Evidence for a compositional boundary within the lithospheric mantle beneath the Kalahari craton from S receiver functions

S and P receiver functions from the Southern African Seismic Experiment are analyzed for lithospheric discontinuities beneath the Kalahari craton. Besides the Moho, the most prominent feature is a discontinuous reduction in seismic velocity of about 4.5% at approximately 150 km depth. The discontinuity appears to have a width of about 10 km. Termed the K-discontinuity, this feature is restricted to the northern half of the array, extending from the Zimbabwe craton south to the TML (Thabazimbi–Murchison lineament), and in the west from Botswana to the edge of the Kalahari craton in the east. It spans several Archean sutures and is thus unlikely related to Archean tectonics. It does, however, appear to be related to subsequent magmatic episodes. The strongest anomaly is coincident with the most intense Karoo volcanism, and it extends to the northern edge of the Bushveld intrusion. From mantle xenoliths and xenocrysts, the entire lithosphere in this region appears to have experienced a long-term infiltration of basaltic melt and metasomatic fluids. We propose that the K-discontinuity reflects the influence of this melt/metasomatic infiltration, which has, over time, intruded and refertilized the lithosphere. Based on kimberlites pipes that show obvious signs of melt metasomatism and likely Karoo influence, the observed reduction in seismic velocity is plausibly consistent with the observed major-element and volatile enrichment at 150 km depth in such kimberlites. If this interpretation is correct, then the high-temperature kimberlite nodules that most clearly reflect this perturbation likely represent the general state of the lower lithosphere, rather than only reflecting local mantle properties in the immediate vicinity of the kimberlite eruption.