ANISOTROPIC TOMOGRAPHY OF THE ATLANTIC-OCEAN FROM RAYLEIGH SURFACE-WAVES

The depth extent of the Mid Atlantic Ridge and the role of hotspots in the Atlantic opening are still a matter of debate. In order to constr ain the structure and the geodynamic processes below the Atlantic Ocea n, we provide the first anisotropic phase velocity maps of this area, obtained at a regional scale. We have determined Rayleigh wave phase v elocities along 1311 direct epicentre to station paths. For each path, phase velocities are calculated by a technique of cross-correlation w ith a synthetic seismogram. These phase velocities are corrected for t he effect of shallow layers. They are then inverted, without a priori constraints, to obtain maps of the lateral variations of the anisotrop ic phase velocities in the period range 50-250 s, The ridge axis corre sponds to a low velocity anomaly, mainly visible at short periods. A g ood correlation between hotspot locations and low velocity anomalies i s obtained for the whole period range. Furthermore, a low velocity ano maly elongated along a North-South direction is visible for every peri od and seems to be correlated with hotspot positions. On average, the North Atlantic is associated with higher velocities than the South Atl antic. The shields below Canada, Brazil and Africa display high veloci ty anomaly at short periods and only the Brazilian and African shields are still visible for a period of 200 s, thus suggesting that the Can adian shield is a shallower structure. The maps of phase velocity anis otropy under the Atlantic Ocean are interpreted in the Mid-Atlantic ar ea, where we have the best resolution, Close to the ridge, the fast ax is of Rayleigh wave phase velocity is found perpendicular to the ridge axis. A comparison of anisotropy directions and plate motion shows th at seismic anisotropy integrates also deeper phenomena such as mantle convection. (C) 1998 Elsevier Science B.V. All rights reserved.