Overview of the Crustal Structure of Sub-Saharan Africa

Book excerpt: In this thesis, new estimates of crustal thickness and structure obtained from H-[kappa] stacking of P-wave receiver functions and the joint inversion of P-wave receiver functions and Rayleigh wave group velocities are presented for 63 seismic stations in Mozambique, Namibia, Uganda, and South Africa. These new estimates are used to characterize the nature of the crust beneath the Mozambique Coastal Plains (MCP) in central and southern Mozambique and, when combined with previous estimates of crustal thickness and structure in sub-Saharan Africa, to investigate if there are secular changes in crustal genesis through the Precambrian. Moho depths beneath 29 broadband seismic stations in Mozambique indicate crustal thicknesses between 23 and 43 km. Moho depths beneath nine stations in Uganda vary between 36 and 43 km. Crustal thickness beneath 19 stations in Namibia ranges from 35 and 48 km, and for six stations in South Africa between 33 and 43 km. The 1-D velocity models for all stations in Precambrian terranes yield an average Vs of 3.7 to 3.8 km/s and show some high velocity (Vs > 3.9 km/s) layers in the lower crust indicating the presence of mafic lithologies. New results reported in this thesis, when combined with results from previous studies in sub-Saharan Africa, yield an average crustal thickness for all Precambrian terranes of 39 ± 4 km. The average crustal thicknesses are found to be almost identical for Archean (39 ± 2 km) and Proterozoic (39 ± 3 km) terranes. The average crustal thicknesses for terranes of all eras fall between 38 and 40 km. Furthermore, the average thickness of the mafic lower crust is almost identical in Archean and Proterozoic terranes (7 ± 4 km and 6 ± 4 km, respectively). In addition, the average crustal shear wave velocities for all terranes fall within one standard deviation of the overall mean velocity of 3.7 km/s. These results are consistent with findings in previous studies highlighting a lack of evidence for secular variation in the crustal structure or composition within Precambrian terranes in eastern and southern Africa, suggesting that secular changes, if they existed, have been obscured by tectonic reworking of crustal structure and composition by later orogenic and/or magmatic events. New results from 14 seismic stations in the MCP reveal sediment thicknesses ranging from 3 to 9 km and Moho depths ranging from 23 to 32 km. An average crustal Vs of 3.6 to 3.7 km/s, together with the lack of high velocity (i.e., Vs > 3.9 km/s) layers in the crust, suggests the crust is stretched continental crust and not oceanic crust.