Geodynamical modeling and multiscale seismic expression of thermo-chemical heterogeneity and phase transitions in the lowermost mantle

The D″ region at the base of the mantle is characterized by seismologically inferred 3D heterogeneity, including multiple interfaces, localized low velocity zones, and anisotropy. The occurrence of the post-perovskite (PPV) phase transition with a steep Clapeyron slope of 11.5-13 MPa/K, close to the core-mantle boundary, is a prime candidate for explaining observed seismic layering in the D″. To examine the effect of the PPV phase transition on seismic structure we have carried out finite-element simulations with high-resolution (up to 3 km) in a cylindrical geometry. The rheology of the mantle has both Newtonian diffusion and non-Newtonian components, with a much greater propensity to non-Newtonian for PPV. From the temperature output we computed the 2D variations in shear wavespeed using a seismic equation of state based on mineral physics data. We then use a wave-packet decomposition of the wavespeed variations, which accounts for the events-to-stations illumination to obtain the seismic expressions of the geodynamically modeled structures. The results reveal lens-shaped PPV structures, much like the patterns obtained from seismic imaging with ScS data. A similar analysis of thermo-chemical anomalies from a subducting slab with crustal material shows that structures with a spatial scale of MORB crustal thickness produce characteristic features reminiscent of the small scale detail in the seismic imaging results. These experiments illustrate the high sensitivity of the seismic expression near the CMB to the wavespeed coefficients of the oceanic crust under high pressure conditions.