WHERE CAN SEISMIC ANISOTROPY BE DETECTED IN THE EARTHS MANTLE - IN BOUNDARY-LAYERS

During the last 30 years, considerable evidence of seismic anisotropy has accumulated demonstrating that it is present at all scales, but no t in all depth ranges. We detail which conditions are necessary to det ect large-scale seismic anisotropy. Firstly, minerals must display a s trong anisotropy at the microscopic scale, and/or the medium must be f inely layered. Secondly, the relative orientations of symmetry axes in the different crystals must not counteract in destroying the intrinsi c anisotropy of each mineral, and there must be efficient mechanisms o f orientation of minerals and aggregates. Finally, the strain field mu st be coherent at large scale in order to preserve long wavelength ani sotropy. Part of shallow anisotropy can be related to the past strain field (frozen-in anisotropy), however the deep anisotropy is due to th e present strain held. All these conditions are fulfilled only in boun dary layers of convective mantle. We review in this paper, the seismic data sets which provide insight into the location at depth of large-s cale anisotropy from the D''-layer up to the lithosphere. In addition to the well-documented seismic anisotropy in the lithosphere and asthe nosphere, there is new evidence of seismic anisotropy in the upper (40 0-660 km) and lower (660-900 km) transition zones and in the D''-layer . Nonetheless the bulk of the lower mantle seems close to isotropy. If we assume the hypothesis that seismic anisotropy is associated with b oundary layers in convective systems, these observations strongly sugg est that the transition zone is a boundary layer which makes the pasag e of matter between the upper and the lower mantle difficult. However, this general statement does not rule out flow circulation between the upper and lower mantles. Finally, the geophysical, mineral physics an d geological applications are briefly reviewed. An intercomparison bet ween surface wave anisotropy and body-wave anisotropy data sets is pre sented. We discuss the scientific potential of seismic anisotropy and how it makes it possible to gain more insight into continental root, d eformation and geodynamics processes.