Lateral variations in CMB heat flux and deep mantle seismic velocity caused by a thermal–chemical-phase boundary layer in 3D spherical convection

Numerical simulations of thermo-chemical, multi-phase, compressible mantle convection in a three-dimensional spherical shell are used to investigate the relationship between lateral variations in seismic shear-wave velocity Vs above the core–mantle boundary (CMB) and lateral variations in heat flux across the CMB (qCMB), when compositional variations and the post-perovskite phase transition are included. For simple thermal convection, the Vs–qCMB relationship is reasonably but not perfectly linear. The post-perovskite transition introduces a non-linearity that amplifies fast Vs anomalies in cold regions, but there is still a unique mapping between δVs and qCMB. Lateral variations in composition such as piles of dense material introduce another non-linearity that affects hot upwelling regions, and introduces a non-uniqueness in δVs–qCMB if the dense material (e.g., MORB) is seismically fast compared to the surrounding material. In this case, dense piles are ringed by sharp, low-Vs anomalies. If the CMB is covered by a global dense layer than variations in δVs and qCMB are reduced but so is the mean value of qCMB. In all cases, the peak-to-peak lateral variation in qCMB is similar to or larger than twice the mean value, which might create problems for generating a dynamo according to existing numerical dynamo simulations. Analytical scalings are developed to explain the observed trends.