The Southeast Indian Ridge between 88°E and 118°E: Variations in crustal accretion at constant spreading rate

The temperature of the mantle and the rate of melt production are parameters which play important roles in controlling the style of crustal accretion along mid-ocean ridges. To investigate the variability in crustal accretion that develops in response to variations in mantle temperature, we have conducted a geophysical investigation of the Southeast Indian Ridge (SEIR) between the Amsterdam hotspot and the Australian-Antarctic Discordance (88°E- 118°E). The spreading center deepens by 2100 m from west to east within the study area. Despite a uniform, intermediate spreading rate (69-75 mm yr- 1), the SEIR exhibits the range in axial morphology displayed by the East Pacific Rise and the Mid-Atlantic Ridge (MAR) and usually associated with variations in spreading rate. The spreading center is characterized by an axial high west of 102°45'E, whereas an axial valley is prevalent east of this longitude. Both the deepening of the ridge axis and the general evolution of axial morphology from an axial high to a rift valley are not uniform. A region of intermediate morphology separates axial highs and MAR-like rift valleys. Local transitions in axial morphology occur in three areas along the ridge axis. The increase in axial depth toward the Australian-Antarctic Discordance may be explained by the thinning of the oceanic crust by ~ 4 km and the change in axial topography. The long-wavelength changes observed along the SEIR can be attributed to a gradient in mantle temperature between regions influenced by the Amsterdam and Kerguelen hot spots and the Australian-Antarctic Discordance. However, local processes, perhaps associated with an heterogeneous mantle or along-axis asthenospheric flow, may give rise to local transitions in axial topography and depth anomalies

Scaling Theory and Numerical Simulations of Aerogel Sintering

A simple scaling theory for the sintering of fractal aerogels is presented. The densification at small scales is described by an increase of the lower cut-off length $a$ accompanied by a decrease of the upper cut-off length $\xi$, in order to conserve the total mass of the system. Scaling laws are derived which predict how $a$, $\xi$ and the specific pore surface area $\Sigma$ should depend on the density $\rho$. Following the general ideas of the theory, numerical simulations of sintering are proposed starting from computer simulations of aerogel structure based on a diffusion-limited cluster-cluster aggregation gelling process. The numerical results for $a$, $\xi$ and $\Sigma$ as a function of $\rho$ are discussed according to the initial aerogel density. The scaling theory is only fully recovered in the limit of very low density where the original values of $a$ and $\xi$ are well separated. These numerical results are compared with experiments on partially densified aerogels.Comment: RevTex, 17 pages + 6 postscript figures appended using "uufiles". To appear in J. of Non-Cryst. Solid

Mise en évidence d'une déformation paléocène en Bolivie du Sud : sa place dans l'évolution géodynamique des Andes Centrales

Cet événement tectonique paléocène et les premiers résultats des études sur les bassins intramontagneux andins remettent en question la conception classique d'une évolution andine crétacée-tertiaire comportant des phases compressives de courte durée séparées par de longues périodes de calme tectonique pendant lesquelles se produisait la sédimentation

« À nos destins promis ce souffle d’autres rives » : la figure du héros dans la recension ε du Roman d’Alexandre

L’article présente brièvement l’histoire du Roman d’Alexandre, depuis ses origines, vers le iiie siècle de notre ère, jusqu’à ses prolongements les plus modernes. Cette œuvre protéiforme n’a cessé d’évoluer au cours des siècles, s’adaptant progressivement au contexte byzantin et chrétien. L’exemple de la recension epsilon montre bien comment, à partir des données historiques mais surtout de l’imaginaire d’une société qui se cherche des héros, le personnage du roi de Macédoine devient le prototype d’une expérience humaine que la mort seule peut arrêter. Participant de différents genres littéraires, le texte d’epsilon transforme Alexandre en une figure qui, au-delà même du héros national, acquiert une dimension universelle, tant par la diversité des pays et des merveilles qu’il a vues, que par la simplicité de sa mort : accompagné d’un deuil cosmique qui rappelle celui de la Passion, mais loin de l’apothéose d’autres versions, Alexandre y meurt en souriant, comme nous aimerions nous-mêmes le faire peut-être.The article gives a brief presentation of the story of the Alexander Romance from its origins (around 3rd century A.D) to its most recent developments. That protean work kept evolving down the centuries, gradually adapting itself to the Byzantine and Christian context. The epsilon review, for instance, does indeed show how from historical data but mostly out of the imaginative need of a society in quest of heroes, the character of the King of Macedonia has become the prototype of a human experience which death alone can bring to a halt. Borrowing from various literary genres, the epsilon text transforms Alexander into a figure that transcends the character of a national hero and assumes a universal dimension, at once through the variety of countries and wonders he saw and the very simplicity of his demise: to the accompaniment of a cosmic sorrow reminiscent of Christ’s passion, but far from the apotheosis of other versions, Alexander dies with a smile on his face, the sort of death we too might wish to die