A Geodynamic Model of Mantle Density Heterogeneity (original) (raw)
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Geophysical Journal International, 1991
In a dynamic Earth, mantle mass heterogeneities induce gravity anomalies, surface velocities and surface topography. These lateral density heterogeneities can be estimated on the basis of seismic tomographic models. Recent papers have described a realistic circulation model that takes into account the observed plate geometry and is able to predict the rotation vectors of the present plates. The relationship between the surface observables and the heterogeneities is sensitive to the viscosity stratification of the mantle. Here we use this model, combined with a generalized least-squares method, in order to infer the viscosity profile of the Earth from the surface observations, and. to get some new insight into the 3-D density structure of the mantle. The computed radial viscosity profile presents a continuous increase of more than two orders of magnitude. The asthenosphere has a viscosity close to 2 x lo2' Pa s. No sharp discontinuity is requested at the upper-lower mantle interface. The largest viscosity 7 X 10" Pas is reached in the middle of the lower mantle. A t greater depth, approaching the core-mantle boundary, the viscosity decreases by one order of magnitude. The model suggests that the well-known degree-2 and order-2 anomaly in the transition zone of the upper mantle is merely the signature of the slabs. It also slightly increases the degree-2 and order-0 in the lower mantle and decreases it in the upper mantle. In other words the inversion requests a hotter lower mantle beneath the equator and a colder upper mantle at the same latitudes.
Journal of Geodynamics, 2008
Lateral heterogeneities in the mantle can be caused by thermal, chemical and non-isotropic pre-stress effects. Here, we investigate the possibility of using observations of the glacial isostatic adjustment (GIA) process to constrain the thermal contribution to lateral variations in mantle viscosity. In particular, global historic relative sea level, GPS in Laurentide and Fennoscandia, altimetry together with tide-gauge data in the Great Lakes area, and GRACE data in Laurentide are used. The lateral viscosity perturbations are inferred from the seismic tomography model S20A by inserting the scaling factorˇto determine the contribution of thermal effects versus compositional heterogeneity and non-isotropic pre-stress effects on lateral heterogeneity in mantle viscosity. Whenˇ= 1, lateral velocity variations are caused by thermal effects alone. Withˇ< 1, the contribution of thermal effect decreases, so that forˇ= 0, there is no lateral viscosity variation and the Earth is laterally homogeneous. These lateral viscosity variations are superposed on four different reference models which differ significantly in the lower mantle viscosity. The Coupled Laplace Finite Element method is used to predict the GIA response on a spherical, self-gravitating, compressible, viscoelastic Earth with self-gravitating oceans, induced by the ICE-4G deglaciation model. Results show that the effect ofˇon uplift rates and gravity rate-of-change is not simple and involves the trade-off between the contribution of lateral viscosity variations in the transition zone and in the lower mantle. Models with small viscosity contrast in the lower mantle cannot explain the observed uplift rates in Laurentide and Fennoscandia. However, the RF3S20 model with a reference viscosity profile simplified from Peltier's VM2 with the value ofˇaround 0.2-0.4 is found to explain most of the global RSL data, the uplift rates in Laurentide and Fennoscandia and the BIFROST horizontal velocity data. In addition, the changes in GIA signals caused by changes in the value ofˇare large enough to be detected by the data, although uncertainty in other parameters in the GIA models still exists. This may encourage us to further utilize GIA observations to constrain the thermal effect on mantle lateral heterogeneity as geodetic and satellite gravity measurements are improved.
Lower Mantle Heterogeneity, Dynamic Topography and the Geoid
Nature, 1985
Density contrasts in the lower mantle, inferred using seismic tomography, drive viscous flow; this results in kilometres of dynamically maintained topography at the core-mantle boundary and at the Earth's surface. The total gravity field due to interior density contrasts and ...
Subduction to the lower mantle - a comparison between geodynamic and tomographic models
2012
It is generally believed that subduction of lithospheric slabs is a major contribution to thermal heterogeneity in Earth's entire mantle and provides a main driving force for mantle flow. Mantle structure can, on the one hand, be inferred from plate tectonic models of subduction history and geodynamic models of mantle flow. On the other hand, seismic tomography models provide important information on mantle heterogeneity. Yet, the two kinds of models are only similar on the largest (1000 s of km) scales and are quite different in their detailed structure. Here, we provide a quantitative assessment how good a fit can be currently achieved with a simple viscous flow geodynamic model. The discrepancy between geodynamic and tomography models can indicate where further model refinement could possibly yield an improved fit. Our geodynamical model is based on 300 Myr of subduction history inferred from a global plate reconstruction. Density anomalies are inserted into the upper mantle beneath subduction zones, and flow and advection of these anomalies is calculated with a spherical harmonic code for a radial viscosity structure constrained by mineral physics and surface observations. Model viscosities in the upper mantle beneath the lithosphere are ∼10 20 Pas, and viscosity increases to ∼10 23 Pas in the lower mantle above D " . Comparison with tomography models is assessed in terms of correlation, both overall and as a function of depth and spherical harmonic degree. We find that, compared to previous geodynamic and tomography models, correlation is improved, presumably because of advances in both plate reconstructions and mantle flow computations. However, high correlation is still limited to lowest spherical harmonic degrees. An important ingredient to achieve high correlation -in particular at spherical harmonic degree two -is a basal chemical layer. Subduction shapes this layer into two rather stable hot but chemically dense "piles", corresponding to the Pacific and African Large Low Shear Velocity Provinces. Visual comparison along cross sections indicates that sinking speeds in the geodynamic model are somewhat too fast, and should be 2 ± 0.8 cm yr −1 to achieve a better fit. Solid Earth, 3, 415-432, 2012 www.solid-earth.net/3/415/2012/ Solid Earth, 3, 415-432, 2012 www.solid-earth.net/3/415/2012/ Solid Earth, 3, 415-432, 2012 www.solid-earth.net/3/415/2012/
Earth and Planetary Science Letters, 2012
Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense 'piles', which have remained isolated from mantle circulation for several hundred million years. This interpretation largely hinges on four seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; (iii) their shear to compressional wave-speed anomaly ratios are high; and (iv) their shear and bulk-sound velocity anomalies appear to be anti-correlated. However, using compressible global mantle circulation models driven by 300 Myr of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale chemical 'piles'. A prescribed core-mantle-boundary temperature of 4000 K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, shear wave velocity anomalies from models that include dense chemical 'piles' at the base of Earth's mantle, where 'piles' correspond to only 3% of the mantle's volume, are substantially stronger than the tomographic model S40RTS, even after accounting for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, elevated ratios between shear and compressional wavespeed anomalies and the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all calculations, an anticorrelation only occurs within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent 'piles' are not required to reconcile the seismic observations examined here. Indeed, our results suggest that if chemical heterogeneity is present in these regions, its dynamical and seismic significance is far less than has previously been inferred.
2011
Mapping the thermal and compositional structure of the upper mantle requires a combined interpretation of geophysical and petrological observations. Based on current knowledge of material properties, we interpret available global seismic models for temperature assuming end-member compositional structures. In particular, we test the effects of modelling a depleted lithosphere, which accounts for petrological constraints on continents. Differences between seismic models translate into large temperature and density variations, respectively, up to 400 K and 0.06 g cm −3 at 150 km depth. Introducing lateral compositional variations does not change significantly the thermal interpretation of seismic models, but gives a more realistic density structure. Modelling a petrological lithosphere gives cratonic temperatures at 150 km depth that are only 100 K hotter than those obtained assuming pyrolite, but density is ∼0.1 g cm −3 lower. We determined the geoid and topography associated with the density distributions by computing the instantaneous flow with an existing code of mantle convection, STAG-YY. Models with and without lateral variations in viscosity have been tested. We found that the differences between seismic models in the deeper part of the upper mantle significantly affect the global geoid, even at harmonic degree 2. The range of variance reduction for geoid due to differences in the transition zone structure (i.e. from 410 to 660 km) is comparable with the range due to differences in the whole mantle seismic structure. Since geoid is dominated by very long wavelengths (the lowest five harmonic degrees account for more than 90 per cent of the signal power), the lithospheric density contrasts do not strongly affect its overall pattern. Models that include a petrological lithosphere, however, fit the geoid and topography better. Most of the long-wavelength contribution that helps to improve the fit comes from the oceanic lithosphere. The signature of continental lithosphere worsens the fit, even in simulations that assume an extremely viscous lithosphere. Therefore, a less depleted, and thus less buoyant, continental lithosphere is required to explain gravity data. None of the seismic tomography models we analyse is able to reproduce accurately the thermal structure of the oceanic lithosphere. All of them show their lowest seismic velocities at ∼100 km depth beneath mid-oceanic ridges and have much higher velocities at shallower depths compared to what is predicted with standard cooling models. Despite the limited resolution of global seismic models, this seems to suggest the presence of an additional compositional complexity in the lithosphere.
Earth and Planetary Science Letters, 2012
Absolute reference frames are a means of describing the motion of plates on the surface of the Earth over time, relative to a fixed point or “frame.” Multiple models of absolute plate motion have been proposed for the Cretaceous–Tertiary period, however, estimating the robustness and limitations of each model remains a significant limitation for refining both regional and global models of plate motion as well as fully integrated and time dependent geodynamic models. Here, we use a novel approach to compare five models of absolute plate motion in terms of their consequences for forward modelled deep mantle structure since at least 140 Ma. We show that the use of hotspots, either fixed or moving, or palaeomagnetics, with or without corrections for true-polar wander, leads to significant differences in palaeo-plate velocities of over 10 cm/yr as well as differences in the location of palaeo-plate boundaries of up to 30° in longitude and latitude. Furthermore, we suggest that first order differences in forward predicted mantle structure between the models are due mostly to differences in palaeo-plate velocities, whereas variation in the location of plate boundaries may contribute to smaller wavelength offsets. We present a global comparison of the absolute reference frames in terms of mantle structure, which we have tomographically filtered to reflect the resolution of the seismic tomography model S20RTS. At very long wavelengths hotspot models best reproduce the mantle structure. However, when geometry and the match of smaller-scale subducted slab volumes are compared, a hybrid model based on moving hotspots after 100 Ma and palaeomagnetic data before (with no corrections for true-polar wander), best reproduces the overall mantle structure of slab burial grounds, even though no single model fits best at all mantle depths. We find also that the published subduction reference frame tested here results in a modelled mantle structure that agrees well with S20RTS for depths > 2500 km, equivalent to subduction before the Cretaceous, but not for shallower depths. This indicates that a careful assimilation of hotspot, palaeomagnetic and seismic tomography data into future absolute plate motion models is required to derive a more robust subduction reference frame.
A comparison of tomographic and geodynamic mantle models
2002
We conduct a comprehensive and quantitative analysis of similarities and differences between recent seismic tomography models of the Earth's mantle in an attempt to determine a benchmark for geodynamic interpretation. After a spherical harmonic expansion, we find the spectral power and radial correlation of each tomographic model as a function of depth and harmonic degree. We then calculate the correlation, at the same depths and degrees, between all possible pairs of models, to identify stable and model-dependent features (the former being usually of longer spatial wavelength than the latter). We can therefore evaluate the degree of robust structure that seismologists have mapped so far and proceed to calculate ad hoc mean reference models. Tomographic models are furthermore compared with two geodynamic subduction models that are based on plate motion reconstructions. We find systematically low intermediate-wavelength correlation between tomography and convective reconstruction models and suggest that the inadequate treatment of the details of slab advection is responsible. However, we confirm the presence of stable, slab-like fast anomalies in the mid-mantle whose geographic pattern naturally associates them with subduction. This finding, in addition to our analysis of heterogeneity spectra and the absence of strong minima in the radial correlation functions besides the one at $700 km, supports the idea of whole mantle convection with slab penetration through the 660 km phase transition, possibly accompanied by a reorganization of flow.
Plateness of the Oceanic Lithosphere and the Thermal Evolution of the Earth’s Mantle
High Performance Computing in Science and Engineering’ 05, 2000
Compared to , the model of the thermal evolution of the Earth's mantle is considerably improved. The temporal development of the radial viscosity profile due to cooling of the Earth could substantially be taken into account by numerical progress using a new variant of the temperature-and pressure-dependence of the shear viscosity of the mantle, namely Eq (5). The laterally averaged heat flow, the Urey number, the Rayleigh number and the volume-averaged temperature as a function of time come up to the expectations that stem from the parameterized evolution models. The mentioned evolution parameters of the present paper better approximate the observational data. Contrary to the parameterized curves, these quantities show temporal variations. This seems to be more realistic for geological reasons. Due to the activation enthalpy, the presented viscosity profile has a highly viscous transition layer (TL) with steep viscosity gradients at the phase boundaries. A low-viscosity zone is situated above and below the TL, each. The lithosphere moves piecewise en bloc. Thin cold sheet-like downwellings have an Earth-like distribution.
Mantle heterogeneities, geoid, and plate motion: A Monte Carlo inversion
Journal of Geophysical Research, 1989
Seismic tomography in both the upper and the lower mantle, as well as subducting oceanic slabs defined by seismicity, has been translated into density heterogeneities to generate models of mantle circulation. These models can predict both the surface velocities and the geoid, which can be compared with plate tectonics and gravity data. A given model is specified by 6 13,739