HAL (Le Centre pour la Communication Scientifique Directe), 1999
The preservation and exhumation of high-pressure rocks is an important observation in understandi... more The preservation and exhumation of high-pressure rocks is an important observation in understanding the geodynamics of orogenic processes. A numerical tool is developed to estimate quantitatively the e¡ect of the complex interplay between the mechanical and thermodynamical behaviour, and to assess under which conditions the preservation of metastable denser phases is possible. A ¢nite di¡erence numerical method is used to solve the continuity, Navier^Stokes and thermal equations for a Newtonian compressible £uid medium. In the model we take into account a typical forcing induced by a subduction process in a collisional environment according to a corner £ow model. We follow the evolution of di¡erent phases in the crust including a pressuret emperature-dependent phase transition in the numerical code. Although eclogite is formed at depth when the phase diagram is only prescribed from thermodynamics, it cannot reach the surface. The kinetic e¡ects of thermally activated di¡usion and of the nucleation processes are taken into account in the modelling of the phase transition. Our simpli¢ed model does not explicitly take into account the presence of water. It assumes that the rate of phase transformation can be computed from a knowledge of pressure, temperature and phase content. The parameters of the kinetic equations are empirically chosen to reproduce qualitatively the typical pressure^temperature^time paths recorded in the Alpine belt. To obtain signi¢cant concentrations of high-pressure phases at the surface, di¡erent activation energies for the prograde and retrograde reactions are needed. This di¡erence may be related to changes in the water content of the crust between its burial and its exhumation.
This chapter presents the fundamental physics necessary to understand the complex fluid dynamics ... more This chapter presents the fundamental physics necessary to understand the complex fluid dynamics of mantle convection. The first section derives the equations of conservation for mass, momentum and energy, and the boundary and interface conditions for the various physical quantities. The thermodynamic and rheological properties of solids are discussed in the second section. The mechanism of thermal convection and the classical Boussinesq and anelastic approximations are presented in the third section. As the subject is not often included in geophysical text books, an introduction to the physics of multicomponent and multiphase flows is given in a fourth section. At last the specific applications to mantle convection are reviewed in the fifth section.
Physics of the Earth and Planetary Interiors, Aug 1, 2023
The radial density of planets increases with depth due to compressibility, leading to impacts on ... more The radial density of planets increases with depth due to compressibility, leading to impacts on their convective dynamics. To account for these effects, including the presence of a quasi-adiabatic temperature profile and entropy sources due to dissipation, the compressibility is expressed through a dissipation number, D, proportional to the planet's radius and gravity. In Earth's mantle, compressibility effects are moderate, but in large rocky or liquid exoplanets (Super-Earths), the dissipation number can become very large. This paper explores the properties of compressible convection when the dissipation number is significant. We start by selecting a simple Murnaghan equation of state that embodies the fundamental properties of condensed matter at planetary conditions. Next, we analyze the characteristics of adiabatic profiles and demonstrate that the ratio between the bottom and top adiabatic temperatures is relatively small and probably less than 2. We examine the marginal stability of compressible mantles and reveal that they can undergo convection with either positive or negative superadiabatic Rayleigh numbers. Lastly, we delve into simulations of convection performed using the exact equations of mechanics, neglecting inertia (infinite Prandtl number case), and examine their consequences for Super-Earths dynamics.
Lithospheric shear localization, as occurs in the formation of tectonic plate boundaries, is ofte... more Lithospheric shear localization, as occurs in the formation of tectonic plate boundaries, is often associated with diminished grainsize (e.g., mylonites). Grainsize reduction is typically attributed to dynamic recrystallization; however, theoretical models of shear-localization arising from this hypothesis are problematic since (1) they require the simultaneous action of two exclusive creep mechanisms (diffusion and dislocation creep), and (2) the grain-growth ("healing")
Cette these de geodynamique comporte deux parties. Dans chacune de ces parties, des processus de ... more Cette these de geodynamique comporte deux parties. Dans chacune de ces parties, des processus de deformation des materiaux manteliques sont mis en jeu. Les donnees gravimetriques, geoide ou anomalies de Bouguer apparaissent comme des observations fondamentales pour contraindre ou guider nos modeles. La premiere partie de cette these presente une analyse spectrale de la topographie et des anomalies de gravite de la province du Basin and Range dans l'Ouest Americain. Des periodicites de differentes longueurs d'onde sont mises en evidence suivant des directions que nous relions a celles des extensions que cette region a subies au Miocene et a l'Actuel. Nous comparons ces observations a des modeles d'instabilite d'etirement. Nous proposons l'existence d'un boudinage a l'echelle lithospherique pour interpreter nos observations dans cette region. La deuxieme partie analyse les relations a grandes longueurs d'onde entre le geoide, les heterogeneites manteliques de densite et la tectonique des plaques. Nous montrons que la prise en compte de la dynamique interne de notre Terre fluide est absolument necessaire pour comprendre ces relations. Nos modeles considerent l'existence de variations laterales des proprietes mecaniques de la Terre. Ces modeles permettent de simuler la presence des plaques tectoniques rigides a la surface d'un manteau convectif homogene.
We present QADR17, a global model of Rayleigh-wave attenuation based on a massive surface wave da... more We present QADR17, a global model of Rayleigh-wave attenuation based on a massive surface wave data set (372 629 frequency-dependent attenuation curves in the period range 50-260 s). We correct for focusing-defocusing effects and geometrical spreading, and perform a stringent selection to only keep robust observations. Then, data with close epicentres recorded at the same station are clustered, as they sample the same Earth's structure. After this preselection, our data set consists of about 35 000 curves that constrain the Rayleigh-wave intrinsic attenuation in the upper mantle. The logarithms of the attenuation along the individual rays are then inverted to obtain global maps of the logarithm of the local attenuation. After a first inversion, outliers are rejected and a second inversion yields a variance reduction of about 45 per cent. Our attenuation maps present strong agreement with surface tectonics at periods lower than 200 s, with low attenuation under continents and high attenuation under oceans. Over oceans, attenuation decreases with increasing crustal ages, but at periods sensitive to the uppermost 150 km, mid-ocean ridges are not characterized by a very localized anomaly, in contrast to what is commonly observed for seismic velocity models. Attenuation is rather well correlated with hotspots, especially in the Pacific ocean, where a strong attenuating anomaly is observed in the long wavelength component of our signal at periods sampling the oceanic asthenosphere. We suggest that this anomaly results from the horizontal spreading of several thermal plumes within the asthenosphere. Strong velocity reductions associated with high attenuation anomalies of moderate amplitudes beneath the East Pacific Rise, the Red Sea and the eastern part of Asia may require additional mechanisms, such as partial melting.
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Copyright
HAL (Le Centre pour la Communication Scientifique Directe), 1999
The preservation and exhumation of high-pressure rocks is an important observation in understandi... more The preservation and exhumation of high-pressure rocks is an important observation in understanding the geodynamics of orogenic processes. A numerical tool is developed to estimate quantitatively the e¡ect of the complex interplay between the mechanical and thermodynamical behaviour, and to assess under which conditions the preservation of metastable denser phases is possible. A ¢nite di¡erence numerical method is used to solve the continuity, Navier^Stokes and thermal equations for a Newtonian compressible £uid medium. In the model we take into account a typical forcing induced by a subduction process in a collisional environment according to a corner £ow model. We follow the evolution of di¡erent phases in the crust including a pressuret emperature-dependent phase transition in the numerical code. Although eclogite is formed at depth when the phase diagram is only prescribed from thermodynamics, it cannot reach the surface. The kinetic e¡ects of thermally activated di¡usion and of the nucleation processes are taken into account in the modelling of the phase transition. Our simpli¢ed model does not explicitly take into account the presence of water. It assumes that the rate of phase transformation can be computed from a knowledge of pressure, temperature and phase content. The parameters of the kinetic equations are empirically chosen to reproduce qualitatively the typical pressure^temperature^time paths recorded in the Alpine belt. To obtain signi¢cant concentrations of high-pressure phases at the surface, di¡erent activation energies for the prograde and retrograde reactions are needed. This di¡erence may be related to changes in the water content of the crust between its burial and its exhumation.
This chapter presents the fundamental physics necessary to understand the complex fluid dynamics ... more This chapter presents the fundamental physics necessary to understand the complex fluid dynamics of mantle convection. The first section derives the equations of conservation for mass, momentum and energy, and the boundary and interface conditions for the various physical quantities. The thermodynamic and rheological properties of solids are discussed in the second section. The mechanism of thermal convection and the classical Boussinesq and anelastic approximations are presented in the third section. As the subject is not often included in geophysical text books, an introduction to the physics of multicomponent and multiphase flows is given in a fourth section. At last the specific applications to mantle convection are reviewed in the fifth section.
Physics of the Earth and Planetary Interiors, Aug 1, 2023
The radial density of planets increases with depth due to compressibility, leading to impacts on ... more The radial density of planets increases with depth due to compressibility, leading to impacts on their convective dynamics. To account for these effects, including the presence of a quasi-adiabatic temperature profile and entropy sources due to dissipation, the compressibility is expressed through a dissipation number, D, proportional to the planet's radius and gravity. In Earth's mantle, compressibility effects are moderate, but in large rocky or liquid exoplanets (Super-Earths), the dissipation number can become very large. This paper explores the properties of compressible convection when the dissipation number is significant. We start by selecting a simple Murnaghan equation of state that embodies the fundamental properties of condensed matter at planetary conditions. Next, we analyze the characteristics of adiabatic profiles and demonstrate that the ratio between the bottom and top adiabatic temperatures is relatively small and probably less than 2. We examine the marginal stability of compressible mantles and reveal that they can undergo convection with either positive or negative superadiabatic Rayleigh numbers. Lastly, we delve into simulations of convection performed using the exact equations of mechanics, neglecting inertia (infinite Prandtl number case), and examine their consequences for Super-Earths dynamics.
Lithospheric shear localization, as occurs in the formation of tectonic plate boundaries, is ofte... more Lithospheric shear localization, as occurs in the formation of tectonic plate boundaries, is often associated with diminished grainsize (e.g., mylonites). Grainsize reduction is typically attributed to dynamic recrystallization; however, theoretical models of shear-localization arising from this hypothesis are problematic since (1) they require the simultaneous action of two exclusive creep mechanisms (diffusion and dislocation creep), and (2) the grain-growth ("healing")
Cette these de geodynamique comporte deux parties. Dans chacune de ces parties, des processus de ... more Cette these de geodynamique comporte deux parties. Dans chacune de ces parties, des processus de deformation des materiaux manteliques sont mis en jeu. Les donnees gravimetriques, geoide ou anomalies de Bouguer apparaissent comme des observations fondamentales pour contraindre ou guider nos modeles. La premiere partie de cette these presente une analyse spectrale de la topographie et des anomalies de gravite de la province du Basin and Range dans l'Ouest Americain. Des periodicites de differentes longueurs d'onde sont mises en evidence suivant des directions que nous relions a celles des extensions que cette region a subies au Miocene et a l'Actuel. Nous comparons ces observations a des modeles d'instabilite d'etirement. Nous proposons l'existence d'un boudinage a l'echelle lithospherique pour interpreter nos observations dans cette region. La deuxieme partie analyse les relations a grandes longueurs d'onde entre le geoide, les heterogeneites manteliques de densite et la tectonique des plaques. Nous montrons que la prise en compte de la dynamique interne de notre Terre fluide est absolument necessaire pour comprendre ces relations. Nos modeles considerent l'existence de variations laterales des proprietes mecaniques de la Terre. Ces modeles permettent de simuler la presence des plaques tectoniques rigides a la surface d'un manteau convectif homogene.
We present QADR17, a global model of Rayleigh-wave attenuation based on a massive surface wave da... more We present QADR17, a global model of Rayleigh-wave attenuation based on a massive surface wave data set (372 629 frequency-dependent attenuation curves in the period range 50-260 s). We correct for focusing-defocusing effects and geometrical spreading, and perform a stringent selection to only keep robust observations. Then, data with close epicentres recorded at the same station are clustered, as they sample the same Earth's structure. After this preselection, our data set consists of about 35 000 curves that constrain the Rayleigh-wave intrinsic attenuation in the upper mantle. The logarithms of the attenuation along the individual rays are then inverted to obtain global maps of the logarithm of the local attenuation. After a first inversion, outliers are rejected and a second inversion yields a variance reduction of about 45 per cent. Our attenuation maps present strong agreement with surface tectonics at periods lower than 200 s, with low attenuation under continents and high attenuation under oceans. Over oceans, attenuation decreases with increasing crustal ages, but at periods sensitive to the uppermost 150 km, mid-ocean ridges are not characterized by a very localized anomaly, in contrast to what is commonly observed for seismic velocity models. Attenuation is rather well correlated with hotspots, especially in the Pacific ocean, where a strong attenuating anomaly is observed in the long wavelength component of our signal at periods sampling the oceanic asthenosphere. We suggest that this anomaly results from the horizontal spreading of several thermal plumes within the asthenosphere. Strong velocity reductions associated with high attenuation anomalies of moderate amplitudes beneath the East Pacific Rise, the Red Sea and the eastern part of Asia may require additional mechanisms, such as partial melting.
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Copyright
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