The African upper mantle and its relationship to tectonics and surface geology

This paper focuses on the upper-mantle velocity structure of the African continent and its relationship to the surface geology. The distribution of seismographs and earthquakes providing seismograms for this study results in good fundamental and higher mode path coverage by a large number of relatively short propagation paths, allowing us to image the SV-wave speed structure, with a horizontal resolution of several hundred kilometres and a vertical resolution of ∼50 km, to a depth of about 400 km. The difference in mantle structure between the Archean and Pan-African terranes is apparent in our African upper-mantle shear wave model. High-velocity (4–7 per cent) roots exist beneath the cratons. Below the West African, Congo and Tanzanian Cratons, these extend to 225–250 km depth, but beneath the Kalahari Craton, the high wave speed root extends to only ∼170 km. With the exception of the Damara Belt that separates the Congo and Kalahari Cratons, any high-speed upper-mantle lid below the Pan-African terranes is too thin to be resolved by our long-period surface wave technique. The Damara Belt is underlain by higher wave speeds, similar to those observed beneath the Kalahari Craton. Extremely low SV-wave speeds occur to the bottom of our model beneath the Afar region. The temperature of the African upper mantle is determined from the SV-wave speed model. Large temperature variations occur at 125 km depth with low temperatures beneath west Africa and all of southern Africa and warm mantle beneath the Pan-African terrane of northern Africa. At 175 km depth, cool upper mantle occurs below the West African, Congo, Tanzanian and Kalahari Cratons and anomalously warm mantle occurs below a zone in northcentral Africa and beneath the region surrounding the Red Sea. All of the African volcanic centres are located above regions of warm upper mantle. The temperature profiles were fit to a geotherm to determine the thickness of the African lithosphere. Thick lithosphere exists beneath all of the cratonic areas; independent evidence for this thick lithosphere comes from the locations of diamondiferous kimberlites. Almost all diamond locations occur where the lithosphere is 175–200 km thick, but they are largely absent from the regions of the thickest lithosphere. The lithosphere is thin beneath the Pan-African terranes of northern Africa but appears to be thicker beneath the Pan-African Damara Belt in southern Africa.