Mantel Convection Models of the D" Region

We have investigated heterogeneity in the bottom D" region of Earth's lower mantle by means of numerical mantel convection modelling using a 2-D cylindrical geometry. The finite element model used, is based on the extended Boussinesq approximation, and includes the 660 km phase transition in the transition zone and the postperovskite transition in the D" region. We apply variable conductivity and phase dependent composite rheology in our model to account for increased conductivity and a higher tendency towards non-linear dislocation creep of postperovskite. We used particle tracers to reveal compositional variations related to compositional layering associated with a subducted oceanic lithospheric slab. The results show lens-like structures of postperovskite directly above the core-mantle boundary with an underlying thin layer of hot perovskite, similar to the structures related to the Cocos subduction, recently resolved in seismological studies (Van der Hilst et al.,2007). Oceanic basaltic crust and a harzburgitic layer become strongly folded, generating small scale structures at the bottom of the mantle. We show model predictions in terms of seismic reflectivity of the complex structures resulting from variations in temperature, phase and composition that will aid in the interpretation of (image-processed) seismic reflectivity cross sections of the D" region.