Effect of lateral viscosity variations in the core-mantle boundary region on predictions of the long-wavelength geoid (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.
Lateral variation in mantle viscosity
Earth and Planetary Sciences Letters
Differences in the viscosity of the earth's upper mantle beneath the western US ( f 10 18 -10 19 Pa s) and global average values based on glacial isostatic adjustment and other data ( f 10 20 -10 21 Pa s) are generally ascribed to differences in temperature. We compile geochemical data on the water contents of western US lavas and mantle xenoliths, compare these data to water solubility in olivine, and calculate the corresponding effective viscosity of olivine, the major constituent of the upper mantle, using a power law creep rheological model. These data and calculations suggest that the low viscosities of the western US upper mantle reflect the combined effect of high water concentration and elevated temperature. The high water content of the western US upper mantle may reflect the long history of Farallon plate subduction, including flat slab subduction, which effectively advected water as far inland as the Colorado Plateau, hydrating and weakening the upper mantle. D
Geophysical Journal International, 2007
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.
Viscosity profile of the lower mantle
Geophysical Journal International, 1985
We determine the variation of effective viscosity q across the lower mantle from models of the Gibb's free energy of activation G* and the adiabatic temperature profile. The variation of G* with depth is calculated using both an elastic strain energy model, in which G* is related to the seismic velocities, and a model which assumes G* is proportional to the melting temperature. The melting temperature is assumed t o follow Lindemann's equation. The adiabatic temperature profile is calculated from a model for the density dependence of the Gnineisen parameter. Estimates of q depend on whether the lower mantle is a Newtonian or power law fluid. In the latter case separate estimates of q are obtained for flow with constant stress, constant strain rate, and constant strain energy dissipation rate. For G* based on the melting temperature, increases in q with depth range from a factor of about 100 for Newtonian deformation or power-law flow with constant stress to about 5 for non-Newtonian deformation with constant strain rate. For G* based on elastic defect energy, increases in q with depth range from a factor of about 1500 for Newtonian deformation or power-law flow with constant stress to about 10 for non-Newtonian deformation with constant strain rate. Among these models, only a non-Newtonian lower mantle convecting with constant strain rate or constant strain energy dissipation rate is consistent with recent estimates of mantle viscosity from post-glacial rebound and true polar wander data.
On lateral viscosity contrast in the mantle and the rheology of low-frequency geodynamics
Geophysical Journal International, 1995
Mantle-wide heterogeneity is largely controlled by deeply penetrating thermal convective currents. These thermal currents are likely to produce significant lateral variation in rheology, and this can profoundly influence overall material behaviour. How thermally related lateral viscosity variations impact models of glacio-isostatic and tidal deformation is largely unknown. An important step towards model improvement is to quantify, or bound, the actual viscosity variations that characterize the mantle. Simple scaling of viscosity to shear-wave velocity fluctuations yields map-views of longwavelength viscosity variation. These give a general quantitative description and aid in estimating the depth dependence of rheological heterogeneity throughout the mantle. The upper mantle is probably characterized by two to four orders of magnitude variation (peak-to-peak). Discrepant timescales for rebounding Holocene shorelines of Hudson Bay and southern Iceland are consistent with this characterization. Results are given in terms of a local average viscosity ratio, Afji, of volumetric concentration, # i. For the upper mantle deeper than 340km the following reasonable limits are estimated for Afj x 0.01 5 # 5 0.15. A spectrum of ratios Afji < 0.1 at concentration level #i x 10-6-10-' in the lower mantle implies a spectrum of shorter timescale deformational response modes for second-degree spherical harmonic deformations of the Earth. Although highly uncertain, this spectrum of spatial variation allows a purely Maxwellian viscoelastic rheology simultaneously to explain all solid tidal dispersion phenomena and long-term rebound-related mantle viscosity. Composite theory of multiphase viscoelastic media is used to demonstrate this effect.
Journal of Geodynamics, 2007
We investigate the effect of lateral viscosity variations (LVV) in the mantle on dynamic geoid and near-surface mantle velocities. Contrary to the previous studies, we analyse the effect not only of the lithospheric keels and asthenosphere but also of the whole mantle. A 3D global viscosity model is constructed using (a) the S20 seismic tomography model converted to temperature and (b) assumptions about homologous temperature in the mantle Paulson et al. . Modeling post-glacial rebound with lateral viscosity variations. Geophys. J. Int. 163, 357-371]. The conversion parameters provide a depthdependent viscosity profile, which is generally consistent with the existing studies. Therefore, the horizontal variations are produced self-consistently within this approach. We estimate them as 'conservative', thus providing a minimum limit for possible horizontal changes; consequently the obtained results show a low limit for dynamic topography, geoid and surface velocities' disturbances. To estimate the effect of LVV we have employed the perturbation method of Zhang and Christensen [Zhang, S., Christensen U., 1993. Some effects of lateral viscosity variations on geoid and surface velocities induced by density anomalies in the mantle. Geophys. J. Int. 114, 547-551], which is modified to implement mantle compressibility. It has been found that the impacts of the upper and lower mantle are comparable in amplitude. The difference between the initial (only radial viscosity) dynamic geoid and the geoid with implemented LVV reaches −27 to +19 m for the whole mantle LVV, while the effects of the upper and lower mantle are equal to −24 to +16 and −20 to +16 m correspondingly. In general these effects are not correlated. The difference in the geoid response leads to substantially altered velocity-to-density scaling factors (up to 30% compared to the initial ones) obtained in least squares inversion. The effect of the lower mantle LVV on surface velocities is also substantial, in particular with respect to the toroidal component, which does not exist in the initial model.
Geophysical Journal International, 2000
We perform a joint inversion of Earth's geoid and dynamic topography for radial mantle viscosity structure using a number of models of interior density heterogeneities, including an assessment of the error budget. We identify three classes of errors: those related to the density perturbations used as input, those due to insuf®ciently constrained observables, and those due to the limitations of our analytical model. We estimate the amplitudes of these errors in the spectral domain. Our minimization function weights the squared deviations of the compared quantities with the corresponding errors, so that the components with more reliability contribute to the solution more strongly than less certain ones. We develop a quasi-analytical solution for mantle¯ow in a compressible, spherical shell with Newtonian rheology, allowing for continuous radial variations of viscosity, together with a possible reduction of viscosity within the phase change regions due to the effects of transformational superplasticity. The inversion reveals three distinct families of viscosity pro®les, all of which have an order of magnitude stiffening within the lower mantle, with a soft Da layer below. The main distinction among the families is the location of the lowest-viscosity regionÐdirectly beneath the lithosphere, just above 400 km depth or just above 670 km depth. All pro®les have a reduction of viscosity within one or more of the major phase transformations, leading to reduced dynamic topography, so that whole-mantle convection is consistent with small surface topography.
Lateral variation in upper mantle viscosity: role of water
Earth and Planetary Science Letters, 2004
Differences in the viscosity of the earth's upper mantle beneath the western US ( f 10 18 -10 19 Pa s) and global average values based on glacial isostatic adjustment and other data ( f 10 20 -10 21 Pa s) are generally ascribed to differences in temperature. We compile geochemical data on the water contents of western US lavas and mantle xenoliths, compare these data to water solubility in olivine, and calculate the corresponding effective viscosity of olivine, the major constituent of the upper mantle, using a power law creep rheological model. These data and calculations suggest that the low viscosities of the western US upper mantle reflect the combined effect of high water concentration and elevated temperature. The high water content of the western US upper mantle may reflect the long history of Farallon plate subduction, including flat slab subduction, which effectively advected water as far inland as the Colorado Plateau, hydrating and weakening the upper mantle. D