Structure Détaillée et Propriétés Sismiques des Diapirs de Manteau dans l'Ophiolite d'Oman

The Oman ophiolite offers a unique opportunity to observe a piece of oceanic lithosphere formed at a paleo fast spreading ridge, obducted on the continent and dissected by erosion, whereas oceanic rocks are difficult to study directly in situ in modern ridges. After considering how to discriminate preserved high temperature, ridge related structures from low temperature structures related to the obduction, we present the general structure of the Oman ophiolite. It shows that the ridge of origin itself was obducted. The paleoridge axis is characterised by an alignment of small mantle diapirs questioning the popular 2D passive mantle model proposed for fast spreading ridges. In the next chapters we present a detailed structural mapping of 2 of these on axis diapirs (Maqsad and Nakhl) and another one which seems aside the paleoridge axis (Mansah diapir). The Maqsad diapir is the best exposed, steep flow lineations are scattered in a 100 km2 area. Cross-sections show that the vertical flow rotates to the horizontal within a 500 meters thick melt impregnated transition zone and that there is a bulge of the Moho above this area. Horizontal radial flow is expelled from the diapir, including in a ridge strike direction. This with other lines of evidence such as the coupling between mantle and lower crustal structures demonstrate the existence of an active mantle flow at the ridge axis. Calculations predict that the diverging flow should become passive 10 km past the diapir limits, as illustrated by field data. Whereas the Nakhl diapir shares similar characteristics, the Mansah diapir is different in several points confirming the idea that this one was off axis. In particular, it displays discontinuous structures and shear zones on its sides, and diabase injections in its periphery, suggesting that it was intruding a pre-existing lithosphere. In a second part of the thesis, we study melt topology fossilised in Oman mantle rocks, with image analysis of several thin sections for each sample to get a 3D view. We show that " melt pockets " can be approximated by ellipsoids with a clear preferred orientation parallel to the lineation, and the aspect ratio between the 3 ellipsoid axis depending on the intensity of the deformation recorded in the olivine matrix. These data combined with measurements of petrofabrics are used to calculate anisotropic seismic properties of oceanic mantle rocks with various melt fractions. These calculations show the importance of the anisotropy in these dry and melt impregnated mantle rocks and suggest that melt fractions at the Moho level might be underestimated by seismic surveys, provided that this zone is studied with horizontal seismic rays. The last chapter of the thesis exposes the preliminary results of a " seismic modelling " of mantle diapirs using our structural observations and the anisotropic seismic velocities calculated. This leads to a discussion of how far our interpretation of Oman is applicable to the East Pacific Rise.L'ophiolite d'Oman permet d'étudier un morceau de lithosphère océanique formé à une dorsale rapide et échoué sur le continent. La structure générale permet de reconnaître la présence de plusieurs diapirs dont certains alimentaient la dorsale d'origine, partiellement obductée avec l'ophiolite. Le diapir de Maqsad est l'exemple type d'un diapir à l'axe. Les linéations verticales couvrent 100 km2 et tournent à l'horizontale au sein d'une zone de transition riche en magma, épaisse de 500 m, juste sous le Moho. Un flux forcé et radial est issu du diapir. Un calcul prédit que ce flux divergent devient passif à 10 km du diapir, comme le suggèrent nos données de terrain. Le diapir de Nakhl a des caractères similaires, tandis que le diapir de Mansah, éloigné de l'axe de la paleodorsale, est bordé par des zones de cisaillement et des injections de diabase qui attestent qu'il poinçonnait une lithosphère refroidie. Dans une seconde partie, les traces de magma dans les péridotites sont étudiées par analyse d'image sur des lames minces. On montre que les poches de liquide ont un allongement préférentiellement parallèle à la linéation et proportionnel à la force de la fabrique cristallographique de l'olivine. Ces données sont utilisées pour calculer les propriétés sismiques des échantillons. Nos résultats suggèrent que la quantité de liquide au Moho de la dorsale Est Pacifique peut être sous estimée à cause de l'anisotropie des roches. Enfin, une modélisation sismique d'un diapir, à partir de nos données structurales et des vitesses sismiques calculées permet de comparer les données d'Oman et de l'EPR