Crustal structure beneath southern Africa: insight into how tectonic events affect the Mohorovičić discontinuity

The long and complex history of southern Africa makes it a geological nexus for understanding how crust forms, evolves and survives plate tectonic processes over billions of years. The goal of this study is to characterize the crustal thickness, composition, and Moho impedance contrasts across the Kaapvaal and Zimbabwe Cratons and surrounding mobile belts, which range in age from Archean to Palaeozoic. We use data gathered from the 1997–1999 Southern Africa Seismic Experiment, the Africa Array (2006–2007) and the Global Seismographic Network (1993–2009) to generate P-wave receiver function Gaussian-weighted common conversion point stacks across the region in order to provide a continuous 3-D image of crustal variations throughout southern Africa. We observe thickened crust associated with mobile belts and the intrusion of the Bushveld Complex relative to the less-deformed cratons. The southern Kaapvaal and eastern Zimbabwe Cratons have a well-defined Moho with an average depth of ∼34 km and Vp/Vs of ∼1.73, indicative of felsic average crustal composition. We explain the felsic composition observed in the Kaapvaal Craton in the context of significant crustal modification related to the deposition of the Ventersdorp lavas. We find that the Bushveld Province, the site of the world's largest layered mafic intrusion, has a thick (>40 km) crust with a Vp/Vs > 1.8, indicative of a mafic average crustal composition. The magnitude of Moho conversions beneath the Bushveld Province is variable, with the lowest amplitude conversion appearing between the eastern and western limbs of the Bushveld Complex, indicative of mafic underplating beneath the region. In the Limpopo Belt and western Zimbabwe Craton, we observe low amplitude Moho conversions beneath the Okavango Dyke Swarm, and attribute this to the reworking of the crust by mafic underplating and intrusion during the Jurassic rifting of Gondwanaland. The Namaqua-Natal event thickened the crust and created a gradational transition from crust to mantle as seen by low amplitude Ps arrivals from receiver functions. Evidence for the presence of a mafic lower crust beneath the Namaqua-Natal Belt is observed in high Vp/Vs values (∼1.8) and a high concentration of granulite xenoliths in kimberlite intrusions. In contrast to past interpretations for craton formation that suggest sharp Moho boundaries and low Vp/Vs ratios are characteristic of undisturbed cratons, we propose that these crustal properties are more controlled by tectonic events that later modify the existing cratonic crust. We cannot rule out secular crustal formation variations in the early Earth, but we propose that the southern African cratonic crust has been too heavily modified by later tectonic events to be used in arguments for secular variation, as may be the case for other cratons as well. Thus, it is important to consider the regional geological history of cratons to ensure that secular variation is not confused with the effects of later tectonic deformation and crustal modification.