Within continental lithosphere, widespread seismic evidence suggests a sharp discontinuous downward decrease in seismic velocity at 60–160 km depth. This midlithospheric discontinuity (MLD) may be due to anisotropy, melt, hydration, and/or mantle metasomatism. We survey global seismologic observations of the MLD, including observed depths, velocity contrasts, gradients, and locales across multiple seismic techniques. The MLD is primarily found in regions of thick continental lithosphere and is a decrease in seismic shear velocity (2–7% over 10–20 km) at 60–160 km depth, the majority of observations clustering at 80–100 km. Of xenoliths in online databases, 25% of amphibole‐bearing xenoliths, 90% of phlogopite‐bearing xenoliths, and none of carbonate‐bearing xenoliths were formed at pressures associated with these depth (2–5 GPa). We used Perple_X modeling to evaluate the elastic moduli and densities of multiple petrologies to test if the MLD is a layer of crystallized melt. The fractional addition of 5–10% phlogopite, 10–15% carbonate, or 45–100% pyroxenite produce a 2–7% velocity decrease. We postulate this layer of crystallized melt would originate at active margins of continents and crystallize in place as the lithosphere cools. The concentration of mildly incompatible elements (Y, Ho, Er, Yb, and Lu) in xenoliths near the MLD is consistent with higher degrees of melting. Thus, we postulate that the MLD is the seismological signature of a chemical interface related to the paleointersection of a volatile‐rich solidus and progressively cooling lithosphere. Furthermore, the MLD may represent a remnant chemical tracer of the lithosphere‐asthenosphere boundary (LAB) from when the lithosphere was active and young.

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The seismic wavefield mainly contains reflected, refracted and direct waves but energy related to elastic scattering can also be identified at frequencies of 1 Hz and higher. The scattered, highfrequency seismic wavefield contains information on the small-scale structure of the Earth's crust, mantle and core. Due to the high thermal conductivity of mantle materials causing rapid dissipation of thermal anomalies, the Earth's small-scale structure most likely reveals details of the composition of the interior, and, is therefore essential for our understanding of the dynamics and evolution of the Earth. Using specific ray configurations we can identify scattered energy originating in the lower mantle and under certain circumstances locate its point of origin in the Earth allowing further insight into the structure of the lowermost mantle. Here we present evidence, from scattered PKP waves, for a heterogeneous structure at the core-mantle boundary (CMB) beneath southern Africa. The structure rises approximately 80 km above the CMB and is located at the eastern edge of the African LLSVP. Mining-related and tectonic seismic events in South Africa, with m b from 3.2 to 6.0 recorded at epicentral distances of 119.3 • to 138.8 • from Yellowknife Array (YKA) (Canada), show large amplitude precursors to PKP df arriving 3-15 s prior to the main phase. We use array processing to measure slowness and backazimuth of the scattered energy and determine the scatterer location in the deep Earth. To improve the resolution of the slowness vector at the medium aperture YKA we present a new application of the F-statistic. The high-resolution slowness and backazimuth measurements indicate scattering from a structure up to 80 km tall at the CMB with lateral dimensions of at least 1200 km by 300 km, at the edge of the African Large Low Shear Velocity Province. The forward scattering nature of the PKP probe indicates that this is velocity-type scattering resulting primarily from changes in elastic parameters. The PKP scattering data are in agreement with dynamically supported dense material related to the Large Low Shear Velocity Province.