Aspherical structure of the core‐mantle boundary from PKP travel times

Maps of aspherical structure in the vicinity of the core‐mantle boundary have been constructed from a tomographic analysis of P′DF and P′AB travel times in the distance range 150°‐180°. These maps reveal long‐wavelength features not previously detected by mantle tomography. The peak‐to‐peak amplitude of these features is on the order of 1 s in vertical travel time, too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core‐mantle boundary. We hypothesize the existence of one or more chemical boundary layers whose large‐scale accumulations on the surface of the core may in some ways be analogous to continents at the earth's surface.