Direct measures of lateral velocity variation in the deep Earth

Current tomographic models of the Earth display perturbations to a radial stratified reference model. However, structures in the deep mantle that are chemically dense with low Rayleigh numbers can develop enormous relief, perhaps with boundaries closer to vertical than to radial. Such features are hard to detect with present tomographic modeling techniques because the timing anomalies are based on long-period filtered waveforms with complexity removed. Here we develop a new tool for processing array data on the basis of a decomposition referred to as a multipath detector, which can be used to distinguish between horizontal structure (in-plane multipathing) and vertical (out-of-plane multipathing) directly from processing array waveforms. A lateral gradient coefficient based on this detector provides a direct constraint on the sharpness of the boundaries and material properties. We demonstrate the usefulness of this approach by processing samples of both P and S data from the Kaapvaal array in southern Africa, which are compared with synthetic predictions from a metastable dynamic model containing sharp edges. Both data and simulations produce timing gradients larger than 2 s/deg in azimuthal changes for S waves, where only minor effects are obtained for P waves. These results further validate the case for distinct chemistry inside the African Low Shear Velocity Province.