Development of Anisotropic Structure by Solid-State Convection in the Earth’s Lower Mantle

Recent seismological observations show patches of highly anisotropic regions in the bottom of an otherwise isotropic lower mantle 1-4. These regions likely correspond to paleo-subduction or plume upwelling, but the exact cause for anisotropy is unknown. Both shape-preferred orientation (SPO) of elastically heterogenous materials 5 and lattice-preferred orientation (LPO) 6-8 have been proposed. Both of these mechanisms imply that large strain deformation occurs in anisotropic regions but the geodynamic implications are different. SPO as a mechanism would imply the presence of large elastic (and hence chemical) heterogeneity whereas LPO as a mechanism implies deformation at high stresses. Here we show, based on numerical modeling incorporating mineral physics of elasticity and LPO development, that the development of LPO associated with slab deformation in the deep lower mantle provides a likely mechanism for the presence of strong anisotropy in the circum-Pacific region. In this model, anisotropy in these regions is caused by high-stress, large-strain deformation due to the collision of subducted slabs with the core-mantle-boundary in the recent past.