Uplift, rifting and related geomorphological evolution of the Ethiopian volcanic province: what do we really know ? (Invited)

The East African Rift System (EARS) is particularly famous because sediments trapped in some of the rift depressions have recorded a major piece of the late Cenozoic mammalian and hominin evolution, as well as associated proxies of climate and environmental changes. The present day landscape along the EARS is spectacular and characterized by axial rift valleys surrounded by variously extended volcanic highlands often culminating at altitude > 3000 m. The morphological development of this unique tectonic and magmatic environment has been increasingly debated these last years and attributed either to (1) the direct and indirect consequences of mantle plume impingement, or to (2) the tectonic and flexural consequences of extensional motion. More than a simple difference in the processes implicated in the genesis of the geomorpholigical characteristics, these two models are opposed by the timing of the topography creation: i.e. early - during the Oligo-Miocene - or late - concomitant with Mio-Pliocene transition and climatic evolution ? In the EARS, the Northern Ethiopian plateau and the associated Afar margin represent an ideal case study to investigate the interplay between uplift and rifting. This margin developed in the heart of the Afar mantle plume related volcanic province and is presently separating the Afar depression from the Ethiopian Highlands with an impressive altitudinal gradient (more than 3000 m in less than 50 km). We review published and present new structural and quantitative data in order to constrain the morpho-tectonic evolution of this system. Based on the following observations we argue that this topographical passive margin started to develop during Miocene times from the top of an already uplifted dome : (1) uplift of the main plateau is broadly distributed with a convex shape, and its maximum does not correspond to the margins location, (2) this "off-rift" maximum is to the contrary located on top of a mantle anomaly revealed by tomography, (3) the resulting centrifuge drainage and the position of the main margin divides are "stable" since Oligocene times, (4) the first "fossil" Miocene rifting episode witch affected the province is perched (2500 m) on top of this dome and represents a limited (25 km wide) basin and range type structure, (5) the marginal morphology started to develop latter on, concomitant with the mid-Miocene significant extension phase and results from the interplay between eastward-tilted blocks and westward sequential propagation of an erosion wave without any evidence of recent east-facing "border fault" activity that could have triggered flexural uplift. Combination of all these available data suggests that most of the elevation of the Ethiopian highlands is inherited from the early Oligocene volcanic and tectonic activities (CFB emplacement + large scale basement doming) triggered by the impingement of the Afar mantle plume under the East African lithosphere. This conclusion disagree with other interpretations for witch significant uplift has been acquired recently since the Pliocene and could have influenced local climate and faunal evolutions.