While the superficial expression of oceanic ridges is generally symmetric, their deeper roots may be asymmetric. Based on a surface wave tomographic, we construct a global cross section parallel to the tectonic equator. VS indicate a difference between the western and eastern flanks of the three major oceanic rift basins. In general, the western limbs have a faster velocity and thicker lithosphere relative to the eastern or northeastern one, whereas the upper asthenosphere is faster in the eastern limb than in the western limb. We interpret the difference between the two flanks as the combination of mantle depletion along the oceanic rifts and of the westward migration of the ridges and the lithosphere relative to the mantle. The low-velocity layer in the upper asthenosphere is assumed to represent the decoupling between the lithosphere and the underlying mantle. These results could be explained in the frame of the westward drift of the lithosphere relative to the underlying mantle.
