Abstract

Perhaps the least ambiguous signal that the mantle is convecting comes from observations of seismic anisotropy---the variation of wave speed with direction---which must arise due to the ordering of material as deformation occurs. Therefore significant effort has been made over many years to infer the direction and nature of mantle flow from these data. Observations have focussed on the boundary layers of the mantle, where deformation is expected to be strongest and where anisotropy is usually present. While prospects for mapping flow seem good, the lack of knowledge of several key issues currently holds progress back. These include the cause of anisotropy in the lowermost mantle, the causative material's response to shear, and the single-crystal or -phase seismic properties of the causative materials. In this chapter we review recent observations of lowermost mantle anisotropy, constraints on mineral elasticity and deformation mechanisms, and challenges in linking geodynamic modelling with seismic observations.