Superplasticity in Earth's Lower Mantle: Evidence from Seismic Anisotropy and Rock Physics

In contrast to the upper mantle, the lower mantle of the Earth is elastically nearly isotropic, although its dominant constituent mineral [(Mg,Fe)SiO 3 perovskite] is highly anisotropic. On the basis of high-temperature experiments on fabric development in an analog CaTiO 3 perovskite and the elastic constants of MgSiO 3 perovskite, the seismic anisotropy was calculated for the lower mantle. The results show that absence of anisotropy is strong evidence for deformation by superplasticity. In this case, no significant transient creep is expected in the lower mantle and the viscosity of the lower mantle is sensitive to grain size; hence, a reduction in grain size will result in rheological weakening.