Three‐dimensional structure of the upper mantle beneath the Atlantic Ocean inferred from long‐period Rayleigh waves: 2. Inversion

The three‐dimensional shear wave velocity structure of the upper mantle beneath the Atlantic Ocean is investigated by inversion of fundamental mode of Rayleigh wave group and phase velocities. The results are reliable down to 300 km depth and improve upon the image of upper mantle structure given by global tomographic models. The inversion of two‐dimensional lateral distributions of group and phase velocities, regionalized without a priori constraints, shows that shear wave velocities are correlated with lithospheric age beneath the North Atlantic. Our data set fails to quantify the correlation between shear velocities and lithospheric age, because of the small lateral extent of young regions (<22 Ma) and of strong transverse gradients. In contrast with similar studies in the Indian and Pacific oceans, slow velocities associated with the location of the Mid‐Atlantic Ridge are restricted to the lithosphere. Therefore the Atlantic mid‐oceanic ridge behaves as a shallow structure, which is weakly related to deep upwelling asthenospheric flows. At depths greater than 200 km, the central part of the Atlantic is characterized by the highest velocity anomaly. This anomaly can be interpreted in different ways. Part of this anomaly might correspond to a negative ( V S H – V S V )/ V S V ratio implicitely included in the isotropic inversion. We also tentatively interpret this anomaly as an important contributor to a shallow degree two pattern of shear wave velocity distribution for the Atlantic area.