Present-day Deformation in the Incipient Strike-slip Basin of the Okavango Delta (Botswana): Impact on the Ecosystem

Abstract

The southward propagation of the East African Rift System inside the Southern African Plateau generates the Okavango Graben, hosting the Okavango Delta, which is one of the largest African endoreic ecosystems associated with an alluvial fan. The deformation and kinematics governing this propagation are still currently being debated, especially whether a strike-slip component is present and whether its impact on the dynamics of ecosystem was ever considered. To go further, we analysed the ground deformation from a network of semi-permanent GNSS stations located inside and around the Okavango Delta. As estimated from previous works, the Okavango Graben is submitted to a right-lateral displacement with highest strain along one border of the graben, where most of the seismicity is located. The inner floor displays a clockwise rotation generating local low intensity compression. This deformation pattern agrees with the asymmetric 3D shape of the basin. The ground subsides with higher rates in the north-east, generating a regional tilt that explains the progressive shift of the annual flood toward the north-east, as described in previous works. This tilting causes some areas to dry up and other areas to become almost permanently flooded. Therefore, we hypothesize that tectonic driving is responsible for the eastward migration of water which could then result in its capture by the Linyanti and Zambezi rivers. The long-term consequence of this deformation is the disappearance of the endorheism of the alluvial fan. This drastic hydrological change will disrupt the ecosystem and impact biodiversity as well as economic resources.