Lowermost mantle deformation beneath Australia linked to deep mantle upwellings and putative remnant slab material

Summary Seismic anisotropy observations can constrain flow and deformation in the lowermost mantle (D″). D″ deformation is often attributed either to changes of mantle flow from mostly horizontal to upwelling near deep mantle plumes or edges of the two antipodal large low-velocity provinces (LLVPs), or to strong deformation in slab-dominated deep mantle regions. A unified understanding of deep mantle flow and its drivers, however, is still developing. We investigate D″ anisotropy beneath Australia using a novel approach combining array processing and shear-wave splitting measurements applied to core-traversing seismic waves which reflect off the underside of the core-mantle boundary up to two times (called SKS, S2KS, and S3KS). Strong differences in splitting between pairs of phases that sample the upper mantle in a similar way but sample different portions of the D″ layer (e.g., SKS-SKKS or SKS-S3KS) can be interpreted as due to a contribution from lowermost mantle anisotropy to the splitting of one or both phases. Using this approach, we detect strong anisotropy in two locations at the southwestern edge of the Pacific LLVP, which we attribute to a change from mostly horizontal to upwelling flow. One of these locations coincides with a previously suggested mantle plume that has not yet reached the surface. We also identify anisotropy south of Australia, where seismic velocities are faster than average and slab remnants may be present. Beneath much of the Australian continent itself, we see little or no evidence for splitting discrepancies, which may be evidence for isotropic, or only weakly anisotropic, lowermost mantle. Our study region thus uniquely showcases seismic anisotropy in diverse deep mantle environments, associated with a possible deep mantle plume, the edge of the Pacific LLVP, and potential remnant slab material.