SEISMIC ANISOTROPY IN OCEAN BASINS - RESISTIVE DRAG OF THE SUBLITHOSPHERIC MANTLE

We use finite-element models to evaluate the effect of asthenospheric strain associated with the motion of an oceanic plate over a presumed stationary mantle on shear wave splitting observations. Modeled veloci ty profiles display a clear strain localization within a horizontal sh ear zone several tens of kilometers wide developed between an almost r igid mechanical lithosphere and a mildly deformed upper mantle. For yo ung oceanic lithosphere, cooling results in migration of the maximum s heer strain rate towards deeper levels and progressive widening of the shear zone. Shear strain accumulates with plate displacement. The def ormed layer thickness depends on plate age and velocity. Seismic aniso tropy depends on the preferred orientations of olivine developed, and thus on the strain field, For a ray propagating vertically (e.g., SKS) the fast wave will be polarized parallel to the Bow direction, i.e., the APM. The delay time will depend on the thickness of the sheared la yer and on its intrinsic anisotropy. It will therefore increase away f rom the ridge at a progressively decreasing rate and depend on the abs olute plate velocity. The estimated anisotropies agree with seismic an isotropy measurements in old domains of ocean basins, suggesting that, away from mid-ocean ridges, this simple model may provide a first app roximation of the mechanism responsible for the observed seismic aniso tropy.