ENIGMATIC REGIONAL STRUCTURAL GRAIN OF THE PRECAMBRIAN CRYSTALLINE ROCKS OF DIRE DAWA AREA, ETHIOPIA

The Precambrian crystalline rocks of Ethiopia represent the interface between the low-grade metamorphic rocks of the Arabian-Nubian Shield in the north and the high-grade metamorphic rocks of the Mozambique Belt to the south (collectively known as the East African Orogen). These rocks crop out in Ethiopia in the north and west as dominantly low-grade volcano-sedimentary rocks, and in the south as interleaving of low-grade volcano-sedimentary and medium- to high-grade gneisses belts. The majority of ages from these terrains are Neoproterozoic to early Paleozoic. Regional structural grain within these Precambrian terrains is N to NNE-trending. These structures are thought be developed through E-W directed shortening associated with collision between East and West Gondwana. Precambrian medium to high-grade migmatitic gneisses are also reported from eastern Ethiopia along the southern margin of the Afar Depression. Zircon ages as old as ~2.5 Ga has been reported from these rocks. This work examined Precambrian outcrops in eastern Ethiopia around the city of Dire Dawa. This region is dominated by migmatitic gneisses showing well-developed gneissic layering. Unlike other Precambrian terrains in Ethiopia, analysis of mesoscopic structures including folds and thrusts revealed the presence of E to ENE-trending regional structural grain within the Dire Dawa Precambrian crystalline rocks. Additionally, kinematic analysis showed consistent top-to-the-south tectonic transport direction. The presence of E-trending regional fabric combined with the top-to-the-south tectonic transport direction cannot be explained as due to E-W directed shortening during the collision between East and West Gondwana. Pre-rift restoration by closing the Gulf of Aden and the Afar Depression puts the Dire Dawa Precambrian crystalline rocks immediately south of the 2.9-2.55 Ga Al-Mahfid terrane in Yemen. ENE-trending structural grain has also been reported from this terrane. Additionally, the Dire Dawa Precambrian crystalline rocks fall within what is suggested to be the north stretching Afif-Azania micro-continent. Hence, it is possible that the Dire Dawa Precambrian crystalline rocks represent part of a pre-Neoproterozoic micro-continent and their E-trending structural grain was developed before the Gondwana collision.