ORIGIN OF MESOSCOPIC FOLDING OF THE TENDAHO-GOBAAT DISCONTINUITY, AFAR DEPRESSION, ETHIOPIA: TECTONIC OR FLOW STRUCTURE?

The Tendaho-Gobaat Discontinuity (TGD) crops out in the center of the Afar Depression, Ethiopia that represents a rift-rift-rift triple junction. The Afar Depression is where the NW-trending Red Sea, the E-W trending Gulf of Aden, and the NE-trending Main Ethiopian Rift meet. The TGD is a NW-trending structure that represents the southern portion of the western branch of the Red Sea rift where it steps onto land at the Gulf of Zula (the SE-propagating Manda Hararo-Gabaad rift). The Manda-Hararo-Gabaat rift bends southeast and becomes the TGD where it meets and truncates NE-trending structures associated with the Main Ethiopian Rift, and continues east to become parallel to E-W trending structures associated with the Gulf of Aden rift. Field studies and fault plane solution data show that the TGD is an oblique sinistral strike-slip fault. Morphologically, the TGD is defined by the fault-bounded Awsa plain which is surrounded by 4-1 Ma old basalts and rhyolites and filled with recent basaltic fissure flows and lacustrine deposits. The rhyolites on both sides of the Awsa plain seem to have erupted within NE-trending grabens which are oriented at high angle to the NW-trending TGD. Associated with these rhyolites are numerous mesoscopic folds and ENE-trending foliation, the origin of which is not unequivocally resolved. Field studies devoted to examining these mesoscopic structures indicate that the majority of these folds are flow structures associated with emplacement of the rhyolite bodies. However, some might be of tectonic origin. This is because: (1) Some of these folds have strong mullion structures developed parallel to their fold axes; (2) Fewer folds have well-developed cleavage developed parallel to their axial planes; and (3) In some places, these folds deform intercalated rhyolitic and basaltic layers indicating that folding is not limited to the rhyolites only. It is not clear what cause such tectonic folding. However, we relate the development of these folds to a transpressional tectonic regime that had affected the rhyolite-filled NE-trending grabens due to oblique sinistral strike-slip faulting along the TGD. Tectonic folding might have occurred during or shortly after the solidification of the rhyolitic lava.