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Abstract 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″ materials are advected toward the LLVPs. The strains of D″ materials generally increase with time along their paths toward the LLVPs and toward deeper depths, but regions far from LLVPs may develop relative high strain as well. Materials in D″ outside the LLVPs mostly undergo lateral stretching, with the stretching direction often aligning with mantle flow direction, especially in fast flow regions. In most models, the depth‐averaged strain in D″ is >0.5 outside the LLVPs, consistent with widespread observations of seismic anisotropy. Flow directions inferred from anisotropy observations often (but not always) align with predictions from geodynamic modeling calculations. 

Abstract The Yellowstone region (western United States) is a commonly cited example of intraplate volcanism whose origin has been a topic of debate for several decades. Recent work has suggested that a deep mantle plume, rooted beneath southern California, is the source of Yellowstone volcanism. Seismic anisotropy, which typically results from deformation, can be used to identify and characterize mantle flow. Here, we show that the proposed plume root location at the base of the mantle is strongly seismically anisotropic. This finding is complemented by geodynamic modeling results showing upwelling flow and high strains in the lowermost mantle beneath the Yellowstone region. Our results support the idea that the Yellowstone volcanism is caused by a plume rooted in the deepest mantle beneath southern California, connecting dynamics in the deepest mantle with phenomena at Earth's surface. 

Abstract Convective flow in the deep mantle controls Earth's dynamic evolution, influences plate tectonics, and has shaped Earth's current surface features. Present and past convection‐induced deformation manifests itself in seismic anisotropy, which is particularly strong in the mantle's uppermost and lowermost portions. While the general patterns of seismic anisotropy have been mapped for the upper mantle, anisotropy in the lowermost mantle (called D′′) is at an earlier stage of exploration. Here we review recent progress in methods to measure and interpret D′′ anisotropy. Our understanding of the limitations of existing methods and the development of new measurement strategies have been aided enormously by the availability of high‐performance computing resources. We give an overview of how measurements of seismic anisotropy can help constrain the mineralogy and fabric of the deep mantle. Specifically, new and creative strategies that combine multiple types of observations provide much tighter constraints on the geometry of anisotropy than have previously been possible. We also discuss how deep mantle seismic anisotropy provides insights into lowermost mantle dynamics. We summarize what we have learned so far from measurements of D′′ anisotropy, how inferences of lowermost mantle flow from measurements of seismic anisotropy relate to geodynamic models of mantle flow, and what challenges we face going forward. Finally, we discuss some of the important unsolved problems related to the dynamics of the lowermost mantle that can be elucidated in the future by combining observations of seismic anisotropy with geodynamic predictions of lowermost mantle flow. 

Abstract We compile and make publicly available a global digital database of body wave observations of seismic anisotropy in the D′′ layer, grouped using the method used to analyze deep mantle anisotropy. Using this database, we examine the global distribution of seismic anisotropy in the D′′ layer, evaluating the question of whether seismic anisotropy is more likely to be located at the edges of the two large‐low velocity provinces (LLVPs) in Earth's mantle than elsewhere. We show that this hypothesis lacks statistical justification if we consider previously observed lowermost mantle anisotropy, although there are multiple factors that are difficult to account for quantitatively. One such factor is the global lowermost mantle ray coverage for different phases that are commonly used to detect deep mantle anisotropy in shear wave splitting studies. We find that the global ray coverage of the relevant seismic phases is highly uneven, with LLVP edges and their interiors less well‐sampled than the global average. 

Abstract Two large low velocity provinces (LLVPs) are observed in Earth's lower mantle, beneath Africa and the Pacific Ocean, respectively. The maximum height of the African LLVP is ∼1,000 km larger than that of the Pacific LLVP, but what causes this height difference remains unclear. LLVPs are often interpreted as thermochemical piles whose morphology is greatly controlled by the surrounding mantle flow. Seismic observations have revealed that while some subducted slabs are laterally deflected at ∼660–1,200 km, other slabs penetrate into the lowermost mantle. Here, through geodynamic modeling experiments, we show that rapid sinking of stagnant slabs to the lowermost mantle can cause significant height increases of nearby thermochemical piles. Our results suggest that the African LLVP may have been pushed more strongly and longer by surrounding mantle flows to reach a much shallower depth than the Pacific LLVP, perhaps since the Tethys slabs sank to the lowermost mantle. 

SUMMARY Mantle plumes form from thermal boundary layers, such as Earth's core–mantle boundary. As plumes rise towards the surface, they are laterally deflected by the surrounding mantle flow that is governed by deep mantle density and viscosity structures. The lateral motions of mantle plumes carry information of deep mantle structure and dynamics and are used to setup reference frames by which absolute plate motions are reconstructed. In this study, we compare two methods to compute deep mantle flow and lateral motion of plumes. In mantle convection (MC) models, the mantle flow field and lateral motions of plumes are determined by solving conservation equations forward-in-time from given initial conditions. In plume advection (PA) models, approximate viscosity and present-day density structures are used to calculate present-day mantle flow which is then propagated backward-in-time assuming zero thermal diffusion, and plume conduits are represented by continuous lines and are passively advected within the background mantle flow. The question is how assumptions in PA models influence the predictions of deep mantle flow and plume lateral motions. Here, we perform purely thermal MC models and thermochemical MC models with intrinsically dense materials in the lowermost mantle. The deep mantle flow and plume lateral motions are determined accurately in each MC model. We also perform PA models using the approximated present-day viscosity and temperature structures in these MC models. We find that PA models without considering temperature-dependence of viscosity and/or only using long wavelength present-day temperature structure (up to degree 20) often lead to an average of ∼50–60 per cent and ∼60–200 per cent differences of present-day mantle flow velocities than purely thermal MC models and thermochemical MC models, respectively. By propagating inaccurate flow fields backward-in-time in PA models often cause even larger errors of mantle flow velocities in the past. Even using the same parameters and starting from the same present-day mantle flow fields as in MC models, the PA models still show an average of ∼10–30 per cent misfit of mantle flow velocities after ∼40 Ma. In addition, we show that errors of mantle flow fields in PA models can cause ∼100–600 per cent differences of plume lateral motions than that constrained in MC models in the past 60 Ma. Even we use the mantle flow in MC models to advected virtual plumes in PA models, the virtual plumes could still show ∼50–300 per cent difference of lateral motions than dynamic plumes in MC models if the virtual plumes do not start with the same locations and/or shapes as plumes in MC models. We also find virtual plumes in PA models initiated at different locations and/or with different shapes can be later advected to similar locations, suggesting that the lateral motions of plumes in PA models can be non-unique. Therefore, it is important to consider the build-in assumptions of PA models when interpreting their predictions on deep mantle flow field and plume lateral motions. The accuracy of PA models would improve as we gain better understanding on Earth's deep mantle structure and dynamics.