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

validation of the ice 3g model of wurm wisconsin deglaciation using a global data base of relative sea level histories a global data base consisting of radiocarbon controlled relative sea level rsl histories from 392 different geographic locations has been compiled for use in analyses of the glacial isostatic adjustment process these data may be employed to constrain both the spatiotemporal characteristics of the last deglaciation event of the current ice age and the rheological structure of the planetary interior in a previous paper in this series tushingham and peltier 1991 we employed the rsl data in this set from sites that were once ice covered to refine the apriori model of deglaciation that had been delivered by past research in the area of pleistocene geomorphology peltier and andrews 1976 wu and peltier 1983 to produce a new global model of this event that we have called ice 3g in the present paper following a description of the full data base and of the methods used to compile it we employ data from the sites that were not ice covered 200 time series to verify the plausibility of this refined model of deglaciation and to test the extent to which the totality of the observations constrain the radial viscoelastic structure of the planet the analysis of solutions to the forward problem presented here reinforce previous inferences of a preference for a thick lithosphere by rsl data from sites along the u s east coast and a preference by data from most near field sites for an increase in viscosity across the 670 km discontinuity in the mantle by a factor of approximately 2 when the viscosity of the upper mantle is fixed to the value of 1021 pa s originally deduced by haskel 1935 on the basis of his analysis of the data from fennoscandia

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

three dimensional finite element modelling of earth s viscoelastic deformation effects of lateral variations in lithospheric thickness summary we have developed a 3 d spherical finite element model to study the dynamic response to surface loads of a self gravitating and incompressible earth with 3 d viscoelastic structure we have forced our model with the ice 3g deglaciation history of tushingham peltier to study the effects of laterally varying lithospheric thickness on observations of post glacial rebound pgr the laterally varying lithospheric thicknesses are derived from estimates of the thermal structure of the oceanic lithosphere and from elastic thicknesses on continents as estimated from studies of long term geological loads our calculations show that the effects of lithospheric structure on the relative sea level change rslc depend on the locations of the observation sites and on the size of loads the rslc at the centre of the north american ice sheet is significantly less sensitive to lithospheric thickness compared with the rslc at the centre of the fennoscandian ice sheet at the peripheral bulges the rslc tends to be more sensitive to lithospheric thickness the rslc is controlled by local lithospheric thickness the rslc at a given location as predicted using models with laterally varying lithospheric thickness can be reproduced using a 1 d model with a uniform lithospheric thickness equal to the local lithospheric thickness coupled with efficient parallel computing we believe that the finite element model that we present here can be used to address a variety of viscoelastic deformation problems in geodynamics

contribution of small glaciers to global sea level observed long term changes in glacier volume and hydrometeorological mass balance models yield data on the transfer of water from glaciers excluding those in greenland and antarctica to the oceans the average observed volume change for the period 1900 to 1961 is scaled to a global average by use of the seasonal amplitude of the mass balance these data are used to calibrate the models to estimate the changing contribution of glaciers to sea level for the period 1884 to 1975 although the error band is large these glaciers appear to account for a third to half of observed rise in sea level approximately that fraction not explained by thermal expansion of the ocean

localization of the gravity field and the signature of glacial rebound the negative free air gravity anomaly centred on hudson bay canada shows a remarkable correlation with the location of the laurentide ice sheet suggesting that this gravity anomaly is the result of incomplete post glacial rebound this region however is also underlain by higher than average mantle seismic velocities suggesting that the gravity low might result instead from dynamic topography associated with convective downwellings here we analyse the global gravity field as a simultaneous function of geographic location and spectral content we find that the hudson bay gravity low is unique with anomalously high amplitude in the spectral band where the power from the laurentide ice load is greatest and the relaxation times predicted for viable models of viscous relaxation are longest we estimate that about half of the hudson bay gravity anomaly is the result of incomplete post glacial rebound and derive a mantle viscosity model that explains both this gravity signature and the characteristic uplift rates for the central laurentide and fennoscandian regions this model has a jump in viscosity at 670 km depth comparable to that in dynamic models of the geoid highs over subducted slabs but lacks a low viscosity asthenosphere consistent with a higher viscosity in the upper mantle beneath shields than in oceanic regions

search for the proverbial mantle osmium sources to the oceans hydrothermal alteration of mid ocean ridge basalt abstract we present os and sr isotopes and mg os and sr concentrations for ridge crest high temperature and diffuse hydrothermal fluids plume fluids and ridge flank warm spring fluids from the juan de fuca ridge the data are used to evaluate the extent to which 1 the high and low temperature hydrothermal alteration of mid ocean ridge basalts morbs provides os to the deep oceans and 2 hydrothermal contributions of non radiogenic os and sr to the oceans are coupled the os and sr isotopic ratios of the high temperature fluids 265 353 c are dominated by basalts 187 os 188 os 0 2 87 sr 86 sr 0 704 but the concentrations of these elements are buffered approximately at their seawater values the 187 os 188 os of the hydrothermal plume fluids collected 1 m above the orifice of hulk vent is close to the seawater value 1 05 the low temperature diffuse fluids 10 40 c associated with ridge crest high temperature hydrothermal systems on average have os 31 fmol kg 1 187 os 188 os 0 9 and sr 86 mol kg 1 87 sr 86 sr 0 709 they appear to result from mixing of a high temperature fluid and a seawater component the ridge flank warm spring fluids 10 62 c on average yield os 22 fmol kg 1 187 os 188 os 0 8 and sr 115 mol kg 1 87 sr 86 sr 0 708 the data are consistent with isotopic exchange of os and sr between basalt and circulating seawater during low temperature hydrothermal alteration the average sr concentration in these fluids appears to be similar to seawater and consistent with previous studies in comparison the average os concentration is less than seawater by more than a factor of two if these data are representative they indicate that low temperature alteration of morb does not provide adequate non radiogenic os and that another source of mantle os to the oceans must be investigated at present the magnitude of non radiogenic sr contribution via low temperature seawater alteration is not well constrained if non radiogenic sr to the oceans is predominantly from the alteration of morb our data suggest that there must be a different source of non radiogenic os and that the os and sr isotope systems in the oceans are decoupled

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

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

global glacial isostatic adjustment palaeogeodetic and space geodetic tests of the ice 4g vm2 model analyses of the global process of glacial isostatic adjustment and post glacial relative sea level change continue to deliver important insights into earth system form and process one successful model of the related phenomenology is based upon a spherically symmetric internal viscoelastic structure for the solid earth which has been denoted vm2 and a model of the most recent deglaciation event of the current ice age denoted ice 4g the primary purpose of this paper is to describe several new a posteriori tests that have recently been performed to further investigate the quality of this global solution to the inverse problem for both mantle viscosity and deglaciation history that is posed by the observables associated with this large scale geodynamic phenomenon i focus especially upon the misfits of observations to the theoretical predictions of this model which i am currently using to further refine its properties and upon predictions made using it of geophysical signals that should soon become visible in the context of the gravity recovery and climate experiment grace satellite mission among the required refinements to ice 4g one that is necessary to eliminate a recently revealed misfit to space geodetic constraints on the present day rate of radial motion at the yellowknife location well to the west of hudson bay and a similar misfit to absolute gravity measurements to the southwest of the bay is the insertion of a keewatin dome of thick ice centred over yellowknife with a ridge of ice extending to the south east in the geomorphological literature the existence of such a keewatin dome previously has been hypothesised but chronological control was lacking on the surface features that suggested its former existence an important additional constraint that requires the late glacial existence of this important feature consists of new inferences of the last glacial maximum lowstand of the sea from sites in the far field of the main concentrations of land ice copyright 2002 john wiley sons ltd

lower mantle seismic discontinuities and the thermal morphology of subducted slabs abstract the nature of the lowermost few 100 km of the mantle holds insights into the scale of convection of the earth s mantle here we investigate the fine structure of this region using seismic migration of shear waves reflecting from velocity discontinuities near the core mantle boundary cmb recorded by a dense network of european seismic stations two main features are found one structure exhibits large scale topography and can be explained by a sharp increase in seismic velocity 206 316 km above the cmb the second structure lies 55 85 km above the cmb and marks a sharp reduction in velocity to explain the origin of these discontinuities we appeal to numerical simulations of three dimensional 3d mantle convection incorporating rigidly moving plates the plates organize the convective flow to allow for the development of large convection cells and cool sheet like downwellings at a convergent plate boundary the results suggest that in places observed seismic complexity in d discontinuity structure may be due to the thermal gradients resulting from the accumulation of cool subducted plate material at the cmb

dynamic mantle density heterogeneities and global geodetic observables summary we investigate the influence of mantle dynamics on low degree deformations of the earth at geological timescale we first compute surface deformations and discuss the analytical form of the tangential surface displacement induced by internal loads in a reference frame related to the centre of mass of the planet we use the theoretical love numbers formalism since the earth has a viscoelastic behaviour at geological timescale then we quantify degree one and degree two deformations induced by upwelling domes and subducted plates sinking into the mantle we use a simple model in which the slabs are modelled as blobs diving vertically through the mantle and in which the domes are assumed to be stable over the last 120 ma their location is modelled from seismic tomography within the lower mantle the temporal evolutions of the j2 gravitational potential coefficient and of the geocentre motion are plotted since 120 ma we find that 1 the mantle density heterogeneities within the mantle can explain the present day non hydrostatic flattening of the earth however they vary at a too slow timescale to significantly perturb the coefficient 2 although there is a significant discrepancy of about a few hundred metres between the centre of figure and the centre of mass of the earth the secular variation of the geocentre motion is one order of magnitude smaller than the one induced by surface loads

glacial isostatic adjustment i the forward problem summary the isostatic adjustment of a radially stratified visco elastic spheroid is treated using space time green functions for the associated surface mass load boundary value problem these impulse response functions are convolved with a heaviside function to give the time dependent deformation of the planet which would be produced by a unit point mass brought up from infinity at t 0 and allowed to remain on the surface the resulting heaviside green functions can be employed to simulate all of the important signatures of glacial isostatic adjustment given a space time dependent surface mass load consisting of ice sheet ablation histories and a model of the simultaneous filling of the ocean basins these source terms are simply convolved with the heaviside green function appropriate to a specific response signature a realistic model of the spatial distribution of the main late pleistocene ice loads and of their temporal disintegration is constructed the response of two visco elastic earth models is computed and compared to a global set of relaxation data relative sea level curves on the basis of this initial comparison of theory and observation the possibility that the lower mantle has a viscosity which is significantly in excess of the viscosity of the upper mantle is excluded in addition clear evidence of the presence of the lithosphere has been found in relaxation data from sites which were near the edge of the laurentide ice sheet such data should therefore prove useful as a basis for analysis of lateral variations in lithosphere thickness further extensions of the calculation are suggested

a reappraisal of postglacial decay times from richmond gulf and james bay canada decay times inferred from relative sea level rsl histories of previously glaciated regions provide a potentially important constraint on mantle rheology we present a new compilation of rsl data from richmond gulf and james bay canada this recompilation reveals errors in previous compilations that led to inaccurate estimates for the richmond gulf decay time in a series of recently published articles we derive updated estimates for the decay time at richmond gulf and james bay using a methodology that incorporates errors in both the age and the height of the sea level markers this exercise is guided by a series of synthetic rsl calculations that show that decay time estimates in the region can be significantly biased if the rsl time series are not corrected for global eustatic sea level trends or if the estimates are based on composite rsl histories derived by combining data from both the richmond gulf and the james bay regions our decay time analysis for richmond gulf applies the pioneering approach of walcott 1980 to a large database and we derive a value of 4 0 6 6 kyr where the range is defined by a misfit tolerance 10 per cent higher than the minimum our analysis for james bay is based on the uplift curve derived by hardy 1976 and we estimate a decay time of about 2 0 2 8 kyr the difference between our estimates for richmond gulf and james bay may be due to errors in the observational record from these regions but could also be influenced by lateral variations in lithospheric structure associated with the assembly of laurentia

modeling dislocation glide in mg2sio4 ringwoodite towards rheology under transition zone conditions abstract deformation resulting from thermally activated plastic slip is modeled in mg2sio4 ringwoodite at 20 gpa for a wide range of temperatures the model relies on the structures of the rate controlling 1 2 1 1 0 screw dislocations which have been modeled using the peierls nabarro galerkin method these calculations are parametrized by density functional theory calculations of surfaces of the 0 0 1 1 1 0 and 1 1 1 planes at finite temperatures dislocation mobility is controlled by kink pair nucleation on the thermally activated 1 4 1 1 0 partial screw dislocations as they occur in ringwoodite single slip critical resolved shear stresses crss corresponding to this mechanism are deduced from orowan s equation the results are found to be in reasonably good agreement with experimental data at 20 gpa which show high effective flow stresses under laboratory conditions finally the crss s are calculated for typical mantle strain rates of 10 16 s 1 at appropriate temperatures expected in the lower transition zone results show that dislocation glide remains difficult and that lattice friction is not yet negligible in ringwoodite under natural conditions

the viscosity of earth s lower mantle inferred from sinking speed of subducted lithosphere abstract the viscosity of the mantle is indispensable for predicting earth s mechanical behavior at scales ranging from deep mantle material flow to local stress accumulation in earthquakes zones but mantle viscosity is not well determined for the lower mantle particularly only few constraints result from elaborate high pressure experiments karato 2008 and a variety of viscosity depth profiles result from joint inversion of the geoid and postglacial rebound data forte and mitrovica 1996 kaufmann and lambeck 2000 mitrovica and forte 2004 here we use inferred lower mantle sinking speed of lithosphere subduction remnants as a unique internal constraint on modeling the viscosity profile this entails a series of elaborate dynamic subduction calculations spanning a range of viscosity profiles from which we select profiles that predict the inferred sinking speed of 12 3 mm yr van der meer et al 2010 our modeling shows that sinking speed is very sensitive to lower mantle viscosity good predictions of sinking speed are obtained for nearly constant lower mantle viscosity of about 3 4 1022 pa s viscosity profiles incorporating a viscosity maximum in the deep lower mantle as proposed in numerous studies only lead to a good prediction in combination with a weak postperovskite layer at the bottom of the lower mantle and only for a depth average viscosity of 5 1022 pa s

mantle viscosity inferences from joint inversions of pleistocene deglaciatio induced changes in geopotential with a new slr analysis and polar wander joint inversions of post glacial rebound induced secular changes in low degree harmonics of the geopotential and true polar wander tpw systematically indicate that the upper mantle viscosity has a value that is considerably smaller than 10 21 pas the lower mantle viscosity is about one order of magnitude larger than the upper mantle viscosity discrepancies in the viscosity profiles inferred from even and odd zonals are likely an indication of recent unmodeled glacial forcings and or insufficient precision in the evaluation of the mass loss from pleistocene ice sheets the influence of systematic errors in slr data reduction due to thermal drag on the lageos satellites for the uneven zonals of the geopotential is avoided by making use of the starlette and stella satellites to recover the uneven zonals and by considering lumped coefficients for these uneven terms

thermal evolution of the mantle following continental aggregation in 3d convection models the claim that supercontinents insulate the mantle is largely based on recognition that seismically slow mantle below the atlantic african geoid high coincides with the former location of pangea we investigate the viability of continental insulation by varying both plate geometries and mantle properties in six three dimensional 3d mantle convection models the efficiency of continental insulation is quantified by calculating the rate of change of the average temperature in the subcontinental mantle our findings generally agree with two dimensional 2d modeling results however we conclude that 2d convection models may exaggerate subcontinental heating particularly in largely bottom heated cases and that subcontinental heating results from an absence of subduction in the subcontinental mantle rather than the insulation of active upwellings correspondingly we find that when large degrees of internal heating are present in the mantle hotter than average regions evolve below large oceanic plates

new perspectives on mantle dynamics from high resolution seismic tomographic model p1200 recently a high resolution tomographic model the p1200 based on p wave travel times was developed which allowed for detailed imaging of the top 1200 km of the mantle this model was used in diverse ways to study mantle viscosity structure and geodynamical processes in the spatial domain there are lateral variations in the transition zone suggesting interaction between the lower mantle plumes and the region from 600 km to 1000 km some examples shown here include the continental region underneath manchuria ukraine and south africa where horizontal structures lie above or below the 660 km discontinuity the blockage of upwelling is observed under central africa and the interaction between the upwelling and the transition zone under the slow icelandic region appears to be complex an expansion of the aspherical seismic velocities has been taken out to spherical harmonics of degree 60 for degrees exceeding around 10 the spectra at various depths decay with a power law like dependence on the degree with the logarithmic slopes in the asymptotic portion of the spectra containing values between 2 and 2 6 these spectral results may suggest the time dependent nature of mantle convection details of the viscosity structure in the top 1200 km of the mantle have been inferred both from global and regional geoid data and from the high resolution tomographic model we have considered only the intermediate degrees l 12 25 in the nonlinear inversion with a genetic algorithm approach several families of acceptable viscosity profiles are found for both oceanic and global data the families of solutions for the two data sets have different characteristics most of the solutions asociated with the global geoid data show the presence of asthenosphere below the lithosphere in other families a low viscosity zone between 400 and 600 km depth is found to lie atop a viscosity jump other families evidence a viscosity decrease across the 660 km discontinuity solutions from oceanic geoid show basically two low viscosity zones one lying right below the lithosphere the other right under 660 km depth all of these results bespeak clearly the plausible existence of strong vertical viscosity stratification in the top 1000 km of the mantle the presence of the second asthenosphere may have important dynamical ramifications on issues pertaining to layered mantle convection numerical modelling of mantle convection with two phase transitions and a realistic temperature and pressure dependent viscosity demonstrates that a low viscosity region under the endothermic phase transition can indeed be generated self consistently in time dependent situations involving a partially layered configuration in an axisymmetric spherical shell model

sea level change and true polar wander during the late cretaceous we demonstrate that rapid changes in the earth s rotation vector can drive relative sea level rsl variations in excess of 100 m in as little as 1 m y stratigraphic constraints on sea level change can thus be used as an independent test of true polar wander tpw events proposed on the basis of paleomagnetic evidence recent estimates of late cretaceous tpw include a period of rapid motion however a corresponding period of large and rapid sea level change is not consistently evident in our preliminary examination of the geological record this suggests that the magnitude of late cretaceous tpw may have been overestimated although a definitive conclusion requires a comprehensive study of high resolution late cretaceous sea level records index terms 1239 1527 4556 8159

modelling antarctic sea level data to explore the possibility of a dominant antarctic contribution to meltwater pulse ia abstract we compare numerical predictions of glaciation induced sea level change to data from 8 locations around the antarctic coast in order to test if the available data preclude the possibility of a dominant antarctic contribution to meltwater pulse ia mwp ia results based on a subset of 7 spherically symmetric earth viscosity models and 6 different antarctic deglaciation histories indicate that the sea level data do not rule out a large antarctic source for this event our preliminary analysis indicates that the weddell sea is the most likely source region for a large 9 m antarctic contribution to mwp ia the ross sea is also plausible as a significant contributor 5 m from a sea level perspective but glacio geological field observations are not compatible with such a large and rapid melt from this region our results suggest that the lambert glacier component of the east antarctic ice sheet experienced significant retreat at the time of mwp ia but only contributed 0 15 m eustatic sea level change all of the ice models considered under predicted the isostatic component of the sea level response in the antarctic peninsula and the soya coast region of the east antarctic ice sheet indicating that the maximum ice thickness in these regions is underestimated it is therefore plausible that ice melt from these areas the antarctic peninsula in particular could have made a significant contribution to mwp ia

the effects of transient rheology on the interpretation of lower mantle viscosity we have reexamined the role played by transient rheology in the interpretation of mantle viscosity this investigation has been carried out by comparing the amplitude responses with the data of secular variation of the relative sea level histories at sites well within the ice margins and at the ice margin like the city of boston a linear burgers body rheology has been assumed in the lower mantle the data near the edge of the ice load proves most sensitive to the transient viscosity structure the non monotonic behavior of sea level data near boston can be explained both by a steady state lower mantle viscosity of 1022p with a thick lithosphere and also by a transient lower mantle rheology but with a thin lithosphere the long term viscosity of the lower mantle in this second model has a steady state value of around 5 1023p

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

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

new constraints on transient lower mantle rheology and internal mantle buoyancy from glacial rebound data a model of mantle rheology which has an explicit transient component of its relaxation spectrum is shown to reconcile two apparently conflicting inferences of lower mantle viscosity if recent analyses of isostatic geoid anomalies are correct in requiring a large increase of viscosity with depth then the weak viscosity stratification demanded by postglacial rebound data implies the importance of such rheological behaviour at least in the earth s lower mantle

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

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

a plume head melting under a rifting margin abstract a large igneous province lip in the form of a long narrow band of thickened oceanic crust runs along the atlantic margin of north america abutting the rifted continental shelf we propose that this like many other lips has a mantle plume origin there is evidence that when the central atlantic ocean opened the rift was underlain by the flattened head of a mantle plume and that the rift site had drifted away from the plume tail by the time of the rifting so that the tail took little part in the formation of the lip we carried out numerical simulations in which we rifted the lithosphere over various model plume heads and calculated the volumes of melt produced we found that the thickness and width of the resulting thickened oceanic crust is very sensitive to the thermal structure directly under the rift and the structure of the lithosphere to fit observations of the lip a thin flat plume head is required such a plume head results when a mantle plume with temperature dependent viscosity passes through a significant step reduction in the background mantle viscosity at 660 km depth however an extensive layer of low viscosity under the rift results in a region of thickened crust much wider than the layer is deep by decoupling the mantle flow from the lithosphere to avoid decoupling we propose that there must be significant topography on the lithosphere and the rift site is a region of thinned lithosphere very thick crust next to the margin can be explained by lithospheric necking and the resulting fast initial upflow under the rift

origin of localized fast mantle flow velocity in numerical models of subduction 1 the origin of localized fast mantle flow near subduction zones found in regional jadamec and billen 2010 2012 and global stadler et al 2010 alisic et al 2010 numerical models using non newtonian rheology is explored using 3d models with simple geometry two suites of models using both a free slab and a slab attached to a lithospheric layer are presented to determine 1 the origin of high magnitudes of mantle flow 50 cm yr and 2 focusing of flow within the mantle wedge with mantle wedge velocities up to two times higher than slab sinking velocities the rheology in the models is either newtonian with a prescribed low viscosity wedge lvw or non newtonian wherein low viscosity regions form in response to high stress surrounding the sinking slab the results show that there are two aspects of the models that lead to localized fast velocities reduction in the mantle viscosity surrounding the sinking slab leads to faster overall flow rates induced by the negative buoyancy of the sinking slab and geometric effects that are caused by lateral variations in viscosity including retrograde pivoting motion of the slab

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

global geoid and sea level changes due to present day ice mass fluctuations we predict gravitationally self consistent global geoid and relative sea level rsl perturbations due to present day melting of ice complexes including the antarctic and greenland ice sheets and a suite of mountain glaciers and ice sheets classic analyses of sea level change indicate that these perturbations will depart significantly from eustatic i e geographically uniform trends e g woodward 1888 farrell and clark 1976 although this result has not always been appreciated in modern analyses mass flux of individual ice reservoirs will produce unique geometries of sea level change and this distinctiveness admits the possibility of using global geoid sea surface and rsl signatures of recent climate change to infer the ongoing mass balance of each reservoir rather than simply the net mass flux as an example we show that perturbations to the geoid arising from noneustatic water loads associated with each ice reservoir are sufficiently large at low degrees to be theoretically measurable within 5 years by the grace satellite mission we complete the study by reanalyzing tide gauge data at 23 sites selected by douglas 1997 in a recent analysis of global rsl rise traditionally estimates of global sea level rise are generated by taking the mean of a set of secular tide gauge trends that have been corrected for the influence of ongoing glacial isostatic adjustment gia related to the late pleistocene glacial cycles the common assumption in such studies is that the geographic scatter in the residual gia corrected trends is due to errors in the gia model or unmodeled processes e g tectonics we consider a large suite of gia model predictions and apply a least squares approach to the gia corrected tide gauge trends to estimate the weighting of various present day sea level signatures we find that the fit to the residual rsl trends is significantly improved and that the procedure is able to resolve a long standing observation of anomalously low sea level rates in europe this preliminary analysis which is relatively insensitive to changes in the assumed geometry of the present day mass balance assumes that ocean thermal expansion is globally uniform however the procedure can be easily extended to incorporate a realistic steric contribution once the geometry of the process is sufficiently well constrained

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

abrupt upper plate tilting during slab transition zone collision abstract the sinking remnant of a surface plate crosses and interacts with multiple boundaries in earth s interior here we specifically investigate the prominent dynamic interaction of the sinking plate portion with the upper mantle transition zone and its corresponding surface elevation signal we unravel for the first time that the collision of the sinking slab with the transition zone induces a sudden dramatic downward tilt of the upper plate towards the subduction trench unraveling this crucial interaction was only possible thanks to state of the art numerical modelling and post processing the new model that is introduced here to study the dynamically self consistent temporal evolution of subduction features accurate subduction zone topography robust single sided plate sinking stronger plates close to laboratory values an upper mantle phase transition and simple continents at a free surface to distinguish the impact of the new physical model features three different setups are used the simplest model setup includes a basic high viscosity lower mantle the second adds a 660 km phase transition and the third includes additionally a continental upper plate common to all models is the clear topographic signal upon slab transition zone interaction the upper plate tilts abruptly towards the subduction trench by about 0 05 and over around 10 ma this dramatic increase in upper plate tilt can be related to the slab induced excitation of the high viscosity lower mantle which introduces a wider flow pattern a large change in horizontal extent of inundation of up to 900 km is observed as a direct consequence of the upper plate tilting such an abrupt variation in surface topography and inundation extent should be clearly visible in temporal records of large scale surface elevation and might explain continental tilting as observed in australia since the eocene and north america during the phanerozoic

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

will earthquake activity in eastern canada increase in the next few thousand years knowledge of whether earthquake activity will increase in the next few thousand years is important for the planning of nuclear waste repositories assuming that fault instability portends earthquak

the influence of a finite glaciation phase on predictions of post glacial isostatic adjustment we predict relative sea level rsl variations present day three dimensional 3d crustal deformation rates and baseline velocities and a set of anomalies in the gravitational field of the planet using a suite of models for the space time geometry of the pleistocene ice sheets these models are distinguished on the basis of the details of the glaciation phase and they include the cases of an infinite glaciation phase i e an assumption of isostatic equilibrium at the time of the last glacial maximum lgm a steady uniform glaciation and a glaciation in which the bulk of the accumulation is limited to the final stages of the growth phase comparison of the observational records of postglacial rsl change with predictions based on the assumption of isostatic equilibrium at lgm have commonly been used to infer both the space time geometry of the final deglaciation event and the mantle viscosity we conclude that both these applications may be influenced significantly by the incorporation of a finite glacial cycle as an example rsl predictions which assume isostatic equilibrium at lgm and those which incorporate a relatively rapid glaciation phase can differ by a factor of 2 or more at some sites within previously glaciated regions when an earth model characterized by a lower mantle viscosity of 1022 pa s is adopted we have also found that differences in the adopted model for the glaciation event are capable of explaining entirely the discrepancies 0 2 mm yr between previously published predictions of present day tangential velocities in north america these discrepancies are characterized by a dominant long wavelength signal and hence they do not necessarily influence strongly predictions of baseline velocities variations in the glaciation model can also have an important effect on predictions of peak gravity anomalies over previously glaciated regions and secular variations in the zonal harmonics of the earth s geoid in both cases the effect may be sufficient to alter significantly the signal from other geophysical processes which may need to be invoked to reconcile the observational constraints

imaging mantle transition zone thickness with sds ss finite frequency sensitivity kernels summary we invert differential sds ss traveltime residuals measured from stacked waveforms and finitefrequency sensitivity kernels for topography on the 410 and 660 km discontinuities this approach yields higher resolution images of transition zone thickness than previous stacking methods which simply average smooth over topographic features apparent structure measured using simple stacking is highly dependent upon the bin size of each stack by inverting for discontinuity topography with a variety of bin sizes we can more accurately calculate the true structure the inverted transition zone model is similar to simple stack models with an average thickness of 242 km but the lateral variations in thickness are larger in amplitude and smaller in scale fast seismic velocities in 3 d mantle models such as sb4l18 correlate with areas of thicker transition zone the elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone the anomalously thin transition zone regions are laterally narrow and not broadly continuous if these variations in transition zone thickness are interpreted as thermal in nature then this model suggests significant temperature variations on small lateral scales

long wavelength gravity field constraint on the lower mantle viscosity in north america the long wavelength negative gravity anomaly over hudson bay coincides with the area depressed by the laurentide ice sheet during the last glacial maximum suggesting that it is at least partly caused by glacial isostatic adjustment gia additional contributions to the static gravity field stem from surface dynamic topography core mantle boundary cmb topography and density anomalies in the subsurface previous estimates of the contribution of gia to the gravity anomaly range from 25 to more than 80 however these estimates did not include uncertainties in all components that contribute to the gravity field in this study we develop a forward model for the gravity anomaly based on density models and dynamic models investigating uncertainty in all components we derive lithospheric densities from equilibrium constraints but extend the concept of lithospheric isostasy to a force balance that includes the dynamic models the largest uncertainty in the predicted gravity anomaly is due to the lower mantle viscosity uncertainties in the ice history the crustal model the lithosphere asthenosphere boundary and the conversion from seismic velocities to density have a smaller effect a preference for lower mantle viscosities 1022 pa s is found in which case at least 60 of the observed long wavelength gravity anomaly can be attributed to gia this lower bound on the lower mantle viscosity has implications for inferences based on models for mantle convection and gia

present day secular variations in the zonal harmonics of earth s geopotential we develop a new formalism for the prediction of secular variations in the gravitational potential field of a spherically symmetric self gravitating maxwell viscoelastic planetary model subjected to an arbitrary surface load which may include a gravitationally self consistent ocean loading component the theory is applied to generate the most accurate predictions to date of the present day secular variations in the zonal harmonics of the geopotential the so called for degree arising as a consequence of the late pleistocene glacial cycles in this respect we use the very recent ice 3g reconstruction of the last late pleistocene deglaciation event tushingham and peltier 1991 a comparison of these predictions with those generated using simplified disk models of the ice sheets which have been used in all previous studies of the harmonics 2 indicates that the disk model approximation introduces unacceptably large errors at all spherical harmonic degrees except perhaps 2 predictions have also been made using a eustatic loading approximation also used in previous studies in place of a gravitationally self consistent ocean loading component and we have found that the resulting discrepancy is largest at degrees 2 8 and 10 in the case of the magnitude of the error incurred using the eustatic approximation can be as large as order 10 15 of the predicted value we have attributed this discrepancy to the present day net flux of water away from the equatorial regions arising from the remnant present day adjustment associated with the late pleistocene glacial cycles the effect represents a heretofore unrecognized contribution to the harmonics or alternatively the nontidal acceleration of earth s axial rate of rotation in terms of the latter the maximum anomaly in the length of day is approximately 1 7 s yr we also consider the sensitivity of the data to variations in the radial mantle viscosity profile by using a suite of forward calculations and an examination of frechet kernels the theory required for the computation of those kernels is described herein we find that the radial variation in sensitivity can be a strong function of the viscosity model used in the calculations for models with a uniform upper mantle viscosity um of 1021 pa s forward predictions of the harmonics exhibit a pronounced peak when a wide enough range of lower mantle viscosities lm are considered we denote the lm value at this peak as at the lowest degrees 4 frechet kernels computed for a series of increasing lm values 1021 pa s lm 1023 pa s indicate a migration of the dominant sensitivity of the data to variations in viscosity from regions below approximately 1200 km depth for to regions above this depth in the lower mantle for the sensitivity of the data to variations in the viscosity profile in the shallowest parts of the lower mantle for the case is also reflected in a set of forward calculations described herein as an example predictions made using earth models in which the viscosity above 1200 km depth is constrained to be 1021 pa s do not exhibit the multiple solutions characteristic of the um 1021 pa s calculations the same is true of earth models in which the upper mantle viscosity is weakened an order of magnitude to 1020 pa s the theory described herein is also applied to compute the signal 10 arising from the retreat of small ice sheets and glaciers described by meier 1984 and also from any potential variations in the mass of the antarctic and greenland ice sheets the present day signal due to the late pleistocene glacial cycles dominates the signal from meier s sources at all degrees except 3 in contrast the signal arising from mass variations in the antarctic and greenland ice sheets is potentially comparable to the former a comparison of observational constraints on the data with predictions of the postglacial rebound signal described in this paper in order to infer mantle rheology cannot proceed until constraints are placed on the present day mass flux of these large polar ice sheets we show that the constraints required are weakest at degrees 2 and 4 finally we outline a potentially important procedure for incorporating predictions of the signal due to the late pleistocene glacial cycles and meier s sources with an observational constraint on the datum to yield bounds on the present day net mass flux from the antarctic and greenland ice sheets a rigorous application of this procedure must wait until observational constraints on are reestablished in the literature

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

role of lateral mantle flow in the evolution of subduction systems insights from laboratory experiments summary we present 3 d laboratory experiments constructed to investigate the pattern of mantle flow around a subducting slab under different boundary conditions in particular we present a set of experiments characterized by different conditions imposed at the trailing edge of the subducting plate that is plate fixed in the far field plate detached in the far field imposed plate motion experiments have been performed using a silicone slab floating inside a honey tank to simulate a thin viscous lithosphere subducting in a viscous mantle for each set we show differences between models that do or do not include the possibility of out of plane lateral flow in the mantle by varying the lateral boundary conditions our results illustrate how a subducting slab vertically confined over a 660 km equivalent depth can be influenced in its geometry and in its kinematics by the presence or absence of possible lateral pathways on the basis of these results we show implications for natural subduction systems and we highlight the importance of suitable simulations of lateral viscosity variations to obtain a realistic simulation of the history of subduction

influence of convergent plate boundaries on upper mantle flow and implications for seismic anisotropy shear wave splitting observations in the region of the upper mantle enveloping subduction zones have been interpreted as showing extensive regions of trench parallel flow despite the difficulty of reconciling such behavior with a sound model based on the forces that drive mantle motion to gain insight into the observations we systematically investigate flow patterns around the cold downwelling sheets associated with consumed plate material in a three dimensional numerical mantle convection model first we compare results from calculations employing prescribed plate geometries and kinematic plate velocities where the convergent plate boundary morphology is varied while keeping the plate velocity and convective parameters fixed subsequently we examine the flow around sheet like downwellings in a number of convection calculations featuring dynamically evolving plate velocities all of the calculations include thick viscous plates and a stratified mantle viscosity in all of the models examined we find that at mid upper mantle depths flow directions no longer align with plate motion and the influence of buoyancy driven downwellings clearly dominates flow solutions in the first models analyzed a pair of plates are included in the calculations and the large scale flow is generally roll like in the final model we investigate the interaction of four plates and a plate geometry characterized by triple junctions we examine a sequence from this calculation that features a triple junction of convergent boundaries in this model large scale flow characterized by convection rolls is superseded by a complex flow solution where flow in the mid upper mantle neither aligns uniformly with the plate motion nor necessarily follows the forcing associated with local buoyancy sources in this setting upper mantle flow in the vicinity of the sheet like downwellings featured in the solution moves orthogonal obliquely and even parallel to different sections of the convergent plate boundaries in the latter case our calculations of the deformation of a fixed volume parcel of upper mantle material suggest that an olivine lattice preferred orientation should develop that would result in a fast polarizing direction for seismic shear waves parallel to the slab our findings have implications for the interpretation of flow in the upper mantle based on seismic anisotropy

a dipping thick segment of the farallon slab beneath central u s it has been hypothesized that much of the crustal deformation attributed to the laramide orogeny of the southwest north american cordillera was caused by dynamic effects induced by the flat subduction of a large oceanic plateau that was embedded within the farallon plate previous studies have identified within the north american mantle a seismic velocity anomaly that plausibly represents the remnants of the subducted plateau coupled plate kinematic and dynamic modeling of the anomaly as well as surface geological findings identify this anomaly as the subducted conjugate to the shatsky rise here we find clear evidence for a northeastward dipping 35 dip thick up to 400 km thick slab like seismic velocity anomaly within the top of the lower mantle below the central united states using a deep focus earthquake below spain we find that the observed seismic waveforms recorded with the dense usarray display multi pathing indicative of sharp surfaces plate tectonic reconstructions in which the anomaly is migrated back in time suggest strong coupling of the plateau thickened slab segment to the overriding north america plate as it was subducted in combination with the reconstructions we interpret the structure as arising from eastward dipping farallon subduction at the western margin of north america during the cretaceous in contrast with some recent interpretations it appears that the plateau area of the slab has been further thickened which might undergo a combination of pure shear bulk shortening during flat slab subduction and or by a folding instability during penetration into the lower mantle

constraints on mantle viscosity based upon the inversion of post glacial uplift data from the hudson bay region summary we outline two parametrizations for post glacial relative sea level rsl histories associated with previously glaciated regions the first parametrization is based on a site dependent normalization of the rsl history while the second involves the estimate of a site dependent logarithm of the inverse decay time for the exponential like form which characterizes these histories both parametrizations are shown to yield data sets which are relatively insensitive to the details of the late pleistocene surface load history and therefore inferences of mantle viscosity based upon them will be particularly robust we apply the two parametrizations to consider the rsl record at a number of sites across the hudson bay region in this regard our inferences which are derived from both forward and inverse calculations are based upon the actual rsl age height pairs obtained by survey rather than the highly subjective set of rsl trends adopted in previous studies one of the main goals of the analysis is to assess the validity of a set of previously published and highly contradictory inferences of the radial profile of mantle viscosity based on the hudson bay rsl record forward analyses using models with isoviscous upper and lower mantle regions as adopted in the vast majority of previous analyses indicate that the parametrized versions of the rsl record in hudson bay excluding data from the cape henrietta maria site are best fitted by a lower mantle viscosity near 1021 pa s the same conclusion holds when data from only northern hudson bay are considered the rsl record in southern hudson bay is not self consistent if the error bars adopted herein are reasonable however the parametrized versions of the rsl curves from each site in this region can be reconciled by a model with a lower mantle viscosity somewhere in the rather moderate range 0 5 3 0 1021 pa s the value of 3 1021 pa s represents a lower bound on the lower mantle viscosity required to fit the rsl records at cape henrietta maria this record is characterized by a longer decay time than those associated with other sites in the data base this lower bound is in contrast with previous suggestions that the rsl record at cape henrietta maria requires a lower mantle viscosity of 30 1021 pa s inverse analyses described herein indicate that the rsl record from the entire hudson bay constrains the average viscosity in the radial region extending from the lower reaches of the upper mantle to mid lower mantle depths the inferences listed above are therefore more properly ascribed to this region rather than the entire lower mantle we have found that the data are sensitive to moderately deeper variations in the radial viscosity profile as one considers sites situated further north in hudson bay this provides one means for a spherically symmetric model to yield decay times which vary across the hudson bay however the data do not provide an unambiguous requirement for such a variation

fluid transport in geological reservoirs with background flow this paper presents fundamental analysis of the injection and release of fluid into porous media or geological reservoirs saturated by a different fluid undergoing a background flow and tests the predictions using analogue laboratory experiments the study reveals new results important for an understanding of the transport of hazardous contaminants through aquifers and the long term fate of carbon dioxide in geological sequestration using numerical and asymptotic analysis we describe a variety of flow regimes that arise and demonstrate an almost instantaneous control of injected fluid by the far field conditions in geological reservoirs for a continuous input the flow develops a horizontal interface between the injected and ambient fluids the background flow thereby effectively caps the height of the injected fluid into a shallower region of vertical confinement for a released parcel of fluid gravitational spreading is found to become negligible after a short time a dominant control of the interface by the background pressure gradient arises and stems from the different velocities at which it drives the injected and ambient fluids individually similarity solutions describing these dynamics show that the parcel approaches a slender triangular profile that grows horizontally as where is time a rate faster than relaxation under gravity shock layers develop at the front or back of the parcel depending on whether it is more or less viscous than the ambient fluid new analytical results describing the long term effects of residual trapping due to capillary retention are developed which yield explicit predictions for the time and length scales on which a parcel of becomes retained we end by applying our results to geological contexts concluding that even slight background motion can have considerable implications for long term transport through the subsurface

mantle plumes and flood basalts enhanced melting from plume ascent and an eclogite component new numerical models of starting plumes reproduce the observed volumes and rates of flood basalt eruptions even for a plume of moderate temperature arriving under thick lithosphere these models follow the growth of a new plume from a thermal boundary layer and its subsequent rise through the mantle viscosity structure they show that as a plume head rises into the lower viscosity upper mantle it narrows and it is thus able to penetrate rapidly right to the base of lithosphere where it spreads as a thin layer this behavior also brings the hottest plume material to the shallowest depths both factors enhance melt production compared with previous plume models the model plumes are also assumed to contain eclogite bodies inferred from geochemistry to be recycled oceanic crust previous numerical models have shown that the presence of nonreacting eclogite bodies may greatly enhance melt production it has been argued that the eclogite derived melt would react with surrounding peridotite and refreeze however recent experimental studies indicate that eclogite derived melts may have reached the earth s surface with only moderate or even minor modification combined with an assumed 15 vol component of eclogite our models yield a sharp peak in melting of about 1 3 myr duration and volumes of melt that encompass those observed in flood basalt provinces

mantle plume heads and the initiation of plate tectonic reorganizations we present results of numerical simulations which examine the plausibility of a reorganization of plate motions brought about by the interaction of a hot low viscosity plume head with the underside of a tectonic plate a numerical model of highly viscous fluid flow driven by thermal buoyancy is employed to examine the interaction of mantle plumes with surface plates in a two dimensional cartesian geometry plate like boundary conditions applied to the upper surface consist of a large active plate with fixed speed plus a smaller passive plate whose speed and direction of motion are permitted to react to tractions induced by the underlying mantle flow driven by the active plate the passive plate is also subject to applied end loads intended to simulate incipient subduction the mantle plume is initiated with a hot patch on the lower boundary and viscosity varies with temperature following an arrhenius rheology the calculations performed with fixed aspect ratio 4 1 and fixed rayleigh number ra 106 where ra refers to the strength of the starting plume were found to depend on three parameters the strength of the viscosity variation the size of the applied end load and the speed of the active plate these calculations indicate that plate motions can be sharply influenced by the lubricating effects of mantle plume heads even to the point of causing a reversal in the direction of plate motion

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