Seismic evidence for accumulated oceanic crust above the 660-km discontinuity beneath southern Africa

[1] High-pressure assemblages of subducted oceanic crust are denser than the normal upper mantle but less dense than the uppermost lower mantle. Thus subducted oceanic crust may accumulate at the base of the upper mantle. Direct observational evidence for this hypothesis, however, remains elusive. We present an analysis of a negativepolarity shear wave converted from compressional wave at a seismic discontinuity near 570–600 km depth beneath southern Africa. The negative polarity of the converted phase indicates a 2.2 ± 0.2% S-velocity decrease with depth at the seismic discontinuity. This velocity reduction is associated, however, with a low velocity contrast at the 660-km discontinuity. The exsolution of Ca-perovskite in former oceanic crust at depth greater than 600 km and the associated small volume fraction of ringwoodite are plausible explanations for the apparent paradox between the negative velocity discontinuity and the low velocity contrast at the 660-km discontinuity.