Mid‐mantle layering from SKS receiver functions

Many seismic data on mid‐mantle discontinuities are related only to subduction zones. Here we use a method that is applicable outside subduction zones. The idea is to use SKS‐to‐P converted phases that are generated in the receiver regions and can be detected in S receiver functions. In two regions (southern Africa and Western Europe) we have detected apparently the same ‘1200‐km’ discontinuity, but at a depth that varies from 1170 km (Africa) to 1240–1270 km (Europe). The S velocity contrast at this discontinuity is ∼0.1 km/s, i.e., half of the value in subduction zones. The ‘1200‐km’ discontinuity may correspond to the phase transition of SiO 2 from stishovite to CaCl 2 structure. The small S velocity contrast is compatible with scenarios in which small remnants of old subducted oceanic lithosphere, thermally equilibrated and partly reacted, are finely dispersed in a bulk mantle matrix.