Uplift of the Ethiopian plateau

Abstract

The topographic configuration of the Earth's surface is the result of the interaction between deep and surface processes. The surface subsides above mantle downwellings and rises up above mantle upwellings forming dynamic topography. Similarly erosion and crustal thickening drive the mean topography to lower and raise respectively because of isostasy. So, in order to reconstruct the topographic evolution of a certain area, it is necessary to distinguish the several components of topography. In this respect, Africa is a perfect natural laboratory to examine the relationships between surface deformation, lithospheric structures and the underlying convecting interior. In particular Ethiopia has long been recognized as an ideal location to study rifting and hot spot tectonism. However, despite all the studies carried out, no consensus exists on several geologic aspects like the timing and pattern of rock uplift, the evolution of the drainage system and the interpretation of deep seismic data. In this thesis the geologic and geomorphologic features of the Ethiopian Plateau have been analyzed in order to reconstruct the topographic evolution of the area since Oligocene. The Ethiopian Plateau is located in the northwest of Ethiopia. It is characterized by up to 2 km thick Oligocene continental flood basalts (CFB) related to the Afar plume activity. Such volcanics deposited on an alternation of Mesozoic sandstones and limestones and on a Paleozoic/Precambrian basement. The plateau is bordered by the Main Ethiopian Rift/Afar escarpment to the east and by the Tana escarpment to the west. The drainage system is characterized by the Blue Nile and Tekeze rivers basins. Here, a multidisciplinary approach has been used combining analytic models, GIS-based analysis, and field work. To characterize the plateau drainage system a quantitative geomorphologic analysis has been performed. The CFB thickness and the amount of the eroded materials have been quantified in order to determine the basalts loading and the erosional unloading. The flexural uplift at the rift and Tana escarpments have been modeled. By subtracting all the isostatic contributions from the present CFB base surface the Oligocene topography has been obtained. Using recent seismic data and crustal models, new residual and dynamic topographies have been elaborated and compared with the pre-CFB one. The CFB geometry analysis shows a rough CFB base surface and an asymmetric basalts distribution respect to the Rift Valley. The thickest deposits concentrate in the Lake Tana region which represented one of the main basalts source areas. The morphometric analysis evidences that the Tekeze and Blue Nile drainages have followed divergent pathways of incision. In particular the Tekeze R. seems to be responding in a more or less steady way to a single, early base level fall whereas the Blue Nile R. is responding in an unsteady way to multiple base level falls that may be both external and internal to the plateau in origin. The regional topographic analysis shows that a dome-shaped feature centered on Addis Ababa is present in both the pre- and post-CFB topography, consistent with the notion that the region has experienced dynamic support since the Oligocene.