Style of faulting and drainage pattern along the Somalian plateau escarpment at the southern Afar margin (Ethiopia) [Stile deformativo e reticolo idrografico lungo la scarpata del Plateau Somalo al margine meridionale dell’Afar (Etiopia)]

An integrated structural and geomorphologic study along the Somalian Plateau escarpment at the southern Afar margin (Dire Dawa area, Ethiopia), provides insights into the rift geometry, timing and evolution of this sector of the East Africa Rift system. Late Triassic-Jurassic sandstones and carbonate sequences lie in nonconformity over the pre-Cambrian metamorphic basement and are in turn disconformably covered by Early Cretaceous continental and transitional sandstones and conglomerates. After another disconformity a thick sequence of Flood Basalt was deposited during the Oligocene and today crop out close to the margin of the Rift at an elevation of about 3500 m in the Somalian Plateau. Rift architecture in the study area is characterized by faulted blocks tilting away from the Afar Depression «domino-style faulting», as already described by MORTON & BLACK (1975). Major faults are ca. E-W oriented and show high-angle geometry. Kinematic indicators measured along the fault planes commonly show a slight left lateral component providing a ca. NNWSSE direction of extension. Faulted monoclines composed of footwall anticline-hanging-wall syncline pairs are observed adjacent to many of the block-bounding and intra-block faults. We interpret this structural style to have formed in response to fault-propagation folding in extensional regime. The drainage pattern in the study area is strongly influenced by the main E-W trending faults and by the NNW-SSE system of fractures and doleritic dikes emplaced during the Oligocene volcanic event. Two main stages of fault activation can be recognised in the area. In fact, the faulted blocks, although usually quite dissected, sometimes have a flat top ridge preserving the remain of a former planation surface, most probably a huge pediment that connected the main escarpment to the Afar. This pediment erase some of the faults and in the distal area is covered by Early-Middle Pleistocene Afar flood basalts. After the modelling of the pediment that also cancelled the previous drainage network, a new phase of fault activity is documented. An even older drainage has been recognised in the Plateau area, where broad paleovalleys cut for over 1400 m below the original plateau in the area which separated the Afar depression from the Somalian lowlands. Originally, they were alimented from the «Afar sector» (i.e., from the north) most probably as a consequence of the upwarping that followed the emplacement of the Trap, and evolved as superimposed rivers. The northward drainage is therefore the result of river reversion as a consequence of the down-faulting associated to the rift activation and evolution. These evidence suggest that the emplacement of the Trap occurred at low elevation and was followed by upwarping and much later by rifting processes and down-faulting.