HR: 08:35h
AN: T31D-03 INVITED     [Abstracts]
TI: Global Model of Seismic Anisotropy and Geodynamics
AU: * Montagner, J
EM: jpm@ipgp.jussieu.fr
AF: Dept. Seismology, IPG Paris, 4 Place Jussieu, case 89, Paris, 75252 France
AU: Stutzmann, E
EM: stutz@ipgp.jussieu.fr
AF: Dept. Seismology, IPG Paris, 4 Place Jussieu, case 89, Paris, 75252 France
AU: Beucler, E
AF: Earth Sciences Dept., University of Oxford, Oxford, Ox1 3PR United Kingdom
AU: Sicilia, D
AF: Dept. Seismology, IPG Paris, 4 Place Jussieu, case 89, Paris, 75252 France
AU: Sebai, A
AF: Dept. Seismology, IPG Paris, 4 Place Jussieu, case 89, Paris, 75252 France
AB: Seismic anisotropy is not a second order effect though it was often neglected due to the inherent heavy mathematical and computational tools needed to describe and model its effects on seismic waves. It turns out that it is a good marker of large scale deformations and that it is reflecting some inherent organization of the matter, contrarily to isotropy. The uppermost mantle down to 410 km is the depth range where the existence of seismic anisotropy is now widely recognized and well documented. Azimuthal variations have been found for body waves and surface waves in different areas of the world. The application of seismic anisotropy to geodynamics in the upper mantle is straightforward, if we assume that fast-polarization axis of mineralogical assemblages (primarily of $\alpha$-olivine) is in the flow plane parallel to the direction of flow. Seismic anisotropy in the mantle is therefore reflecting the strain field prevailing in past (frozen-in anisotropy) for shallow layers or present convective processes in deeper layers. From the global geodynamics point of view, seismic anisotropy makes it possible to define the root of continents, to investigate the coupling between the lithosphere and the rest of the mantle and more generally to gain insight into mantle convection. Oceans are the areas where Plate tectonics applies almost perfectly. In the Pacific ocean, the map of the azimuthal anisotropy at 100 km shows that it is very large along spreading ridges with a large asymmetry for the East Pacific rise. The direction of anisotropy is in very good agreement with plate motion. The anisotropy is large as well in the middle of the Pacific plate, but it can be observed that there is a line of very small azimuthal anisotropy almost parallel to the EPR and another one between EPR and Tonga- Kermadec subduction zone. These linear areas of small anisotropy were coined Low Anisotropy Channel by Montagner (EPSL, 2002). They are presumably related to cracking within the Pacific plate and/or to secondary convection within and below the rigid lithosphere, predicted by numerical and analog experiments. These new features provide strong constraints on the decoupling between plate and asthenosphere. The existence and location of these LACs might be related to the current active volcanoes and hotspots (possibly plumes) in Central Pacific. LACs, which are dividing the Pacific Plate into smaller units, might indicate a future reorganization of plates with ridge migrations in the Pacific Ocean. Below continental lithosphere, it is also observed significant azimuthal anisotropy which is reflecting asthenospheric flow. Some recent results beneath the eastern Africa will be presented and also show that this flow is highly perturbed by the presence of the Afar hotspot upwelling and might induce upwellings (with babyplumes) and downwellings at large distance (several thousands kilometers) from Afar. In conclusion, the scientific potential of seismic anisotropy is enormous and largely unexploited. It provides a new dimension in the investigation of processes of our dynamic Earth.
DE: 8180 Tomography
DE: 7255 Surface waves and free oscillations
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting