Exploring the Development of Shear Wave Radial Anisotropy in the Lower Mantle due to Slab-induced Plume Generation from LLSVPs

Seismic anisotropy, which involves directionally dependent wave propagation, is likely to occur in the lowermost few hundreds km of the mantle, especially at the edges of Large Low Shear Velocity Provinces (LLSVPs). This anisotropy may be indicative of significant deformation, potentially due to mantle flow interacting with the sides of these provinces or the generation of mantle plumes. In this study, we investigate subducted slab induced plume generation from an LLSVP boundary and the flow behaviour of the lower mantle using compressible 2-D and 3-D mantle convection models in the geodynamic modeling software ASPECT combined with mantle fabric simulation in ECOMAN. In our geodynamic simulation, we assume that the LLSVPs are chemically distinct piles with intrinsically high viscosity. We use the Clapeyron slope of the phase transition from Bridgmanite to post-Perovskite from the previous mineralogical study by Oganov Ono (2004) in the mantle fabric calculation. Modeling lattice preferred orientation of Bridgmanite and post-Perovskite in the lower mantle reveals that the lower mantle is overall isotropic except the regions of plume conduits and the surroundings of the subducted slab where vertically polarized shear wave (Vsv ) is faster. The generation of anisotropy are caused by the accumulation of high finite strain in these regions. The bottom 300 km of the lower mantle is characterized by fast horizontally polarized shear wave (Vsh ) beneath the subducted slab which deflects to fast Vsv at the margins of the LLSVPs due to the rheological contrast between the highly viscous LLSVP and less viscous ambient mantle. Our result shows that six possible slip systems 100(010), 100(001), 010(100), 001(100), 110(-110) and -110(110) of Bridgmanite and the slip system 100(001) of post-Perovskite can produce a fast Vsv in the plume generation zones where post-Perovskite transforms to Bridgmanite and fast Vsh at the base of the subducted slab where post-Perovskite is preserved in the D. However, our models do not show anisotropy inside of the LLSVPs and the subducted slab, possibly because of their high viscosity. Our findings are comparable with the previous seismic observations beneath the Iceland plume where Vsv > Vsh and the slab-driven flow at the base of the mantle beneath the northeastern Pacific Ocean where Vsh > Vsv .