Seismic constraints on the depth and composition of the mantle keel beneath the Kaapvaal craton

S–P travel-time residuals and receiver-function images are used to infer the Vp/Vs (compressional to shear wave velocity) ratio of the lithospheric mantle beneath southern African and the topography of the underlying 410-km discontinuity. Low Vp/Vs ratios provide evidence independent of geochemical observations for a highly depleted root (Mg#∼92–94) beneath the Kaapvaal craton. The receiver-function images, on the other hand, consistently show a flat 410-km discontinuity beneath the entire array. This observation, after combined with the results of geodynamical modeling, allows us to place limits on the thickness of this chemical boundary layer, which is between ∼160 and ∼370 km.