The role of a pressure-dependent rheology in the dynamics of mantle circulation

A thermomechanical model for upper mantle convection was constructed such that the thickness and the structure of the lithosphere are determined self consistently by the heat transported by convection. In this study of the interaction between the lithosphere and upper mantle, strongly temperature and pressure dependent rheologies for both Newtonian and non-Newtonian creep mechanisms are employed. For a strictly temperature dependent rheology an insignificant amount of heat, less than 12.5 mW/sq m, can be transported convectively for an interior viscosity, 0(10 sup 21 Pas), compatible with post glacial rebound. On the other hand, for similar values of the interior viscosity, steady heat fluxes between 20 and 40 mW/sq m are produced by introducing pressure dependence into the rheology. For the temperature and pressure dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated, once all of the rheological parameters are fixed. This finding may have important ramifications for parameterized convection

Origine des contraintes tectoniques déviatoriques dans la lithosphère terrestre

Les contraintes associées aux déformations tectoniques de grande ampleur ont trois origines possibles : 1. les forces appliquées en bout de plaque contrebalancées par la résistance visqueuse sous les plaques, 2. les hétérogénéités de densité à l'intérieur des plaques, à moins de 1 50 km de profondeur, 3. les hétérogénéités de densité situées profond dans le manteau terrestre. Dans le cas d'une convection à deux couches dans le manteau terrestre, cette dernière source de contrainte est moins importante que les deux premières

Global plate motion and the geoid: a physical model

cited By 33International audienceThe effect of lateral variations in the sub-lithospheric viscosity, i.e. in the lithosphere-asthenosphere coupling, is incorporated in dynamic earth models driven by plate velocities. If the coupling is stronger under fast-moving continents, it is shown that a geoid low will develop in their wake and a high in front of them. Thus the well-known low in the Indian ocean could be explained, at least in part, in terms of induced upper-mantle dynamics. Similarly, if the thickness of oceanic plates increases with age the models show that trenches should be associated with marked geoid highs, whereas ridges could correspond to much weaker geoid lows. This also seems to agree with some features of the observed geoid at very long wavelengths. The mathematical framework of such dynamic earth models is developed extensively here. These models are characterized by lateral viscosity variations inside an outer shell and a purely radial viscosity structure at greater depth. Their internal flow patterns are only driven by imposed surface velocities, not by internal loads. Copyright © 1988, Wiley Blackwell. All rights reserve

Geoid heights and lithospheric stresses for a dynamic Earth.

cited By 237International audienceMass heterogeneities in the Earth's crust and mantle are known to exist at all depths and for a large range of wavelengths characteristic of their lateral extent. They are the sources of measurable quantities like topography, tectonic stresses and the geoid. Quantitative relationships between these surface observables and deep sources are established for various Earth viscosity structures with spherical symmetry. For an homogeneous mantle, the surface stresses and the geoid height increase with the depth of the perturbing heterogeneity but decrease markedly beyond a critical depth proportional to the wavelength. It is shown that upper mantle return flow is also capable of generating the geoid without producing too large a topography and too strong tectonic stresses. The two types of contribution are expected to occur in the Earth. Further progress will require careful data analysis and correlation between topography, stresses, geoid and deep density structures. -from Author

On the interpretation of linear relationships between seafloor subsidence rate and the height of the ridge

International audienc

EPSL Thermal evolution of the oceanic lithosphere: an alternative view Abstract

The most common model used for representing the evolution with age of the oceanic lithosphere is the ‘plate model’ where the temperature is set at a fixed depth, called the base of the plate. This ‘base of the plate ’ has no physical meaning but this model provides a mathematical substitute for a system where small-scale convection occurs through instabilities growing at the base of the cooling lithosphere and becomes effective only below old ocean. Another possible view is that convection provides heat at the base of the lithosphere whatever the age of the overlying plate. This last process can be modeled by a Constant Heat flow Applied on the Bottom Lithospheric ISothetm (CHABLIS model). A good fit to the observables (bathymetry and geoid as function of age, and old age heat-flow) can be obtained both for plate and CHABLIS models in spite of an experimentally determined thermal expansion coefficient much larger than assumed in previous plate models. These models have important consequences for several geodynamic processes. The plate, at an age of 100 Ma is only 80 km thick for both models: melting above a hot-snot can then occur in the garnet-spine1 transition field without much plate thinning. In the plate model the subsidence is stopped at an age of about 80 Ma while, according to the CHABLIS model, several hundred meters of subsidence are expected after 100 Ma. Thus the two models predict quite a different long-term pattern of subsidence in the sedimentary basins. Finally, in the CHABLIS model, the global cooling of the mantle coming from cold material eroded by secondary convection at the base of the plates is considerably larger than in plate models: it amounts to 40%, the remaining 60 % being due to the subduction process

Linear stability of a double diffusive layer of an infinite prandtl number fluid with temperature-dependent viscosity

Studia Geophysica et Geodaetica, v. 48, n. 3, p. 519-537, 2004. http://dx.doi.org/10.1023/B:SGEG.0000037470.80659.e5International audienc

Inverting Glacial Isostatic Adjustment signal using Bayesian framework and two linearly relaxing rheologies

International audienceGlacial Isostatic Adjustment (GIA) models commonly assume a mantle with a viscoelastic Maxwell rheology and a fixed ice history model. Here, we use a Bayesian Monte Carlo approach with a Markov chain formalism to invert the global GIA signal simultaneously for the mechanical properties of the mantle and the volumes of the ice sheets, using as starting ice models two previously published ice histories. Two stress relaxing rheologies are considered: Burgers and Maxwell linear viscoelasticities. A total of 5720 global palaeo sea level records are used, covering the last 35 kyr. Our goal is not only to seek the model best fitting this data set, but also to determine and display the range of possible solutions with their respective probability of explaining the data. In all cases, our a posteriori probability maps exhibit the classic character of solutions for GIA-determined mantle viscosity with two distinct peaks. What is new in our treatment is the presence of the bi-viscous Burgers rheology and the fact that we invert rheology jointly with ice history, in combination with the greatly expanded palaeo sea level records. The solutions tend to be characterized by an upper-mantle viscosity of around 5 × 1020 Pa s with one preferred lower-mantle viscosities at 3 × 1021 Pa s and the other more than 2 × 1022 Pa s, a rather classical pairing. Best-fitting models depend upon the starting ice history and the stress relaxing law. A first peak (P1) has the highest probability only in the case with a Maxwell rheology and ice history based on ICE-5G, while the second peak (P2) is favoured for ANU-based ice history or Burgers stress relaxation. The latter solution also may satisfy lower-mantle viscosity inferences from long-term geodynamics and gravity gradient anomalies over Laurentia. P2 is also consistent with large Laurentian and Fennoscandian ice-sheet volumes at the Last Glacial Maximum (LGM) and smaller LGM Antarctic ice volume than in either ICE-5G or ANU. Exploration of a bi-viscous linear relaxing rheology in GIA now seems logical due to a new set of requirements to satisfy observations of transient post-seismic flow seen so ubiquitously in space gravimetry and other global geodetic data