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The mineral olivine dominates the composition of the Earth's upper mantle and hence controls its mechanical behaviour and seismic anisotropy. Experiments at high temperature and moderate pressure, and extensive data on naturally deformed mantle rocks, have led to the conclusion that olivine at upper-mantle conditions deforms essentially by dislocation creep with dominant [100] slip. The resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth1-4. The rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle5. But new high-pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle, but with a different slip direction. Here we show that this high-pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy, consistent with seismological observations below 250km depth. These results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below.
Despite the considerable effort to characterize olivine's deformation mechanisms over the past 30 yr, it is only recently that deformation experiments could be conducted at pressure-temperature conditions of the entire upper mantle6-8. New simple-shear experiments on olivine aggregates at 11 GPa and 1,400°C, conditions equivalent to those at depths of 330 km, have shown that deformation takes place by dislocation creep, with dominant activation of [001]{hk0} slip systems9, suggested by the concentration of [001] parallel to the shear direction and of [100] and [010] normal to the shear plane (Fig. 1). Transmission electron microscopy shows the exclusive presence of dislocations with [001] Burgers vectors in a screw orientation, compatible with [001](hk0) slip. Dominant [001] slip in the deep upper mantle requires re-evaluation of the interpretation of anisotropic physical properties. For instance, the fastest P-wave velocity will no longer parallel the shear direction as in an upper mantle deforming by dominant [100](010) slip, which is the assumption traditionally used in relating flow and seismic anisotropy in the mantle10,11.
Several lines of evidence point to seismic anisotropy decreasing with depth in the upper mantle. Most global one-dimensional models (PREM, IASP, AK135 and AK303) show horizontally propagating P waves travelling (at velocity v^sub PH^)...