Lowermost Mantle Flow at Thermochemical Piles Constrained by Shear Wave Anisotropy: Insights From Combined Geodynamic and Mantle Fabric Simulations at Global Scale

Abstract Seismic anisotropy is observed in the lowermost few hundred kilometers of the mantle. This anisotropy likely signifies strong deformation, possibly caused by mantle flow interacting with the edges of Large Low‐Shear‐Velocity Provinces (LLSVPs) or by the ascent of mantle plumes originating near these regions. In this study, we explore generation of seismic anisotropy as a result of deformation at LLSVPs and the flow behavior of the lowermost mantle using 3‐D global models of compressible mantle convection in the geodynamic modeling software ASPECT, coupled with the mantle fabric simulations code ECOMAN. In our simulations, we initiate the LLSVPs as a 100‐km thick chemically flat layer in the lowermost mantle. Our models include a plate reconstruction spanning the Pangea breakup over the past 250 million years. We tested several geodynamic models with varying compositional densities and viscosity ranges for the LLSVPs and thermal conductivities of the ambient mantle and computed the mantle fabrics for each model. Our findings align with previous shear wave radial anisotropy seismic tomography models at the lowermost mantle, where fast vertically polarized shear waves are observed near the LLSVPs. Our modeled anisotropy is mostly accumulated at the edges of the LLSVPs, which is consistent with previous regional seismic anisotropy observations. Our preferred seismic anisotropy results are for LLSVPs compositionally 100 times more viscous and 2% denser than the surrounding mantle. An increased compositional viscosity within the LLSVP strengthens its margins, leading to vertical deflection of mantle flow along its boundaries.