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Structure et deformation du manteau continental sud americain : apport de la tomographie en ondes de surface et de l'anisotropie sismique

Abstract : The formation of the South American continent involved the entire lithosphere and lasted more
than 3.5 Gy. The structure and deformation of the upper continental mantle have been studied in the present thesis at the continental scale, using surface wave tomography, and at the local scale of south-east Brazil, using shear-wave splitting of teleseismic SKS waves.
The Rayleigh waves anisotropic tomography has been used to determine the structure of the upper mantle beneath the continent and the surrounding oceanic basins. A good correlation exists between geological structures and velocity anomalies. The azimuthal anisotropy of Rayleigh waves suggests that
there is no evidence for large scale deformation below 200 km depth. This result contributes to constrain the anisotropy highlighted beneath south-east Brazil. In this region, shear-wave splitting revealed a direction of the fast S -polarisation plane parallel to the structural trend of both belts bounding the cratonic root. Delay times greater than 2s have been measured within the Alem Paraiba shear zone, east of the southern tip of the São Francisco craton. Such high delay times suppose : 1) the shear zone cuts through the entire lithosphere, 2) the lithosphere is characterised by an important intrinsic anisotropy, 3) the asthenosphere contributes to the delay time by the existence of two layers of anisotropy or consistent tectonic fabric between the lithosphere and the asthenosphere, suggesting an absence of coupling since neoproterozoic times.
The last part of this study deals with three-dimensional numerical modeling of the lithospheric deformation to study the mechanical coupling between the crust and the upper mantle, and the localisation of the deformation within shear zones.
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Submitted on : Thursday, September 23, 2004 - 4:00:25 AM
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Maggy Heintz. Structure et deformation du manteau continental sud americain : apport de la tomographie en ondes de surface et de l'anisotropie sismique. Géologie appliquée. Université Montpellier II - Sciences et Techniques du Languedoc, 2003. Français. ⟨tel-00006948⟩

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