Advances in Mapping Lowermost Mantle Convective Flow With Seismic Anisotropy Observations

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. Earthquakes cause waves that travel through Earth's interior and are recorded by distant seismometers. These seismic waves behave differently depending on the material that they pass through, revealing Earth's material properties. At the very bottom of the mantle, seismic waves sometimes travel at different speeds depending on their direction. This material property is called seismic anisotropy and is caused by material deformation and flow. Global patterns of mantle flow, which is directly connected to surface processes such as the movement of tectonic plates or hotspot volcanism, can therefore be inferred from seismic anisotropy. Recent years have seen advances in anisotropy imaging in the lowermost mantle as well as in numerical calculations of flow in Earth's lowermost mantle. We review the methods that are used to infer lowermost mantle anisotropy. We further give an overview of previous results and interpretations, which include seismic anisotropy caused by upwelling plumes and ancient slab remnants. Future improvements in the fields of seismology, geodynamics, and mineral physics are needed to improve our understanding of global deep mantle flow. Measurement techniques for deep mantle anisotropy have been improved substantially in the last decade These improvements enable inferences of deep mantle flow with increasing confidence Knowledge of mantle dynamics elucidates the drivers of flow, relationships among structures, and Earth's dynamic evolution