New Seismic Images of Crustal Structure Beneath Southern Africa

The deep 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 provide new constraints on crustal thickness and composition across the Kaapvaal and Zimbabwe Cratons and surrounding mobile belts across a range of geologic terranes ranging in age from the Archean to the Paleozoic. We use data gathered from the 1997-1999 Southern Africa Seismic Experiment (SASE), a broadband seismometer deployment with ~75 km station spacing. We generated P-wave receiver functions for 79 stations using the iterative deconvolution method of Ligorria and Ammon (1999) with a Gaussian pulse width of 2.5, which corresponds to a dominant period of ~2 sec in the receiver functions. We utilized the Funclab receiver function analysis software of Eagar and Fouch (2012) to trace edit receiver functions, and subsequently perform H-κ stacking and Common Conversion Point (CCP) imaging. The use of CCP stacking differentiates our study from previous studies using these data (e.g. Nguuri et al., 2001, Nair et al., 2006), as it provides us with a continuous three-dimensional image of crustal variations throughout southern Africa. We find that crustal thickness in mobile belt regions is thick compared to the cratons, with the exception of the area affected by the Bushveld Complex. The Kaapvaal and eastern Zimbabwe Cratons have a well-defined average Moho depth of ~34 km and Vp/Vs of ~1.73, indicative of intermediate average crustal composition. These results are consistent with a relatively unmodified continental crust due to limited deformation history of these regions since formation. Conversely, the Archean Okwa-Magondi belt, western Zimbabwe Craton, Proterozoic Kheiss thrust belt and Namaqua-Natal, and Paleozoic Cape-Fold belts have Moho depths with a wide range of values from ~36 km to ~47 km, and Vp/Vs values ranging from 1.74 to 1.85 which seem to depend upon proximity to cratons. These regions are likely thicker due to collisional tectonic processes during formation. We find that the Bushveld Complex, the site of the world's largest layered mafic intrusion, has a thick (~39 km) and less prevalent Moho with a Vp/Vs ~1.8, indicative of a more mafic average crustal composition. We attribute the weak and deeper Moho and Vp/Vs anomaly to crustal underplating of mafic/ultramafic composition during the emplacement of the Bushveld Complex.