Observations of seismic anisotropy are a powerful tool to explore deformation and flow in the deep mantle. Recent work has explored how flow in the deepest mantle interacts with major structures such as subducting slab remnants, large low velocity provinces (LLVPs), and mantle plumes. However, a comprehensive framework describing the patterns and drivers of flow in the mantle’s bottom boundary layer is only starting to emerge. Here we target the lowermost mantle beneath Australia and the surrounding region, which encompasses slab remnants, the edge of the Pacific LLVP, and a previously identified possible mantle plume that has not yet reached the surface. We apply a recently developed approach that relies on array processing of SmKS phases, which increases signal-to-noise ratios and enables analysis of low-amplitude phases such as S3KS. We supplement our differential SmKS splitting measurements with analyses of ScS phases that sample our study area. We infer strong seismic anisotropy in localized regions, including along the southwestern edge of the Pacific LLVP and in a region south of Australia that is dominated by high seismic velocities. To provide an interpretive framework for our observations, we compare them with the results of instantaneous mantle flow models and with whole-mantle S wave tomography models. Our results support an emerging view of lowermost mantle dynamics that involves slab-driven flow, interactions between mantle flow and structures such as LLVPs, and strong deformation at the root of mantle plumes, including a plume that has not yet reached the surface.

The dynamics of Earth’s D” layer at the base of the mantle plays an essential role in Earth’s thermal and chemical evolution. Mantle convection in D” is thought to result in seismic anisotropy; therefore, observations of anisotropy may be used to infer lowermost mantle flow. However, the connections between mantle flow and seismic anisotropy in D” remain ambiguous. Here we calculate the present-day mantle flow field in D” using 3D global geodynamic models. We then compute strain, a measure of deformation, outside the two large-low velocity provinces (LLVPs) and compare the distribution of strain with previous observations of anisotropy. We find that, on a global scale, D” material is advected towards the LLVPs. Strain is highest at the core-mantle boundary (CMB) and decreases with height above the CMB. Material outside the LLVPs mostly undergoes lateral stretching, with the stretching direction often, but not always, aligning with mantle flow direction. Strain generally increases towards the LLVPs and reaches a maximum at their edges, although models that consider recrystallization suggest that anisotropy may actually be weaker near LLVP edges. The depth-averaged strain in D” is >1.5 in almost all regions, consistent with widespread observations of seismic anisotropy. The mantle flow field and strain in D” outside of LLVPs are not very sensitive to LLVP density but are strongly controlled by local density and viscosity variations outside the LLVPs. Flow directions inferred from anisotropy observations often (but not always) align with predictions from geodynamic modeling calculations.

The seismic structure of the Earth’s lowermost mantle is characterized by two large low shear provinces (LLSVPs) beneath the Pacific and Africa. Surrounding the LLSVPs are regions with generally higher-than-average seismic velocities which are likely caused by ancient subducted slabs. However, seismic observations have also revealed relatively low-velocity structures in the subducting regions outside of the LLSVPs in the lowermost mantle. The question is what cause these low velocity structures? Here, three-dimensional, high-resolution geodynamic models are performed to study the thermal structure and dynamics of the Earth’s lowermost mantle outside of the LLSVPs. Widespread hot thermal anomalies, or thermal ridges as named here, are found in the relatively cold, downwelling regions of the lowermost mantle in our models, with linear, ridge-like morphology, elongated in the lateral mantle flow directions. A systematic parameter-space exploration shows that thermal ridges only form when there are gaps and thermal heterogeneities in the cold downwellings, and their formation is promoted by an increase of the core-mantle boundary (CMB) heat flux and Rayleigh number and by a reduction of the lowermost mantle viscosity. The low seismic velocity anomalies outside of the LLSVPs above the CMB may be related to the formation of thermal ridges in the lowermost mantle.