Deformation of the lowermost mantle from seismic anisotropy

Understanding the lowermost part of the Earth’s mantle—known as D''—can help us investigate whole-mantle dynamics, core-mantle interactions and processes such as slab deformation in the deep Earth. D'' shows significant seismic anisotropy, the variation of seismic wave speed with direction. This is likely due to deformation- induced alignment of MgSiO3-post-perovskite (ppv), believed to be the main mineral phase present in the region; however if this is the case, then previous measurements of D'' 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 D'' beneath North and Central America, where slab material impinges6 on the core-mantle boundary (CMB), using shallow as well as deep earthquakes to increase the azimuthal coverage in D!. We make >700 individual measurements of shear wave splitting in D'' in three regions from two different azimuths in each case, and we show that the previously-assumed case of vertical transverse isotropy (VTI, where wave speed shows no azimuthal variation) is not possible; more complicated mechanisms must be involved. We test the fit of different MgSiO3-ppv deformation mechanisms to our results and find that shear on (001) is most consistent with observations and expected shear above the CMB beneath subduction zones. With new models of mantle flow, or improved experimental evidence of which ppv slip systems dominate, this method will allow us to map deformation at the CMB and link processes in D'', such as plume initiation, to the rest of the mantle.