Morpho-tectonic Evolution of the Western Afar Margin (southern Red Sea, Ethiopia)

The western Afar margin represents the southern end of the Red Sea rift system. It developed in the heart of the Afar plume related volcanic province which lies at the famous triple junction connecting the East African, the Gulf of Aden and the Red Sea rifted systems. This margin is presently separating the Afar depression from the Ethiopian Highlands and is topographically expressed by an impressive altitudinal gradient (more than 3000 m in less than 50 km). This topographical passive margin has been developed during Miocene times from the top of an uplifted dome, which has been related principally to the Afar plume impingement and the associated extrusion and differentiation of Ethiopian Continental Flood Basalts 30 Ma ago (Pik et al. 2003). This margin is also typically separating the edge of a thick continental block (below the Oligocene CFB) from an extremely thinned domain (the Afar depression) which most probably represents one of the rare worldwide rifting step corresponding to the Ocean Continent Transition zone at the rift to drift transition. Mechanisms invocated to explain this geodynamical phenomenon of extreme lithospheric thinning, predating continental break-up and sea floor spreading, are up to know controversial and do not really satisfy geological and geophysical observations along old passive margins bounding well developed oceanic domains. In this context, the on-shore western Afar margin is an ideal case study to try to precise geodynamic and structural thinning processes because time lapse since the initiation of rifting is so reduce (less than 30 Ma) that outcropping morphological expression of implicated geological events has been preserved and can be easily documented and investigated with traditional approaches. A particular geological characteristic of this southern part of the Red Sea margin resides in the well developed marginal basins, morphologically expressed (but at various altitude) all along the topographical gradient from the Eritrean margin to the heart of the volcanic province at the transition with the East African Rift branch. Such morphological steps have been variously described and interpreted in the past. In this study we will present a new geomorphological view and interpretation of these marginal basins development, as well as the potential structural and geodynamic implications of this new interpretation in the margin development and thinning.