Evidence for small-scale thermal convection near the CMB from joint multi-scale analysis of seismic images and finite element modelling

The discovery of the post-perovskite phase near the CMB has stirred a debate about the relative effects of thermo-chemical changes and distortion of the phase boundary by thermal convection. Previous work on chemical buoyancy in the D" layer considered relatively long wavelength solutions with horizontal wavelengths greater than 600 km. We consider horizontal (vertical) wavelengths as short as 10 km (4 km) to determine if thermal convection with a post-perovskite transition is compatible with recent multi-scale seismic imaging with ScS and SKKS precursors and coda, or if composition must be considered. We investigate the interaction of subducting lithospheric slabs with the D" region, accounting for the post-perovskite phase transition, by numerical simulation with a cylindrical finite element convection model. The nodal point resolution of the model is about 3 km (10 km) in the vertical (horizontal) direction (which is slightly smaller than the spatial resolution afforded by ScS and SKKS scattering). The slope and CMB intercept of the ppv Clapeyron curve are taken to be 11 MPa/K and 3500 K, respectively, while the CMB temperature is set to 4000 K. Phase-dependent composite non-linear rheology is used with a greater propensity for non-linear creep in ppv regions. Our results show ppv lens-like structures of ~250 km thickness and > 1000 km lateral extent near remnants of cold downwellings; mantle regions where hot plumes emerge from the CMB are pv- rich. The cold ppv regions deform according to non-Newtionian flow and have lower viscosity than the hotter pv plumes, which have linear (Newtonian) rheology. In the numerical models, the base of the ppv lenses is inside the thermal boundary layer near the CMB and is marked by a stronger S velocity gradient than the top. The evidence is built by subjecting the finite element modelling - upon subtracting a tomographic S velocity background model - to a decomposition into wave packets, restricted to (about 6) scales and orientations proven to be resolved by the multi-scale imaging of large sets of global network data. This resolution is derived from a matrix representation of the generalized Radon transform, making use of the concentration of wave packets, and opens new pathways for joint seismic and geodynamic analysis of the D" region.