Stacking of approximately 1500 radial receiver functions recorded at about 80 broadband seismic stations deployed in southern Africa reveals systematic spatial variations in the ratio of crustal P and S wave velocities ($phi$), crustal thickness (H), and the amplitude of the converted Moho phases (R). The eastern Zimbabwe and the southern Kaapvaal cratons are characterized by small H (~38 km), small $phi$ (~1.73), and large R (~0.15) values, suggesting that the relatively undisturbed Archean crust beneath southern Africa is separated from the mantle by a sharp Moho and is felsic in composition. The Limpopo belt, which was created by a collisional event at 2.7 Ga, displays large H (~43 km) but similar $phi$ and R values relative to the cratonic areas. The Bushveld Mafic Intrusion Complex and its surrounding areas show large $phi$ (~1.78), large H (~43 km), and small R (~0.11) values, reflecting the intrusion of mafic material into the original crust as a result of the Bushveld event at 2.05 Ga. Excluding the Bushveld, the spatially consistent and age-independent low $phi$ accentuate the difference between felsic crustal composition and more mafic island arcs that are thought to be the likely source of continental material. Within such an island arc model, our data, combined with xenolith data excluding mantle delamination in cratonic environments, suggest that the modification to a felsic composition (e.g., by the partial melting of basalt and removal of residue by delamination) is restricted to have occurred during the collision between the arcs and the continent.