Résumé
The results are presented of new simple-shear experiments on olivine aggregates at 11 GPa and 1400 degrees C (equivalent to 330 km depth), and show that deformation occurs by dislocation creep with dominant activation of [001] {hko} slip systems. These systems are indicated by the concentration of [001] parallel to the shear direction and of [100] and [010] perpendicular to the shear plane. TEM show dislocations with [010] Burgers vectors in a screw orientation compatible with [010] (hko) slip. Modelling results show that this dominant activity of olivine at high P is compatible with seismic data indicating weak anisotropy in the upper mantle below 330 km, and that this upper mantle layer below 250 km is actively deforming by dislocation creep while bottom layers may be coupled down to 400 km depth.