A review of mechanisms generating seismic anisotropy in the upper mantle

Seismic anisotropy is a key observational tool for mapping flow in Earth's upper mantle. However, interpreting patterns of anisotropy relies on a firm understanding of the microphysical mechanisms that generate anisotropy. Here we discuss our current understanding of the generation of intrinsic and extrinsic seismic anisotropy in upper-mantle rocks. Intrinsic anisotropy results from the elastic anisotropy of the constituent minerals. We address the role of thermochemical conditions in modifying the manner in which these minerals align with the deformation reference frame, controlling the macroscopic anisotropy. Extrinsic anisotropy results from the composite behavior of a material composed of multiple phases. We examine the influence of mineralogical layering and the presence of a melt phase on anisotropy, including the interaction between extrinsic and intrinsic anisotropy. Finally, we compare and contrast existing methods to forward model the development of anisotropy in the upper mantle and demonstrate that current predictions suggest most observed anisotropy results from intrinsic anisotropy when fluids are not present.