Upper mantle seismic anisotropy resulting from pressure-induced slip transition in olivine

Seismic anisotropy in the oceanic lithosphere results from flow-induced crystallographic preferred orientation of dry olivine during lithosphere creation. Recent experiments, however, showed that high water activity changes the flow mechanisms of olivine and hence the crystallographic preferred orientation, better explaining the seismic anisotropy in the mantle wedge above subduction zones. Whether changes in the crystallographic preferred orientation of olivine are unique to the effects of water has become controversial and is critical to resolve. Here we report low-stress, high-strain experiments on typical dry mantle rock at high pressures and temperatures, showing that at similar to 3 GPa, pressure induces the same profound transition in olivine crystallographic preferred orientation that is produced by high water activity at lower pressure. One important consequence for global tectonics is that alignment of fast seismicwaves parallel to trenches beneath subducting slabs probably reflects entrainment of sub-lithospheric mantle in the direction of subduction, rather than trench-parallel flow as interpreted at present. From the large variety of crystallographic preferred orientations in olivine in both experiments and natural rocks, we infer that in addition to the pressure-induced changes in olivine slip systems implied here, there are probably further changes in slip systems at higher pressure and temperature.