of the observed topography and are highly correlated with observed gravity anomalies for harmonic degrees larger than 10 comparisons of our residual topography with geodynamical calculations of dynamic topography based on mantle circulation are rather poor

present day uplift of the european alps evaluating mechanisms and models of their relative contributions abstract recent measurements of surface vertical displacements of the european alps show a correlation between vertical velocities and topographic features with widespread uplift at rates of up to 2 2 5 mm a in the north western and central alps and 1 mm a across a continuous region from the eastern to the south western alps such a rock uplift rate pattern is at odds with the horizontal velocity field characterized by shortening and crustal thickening in the eastern alps and very limited deformation in the central and western alps proposed mechanisms of rock uplift rate include isostatic response to the last deglaciation long term erosion detachment of the western alpine slab as well as lithospheric and surface deflection due to mantle convection here we assess previous work and present new estimates of the contributions from these mechanisms given the large range of model estimates the isostatic adjustment to deglaciation and erosion are sufficient to explain the full observed rate of uplift in the eastern alps which if correct would preclude a contribution from horizontal shortening and crustal thickening alternatively uplift is a partitioned response to a range of mechanisms in the central and western alps the lithospheric adjustment to deglaciation and erosion likely accounts for roughly half of the rock uplift rate which points to a noticeable contribution by mantle related processes such as detachment of the european slab and or asthenospheric upwelling while it is difficult to independently constrain the patterns and magnitude of mantle contributions to ongoing alpine vertical displacements at present future data should provide additional insights regardless interacting tectonic and surface mass redistribution processes rather than an individual forcing best explain ongoing alpine elevation changes

accurate measurements of residual topography from the oceanic realm in the oceans our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped these anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle here we analyze a global database of seismic reflection and wide angle profiles from heavily sedimented oceanic crust which abuts continental lithosphere at 449 locations we calculated water loaded subsidence compared it with a reference age depth relationship and determined residual depth we then combined these spot measurements of residual depth with observations from mid oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth our results suggest that the amplitude of residual depth varies by up to 1 km with wavelengths of order 103 km we compare our residual depths with free air gravity and seismic tomographic anomalies our results show that residual depths correlate with long wavelength gravity anomalies in contrast correlations between residual depths and vertically averaged shear velocity anomalies within the upper and or the lower mantle are weaker the largest discrepancies occur at short 1000 km wavelengths these combined observations suggest that residual depth anomalies could be generate by density variations within a thin 102 km low viscosity layer beneath the lithosphere our global compilation should play a significant role in helping to refine predictive geodynamical models

modeling the dynamic component of the geoid and topography of venus 1 we analyze the venusian geoid and topography to determine the relative importance of isostatic elastic and dynamic compensation mechanisms over different degree ranges the geoid power spectrum plotted on a log log scale shows a significant change in its slope at about degree 40 suggesting a transition from a predominantly dynamic compensation mechanism at lower degrees to an isostatic and or elastic mechanism at higher degrees we focus on the dynamic compensation in the lower degree interval we assume that 1 the flow is whole mantle in style 2 the long wavelength geoid and topography are of purely dynamic origin and 3 the density structure of venus mantle can be approximated by a model in which the mass anomaly distribution does not vary with depth solving the inverse problem for viscosity within the framework of internal loading theory we determine the families of viscosity models that are consistent with the observed geoid and topography between degrees 2 and 40 we find that a good fit to the data can be obtained not only for an isoviscous mantle without a pronounced lithosphere as suggested in some previous studies but also for models with a high viscosity lithosphere and a gradual increase in viscosity with depth in the mantle the overall viscosity increase across the mantle found for the latter group of models is only partially resolved but profiles with a 100 km thick lithosphere and a viscosity increasing with depth by a factor of 10 80 hence similar to viscosity profiles expected in the earth s mantle are among the best fitting models

topography of the ocean floor thermal evolution of the lithosphere and interaction of deep mantle heterogeneities with the lithosphere topography associated with large wave length geoid is derived by two methods first the bathymetry corrected for sediment loading is decomposed using a least squares method as a function of age plus terms proportional to the geoid the topography at old ages is flatter predicted by the law at degrees 2 and 3 very little topography is associated with the geoid the transfer function is not even certain in sign but is never smaller than 0 6 more topography seems associated with higher degree harmonics 6 and 7 second the function of age alone best fitting the topography is determined the correlation coefficients of the residual topography with the geoid are then computed a slight correlation appears for degree 6 but not for degrees 2 and 3 these results suggest a mechanical decoupling between the lower mantle mass heterogeneities and the lithosphere

neogene residual subsidence and its response to a sinking slab in the deep mantle of eastern china abstract mantle convection could have a significant effect on basin evolution however research quantifying this relationship is controversial to understand the formation mechanism and evolution of the cenozoic rift basins in eastern china we applied the back stripping technique and strain rate inversion modeling to 119 wells from sangjiang basin bohai bay basin north yellow sea basin south yellow sea basin east china sea shelf basin and four basins within the northern south china sea margin the modeled results can be used to reconstruct the tectonic subsidence history and further assess the potential subsidence mechanisms of eastern china residual subsidence is defined as the difference between the theoretical and observed tectonic subsidences our results show that the residual subsidence since 20 ma in eastern china generally increases from 100 to 300 m in terrestrial areas to 1 2 1 8 km on the continental shelf our observed residual subsidence is generally consistent with the present day dynamic topography that is predicted from mantle flow models and it is associated with a stagnant slab within the mantle that has been observed by seismic tomography the migration pathway of the residual subsidence since 20 ma is consistent with the movement direction of the pacific and philippine plates therefore we suggest that the residual subsidence might be a dynamic subsidence induced by a negative buoyancy of the sinking slab in the deep mantle beneath eastern china

partial advection of equidensity surfaces a solution for the dynamic topography problem although one layer dynamic models of the earth s mantle have successfully explained the geoid they generate a surface dynamic topography that seems too large relative to geological observations in this study we hypothesize the possibility of partial advection of mantle equidensity surfaces by vertical motion induced by driving loads these large scale flow dependent loads would greatly reduce the dynamic topography amplitude while preserving a good fit to the observed geoid various physical processes related to nonequilibrium phase changes or to the existence of chemical heterogeneity in the mantle could justify a partial advection of the mean density in this paper we simply consider the flow dependent loads as proportional to the vertical flow velocity two density mantle models are considered one from subduction reconstruction ricard et al 1993 and one from seismic tomography li and romanowicz 1995 we show that a very moderate entrainment a few kilometers of the equidensity surfaces in the transition zone is sufficient to reduce dynamic topography amplitude by a factor of 2 or 3 the seismic velocity signal associated with this entrainment would be hidden by the signal of thermal origin using this new hypothesis we compute sea level changes associated with epeirogeny for the cretaceous paleocene and oligocene periods the amplitude and phase of these changes are in fairly good agreement with geological hypsometric curves our results suggest that not only the thermodynamics but also the kinetics of mineralogical phase changes in the transition zone are of crucial importance

dynamic surface topography a new interpretation based upon mantle flow models derived from seismic tomography the very long wavelength i e in the degree range l 1 8 dynamic topography at the earth s surface estimated by isostatic reduction of the observed topography is shown to be in good agreement with the topography predicted by a model of viscous flow in the mantle this flow model was previously derived by independent consideration of the observed nonhydrostatic geoid and is based upon a recent model of large scale seismic shear velocity heterogeneity in the mantle our estimate and prediction of the long wavelength dynamic surface topography reveals significant depressions 2 3 km of the continental shields relative to their hydrostatic reference positions

oceanic residual topography agrees with mantle flow predictions at long wavelengths dynamic topography the surface deflection induced by sublithosheric mantle flow is an important prediction made by geodynamic models but there is an apparent disparity between geodynamic model predictions and estimates of residual topography total topography minus lithospheric and crustal contributions we generate synthetic global topography fields with different power spectral slopes and spatial patterns to investigate how well the long wavelength spherical degrees 1 to 3 components can be recovered from a discrete set of samples where residual topography has been recently estimated an analysis of synthetic topography along with observed geoid and gravity anomalies demonstrates the reliability of signal recovery appropriate damping factors which depend on the maximum degree in the spherical harmonic expansion that is used to fit the samples must be applied to recover the long wavelength topography correctly large damping factors smooth the model excessively and suppress residual topography amplitude and power spectra unrealistically recovered long wavelength residual topographies based on recent oceanic point wise estimates with different spherical expansion degrees agree with each other and with the predicted dynamic topography from mantle flow models the peak amplitude of the long wavelength residual topography from oceanic observations is about 1 km suggesting an important influence of large scale deep mantle flow

the behavior of the lithosphere on seismic to geologic timescales the strength of the lithosphere and how it responds to loading and unloading are fundamental problems with wide implications flexure studies suggest that the elastic thickness a proxy for the strength of the lithosphere increases with plate age but decreases with load age the elastic thickness is significantly less than the seismic thickness of the lithosphere as indicated by the depth to the low velocity zone suggesting that the lithosphere is a strong structure at short seismic timescales and a weak one at long timescales the mechanism controlling this weakening is not known but it probably involves some form of load induced stress relaxation despite weakening the lithosphere is capable of retaining its strength and supporting loads such as volcanoes and sediments for long periods of geological time lithosphere relaxation should be included in geodynamical models especially as it has impacts on stratigraphy sea level change and dynamic topography

variation of the subsidence parameters effective thermal conductivity and mantle dynamics abstract the subsidence of young seafloor is generally considered to be a passive phenomenon related to the conductive cooling of the lithosphere after its creation at mid oceanic ridges recent alternative theories suggest that the mantle dynamics plays an important role in the structure and depth of the oceanic lithosphere however the link between mantle dynamics and seafloor subsidence has still to be quantitatively assessed here we provide a statistical study of the subsidence parameters subsidence rate and ridge depth for all the oceans these parameters are retrieved through two independent methods the positive outliers method a classical method used in signal processing and through the mifil method from the subsidence rate we compute the effective thermal conductivity k eff which ranges between 1 and 7 w m 1 k 1 we also model the mantle flow pattern from the s40rts tomography model the density anomalies derived from s40rts are used to compute the instantaneous flow in a global 3d spherical geometry we show that departures from the k eff 3 w m 1 k 1 standard value are systematically related to mantle processes and not to lithospheric structure regions characterized by k eff 3 w m 1 k 1 are associated with mantle uplifts mantle plumes or other local anomalies regions characterized by k eff 3 w m 1 k 1 are related to large scale mantle downwellings such as the australia antarctic discordance aad or the return flow from the south pacific superswell to the east pacific rise this demonstrates that mantle dynamics plays a major role in the shaping of the oceanic seafloor in particular the parameters generally considered to quantify the lithosphere structure such as the thermal conductivity are not only representative of this structure but also incorporate signals from the mantle convection occurring beneath the lithosphere the dynamic topography computed from the s40rts tomography model reproduces the subsidence pattern observed in the bathymetry overall we find a good correlation between the subsidence parameters derived from the bathymetry and the dynamic topography this demonstrates that these parameters are strongly dependent on mantle dynamics

the effect of composition of mantle fluids melts on diamond formation processes abstract experimental modeling of natural diamond forming processes is performed at high p t conditions in the k 2 co 3 mg 2 sio 4 kcl h 2 o c system in order to establish the influence of the crystallization environment composition on diamond nucleation and growth most of the experiments were performed at fixed p t parameters p 7 5 gpa t 1600 c 15 h and only the compositions of melts and fluids was varied the degree of the graphite to diamond conversion which is the quantitative indicator of diamond forming capability of the medium was measured in each experiment the results of the present study show that in fluid free carbonate carbonate silicate and carbonate chloride melts diamond crystallize mainly by a fg film growth mechanism in h 2 o containing melts and fluids only the tgg temperature gradient growth mechanism of diamond crystallization was observed increasing water content in all water containing melts and fluids of the studied system considerably enhances diamond nucleation and crystallization in the following sequence k 2 co 3 h 2 o kcl h 2 o mg 2 sio 4 h 2 o the intensity of formation of metastable graphite increases with decreasing temperature and h 2 o content it was shown that diamond crystallization mechanism similar to tgg can have more considerable distribution in nature than fg based on the experimental data we considered possible scenarios of diamond and graphite behavior in nature in the closed systems with excess carbon a decrease in temperature leads to diamond crystallization changes with joint crystallization of diamond and graphite and afterward only metastable graphite crystallization takes place at carbon excess the increasing of p and t lead to increasing of the intensity of diamond crystallization right up to formation of polycrystalline aggregates and at deficiency in carbon only carbon dissolution occurs in the open systems without the additional source of carbon reducing of h 2 o content and changing media from carbon saturated aqueous fluid to fluid free melt result in first to the diamond nucleation then in the joint crystallization of diamond and graphite and finally in fg diamond synthesis the evolution of media composition from fluid free melt to aqueous fluid at the constant excess of carbon results in fg diamond synthesis then joint crystallization of diamond and graphite and finally diamond nucleation similar to tgg mechanism

locally layered convection inferred from dynamic models of the earth s mantle the structure of convection in the mantle is still the subject of considerable debate the now standard modelling of the convective flow as driven in a viscous mantle by density anomalies derived from seismic tomography has successfully explained the longest wavelength degree 2 to 8 geoid anomalies and provided important information concerning the viscosity structure of the mantle1 8 with this approach however the predicted response of surface topography to convective stresses the dynamic topography has a typical magnitude of several kilometres which does not conform with observations9 11 a possible source of this discrepancy lies in the severe underestimation by tomography of density anomalies due to deflections of the boundary between the upper and lower mantle at 660 km depth here we model the mantle flow implied by seismically derived density heterogeneities using an empirical method to account for the 660 km boundary topography the predicted dynamic surface topography thus obtained is significantly reduced to values that conform with the observations in addition the 660 km boundary topography appears to have a strong influence on the computed mantle circulation inducing local layering of the convective flow

cenozoic epeirogeny of the indian peninsula peninsular india is a cratonic region with asymmetric relief manifest by eastward tilting from the 1 5 km high western ghats escarpment toward the floodplains of eastward draining rivers oceanic residual depth measurements on either side of india show that this west east asymmetry is broader scale occurring over distances of 2 000 km admittance analysis of free air gravity and topography shows that the elastic thickness is 10 3 km suggesting that regional uplift is not solely caused by flexural loading to investigate how indian physiography is generated we have jointly inverted 530 river profiles to determine rock uplift rate as a function of space and time key erosional parameters are calibrated using independent geologic constraints e g emergent marine deposits elevated paleosurfaces uplifted lignite deposits our results suggest that regional tilt grew at rates of up to 0 1 mm a 1 between 25 ma and the present day neogene uplift initiated in the south and propagated northward along the western margin this calculated history is corroborated by low temperature thermochronologic observations by sedimentary flux of clastic deposits into the krishna godavari delta and by sequence stratigraphic architecture along adjacent rifted margins onset of regional uplift predates intensification of the indian monsoon at 8 ma suggesting that rock uplift rather than climatic change is responsible for modern day relief a positive correlation between residual depth measurements and shear wave velocities beneath the lithosphere suggests that regional uplift is generated and maintained by temperature anomalies of 100 c within a 200 25 km thick asthenospheric channel this article is protected by copyright all rights reserved

seasonal variability of grain size weathering intensity and provenance of channel sediments in the alaknanda river basin an upstream of river ganga india the ganga river in the indian sub continent is amongst the highest sediment transporting rivers to the world s oceans previous studies estimated that ganga river derives 65 of its sediment load from the higher himalayas the alaknanda river is a headwater stream of the ganga river and has a high sediment flux in the present study freshly deposited channel sediments were collected from the alaknanda river and its tributaries during the post monsoon pre monsoon and monsoon seasons and were analyzed for grain size distribution major oxides composition magnetic susceptibility and heavy mineral assemblages the results show that the channel sediments are mostly medium to coarse sand 56 4 930 m unimodal and symmetrical to fine skewed in distribution the presence of dams and reservoirs seemed to affect the natural grain size distribution in the alaknanda river the average value of the chemical index of alteration of 56 and weathering index of parker of 43 indicates intense physical weathering in the basin the magnetic susceptibility and heavy mineral assemblages showed the effect of local lithology in controlling sediment composition the principal component analysis on the major oxide composition of channel sediment shows that felsic mafic and carbonate weathering contribute to the high sediment load of the alaknanda river this study established that along with weathering of higher himalayan crystallines weathering of garhwal group enriched in magnesite dolomites and mafic intrusion is also a significant source of channel sediments in the upper ganga basin

joint modeling of lithosphere and mantle dynamics sensitivity to viscosities within the lithosphere asthenosphere transition zone and d layers abstract although mantle rheology is one of the most important properties of the earth how a radial mantle viscosity structure affects lithosphere dynamics is still poorly known particularly the role of the lithosphere asthenosphere transition zone and d layer viscosities using constraints from the geoid plate motions and strain rates within plate boundary zones we provide important new refinements to the radial viscosity profile within the key layers of the lithosphere asthenosphere transition zone and d layer we follow the approach of the joint modeling of lithosphere and mantle dynamics ghosh and holt 2012 ghosh et al 2013b 2019 wang et al 2015 to show how the viscosities within these key layers influence lithosphere dynamics we use the viscosity structure sh08 steinberger and holme 2008 as a starting model the density variations within the mantle are derived from the tomography models which based on prior modeling had provided a best fit to the surface observables wang et al 2015 our results show that narrow viscosity ranges of moderately strong lithosphere 2 6 5 6 1022 pa s and moderately weak transition zone 5 9 3 1020 pa s as well as slightly large ranges of moderately weak asthenosphere 5 34 1019 pa s and d layer 4 8 18 1020 pa s are necessary to match all the surface observables we also find that a very strong lithosphere 8 6 1022 pa s along with a weak asthenosphere

global isostatic geoid anomalies for plate and boundary layer models of the lithosphere abstract isostatic geoid anomalies are usually interpreted using a flat earth one dimensional idealization isostatic anomalies on the spherical self gravitating earth differ from this idealization because 1 degree one terms in the spherical harmonic expansion vanish 2 each term in the spherical harmonic expansion is multiplied by l 2 l 0 5 relative to the flat earth case 3 mass in cones rather than straight sided columns is constant and 4 further deformation of the earth is induced by the gravitational attraction of the deformation caused by the isostatic potential anomaly when the effect of each of these is quantified the second third and fourth nearly cancel leaving the degree one over the horizon effect providing the most important difference calculations of model isostatic geoid anomalies for the spherical analogues developed here of the plate and boundary layer thermal models show that this effect can bias estimates of geoid slopes by over 20 although the effect is usually less than 5 the geoid anomalies for these two models are quite different over old ocean basins but they are unlikely to be distinguishable on the basis of global geoid observations owing to the presence of other larger perturbations in the geoid stripping the effects of plate aging and a hypothetical uniform 35 km thick isostatically compensated continental crust from the observed geoid emphasizes that the largest amplitude geoid anomaly is the geoid low of almost 120 m over west antarctica this anomaly is a factor of two greater in amplitude than the low of 60 m over sri lanka

global models from sparse data a robust estimate of earth s residual topography spectrum a significant component of earth s surface topography is maintained by stresses induced by underlying mantle flow this dynamic topography cannot be directly observed but it can be approximated

on the observability of epeirogenic movement in current and future gravity missions abstract next generation gravity missions are expected to improve the accuracy of temporal gravity models significantly the periodic signals and trends retrieved by these missions are induced by mass redistribution in the earth system carrying essential information on dynamic processes in the atmosphere cryosphere continental hydrosphere the oceans and the solid earth while temporal gravity signals induced by deep earth s processes are commonly thought to lie below the observational threshold of satellite gravity missions as one assumes them to be small in amplitude and restricted to the longest spatial and temporal scales there exists evidence from the geologic record for rapid uplift and subsidence events at regional scales especially along passive continental margins related to flow in the underlying mantle here we explore novel mantle flow retrodictions for geodynamically plausible compressible high resolution earth models with 670 million finite elements these time dependent earth models link to geologic observables in the late paleogene that can be tested independently while at the same time indicating mantle flow induced geoid rates on the order of 5 m year at spatial scales of 1000 km we assess the signal retrievability of the modeled rates in closed loop numerical simulations with different satellite gravity retrieval mission assumptions including grace grace fo and a next generation gravity mission we find the modeled deep earth signal to be on the edge of detectability but coming into the range of detectability in future temporal gravity field solutions suggesting the use of satellite gravity data to validate geodynamic earth models importantly the application of forward modeled dynamic mantle signals seems to be essential for improved de aliasing and signal separation in future gravity missions

layered mantle convection a model for geoid and topography the long wavelength geoid and topography are dynamic effects of a convecting mantle the long wavelength geoid of the earth is controlled by density variations in the mantle and has been explained by circulation models involving whole mantle flow however the relationship of long wavelength topography to mantle circulation has been a puzzling problem in geodynamics we show that the dynamic topography is mainly due to density variations in the upper mantle even after the effects of lithospheric cooling and crustal thickness variation are taken into account layered mantle convection with a shallow origin for surface dynamic topography is consistent with the spectrum small amplitude and pattern of the topography layered mantle convection with a barrier about 250 km deeper than the 670 km phase boundary provides a self consistent geodynamic model for the amplitude and pattern of both the long wavelength geoid and surface topography

a global isostatic gravity model of the earth the long wavelength features of the external gravity field of the earth contain the gravitational signal from deep seated lateral mass and density inhomogeneities sustained by dynamic earth mantle processes to interpret the observed gravity field with respect to mantle dynamics and structures it is essential first to remove the lithosphere induced anomalous gravitational potential which is generated by the topographic surface load and its isostatically compensating masses based upon the most recent global compilation of crustal thickness and density data and the age distribution of cooling oceanic lithosphere residual topography and gravity are calculated by subtracting the known crustal and oceanic lithosphere compensating masses and gravitational effects from the surface fields empirical admittances between residual topography and gravity are then computed to estimate the effective depths of the remaining compensating masses which are not explained by the initial data and model assumptions this additional compensation is eventually placed by adjusting the density in the uppermost mantle between the moho and on average 70 km depth with a maximum of 118 km under tibet the lithospheric mass distribution is used in a subsequent forward computation to create a global model of the lithosphere induced gravitational potential the resulting isostatic model is considered to be valid for spatial wavelengths longer than 500 km the isostatic lithosphere model field expressed in terms of both gravity and geoid heights is subtracted from the observed free air gravity field to yield a global set of 1 1 isostatic gravity disturbances and from a satellite derived long wavelength geoid to yield the isostatic residual geoid the comparison of residual mantle gravity residual topography and isostatic corrected gravity allows us to identify the main characteristics of the underlying mantle for example dynamic support by mantle flow of the north atlantic topographic high applying the isostatic correction the overall pattern of the geoid becomes smoother and the most pronounced features which are separated in the observed geoid tend to get connected to larger structures these results stress the importance of separation of the lithospheric gravitational impact for a correct interpretation of the external gravity field even in its very long wavelength constituents also the isostatic corrected geoid spectrum reveals a stronger decrease in power from degree 3 to degree 4 and degree 5 to degree 6 which is in accordance with seismological models of deep mantle structures

diamond formation in the system mgo sio2 h2o c at 7 5 gpa and 1 600 c diamond crystallization has been studied in the sio2 h2o mg2sio4 h2o and h2o subsystems at 7 5 gpa and 1 600 c we found that dissolution of initial graphite is followed by spontaneous nucleation of diamond and growth of diamond on seed crystals in 15 h runs the degree of graphite to diamond transformation mdm mdm mgr 100 where mdm is mass of obtained diamond and mgr mass of residual graphite reached 100 in h2o rich fluids but was only 35 50 in water saturated silicate melts in 40 h runs an abrupt decrease of has been established at the weight ratio h2o h2o sio2 0 16 or h2o h2o mg2sio4 0 15 our results indicate that is a function of the concentration of water which controls both the kinetics of diamond nucleation and the intensity of carbon mass transfer in the systems the most favorable conditions for diamond crystallization in the mantle silicate environment at reliable pt parameters occur in the fluid phase with low concentration of silicates solute in h2o poor silicate melts diamond formation is questionable

inferring radial models of mantle viscosity from gravity grace data and an evolutionary algorithm convective flow in the mantle can be thought of and modeled as exclusively driven by density heterogeneities in the mantle itself and the resulting lateral variations in the earth s gravity field with this assumption and a model of mantle rheology a theoretical relationship can be found between 3d mantle structure and flow related quantities that can be measured on the earth s surface like free air gravity anomalies this relationship can be used to set up an inverse problem with 1d mantle viscosity as a solution in the assumption that seismic velocity anomalies be of purely thermal origin and related to density anomalies by a simple scaling factor we invert the large scale length component of the above mentioned measurements jointly with seismic observations waveforms and or travel times to derive an accurate 5 layer spherically symmetric model of upper and lower mantle viscosity we attempt to account for non uniqueness in the inverse problem by exploring the solution space formed of all possible radial profiles of earth viscosity by means of a non deterministic global optimization method the evolutionary algorithm ea for each sampled point of the solution space a forward calculation is conducted to determine a map of gravity anomalies whose similarity to grace gravity recovery and climate experiment is then measured the procedure is iterated to convergence according to ea criteria the robustness of the inversion is tested by means of synthetic tests indicating that our gravity data set is able to constrain less than 6 radial layers each with uniform viscosity independently of the tomographic model or the scaling factor adopted to convert seismic velocity into density structure the ea optimization method finds viscosity profiles characterized by low viscosity in a depth range corresponding to the transition zone and relatively uniform elsewhere

understanding the effects of mantle compressibility on geoid kernels summary we develop a quasi analytical solution to compute geoid kernels for a compressible mantle with newtonian rheology by separating the stresses induced by self gravitation from the stresses resulting from viscous flow we simplify the equations and gain some insight for realistic variations in the background density field po r the solution obtained using propagator matrices converges rapidly compressibility enters into the flow problem directly through the continuity equation and indirectly by influencing parameters such as gravitational acceleration g r and density contrasts across compositional boundaries in order to understand all these effects we introduce them sequentially starting with an incompressible earth model and ending up with a realistic compressible model that includes a compressible inner and outer core phase changes in the transition zone and an ocean the largest effects on geoid kernels are from different assumptions for g r possible effects of transformational superplasticity and differences in assumptions for density contrasts at the surface and at the core mantle boundary are next in importance the effects of compressibility on the flow itself are somewhat smaller followed by the effect of compressibility of the outer core a gravitationally consistent treatment of the ocean layer yields geoid kernels that are very similar to those for a dry planet the compressibility of the inner core has a negligible impact on the geoid kernels the largest effects from compressibility are comparable to the effects of a moderate 40 per cent change in the viscosity contrast between the upper and lower mantle

formation of various types of graphite inclusions in diamond experimental data abstract as a result of experiments on diamond crystallization and annealing diamond crystals with graphite inclusions were produced crystallization of diamond was realized in dry and water containing chloride carbonate and silicate systems at p 5 7 7 5 gpa and t 1400 1800 c using high pressure equipment of split sphere type annealing of diamond crystals in the field of thermodynamic stability of graphite was carried out at p 7 0 gpa and t 2500 c as well as at 1500 c in vacuum 10 5 atm as a result we produced diamond crystals with graphite inclusions of three genetic types protogenetic syngenitic and epigenetic initial graphite and newly formed diamond and graphite were studied using optical and scanning electron microscopes as well as micro raman spectroscopy it was shown experimentally that graphite inclusions in diamond crystals could form not only within the graphite stability field prior or after diamond crystallization but also within the diamond stability field as a result of joint crystallization of diamond and graphite the features of graphite inclusions morphology raman spectra and strain patterns around the inclusions could be indicators of the mechanism of inclusion formation

spatial and temporal patterns of cenozoic dynamic topography around australia 1 despite its importance the spatial and temporal pattern of dynamic topography generated by mantle convective circulation is poorly known we present accurate estimates of dynamic topography from oceanic basins and continental margins surrounding australia our starting point is measurement of residual depth anomalies on the oldest oceanic floor adjacent to the continental shelf these anomalies were determined from a combined dataset of 200 seismic reflection and wide angle images of well sedimented oceanic crust they have amplitudes of between 1 km and 0 5 km and their spatial variation is broadly consistent with long wavelength free air gravity and shallow seismic tomographic anomalies along the northwest shelf a regional depth anomaly of 300 to 700 m intersects the adjacent continental shelf the temporal evolution of this anomaly was determined by analyzing the stratigraphic architecture of an extensive carbonate platform which fringes the shelf and records a dramatic switch from progradation to aggradation during neogene times three dimensional seismic mapping calibrated by boreholes was used to calculate water loaded subsidence histories at rollover points of clinoforms along the shelf at 9 3 ma the rate of subsidence increases from 5 to up 75 m myr 1 generating a subsidence anomaly of 300 to 700 m the amplitude of this anomaly varies along the shelf and cannot be generated by glacio eustatic sea level variation instead we propose that a regional subsidence episode which affects both the proximal shelf and the distal oceanic basin was generated by convective drawdown by combining our results with other published estimates of uplift and subsidence a map of australia which shows the spatial and temporal pattern of dynamic topography is presented most but not all of australia s epeirogeny can be attributed to rapid northward motion of the australian plate over a pre existing pattern of convective circulation

the combined inversion of seismological and goce gravity data new insights into the current state of the pacific lithosphere and upper mantle abstract in this study we combine seismological and goce satellite gravity information by using a bayesian like technique with the aim of inferring the density structure of the pacific 90 n 90 s 121 e 60 w lithosphere and upper mantle we recover a 1 1 3 d density model down to 300 km depth which explains gravity observations with a variance reduction of 67 41 the model with an associated a posteriori standard deviation provides a significant contribution to understanding the evolution of the pacific lithosphere and answers to some debated geodynamic questions our methodology enables us to combine the recovery of density parameters with the optimum density v sv scalings the latter account for both seismological and gravity observations in order to identify the regions characterized by chemically induced density heterogeneities which add to the thermally induced anomalies chemically modified structures are found west of the east pacific rise epr and are of relevant amplitude both below the north western side of the pacific plate at the base of the lithosphere and up to 100 km depth beneath the hawaiian and super swell regions thus explaining the anomalous shallow regions without invoking the thermal buoyancy as the sole justification coherently with the chemically modified structures our results a support a lighter and more buoyant lithosphere than that predicted by the cooling models and b are in favor of the hypothesized crustal underplating beneath the hawaiian chain and beneath the volcanic units in the southern branch of the super swell region the comparison between calculated mantle gravity residuals and residual topography a suggests a lateral viscosity growth associated with the increasing thickness and density of the plate and b correlates well with sub lithospheric mantle flow from the epr towards west up to the kermadec and tonga trench in the south and the kuril kamchatka trench in the north

topographic fingerprint of deep mantle subduction the dynamic topography links with the mantle structures at various temporal and spatial scales however it is still unclear how it relates to the dynamics of subducting lithosphere when plates reach the mantle transition zone and lower mantle seismic tomography images show how slab morphologies vary from sinking sub vertically into the lower mantle to lying flat above the upper lower mantle discontinuity to thickening in the shallow lower mantle these slab shapes have been considered to be the result of variable interaction of the slab with the upper lower mantle discontinuity at 670 km depth previous studies show that periodic deep slab dynamics can explain a variety of enigmatic geological and geophysical observations such as periodic variations of the plate velocities trench retreat and advance episodes and the scattered distribution of slab dip angle in the upper mantle in this study we use two dimensional subduction models to investigate the surface topography expression and its evolution during slab transition zone interaction our models show that topography does not depend on slab morphology indeed the dynamic topography cannot distinguish between a slab sinking straight into the lower mantle and slab stagnation at the upper lower mantle boundary however topographic oscillations are related to episodes of the trench advance and retreat which in turn are linked to the slab folding behaviour at transition zone depths our results suggest that the surface transient signal observed by geological studies could help to detect deep subduction dynamics

static and dynamic support of western united states topography article history accepted 7 october 2013 available online 31 october 2013 editor p shearer isostatic and dynamic models of earth s surface topography can provide important insights into the driving processes of tectonic deformation we analyze these two estimates for the tectonically active western united states using refined structural models derived from earthscope usarray for the crust use of recent moho depth measurements and crustal density anomalies inferred from passive source seismology improve isostatic models however seismically determined lithospheric thickness variations from lithosphere asthenosphere boundary lab maps and lithospheric and mantle density anomalies derived from heat flow or uppermost mantle tomography do not improve isostatic models substantially perhaps this is a consequence of compositional heterogeneity a mismatch between thermal and seismological lab and structural complexity caused by smaller scale dynamics the remaining non isostatic dynamic component of topography is large topography anomalies include negative residuals likely due to active subduction of the juan de fuca plate and perhaps remnants of formerly active convergence further south along the margin our finding of broad scale positive residual topography in the basin and range substantiates previous results implying the presence of anomalous buoyancy there which we cannot fully explain the colorado plateau does not appear dynamically anomalous at present except at its edges many of the residual topography features are consistent with predictions from mantle flow computations this suggests a convective origin and important interactions between vigorous upper mantle convection and intraplate deformation

superweak asthenosphere in light of upper mantle seismic anisotropy earth s upper mantle includes a 200 km thick asthenosphere underneath the plates where viscosity and seismic velocities are reduced compared to the background this zone of weakness matters for plate dynamics and may be required for the generation of plate tectonics itself however recent seismological and electro magnetic studies indicate strong heterogeneity in thinner layers underneath the plates which if related to more extreme global viscosity reductions may require a revision of our understanding of mantle convection here i use dynamically consistent mantle flow modeling and the constraints provided by azimuthal seismic anisotropy as well as plate motions to explore the effect of a range of global and local viscosity reductions the fit between mantle flow model predictions and observations of seismic anisotropy is highly sensitive to radial and lateral viscosity variations i show that moderate sub oceanic viscosity reductions to 0 01 to 0 1 times the upper mantle viscosity are preferred by the fit to anisotropy and global plate motions depending on layer thickness lower viscosities degrade the fit to azimuthal anisotropy localized patches of viscosity reduction or layers of subducted asthenosphere however have only limited additional effects on anisotropy or plate velocities this indicates that it is unlikely that regional observations of sub plate anomalies are both continuous and indicative of dramatic viscosity reduction locally such weak patches may exist and would be detectable by regional anisotropy analysis for example however large scale plate dynamics are most likely governed by broad continent ocean asthenospheric viscosity contrasts rather than a thin possibly high melt fraction layer

dynamic uplift during slab flattening subduction exerts a strong control on surface topography and is the main cause of large vertical motions in continents including past events of large scale marine flooding and tilting the mechanism is dynamic deflection the sinking of dense subducted lithosphere gives rise to stresses that directly pull down the surface here we show that subduction does not always lead to downward deflections of the earth s surface subduction of young lithosphere at shallow angles flat subduction leaves it neutrally or even positively buoyant with respect to underlying mantle because the lithosphere is relatively warm compared with older lithosphere and because the thickened and hence drier oceanic crust does not undergo the transformation of basalt to denser eclogite accounting for neutrally buoyant flat segments along with large variations in slab morphology in the south american subduction zone explains along strike and temporal changes in dynamic topography observed in the geologic record since the beginning of the cenozoic our results show that the transition from normal subduction to slab flattening generates dynamic uplift preventing back arc marine flooding