Control of Pre-rift Lithospheric Structure on the Architecture and Evolution of Continental Rifts: Insights From the Main Ethiopian Rift, East Africa

We investigate the along-axis variations in architecture, segmentation, and evolution of the Main Ethiopian Rift (MER), East Africa, and relate these characteristics to the regional geology, lithospheric structure, and surface processes. We first illustrate significant along-axis variations in basin architecture through analysis of simplified geological cross sections in different rift sectors. We then integrate this information with a new analysis of Ethiopian topography and hydrography to illustrate how rift architecture (basin symmetry/asymmetry) is reflected in the margin topography and has been likely amplified by a positive feedback between tectonics (flexural uplift) and surface processes (fluvial erosion and unloading). This analysis shows that similar to 70% of the 500km long MER is asymmetric, with most of the asymmetric rift sectors being characterized by a master fault system on the eastern margin. We finally relate rift architecture and segmentation to the regional geology and geophysical constraints on the lithosphere. We provide strong evidence that rift architecture is controlled by the contrasting nature of the lithosphere beneath the homogeneous, strong Somalian Plateau and the weaker, more heterogeneous Ethiopian Plateau, differences originating from the presence of pre-rift zones of weakness on the Ethiopian Plateau and likely amplified by surface processes. The data provided by this integrated analysis suggest that asymmetric rifts may directly progress to focused axial tectonic-magmatic activity, without transitioning into a symmetric rifting stage. These observations have important implications for the asymmetry of continental rifts and conjugate passive margins worldwide. Plain Language Summary The Ethiopian Rift Valley is a classic example of an area where a continent is splitting apart. Here active volcanism, earthquakes, and fracturing of the Earth's surface break continents and form new oceans. In this paper we analyze the shape and size of the faults and fractures combined with the relief and river drainage of the rift valley, in order to interpret which faults control the shape of the rift and how they have evolved through time. We find that more than half of the rift is defined by a large fault escarpment along the eastern side of the rift, with the western side defined by a gradual slope (an asymmetric rift). Less than half is a classical rift with clear fault escarpments on both sides (a symmetric rift), and we find no evidence for the previously held view of progressive evolution from an asymmetric to symmetric rift. Instead, we find that the morphology of the rift is primarily controlled by the contrasting properties of the rocks beneath the two sides of the rift, with major fault escarpments forming only where the rocks are strong.