Patterns of deformation in the deepest mantle linked to ancient subduction

Shear wave speeds in Earth's deepest mantle (D") that vary with wave propagation and polarization direction -- a property called seismic anisotropy -- offer insights into mantle convection. To date, global patterns of D" anisotropy have been mostly derived from long wavelength radially anisotropic tomography models, which often disagree except for the large-scale degree-2 pattern. Here, we present 70,000 differential splitting measurements from seismic waves that traverse Earth's mantle and core, sampling nearly 75% of D", including most seismically faster regions. We conduct detailed synthetic tests to demonstrate which splitting measurements indicate the presence of lowermost mantle anisotropy. Evidence for D" anisotropy is found in about two thirds of our sampled area, more than doubling the area in which seismic anisotropy has been detected using shear-wave splitting measurements. Inferred deformation is strong within and around ancient slab remnants, which likely have lower temperatures than the ambient mantle. This is consistent with the crystallographic orientation of postperovskite (pPv) being an important contributor to the new maps of lowermost mantle anisotropy. Our observations suggest a close link between the subduction of tectonic plates and convective flow in the deepest mantle.