TWO-DIMENSIONAL (2D) LITHOSPHERIC STRUCTURE BENEATH THE SOUTHWESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM

Abstract

To better understand the Precambrian lithospheric structure beneath the Southwestern Branch (SWB) of the East African Rift System (EARS) and their influence on the development of this rift system, we developed a two-dimensional (2D) forward gravity model across Congo and Zambia that is constrained by the surface geology and results from previous magnetotelluric (MT) lithospheric imaging study. We constructed the 2D forward gravity model from the Bouguer gravity anomalies of the European Improved Gravity Model of the Earth by New Techniques (EIGEN – 6C4), which has spatial resolution of ~15 km X 15 km. Most of the rift basins of the SWB were developed during the Permian – Triassic Karoo rifting event during the early stages of the dispersal of Gondwana, but these were reactivated during the Paleogene – Quaternary rifting event of the EARS. Our results showed that the Archean – Paleoproterozoic Congo craton, the Bangweulu cratonic block and the Niassa craton, and the Mesoproterozoic – Neoproterozoic Southern Irumide orogenic belt surrounding the Niassa craton have thick sub-continental lithospheric mantle (SCLM) with lithospheric thickness reaching ~250 km . This suggests that the Niassa craton underlies the entire surface extent of what has been referred to as the “Southern Irumide orogenic belt”. No rift basins of the SWB have been observed extending within these cratonic blocks with the exception of the Mweru – Wantipa rift found on the northeastern edge of the Bangweulu cratonic block. Our results also found a relatively thinner (~160 km) and uniform lithosphere beneath the Paleoproterozoic – Mesoproterozoic Kibaran orogenic belt and the Neoproterozoic Lufilian foreland basin, which underlie the Upemba, Kundelungu, and Mweru rift basins of the SWB. This suggests that the Lufilian foreland basin was built on a “Kibaran lithosphere” rather than on a “Bangweulu lithosphere”. Further, our results showed that thinner lithosphere (~100 km thick) is present beneath the Mesoproterozoic – Neoproterozoic Irumide metacraton and the Mwembashi shear zone, which represents the suture zone between the metacraton and the Niassa craton. The presence of thinner lithosphere beneath the Irumide metacraton and the Mwembashi shear zone seems to have significantly controlled the development of the Luangwa rift basin of the SWB.