1,721,080 research outputs found
Perturbations in the Earth's rotation induced by internal density anomalies: implications for sea-level fluctuations
No full textThe effects of internal mass anomalies on the Earth's rotation are analyzed within the framework of linearized Liouville equations and Maxwell rheology for the mantle. Our approach is appropriate for a simplified modeling of subduction. Sea-level fluctuations induced by long-term rotational instabilities are also considered. -from Author
Deep continental roots: the effects of lateral variations of viscosity on post-glacial rebound
No full textThe existence of lateral viscosity variations in the earth mantle could be inferred by recent tomographic results. This can potentially cause substantial changes in the interpretation of the results from usual postglacial uplift modelling which assumes a uniform mantle. In this work we study the impact of a high viscosity craton located below the lithosphere in Fennoscandia. The solution is obtained using a finite element code which treats the surface rebound of an axisymmetrical viscoelastic half-space. Our findings are potentially very important in the interpretation of GPS data. -from Author
Finite element modeling of lateral viscosity heterogeneities and post-glacial rebound
No full textThe existence of lateral viscosity heterogeneities of a few orders of magnitude in the Earth mantle has recently been proposed by several authors, although classical post-glacial rebound models are consistent with a rather uniform structure. Tomographic results reveal the presence of lateral heterogeneities in seismic velocities within the mantle which can be correlated with temperature and viscosity variations. Our aim is to analyze the uplift in deglaciated areas, using a realistic horizontal viscosity profile of the mantle, in order to determine the influence of such models on global mantle viscosity evaluation. Since analytical models cannot handle lateral heterogeneities, we used a finite-element numerical approach. The code is a version of TECTON program (Melosh and Raefsky, 1983), suitable for treating the axisymmetric case and modified to take isostatic compensation of the surface load into account. We found that the average mantle viscosity values deviate from the "canonical" value of 1021 Pa · s when lateral variations are taken into consideration. The displacement in the center is controlled by the average viscosity underlying the load. For long wavelengths (heterogeneities of two orders of magnitude), the estimated average viscosity may differ by a factor 7 from the results obtained using radial models. For stronger variation (of four orders of magnitude) the deviation is a factor of 30, although such strong viscosity seems to be inconsistent with the data. Difficulties in data fitting are also encountered for smaller variations when short wavelengths are considered. When the interplay of vertical and lateral stratification is considered, we find that average viscosities in the upper and lower mantle depend on the magnitude and pattern of the heterogeneities in each layer. In particular, strong lateral viscosity contrasts in the upper mantle can overlie a uniform lower mantle. © 1990
Mantle rheology and satellite signatures from present-day glacial forcings
No full textStudies the changes of the long-wavelength portion of the Earth's gravity field as a result of present-day glacial discharges and the possible growth of the Antarctic ice sheet. We employ both Maxwell and Burgers' body rheologies and find that there are significant differences in the responses between the 2 rheologies for time spans less than a century. Both present-day glacial forcing and the ice buildup on Antarctica can cause non-negligible perturbations in j2 of approx 1/3 the observed amount. It is proposed that detailed satellite monitoring of present-day ice movements in conjunction with geodetic satellite missions will eventually provide a viable alternative for estimating deep mantle viscosity, useful in mantle convection models. -from Author
Glacial isostasy and the interplay between upper and lower mantle lateral viscosity heterogeneities
No full textFrom the analysis of sea‐level data near the centre of the former Fennoscandian ice sheet and the theoretical predictions of vertically and laterally stratified Earth models, we explore the extent to which lateral viscosity variations could have influenced the estimate of long term mantle viscosity. We follow a finite element scheme in cylindrical symmetry, focusing on the effects of lateral viscosity contrasts of different magnitudes and wavelengths. Sea‐levels are consistent with long wavelength lateral viscosity contrasts of at most 2 orders of magnitude; short wavelength variations, comparable with the lateral extension of the surface load, should be ruled out. Differences in the average viscosities inferred from laterally stratified and uniform mantle models, may be as high as 1 order of magnitude. If we allow for different lateral viscosity patterns in the upper and lower mantle, we find that sea‐levels are consistent with short wavelength variations in the upper mantle of around 2 orders of magnitude, coupled with smoother lateral viscosity contrasts in the lower mantle. Copyright 1989 by the American Geophysical Union
Lateral heterogeneities in mantle viscosity and post‐glacial rebound
No full textSeismic tomographic results and convection calculations support the evidence of horizontal temperature variations in the mantle. On the basis of commonly accepted rheological laws, we thus expect lateral variations in the viscosity of several orders of magnitude. This paper is concerned with the effects of lateral viscosity variations on vertical displacement induced by Pleistocenic deglaciation. A finite‐element scheme in axial symmetry mimics the relaxation process of a flat Earth model characterized by a linear Maxwell rheology. We follow a spectral approach to analyse the impact of lateral viscosity variations with different amplitudes and wavelengths. The potential impact of lithospheric thickening and viscosity increase in stable continental regions on the interpretation of sea level data is also analysed. Lateral viscosity heterogeneities are found to have strong influence on ground deformations induced in deglaciated areas. From the analysis of sea level data near the centre of the former Fennoscandian ice sheet and the theoretical predictions of radially and laterally stratified Earth models, we explore the extent to which lateral viscosity contrasts may have influenced the inference of long‐term mantle viscosity. While radially stratified models predict a rather uniform mantle viscosity, we show that lateral viscosity contrasts of 1 or 2 orders of magnitude are consistent with sea level data if the wavelength of the heterogeneity is comparable with the dimension of a convecting cell. In this case, the average viscosity can deviate by around an order of magnitude from the ‘canonical’ value of 1021 Pa s predicted by rebound models in the past. Viscosity values close to 1021 Pa s are found to be appropriate for the mantle region underlying the load. Long wavelength viscosity variations of 3 or 4 orders of magnitude degrade our ability to reproduce the observed uplift in the centre of the ice sheet and must be ruled out. For wavelengths comparable with the horizontal extension of the surface load and viscosity contrasts of 1 or 2 orders of magnitude, model results are found to be inconsistent with sea level data. These findings suggest that rebound modelling can become a useful tool to constrain the magnitude and wavelength of viscosity contrasts. Comparison between horizontally and vertically varying models indicates that lateral viscosity heterogeneities could have been interpreted as radial variations in previous rebound studies. Copyright © 1989, Wiley Blackwell. All rights reserve
The dynamical influences of a hard transition zone on post-glacial uplifts and rotational signatures
No full textRecent investigations from laboratory and Monte-Carlo inversion of geophysical signatures have suggested that the transition zone of the mantle between 400 and 670 km depth may be stiffer than the lower and the upper mantle. By means of a five-layer viscoelastic spherical Earth model, we have calculated the displacement fields associated with post-glacial rebound, the induced polar motions, the temporal variations of the coefficients of the geopotential up to degree eight and the stress fields induced by deglaciation in the lithosphere and the upper mantle. Temporal variations of stress fields in the lithosphere reveal a non-monotonic behaviour due to the viscosity stratification. The results demonstrate the importance of the coming LAGEOS II geodetic satellite mission on constraining the rheological nature of the transition zone in the mantle. -from Author
The dynamical influences of a hard transition zone on post-glacial uplifts and rotational signatures
No full textRecent investigations from laboratory and Monte-Carlo inversion of geophysical signatures have suggested that the transition zone of the mantle between 400 and 670 km depth may be stiffer than the lower and the upper mantle. By means of a five-layer viscoelastic spherical Earth model, we have calculated the displacement fields associated with post-glacial rebound, the induced polar motions, the temporal variations of the coefficients of the geopotential up to degree eight and the stress fields induced by deglaciation in the lithosphere and the upper mantle. Temporal variations of stress fields in the lithosphere reveal a non-monotonic behaviour due to the viscosity stratification. The results demonstrate the importance of the coming LAGEOS II geodetic satellite mission on constraining the rheological nature of the transition zone in the mantle. -from Author
The effects of transient rheology on the interpretation of lower mantle viscosity
No full textWe 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. Copyright 1985 by the American Geophysical Union
Effects of lateral viscosity variations on postglacial rebound: Implications for recent sea‐level trends
No full textGlobal sea‐level changes have been occurring since the last Pleistocene deglaciation. Recently the contribution of current postglacial rebound to local sea‐level variations has been removed by means of a radially stratified earth model in order to assess the amount of sea‐level rise due to recent climatic changes. We have studied the effects of lateral variations in the viscosity structure on the postglacial uplift rates along the continental margins. Finite‐element calculations in cylindrical geometry show that the spread in the cumulative vertical displacement may be affected by 20% in the presence of lateral rheological contrasts. But the vertical rates of deformation can attain differences of up to 50% for the different models. Horizontal deformation rates along continental margins are much more affected by the lateral variations in viscosity and can be used in the future as constraints on mantle rheology. Copyright 1990 by the American Geophysical Union
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