THE BIRTH OF A PLATE BOUNDARY: THE BOUNDARY BETWEEN THE NUBIAN/SOMALIAN AND ARABIAN PLATES WITHIN THE AFAR DEPRESSION, ETHIOPIA

The Afar Depression in Ethiopia, Eritrea and Djibouti in the horn of Africa is a rift-rift-rift triple junction where the NW-trending Red Sea, the E-trending Gulf of Aden and the NE-trending Main Ethiopian Rift approaches each other and form a 20,000 km2 zone of diffused deformation. It has been tentatively agreed that the Red Sea separates the Arabian Plate from the Nubian Plate, the Gulf of Aden separates the Arabian Plate from the Somalian Plate, and the Main Ethiopian Rift separates the Nubian Plate from the Somailan Plate. The Afar triple junction has been considered as the best example of rift-rift-rift triple junction that exposed on land. However, the Red Sea penetrates the Afar Depression and propagates SSE along the Manda Hararo – Gobaad trend. The Gulf of Aden steps onto land and continues NW within the Afar Depression along the Asal – Manda Inakir rift. Deep grabens and Quaternary shield volcanoes characterize both rifts. The Main Ethiopian Rift enters the Afar Depression along a N-trend and a NNE-trend that continues up to the triple junction. The SE-propagating Manda Hararo – Gobaad and the NW-propagating Asal – Manda Inakir rifts overlap and enclose the intensely faulted East - Central Block. This block is characterized by NW-trending normal and strike-slip faults and deep NW-trending grabens. The location and nature of the plate boundary between the Arabian Plate on one hand and the Nubian and Somalian Plates on the other hand have not been defined since the two overlapping rifts do not connect. We have used RADARSAT images and previously published scaled physical models in conjunction with field studies and fault plane solutions from seismic data to show that the plate boundary separating the Arabian plate from the Nubian – Somalian Plates within the Afar Depression is a seismically-active high strain zone within the East-Central Block (overlap zone) following the Asal rift – Der Ela - Gaggade grabens - Dobi-Hanle transfer zone - Dobi graben.