THE TRI-TECTONIC DANCE OF SOUTH-CENTRAL AFRICA: CRATONS, OROGENIC BELTS, AND CONTINENTAL RIFT SYSTEMS

Understanding what controls the pathways of the propagation of continental rift systems (why here and not there?) is challenging. One handy explanation has been that continental rift systems follow zones of thin lithosphere largely presented by orogenic belts of different ages extending around Archean – Paleoproterozoic cratons. Here, we examine the complex nature of the tectonic interplay between cratons, orogenic belts, and continental rift systems by focusing on the Southwestern Branch (SWB) of the East African Rift System (EARS). The SWB hosts what is considered to be the youngest and the southwestern-most Cenozoic-age rift segment of the EARS (Okavango). It also hosts Paleozoic – Mesozoic Karoo-age rift segments that might have been reactivated by extension associated with the EARS (e.g. Luangwa). The first order regional observation from remote sensing and surface geology data is that the SWB is defined by NE-trending individual rift basins stretching within a collage of Paleoproterozoic – Neoproterozoic orogenic belts and metacratons (Magondi, Ghanzi-Chobe, Damara, Zambezi, Irumide, and Lufilian) bonded between the Congo-Tanzania-Bangweulu craton in the north and the Zimbabwe-Kaapvaal-Niassa craton to the south. The orogenic belts and metacratons are stitched together and with the cratons by Precambrian lithospheric-scale sutures. Geophysical surveys indicate that the sub-continental lithospheric mantle (SCLM) beneath some of these sutures has been weakened repeatedly through delamination and metasomatism. The most prominent of these sutures is the Mwembeshi shear zone which represents in Zambia the suture between the Niassa craton and the Irumide metacraton, and possibly in Botswana, over 600 km away to the southwest, the suture between th Ghanzi – Chobe and the Damara Belt. It can be observed that the rift border faults of Luangwa in northern Zambia, Lukusashi-Luano in central Zambia, Kafu in southwestern Zambia, and Okavango in northwest Botswana are localized within this suture. We suggest that where the presence of the thinner lithosphere of orogenic belts is important for providing favorable broad sites for the development of continental rifts, the presence of sutures with significantly weakened SCLM is critical for providing narrow pathways for the propagation of these rifts.