Origin of ULVZs near the African LLSVP: Implications from their distribution and characteristics

Ultra-low velocity zones (ULVZs) provide important information on the composition and dynamics of the core-mantle boundary (CMB). However, their global distribution and characteristics are not well constrained, especially near African large low-shear velocity provinces (LLSVPs). Here, we used ScS precursor (SdS) and postcursor (ScscS) phases recorded by various seismic networks in Africa and South America to investigate the ULVZ characteristics underlying the South Atlantic Ocean. We found no evidence of ULVZs near the SE boundary of South America, but an ULVZ was found within the SW boundary of the African LLSVP, with thicknesses ranging from 11–18 km and reductions in S-wave velocities of 18%–34%. Our results, combined with the global distribution of ULVZs, suggest that thermal activity may be essential to ULVZ formation. Moreover, subducted slab and mantle flow may also play a key role, depending on the location of the ULVZs.