Spatial Variations in Crustal Thickness and Vp/Vs Ratio in Southern Africa and Their Geological Implications

Measurements of crustal thickness (H) and VP//VS ratio (R) at about 80 broadband seismic stations in southern Africa were made using data from the Southern African Seismic Experiment, by stacking receiver functions. Our results suggest systematic spatial variations in both H and R that appear to be geologically controlled. Regarding H, we observe relatively thick crust beneath the Limpopo belt, younger collisional belts to the south of the Kaapvaal craton, and beneath the Bushveld intrusion, compared to the crust beneath the Kaapvaal and Zimbabwe cratons, a result that is consistent with previous studies. There are noteworthy spatial variations in R. The most dramatic change in R actually occurs within the Kaapvaal craton. In particular, going from the Neo-Archean Kimberly block to the Meso-Archean Witswatersrand block (east), the average value of R increases systematically from 1.76 to 1.81. Taken at face value, this suggests that, on average, the Meso-Archean crust is more mafic than the Neo-Archean crust, which may indicate temporal differences in crustal formation processes. R is anomalously high in the general region of the Bushveld intrusion. Indeed the highest value in the entire data set is found at the western edge of the Bushveld, and suggests a more mafic crust, consistent with the characteristics of the intrusion. Finally, there is a region of very low R (\u3c1.7) at the southern edge of the Kaapvaal craton, that may reflect processes active during accretion at this margin. These observations, and particularly the values of R, provide important constraints on the processes that have formed and modified the composition of the ancient southern African crust