ABSTRACT
The lowermost part of the Earth's mantle-known as Dâ³-shows significant seismic anisotropy, the variation of seismic wave speed with direction. This is probably due to deformation-induced alignment of MgSiO(3)-post-perovskite (ppv), which is believed to be the main mineral phase present in the region. If this is the case, then previous measurements of Dâ³ anisotropy, which are generally made in one direction only, are insufficient to distinguish candidate mechanisms of slip in ppv because the mineral is orthorhombic. Here we measure anisotropy in Dâ³ beneath North and Central America, where material from subducting oceanic slabs impinges on the core-mantle boundary, using shallow as well as deep earthquakes to increase the azimuthal coverage in Dâ³. We make more than 700 individual measurements of shear wave splitting in Dâ³ in three regions from two different azimuths in each case. We show that the previously assumed case of vertical transverse isotropy (where wave speed shows no azimuthal variation) is not possible, and that more complicated mechanisms must be involved. We test the fit of different MgSiO(3)-ppv deformation mechanisms to our results and find that shear on (001) is most consistent with observations and the expected shear above the core-mantle boundary beneath subduction zones. With new models of mantle flow, or improved experimental determination of the dominant ppv slip systems, this method will allow us to map deformation at the core-mantle boundary and link processes in Dâ³, such as plume initiation, to the rest of the mantle.
