a geodynamic model of mantle density heterogeneity using cenozoic and mesozoic plate motion reconstructions we derive a model of present day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle the thermal buoyancy of these slabs is estimated from the observed thermal subsidence cooling of oceanic lithosphere slab velocities in the upper mantle are computed from the local convergence rate we assume that slabs cross the upper lower mantle interface and continue sinking into the lower mantle with a reduced velocity for a velocity reduction factor between 2 and 5 our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other we have also computed a synthetic geoid from our density model for a viscosity increase of about a factor of 40 from the upper to lower mantle our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95 and explains 84 of the observed geoid assuming that the model c21 and s21 terms are absent due to a long relaxation time for earth s rotational bulge otherwise 73 of the geoid variance is explained the viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle since downwelling velocities are expected to scale roughly as the logarithm of viscosity loge 40 3 7 these results show that the history of plate tectonics can explain the main features of the present day structure of the mantle the dynamic topography induced by this heterogeneity structure consists mainly of about 1 km amplitude lows concentrated along the active continental margins of the pacific basin our model can also be used to predict the time variation of mantle heterogeneity and the gravity field we find that the age of the geoid defined as the time in the past before which the geoid becomes uncorrelated with the present geoid is about 50 m y our model for the history of the degree 2 geoid which is equivalent to the history of the inertia tensor should give us a tool to study the variations in earth s rotation pole indicated in paleomagiietic studies

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

temporal and spatial evolution of dynamic support from river profiles a framework for madagascar 1 we present a strategy for calculating uplift rates as a function of space and time from large sets of longitudinal river profiles this strategy assumes that the shape of a river profile is controlled by the history of uplift rate and moderated by the erosional process we assume that upstream drainage area is invariant the algorithm was tested on a set of 100 river profiles which were extracted from a digital elevation model of madagascar this set of profiles was simultaneously inverted to obtain uplift rate as a smooth function of space and time the fit between observed and calculated profiles is excellent and suggests that madagascar was uplifted by 1 2 km at rates of 0 2 0 4 mm yr during the last 15 myrs the location of madagascar suggests that its topographic elevation is maintained by convective circulation of the sub lithospheric mantle residual depth anomalies of oceanic fragments encompassing the island show that the island straddles a dynamic topographic gradient which generates asymmetric neogene uplift volcanism warped peneplains and uplifted marine terraces corroborate the existence of youthful uplift we suggest that sets of longitudinal river profiles contain useful information about the history of regional uplift which can be extracted by inverse modeling and calibrated by independent geologic observations

small scale mantle convection produces stratigraphic sequences in sedimentary basins cyclic sedimentary deposits link stratigraphic sequences that are now geographically distant but were once part of the same depositional environment some of these sequences occur at periods of 2 to 20 million years and eustatic sea level variations or regional tectonic events are likely causes of their formation using numerical modeling we demonstrate that small scale mantle convection can also cause the development of stratigraphic sequences through recurrent local and regional vertical surface movements small scale convection driven stratigraphic sequences occur at periods of 2 to 20 million years and correlate only at distances up to a few hundred kilometers these results suggest that previous sequence stratigraphic analyses may contain erroneous conclusions regarding eustatic sea level variations

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

constraints on dynamic topography from asymmetric subsidence of the mid ocean ridges abstract stresses from mantle convection deflect earth s surface vertically producing dynamic topography that is important for continental dynamics and sea level change but difficult to observe due to overprinting by isostatic topography for long wavelengths 10 4 km the amplitude of dynamic topography is particularly uncertain with mantle flow models typically suggesting larger amplitudes 1000 m than direct observations here we develop a new constraint on the amplitude of long wavelength dynamic topography by examining asymmetries in seafloor bathymetry across mid ocean ridges we compare bathymetric profiles across the mid atlantic ridge mar and the east pacific rise epr and we find that the south american flank of both ridges subsides faster than its opposing flank this pattern is consistent with dynamic subsidence across south america supported by downwelling in the lower mantle to constrain the amplitude of dynamic topography we compare bathymetric profiles across both ridges after correcting bathymetry for several different models of dynamic topography with varying amplitudes and spatial patterns we find that long wavelength dynamic topography with an amplitude of only 500 m explains the observed asymmetry of the mar a similar model can explain epr asymmetry but is complicated by additional asymmetrical topography associated with tectonic crustal thickness and or asthenospheric temperature asymmetries across the epr after removing 500 m of dynamic topography both the mar and epr exhibit a slower seafloor subsidence rate 280 290 m myr 1 2 than previously reported our finding of only 500 m of long wavelength dynamic topography may indicate the importance of thermochemical convection and or large viscosity variations for lower mantle dynamics

dynamic causes of the relation between area and age of the ocean floor estimates for the area and age of the ocean floor are at odds with assumptions for mantle convection which imply that an older sea floor rather than a new one would be preferentially subducted over time previous efforts to explain these relationships have been based on geologic evidence and simple models coltice et al p 335 1 created numerical three dimensional convection models representing more realistic physical boundaries including a spherical earth the existence of continents and supercontinents over time and realistic rheologies a combination of continents and plate like behavior of the ocean floor sufficed to produce the observed relationship between plate area and plate age which explains why some old oceanic crust still remains 1 lookup doi 10 1126 science 1219120

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

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

transient convective uplift of an ancient buried landscape sedimentary basins in the north atlantic ocean preserve a record of intermittent uplift during cenozoic times1 these variations in elevation are thought to result from temperature changes within the underlying icelandic mantle plume2 when parts of the european continental shelf were episodically lifted above sea level new landscapes were carved by erosion but these landscapes then subsided and were buried beneath marine sediments3 here we use three dimensional seismic data to reconstruct one of these ancient landscapes that formed off the northwest coast of europe during the palaeocene eocene thermal maximum we identify a drainage network within the landscape and by modelling the profiles of individual rivers within this network we reconstruct the history of surface uplift we show that the landscape was lifted above sea level in a series of three discrete steps of 200 400 m each after about 1 million years of subaerial exposure this landscape was reburied we use the magnitude and duration of uplift to constrain the temperature and velocity of a mantle plume anomaly that drove landscape formation we conclude that pulses of hot chemically depleted mantle material spread out radially beneath the lithospheric plate at velocities of 35 cm yr 1

reconstructing farallon plate subduction beneath north america back to the late cretaceous using an inverse mantle convection model that assimilates seismic structure and plate motions we reconstruct farallon plate subduction back to 100 million years ago models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy requiring that the farallon slab was flat lying in the late cretaceous consistent with geological reconstructions the simulation predicts that an extensive zone of shallow dipping subduction extended beyond the flat lying slab farther east and north by up to 1000 kilometers the limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten the results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

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

global dynamic topography observations reveal limited influence of large scale mantle flow mantle flow causes earth s surface to uplift and subside global analysis of dynamically generated topography suggests that temperature induced small scale mantle flow has a bigger influence on surface topography than large scale mantle flow

subduction controls the distribution and fragmentation of earth s tectonic plates the theory of plate tectonics describes how the surface of the earth is split into an organized jigsaw of seven large plates 1 of similar sizes and a population of smaller plates whose areas follow a fractal distribution 2 3 the reconstruction of global tectonics during the past 200 my 4 suggests that this layout is probably a long term feature of our planet but the forces governing it are unknown previous studies 3 5 6 primarily based on statistical properties of plate distributions were unable to resolve how the size of plates is determined by lithosphere properties and or underlying mantle convection here we demonstrate that the plate layout of the earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere using 3d spherical models of mantle convection with plate like behaviour that match the plate size frequency distribution observed for earth we show that subduction geometry drives the tectonic fragmentation that generates plates the spacing between slabs controls the layout of large plates and the stresses caused by the bending of trenches break plates into smaller fragments our results explain why the fast evolution in small back arc plates 7 8 reflects the dramatic changes in plate motions during times of major reorganizations our study opens the way to use convection simulations with plate like behaviour to unravel how global tectonics and mantle convection are dynamically connected

a comparison of numerical surface topography calculations in geodynamic modelling an evaluation of the sticky air method summary calculating surface topography in geodynamic models is a common numerical problem besides other approaches the so called sticky air approach has gained interest as a free surface proxy at the top boundary the often used free slip condition is thereby vertically extended by introducing a low density low viscosityfluid layer this allows the air crust interface to behave in a similar manner to a true free surface we present here a theoretical analysis that provides the physical conditions under which the sticky air approach is a valid approximation of a true free surface two cases are evaluated that characterize the evolution of topography on different timescales 1 isostatic relaxation of a cosine perturbation and 2 topography changes above a rising plume we quantitatively compare topographies calculated by six different numerical codes using finite difference and finite element techniques using three different topography calculation methods i direct calculation of topography from normal stress ii body fitting methods allowing for meshing the topography and iii lagrangian tracking of the topography on an eulerian grid it is found that the sticky air approach works well as long as the term st ch hst l 3 is sufficiently small where st and hst are the viscosity and thickness of the sticky air layer and ch and l are the characteristic viscosity and length scale of the model respectively spurious lateral fluctuations of topography as observed in some marker based sticky air approaches may effectively be damped by an anisotropic distribution of markers with a higher number of markers per element in the vertical than in the horizontal direction

sea level and vertical motion of continents from dynamic earth models since the late cretaceous dynamic earth models are used to better understand the impact of mantle dynamics on the vertical motion of continents and regional and global sea level change since the late cretaceous a hybrid approach combines inverse and forward models of mantle convection and accounts for the principal contributors to long term sea level change the evolving distribution of ocean floor age dynamic topography in oceanic and continental regions and the geoid we infer the relative importance of dynamic versus other factors of sea level change determine time dependent patterns of dynamic subsidence and uplift of continents and derive a sea level curve we find that both dynamic factors and the evolving distribution of sea floor age are important in controlling sea level we track the movement of continents over large scale dynamic topography by consistently mapping between mantle and plate frames of reference and we find that this movement results in dynamic subsidence and uplift of continents the amplitude of dynamic topography in continental regions is larger than global sea level in several regions and periods so that it has controlled regional sea level in north and south america and australia since the late cretaceous northern africa and arabia since the late eocene and southeast asia in the oligocene miocene eastern and southern africa have experienced dynamic uplift over the last 20 to 30 m y whereas siberia and australia have experienced cenozoic tilting the dominant factor controlling global sea level is a changing oceanic lithosphere production that has resulted in a large amplitude sea level fall since the late cretaceous with dynamic topography offsetting this fall

on topography and geoid from 2 d stagnant lid convection calculations 1 temperature dependent convection in the stagnant lid regime with a frank kamenetskii and arrhenius viscosity formulation are compared when properly scaled the nusselt number and root mean square velocity of the fluid for the two viscosity formulations are similar but the surface stresses and hence predicted dynamic topography and geoid differ significantly the arrhenius viscosity formulation results are insensitive to a viscosity cutoff at the high viscosity limit as long as the cutoff is 104 times the basal viscosity the condition number for stagnant lid convection matrices using a penalty formulation is smaller than the penalty number times viscosity contrast value a reasonable estimate of the condition number explaining why large viscosity contrast convection problems with the penalty method agree with other formulations and analytic solutions the difference in magnitude between the maximum and minimum dynamic topography could be used to constrain the magnitude of the viscosity contrast across the stagnant lid in planetary bodies

phanerozoic marine inundation of continents driven by dynamic topography above subducting slabs a spherical model of mantle flow constrained by the locations of trenches can be used to predict the dynamic topography of the earth s surface and hence the marine inundation of continents for past periods of high sea level the predicted geographical pattern of flooding correlates well with the geological record the high spatial correlation may result from increased plate velocities at these times leading to increased rates of subduction subsidence and inundation at convergent margins

scales of transient convective support beneath africa beneath sub equatorial africa there is convincing evidence for a gigantic slow velocity anomaly within the lower mantle that is inferred to be a thermochemical superplume here we define the spatial and temporal pattern of uplift across the northern edge of this putative superplume by analyzing stratal geometries and stacking velocities of neogene sedimentary rocks these marine deposits are imaged on a dense grid of seismic reflection profiles that straddle the southwest african coastal shelf inverse modeling of stacking velocity profiles demonstrates that there have been at least two significant phases of uplift a post pliocene uplift event increases from 0 to 500 m elevation over a distance of 1000 km in close agreement with geophysical estimates of the present day variation of dynamic topography an earlier phase of uplift occurred in middle oligocene time ca 30 35 ma ago we propose a two stage development of epeirogeny in which a middle oligocene phase of uplift may be related to initiation and growth of the african superplume post pliocene denudation records rapid 100 250 m ma vertical motions generated by upper mantle convection which vary on length scales of 500 1000 km our results support the existence of a convective circulation system that has operated on different temporal and spatial scales beneath the african plate

environmental processes of the ice age land oceans glaciers epilog knowledge of the state of the earth at the last glacial maximum lgm an interval around 21 000 years ago is an important benchmark for understanding the sensitivity of global environmental systems to change much progress in understanding climates of the lgm has occurred in the 20 years since the end of the climap project of the 1970s climate long range investigation mapping and prediction here we review this progress based on presentations and discussion at a rst open science meeting of the epilog project environmental processes of the ice age land oceans glaciers held in delmenhorst germany may 1999 we outline key controversies and document protocols for epilog contributions so that a new synthesis of the lgm earth can emerge as an open project of the world s community of scientists 2001 elsevier science ltd all rights reserved

mantle dynamics isostasy and the support of high terrain dynamic topography is commonly understood to be deflection of the earth s surface that results from convection of the mantle because different authors use the words dynamic topography differently topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m for most regions however surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography the small free air gravity anomalies and still smaller isostatic anomalies 2000 m of dynamic topography moreover simple examples show that if flow induced stresses maintain a given density distribution then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium in light of confusion over the term dynamic topography we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow induced stresses and to distinguish it from topography that is compensated isostatically

variation of summer monsoon intensity in the north china plain and its response to abrupt climatic events during the early middle holocene abstract ningjinpo paleolake was the largest lake in the north china plain during the holocene and its continuous sedimentary record has the potential to reconstruct the summer monsoon intensity and determine the limnological response to abrupt climatic events at a high resolution here we use ams 14c dating and analyses of grain size and major element contents and the results show that 1 during 11 5 10 0 cal kyr bp the geochemical climate change index c 1 16 sediment accumulation rate sar 0 007 cm yr and median particle diameter md 20 6 m are the lowest within the section while the geochemical drought index 0 0323 fe mn ratio 0 0088 and clay content 17 3 are the highest which indicate that the north china plain received less precipitation and chemical weathering and erosional intensity in the ningjinpo paleolake catchment were weak 2 during 10 0 7 7 cal kyr bp the c index 1 36 content of sand 19 1 and silt 68 0 md 27 2 m and sar 0 019 cm yr increase significantly these characteristics indicate that the climate gradually became wetter chemical weathering and erosion intensity increased and there was increased runoff to the lake which raised the lake level 3 during 7 7 5 5 cal kyr bp the c index 1 56 silt content 71 8 and sar 0 030 cm yr increase to the highest while the fe mn ratio 0 0068 sand content 14 4 and md 21 4 m decrease to their lowest levels these suggest the climate was warm and humid the runoff to the lake was large chemical weathering in the catchment was strong and the water level of the lake reached the maximum for the holocene 4 during 5 5 3 3 cal kyr bp the c index 1 26 and sar 0 011 cm yr decrease substantially whereas the sand content 28 2 and md 18 3 m increase to the highest levels these characteristics demonstrate that the climate gradually became arid the runoff to ningjinpo paleolake was reduced the intensity of chemical weathering and erosion in the catchment weakened and the lake level decreased although the high frequency of floods in this period 5 in addition to the globally recorded abrupt climatic events at 8 2 cal kyr bp 5 3 cal kyr bp and 4 2 cal kyr bp which were recorded in the northern monsoon region of china an abrupt climatic event in the middle holocene at 7 2 cal kyr bp was also recorded in the northern monsoon region of china these events may have been amplified by unusually active enso since the middle holocene

seismic tomography surface uplift and the breakup of gondwanaland integrating mantle convection backwards in time mantle density heterogeneities imaged using seismic tomography contain information about time dependent mantle flow and mantle structures that existed in the past we model the history of mantle flow using a tomographic image of the mantle beneath southern africa as an initial condition while reversing the direction of flow and analytically incorporating cooling plates as a boundary condition if the resulting backwards integrated model for structures is used as a starting point for a forwards convection model today s mantle can be adequately reconstructed if we do not integrate backwards more than than about 50 75 ma flow can also be reliably reversed through the mesozoic but only if instability of the lower boundary layer can be suppressed our model predicts that the large seismically slow and presumably hot structure beneath southern africa produced 500 700 m of dynamic topography throughout the cenozoic since 30 ma uplift has moved from eastern to southern africa where uplift rates are 10 m myr consistent with observations during the mesozoic the modeled topographic high is situated near gondwanaland rifting raising the possibility that this buoyant structure may have been involved with this breakup

mantle control of the geodynamo consequences of top down regulation the mantle global circulation including deep subduction and lower mantle superplumes exerts first order controls on the evolution of the core the history of the geodynamo and the structure of the geomagnetic field mantle global circulation models that include realistic plate motions deep subduction and compositional heterogeneity similar to the observed large low seismic velocity provinces in the lower mantle predict that the present day global average heat flux at the core mantle boundary cmb exceeds 85 mw m 2 this is sufficient to drive the present day geodynamo by thermochemical convection and implies a very young inner core with inner core nucleation between 400 and 1100 ma the mantle global circulation also generates spatially heterogeneous heat flux at the cmb with peak to peak lateral variations exceeding 100 mw m 2 such extreme lateral variability in cmb heat flux in conjunction with the high thermal conductivity of the core implies that the liquid outer core is thermally unstable beneath the high seismic velocity regions in the lower mantle but thermally stable beneath the large low seismic velocity provinces numerical dynamo simulations show how this pattern of heterogeneous boundary heat flux affects flow in the outer core producing localized circulation patterns beneath the cmb tied to the mantle heterogeneity and long lived deviations from axial symmetry in the geomagnetic field

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

evidence for small scale mantle convection from seasat altimeter data evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from seasat altimeter data subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast moving pacific and indo australian plates in particular for the east central pacific ocean the seasat derived data sets reveal lineations with 1 wavelengths in the range 150 500 km perhaps increasing with plate age and 2 peak to trough amplitudes of 5 20 mgal for gravity anomalies the lineated pattern over the pacific plate which first becomes discernable over seafloor 5 to 10 m y old west of the east pacific rise is clearly oblique to the trends of the prominent pacific farallon fracture zones onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 1018 pa s 1019 p which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies recent numerical studies of convection in fluids with a temperature and pressuredependent viscosity support an early onset time for convective instability and low viscosity zones in the upper mantle beneath young lithosphere such low viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves

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

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

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

uplift histories from river profiles 1 longitudinal river profiles where elevation of a river bed is plotted as a function of distance along the river bed contain information about uplift rate when a region adjacent to a reference level e g sea level is uplifted a rapid change in gradient occurs near the river mouth the erosional process causes this change in gradient to migrate upstream thus a river profile is effectively a tape recording of the uplift rate history provided that the erosional process can be adequately parameterized here we use a non linear equation to relate the shape of a river profile z x to uplift rate history u t if erosion is assumed to be dominated by knickpoint retreat an inverse model can be formulated and used to calculate uplift rate histories our model builds upon standard stream profile analysis which focuses on the relationship between profile slope and drainage area we have applied this analytical approach to river profiles from the bie dome angola calculated uplift rate histories agree with independent geologic estimates

unraveling topography around subduction zones from laboratory models the relief around subduction zones results from the interplay of dynamic processes that may locally exceed the iso static contributions the viscous dissipation of the energy in and around subduction zones is capable of generating kilometer scale vertical ground movements in order to evaluate dynamic topography in a selfconsistent subduction system we carried out a set of laboratory experiments wherein the lithosphere and mantle are simulated by means of newtonian viscous materials namely silicone putty and glucose syrup models are kept in their most simple form and are made of negative buoyancy plates of variable width and thickness freely plunging into the syrup the surface of the model and the top of the slab are scanned in three dimensions a forebulge systematically emerges from the bending of the viscous plate adjacent to the trench with a large wavelength dynamic pressure offsets the foreside and backside of the slab by 500 m on average the suction that accompanies the vertical descent of the slab depresses the surface on both sides at a distance equal to the half width of the slab the topographic depression amounts to 500 m on average and becomes negligible at a distance that equals the width of the slab in order to explore the impact of slab rollback on the topography the trailing edge of the plates is alternatively fixed to fixed mode and freed from free mode the end wall of the tank both the pressure and suction components of the topography are 30 lower in the free mode indicating that slab rollback fosters the dynamic subsidence of upper plates our models are compatible with first order observations of the topography around the east scotia tonga kermadec and banda subduction zones which exhibit anomalous depths of nearly 1 km as compared to adjacent sea floor of comparable age

topography and geoid induced by a convecting mantle beneath an elastic lithosphere summary in the absence of seismological measurements observations of the topography and gravity fields of solid planets are the primary constraints on their internal structure to compute the synthetic geoid and topography induced by the dynamics of planetary interiors we introduce a 3 d numerical tool describing mantle convection beneath an elastic lithosphere although the energy conservation is treated in the whole spherical domain the deformation aspect is solved using a hybrid technique finite volume method for the viscous flow spectral method for elastic deformation the mechanical coupling is achieved via the imposition of the traction at the surface of the viscous flow as a basal boundary condition for the elastic deformation we present both response functions and full thermal convection cases computed with our new method for planetary bodies of varying dimensions the filtering effect of the lithosphere on the dynamic topography and geoid is specific for each planetary body justifying the importance of such a tool furthermore since our approach specifically focuses on the mechanical coupling at the base of the lithosphere it will permit future more elaborate rheological treatments it also enables to discriminate between the radial and tangential components of the viscous traction the latter is found to have a significant influence on the elastic deformation the effect on geoid is prominent more specifically while a thin elastic lithosphere is usually considered to play little role on the dynamic topography and geoid of venus a 35 per cent reduction is obtained for geoid height in the numerical example we propose on a planet with thicker elastic lithosphere such as mars the consequence of this filtering effect is to rule out the possibility of a dynamical support for the tharsis rise even for the lowest admissible values of elastic thickness in this region

impact of the lithosphere on dynamic topography insights from analogue modeling density anomalies beneath the lithosphere are expected to generate dynamic topography at the earth s surface due to the induced mantle flow stresses which scale linearly with density anomalies while the viscosity of the upper mantle is expected to control uplift rates however limited attention has been given to the role of the lithosphere here we present results from analogue modeling of the interactions between a density anomaly rising in the mantle and the lithosphere in a newtonian system we find that for instabilities with wavelengths of the same order of magnitude as lithosphere thickness the uplift rate and the geometry of the surface bulge are inversely correlated to the lithosphere thickness we also show that a layered lithosphere may modulate the topographic signal with respect to previous approaches our models represent a novel attempt to unravel the way normal stresses generated by mantle flow are transmitted through a rheologically stratified lithosphere and the resulting topographic signal

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

interactions of scales of convection in the earth s mantle abstract the existence of undulations of the geoid gravity and bathymetry in ocean basins as well as anomalies in heat flow point to the existence of small scale convection beneath tectonic plates the instabilities that could develop at the base of the lithosphere are sufficiently small scale 60 ma they form networks that are shaped by the plate evolution slabs plumes and the geometry of continental boundaries plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere they create a wake in which new instabilities form downstream these wakes form channels that are about 1000 km wide and thus are possibly detectable by seismic tomography beneath fast plates cold sinking instabilities are tilted in the direction opposite to plate motion while they sink vertically for slow plates these instabilities are too small to be detected by usual seismic methods since they are about 200 km in lateral scale however this preferred orientation of instabilities below fast plates could produce a pattern of large scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models

isostasy flexure and dynamic topography abstract a fundamental scientific question is what controls the earth s topography although the theoretical principles of isostasy flexure and dynamic topography are widely discussed the parameters needed to apply these principles are frequently not available isostatic factors controlling lithospheric buoyancy are frequently uncertain and non isostatic factors such as lithospheric bending towards subduction zones and dynamic topography are hard to distinguish the question discussed here is whether a set of simple rules that relate topography to lithospheric structure in various tectonic environments can be deduced in a way that missing parameters can be approximated or does each area behave differently making generalizations problematic we contribute to this issue analyzing the asia africa arabia europe domain following a top down strategy we compile a new crustal thickness map and remove the contribution of the crust from the observed elevation then the challenge is to interpret the residual topography in terms of mantle lithosphere buoyancy and dynamics based on systematic relationships between tectonic environments and factors controlling topography we argue that crustal buoyancy and mantle lithospheric density can be approximated from available geological data and that regions near mantle upwelling or downwelling are easily identified by their extreme residual topography yet even for other areas calculating lithospheric thickness from residual topography is problematic because distinguishing variations in mantle lithosphere thickness from sub lithospheric dynamics is difficult fortunately the area studied here provides an opportunity to examine this issue based on the conjunction between the afar plume and the mid ocean ridge in the nearby gulf of aden and southern red sea we constrain the maximal amplitude of dynamic topography to 1 km this estimate is based on a narrow definition of dynamic topography that only includes sub lithospheric processes and using mid ocean ridges as reference where mantle lithosphere buoyancy is zero