Thermal and Chemical Structures at the Core-Mantle Boundary: Implications for the Mantle Dynamics

The core-mantle boundary (CMB) represents the lower boundary layer of the actively convecting Earth's mantle and is structurally very complex. For example, large low shear wave velocity provinces (LLSVPs) but also small-scale ultra-low velocity zones (ULVZs) have been detected seismically. Thermal and chemical structures such as thermal plumes and thermochemical piles have been considered to explain the complexities. Both affect the dynamics of the Earth's mantle and its temporal evolution. But also the surface plates are an essential aspect of mantle convection that strongly influence the dynamics of the interior. Cold subducting slabs penetrating the lower boundary layer will also affect the CMB topography. To study the structure and dynamics of the lower mantle we use numerical thermochemical models of mantle convection with a complex rheological approach, including a strong temperature-, stress- and pressure-dependent viscosity. This allows for the investigation of thermal plumes and thermochemical piles in combination with plate-like surface motion and deep subduction. In thermochemical convection dense material is viscously trapped by the flow and piled beneath plumes. The presence of the dense layer reduces the mobility of the surface plates but during plate evolution we find a variety of plume classes (plumes, thermals, line-plumes) leaving a complex structure in the CMB topography.