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  • 1
    Electronic Resource
    Electronic Resource
    On A Particular Solution of the Non-Linear Liouville Equations (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 114 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The rotational behaviour of a stratified viscoelastic planet is analysed by means of a quasi-analytical method. Our approach is particularly appropriate to study the long-term polar wander induced by internal loads, and in particular to study the effects due to time-dependent mantle convection. We focus on a simple explicit solution of the Liouville non-linear equations, in order to establish the relationships between internal rheological constitution of the planet and polar motion. Both the rate and the direction of polar wander are found to be extremely sensitive to the mantle stratification and in particular to the nature of the 670km depth seismic discontinuity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb03927.x
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  • 2
    Electronic Resource
    Electronic Resource
    Mantle dynamics and geoid Green functions (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 120 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We compute Green functions relating mantle mass heterogeneities to geoid undulations using both incompressible and radially compressible mantle rheologies. Our results differ from those previously published by Forte & Peltier (1991) and Thoraval, Machetel & Cazenave (1994). This is due to two factors. Instead of taking gravity as constant throughout the mantle, we compute it self-consistently with the radial density profile. Secondly, we show that in the mathematical formulation of the compressible case, stresses are not continuous through a density jump. We then point out that the presence of a surface ocean has no additional effect on the geoid signal associated with internal mantle mass anomalies. We apply this formalism to a geodynamic model of mantle density variations and show that an excellent fit to the observed geoid can be obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb01835.x
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  • 3
    Electronic Resource
    Electronic Resource
    Effects on post-glacial rebound from the hard rheology in the transition zone (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 109 (1992), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We analyse the influences of a viscosity increase in the transition zone between 420 and 670 km on the geophysical signatures induced by post-glacial rebound, ranging from the perturbations in the Earth's rotation to the short wavelength features associated with the migration of the peripheral bulge. A self-gravitating model is adopted, consisting of an elastic lithosphere, a three-layer viscoelastic mantle and an inviscid core.The horizontal displacements and velocities and the stress pattern are extremely sensitive to the viscosity increase and to the chemical stratification of the transition zone. The hardening of the upper and the chemical density jumps in mantle below the 420 discontinuity induces a channel effect which contaminates the horizontal deformation both in the near-field and in the far-field from the ice-sheets. These findings indicate that intraplate geodetic data can be used to put bounds on the viscosity increase in the transition zone and on the amount of chemical stratification in the mantle.The stress field induced in the lithosphere by the Pleistocenic ice-sheet disintegration is a very sensitive function of mantle viscosity stratification. The existence of seismic activity along passive continental margins of previously glaciated areas requires a substantial viscosity increase in the mantle, with the viscosity of the transition zone acting as a controlling parameter. A viscously stratified mantle is responsible for a delayed upward migration of stress in the lithosphere which can account for the seismicity today.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1992.tb00125.x
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  • 4
    Electronic Resource
    Electronic Resource
    Polar wandering of a dynamic earth (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 113 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The rotational behaviour of a stratified visco-elastic planet submitted to changes in its inertia tensor is studied in a viscous quasi-fluid approximation. This approximation allows for large displacements of the Earth rotation axis with respect to the entire mantle but is only valid for mass redistribution within the planet occurring on the time scale of a few million years. Such a motion, called true polar wander (TPW), is detected by palaeomagneticiens assuming that the Earth's magnetic field remains on average aligned with the spin axis. Our model shows that a downgoing cold slab induces a TPW which quickly brings this slab to the pole for a mantle of uniform viscosity. The same slab is slowly moved toward the equator when a large viscosity increase with depth takes place in the mantle. Our model is also suitable to investigate the effects of a non-steady-state convection on the Earth's rotation. We discuss these effects using a simple mass redistribution model inspired by the pioneering paper of Goldreich & Toomre (1969). It consists of studying the TPW induced by a random distribution of slabs sinking into the mantle. For such a mass redistribution, only a strongly stratified mantle can reduce the Earth's pole velocity below 1d̀ Ma-1, which is the upper bound value observed by palaeomagnetic investigations for the last 200 Ma. Our model also shows that when corrected for the hydrostatic flattening, the Earth's polar inertia generally corresponds to the maximum inertia, as it is presently observed. However, this may not be the case during some short time periods. We also discuss the amount of excess polar flattening that can be related to tidal deceleration. This frozen component is found to be negligible.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb00888.x
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  • 5
    Electronic Resource
    Electronic Resource
    Topography and geoid due to lithospheric mass anomalies (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 122 (1995), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A model of lithospheric thickness and a recent compilation of Moho depths are used to compute the Earth's isostatic surface topography and associated gravity anomalies. The results are strongly influenced by the uncertainties in lithospheric depth and crustal density profiles. The preferred models explain most of the observed topography and are highly correlated with observed gravity anomalies for harmonic degrees larger than 10. Comparisons of our residual topography with geodynamical calculations of dynamic topography based on mantle circulation are rather poor.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1995.tb06850.x
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  • 6
    Electronic Resource
    Electronic Resource
    Comparison Between Newtonian and Non-Newtonian Flow Driven By Internal Loads (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 112 (1993), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The interpretation of long wavelength geoid and plate motions on the basis of dynamic Earth models has usually been done assuming linear viscous rheologies in the mantle. In this paper, we develop spherical 3-D models of mantle circulation using power-law creep rheologies with an exponent n= 3. We found that the stress-dependent rheologies only modify the amplitude of the topography supported by an internal load, by a small percentage with respect to the linear predictions. the geoid anomalies induced by internal loads can be affected by around 20 per cent. These changes are also occurring at degrees and orders different from those of the mass anomaly itself. As the geoid spectrum is strongly decreasing with degree, the dynamic topography induced at high degrees can be contaminated in a nonnegligible way by the low degree loads. the main contamination occurs at a harmonic triple of that of the most important load. the flow structure is much more dependent on the creep law than are the dynamic topography and the geoid. In contrast to linear rheology, a power-law creep is able to sustain a toroidal velocity field. However, this toroidal component only carries a small percentage of the kinetic energy and thus the non-linear creep with n= 3 cannot by itself explain the observed quasi-equipartition of plate tectonic energy between toroidal and poloidal components.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1993.tb01440.x
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  • 7
    Electronic Resource
    Electronic Resource
    Inferring the viscosity and the 3-D density structure of the mantle from geoid, topography and plate velocities (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 105 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: 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 prfile 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 × 1020 Pa s. No sharp discontinuity is requested at the upper-lower mantle interface. The largest viscosity 7 × 1022 Pa s is reached in the middle of the lower mantle. At 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb00796.x
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  • 8
    Electronic Resource
    Electronic Resource
    Excitation of true polar wander by subduction (1992)
    [s.l.] : Nature Publishing Group
    Nature 360 (1992), S. 452-454 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The 1969 paper of Goldreich and Toomre11 renewed the interest of the geophysical community12 in the role of polar wandering in Earth's dynamics. Their example of a colony of beetles crawling on the surface of a quasi-rigid Earth suggests that large excursions of the axis of rotation with respect to ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/360452a0
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  • 9
    Electronic Resource
    Electronic Resource
    Time-dependent density anomalies in a stratified, viscoelastic mantle: Implications for the geoid, Earth's rotation and sea-level fluctuations (1993)
    Springer
    Surveys in geophysics 14 (1993), S. 537-553 
    Details
    ISSN: 1573-0956
    Keywords: Rheology ; viscosity and density jumps ; geoid ; rotation ; sea levels
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The effects on the ℓ = 2 geoid component and Earth's rotation due to internal mass anomalies are analyzed for a stratified viscoelastic mantle described by a Maxwell rheology. Our approach is appropriate for a simplified modeling of subduction. Sea-level fluctuations induced by long-term rotational instabilities are also considered. The displacement of the Earth's axis of rotation, called true polar wander (TPW) and the induced eustatic sea-level fluctuations, are extremely sensitive to viscosity and density stratification at the 670 km seismic discontinuity. Phase-change models for the transition zone generally allow for huge amount of TPW, except for large viscosity increases; the dominant contribution in Liouville equations comes from a secular term that reflects the viscous behaviour of the mantle. In chemically stratified models, TPW is drastically reduced due to dynamic compensation of the mass anomalies at the upper-lower mantle interface. When the source is embedded in the upper mantle close to the chemical density jump, transient rotational modes are the leading terms in the linear Liouville equations. Long-term rotation instabilities are valuable contributors to the third order cycles in the eustatic sea-level curves. Rates of sea-level fluctuations of the order of 0.05–0.1 mm/yr are induced by displacements of the Earth's axis of rotation compatible with paleomagnetic data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00690576
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  • 10
    Electronic Resource
    Electronic Resource
    Relative Sea Level Variations Caused by Subduction (1997)
    Springer
    Surveys in geophysics 18 (1997), S. 225-238 
    Details
    ISSN: 1573-0956
    Keywords: Dynamic topography ; geoid ; sea level ; subduction ; viscosity profile
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract By means of a stratified Earth model with viscoelastic rheology, we have studied the long-term global fluctuations of Relative Sea Level (RSL) induced by subducting slabs. We have computed RSL variations for both a single subduction and a realistic distribution of slabs by a numerical simulation based on a simplified model of the subduction process. RSL is determined by the offset between the geoid and the dynamic topography; our analysis demonstrates that the latter provides the prevailing contribution. We have studied, in addition, the effects of rheological stratification upon the amplitude and time-evolution of these two quantities and, consequently, of RSL fluctuations. According to our results, an upper bound for the rate of RSL associated with subduction is of the order of 0.1 mm/yr, in agreement with previous studies. This rate of sea level variation is comparable with that attributed to changes in the tectonic regime on a large scale. This preliminary result corroborates the suggestion by other authors to include subduction in the list of geophysical mechanisms which contribute to long-term RSL fluctuations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1006540126947
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