The Origins of Africa's Upper Mantle Discontinuities

Enigmatic discontinuities in the upper mantle beneath Africa, such as the mid-lithosphere and X-discontinuities, have prompted various theories regarding their causes. However, most studies rely on S-to-P receiver functions, raising concerns about resolution. In this study, we enhance the depth-resolution of mantle discontinuities using high-resolution P-to-S receiver functions that are free from crustal reverberations. Our analysis reveals the depths and the polarities of both single and multiple discontinuities. The most prevalent observation is an unpaired (i.e., no high velocity discontinuity beneath or above) low-velocity discontinuity at depths between 50 and 100 km, which shows minimal secular variation. Additionally, paired discontinuities are detected either within or below the heterosphere. Comparisons with thermal, chemical and geophysical models that predict rapid seismic velocity gradients due to partial-melting (induced by volatiles or sub-solidus grain-boundary mechanisms) suggest that the common sharp velocity drop is most likely caused by a sub-solidus mechanism, while the paired discontinuities, including a newly resolved high-velocity discontinuity, may indicate the base of a melt layer. This mantle stratification might be linked to magmatism or remnants of past continental assembly.