THE MAKING OF A DIVERGENT PLATE BOUNDARY: THE ARABIA-AFRICA PLATE BOUNDARY WITHIN AFAR, ETHIOPIA

The triangular-shaped Afar Depression in Ethiopia, Eritrea and Djibouti is considered from global plate kinematics a classical rift-rift-rift triple junction where the Red Sea, the Gulf of Aden and the Main Ethiopian Rift meet and separate the Arabian, Nubian and Somali Plates. This depression is surrounded by the rising escarpments of the Ethiopian plateau in the west and the Somali plateau in the south where elevation drops from ~2 km to reach -160 m below sea level in northern Afar. The Afar triple junction was formed in response to extension following the out-pouring of ~30 Ma basalts when the Afar mantle plume reached the base of the Arabian-Nubian Shield. Subsequently, the Red Sea and the Gulf of Aden were formed as continental rifts at ~24 Ma but transitioned into oceanic spreading centers at ~10 and ~5 Ma, respectively. The Main Ethiopian Rift joined the two oceanic rifts within Afar ~10 Ma. However, the two oceanic rifts (the Red Sea and the Gulf of Aden) do not directly connect within Afar. Rather, the Red Sea spreading center stepped onto Afar at the Gulf of Zula forming the Red Sea propagator which is manifested by spectacular active rifts and shield volcanoes. Similarly, the Gulf of Aden spreading center stepped onto Afar at the Gulf of Tadjoura forming the Gulf of Aden propagator which is equally characterized by active rifts and shield volcanoes. The Red Sea and the Gulf of Aden propagators (respectively propagating in a southeast and northwest directions) enclose a ~120 km wide overlap zone characterized by deep northwest-trending grabens (such as the Dobe graben) surrounded by horst structures. The horst structures within the overlap zone are currently rotating in a clockwise direction due to dextral shear exerted by differential spreading of the Red Sea and the Gulf of Aden propagators. The southern extension of the Red Sea propagator in Afar is represented by the ~50 km wide, northwest-trending Tendaho graben which is surrounded by northwest-trending normal faults deforming ~2 Ma old basaltic rocks. The age of the basaltic flows becomes progressively younger towards the center of the graben reaching ~35 ka. Recent magnetic and gravity studies show that the extension within the Tendaho Graben is magma-assisted and it is maintained by a ~10 km wide zone of basaltic diking. The magnetic pattern across the Red Sea propagator is similar to those observed on mid-ocean ridges where the magmatic diking zone is characterized by normal polarity flanked by reverse polarity. Correlation of the magnetic pattern to geochronological data suggests that the magmatic dikes were emplaced between ~800 ka and ~35 ka. Additionally, these data suggest that extension across the Tendaho graben has slowed considerably in the past ~800 ka and the bulk of the strain might have been transferred to the northeast to be accommodated within the northwest-trending grabens of the overlap zone as indicated by the cumulative ~12 mm/year extension. Magnetic and gravity surveys across the Dobe graben show a pattern that is fundamentally different from that across the Tendaho graben in which these data do not support the presence of magmatic diking under Dobe. It is possible that the Dobe graben is currently mechanically stretching as supported by continuous occurrence of earthquakes indicating northeast-southwest extension. Hence, we suggest that the plate boundary between Arabia and Africa (Nubia and Somalia) within Afar is currently in the make and it is characterized by a defused zone of extension passing through the Red Sea propagator, the Dobe graben, and the Gulf of Aden propagators. Extension within the Red Sea and the Gulf of Aden propagators evolved into magma-assisted extension whereas the extension within the Dobe graben is still maintained through mechanical stretching.