Building of the Amsterdam-Saint Paul plateau: A 10 Myr history of a ridge-hot spot interaction and variations in the strength of the hot spot source

International audienceThe Amsterdam-Saint Paul plateau results from a 10 Myr interaction between the South East Indian Ridge and the Amsterdam-Saint Paul hot spot. During this period of time, the structure of the plateau changed as a consequence of changes in both the ridge-hot spot relative distance and in the strength of the hot spot source. The joint analysis of gravity-derived crust thickness and bathymetry reveals that the plateau started to form at ~10 Ma by an increase of the crustal production at the ridge axis, due to the nearby hot spot. This phase, which lasted 3-4 Myr, corresponds to a period of a strong hot spot source, maybe due to a high temperature or material flux, and decreasing ridge-hot spot distance. A second phase, between ~6 and ~3 Ma, corresponds to a decrease in the ridge crustal production. During this period, the hot spot center was close to the ridge axis and this reduced magmatic activity suggests a weak hot spot source. At ~3 Ma, the ridge was located approximately above the hot spot center. An increase in the hot spot source strength then resulted in the building of the shallower part of the plateau. The variations of the melt production at the ridge axis through time resulted in variations in crustal thickness but also in changes in the ridge morphology. The two periods of increased melt production correspond to smooth ridge morphology, characterized by axial highs, while the intermediate period corresponds to a rougher, rift-valley morphology. These variations reveal changes in axial thermal structure due to higher melting production rates and temperatures

Starting laminar plumes: Comparison of laboratory and numerical modeling

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94953/1/ggge1631.pd

Dynamique des panaches thermiques laminaires (application aux panaches mantelliques)

En géosciences, les points chauds, îles volcaniques intraplaques telles que Hawaii ou la Réunion, pourraient être l expression en surface de panaches qui monteraient depuis la base du manteau, à 2900 km de profondeur. Les panaches, instabilités de la convection de Rayleigh-Bénard, se développeraient dans le manteau avec un nombre de Prandtl (Pr) de 1023 mais peuvent aussi être modéisés à partir d un chauffage de petite taille, numériquement avec Pr infini, ou en laboratoire avec Pr ~103 - 106. Dans cette thèse nous étudions les caractéristiques d un panache isolé dans un fluide visqueux, à viscosité constante. Nous utilisons des expériences de laboratoire et des modèles numériques. Les techniques de visualisation donnent accès aux champs de température et de vitesse pour une section 2-D de la cuve. Les simulations numériques utilisent une méthode d éléments finis à symétrie cylindrique qui reproduisent les conditions de laboratoire avec les propriétés mesurées des fluides et du chauffage. On obtient un excellent accord entre ces deux approches indépendantes. Cela permet de proposer des lois d échelles simples pour la dynamique de la tête et du conduit du panache et de les appliquer au cas de la Terre. Nous montrons notamment que pour des Pr supérieurs à 1000 l effet de confinement l emporte largement sur les effets inertiels et que la dynamique du panache est bien décrite par l approximation Pr infini. Ceci est en particulier vrai pour les manteaux planétairesIn Earth Sciences, hot upwelling plumes are thought to develop from the base of the 2900 kmthick solid mantle of our planet and to generate hotspots, i.e. intraplate volcanic islands such as Hawaii and La Reunion. Although generated through chaotic Rayleigh-B enard instabilities at a Prandtl number (Pr) around 1023, they can be modelled with the simpler case of starting plumes out of a finite-size heater, either numerically for infinite Prandtl number, or in the laboratory with fluids with Pr ~ 103 - 106. Hence, the question is to find simple scaling laws for isolated rising plumes and apply them to the Earth s mantle case. In this thesis we study the characteristics of an isolated plume growing in a viscous fluid with constant viscosity. We use both laboratory experiments and numerical models : the visualization techniques give us access to the growing plume temperature and velocity fields, on a 2-D section of the tank, whereas the numerical simulations are axisymmetric finite element simulations that attempt to reproduce the laboratory conditions as closely as possible. We find excellent quantitative agreement between the two fully independent approaches. This is used to derive scaling laws for the dynamics of the plume head and stem, and apply them to the Earth s case. We further show that for Pr larger than 1000, confinement effects are more important than inertial effects and that plumes dynamics are well described by the approximation if infinite Prandlt number. This is especially true for planetary mantlesPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Anatomy of a laminar starting thermal plume at high Prandtl number

We present an experimental study of the dynamics of a plume generated from a small heat source in a high Prandtl number fluid with a strongly temperature-dependent viscosity. The velocity field was determined with particle image velocimetry, while the temperature field was measured using differential interferometry and thermochromic liquid crystals. The combination of these different techniques run simultaneously allows us to identify the different stages of plume development, and to compare the positions of key-features of the velocity field (centers of rotation, maximum vorticity locations, stagnation points) respective to the plume thermal anomaly, for Prandtl numbers greater than 103. We further show that the thermal structure of the plume stem is well predicted by the constant viscosity model of Batchelor (Q J R Met Soc 80: 339-358, 1954) for viscosity ratios up to 50. © 2010 Springer-Verlag