Northwest to north-northwest extension direction in the Ethiopian rift deduced from the orientation of extension structures and fault-slip analysis

Abstract

Northeast to north-northeast trends of eruptive centers, some of which formed at the termination of faults, and en echelon extension fractures—the largest one of which is 0.1 km wide, up to 1 km long, and at least 50 m deep—exist between lat. 7°N and 9°N in the Ethiopian rift on Systeme Probatoire d9Observation de la Terre (SPOT) imagery. The extension fractures form scars in the rift floor. Border faults of the rift are generally oriented northeast to north-northeast. Two fault systems observed on Land-sat Multi-spectral Scanner (MSS) cross the rift, the Bilate-Weyira fault with a north-northwest strike and the Wonji fault belt with a north-northeast trend. North-northwest-striking faults cutting across the rift are interpreted to be transfer faults that cut the rift into rhomboid-shaped basins. Extension fractures and rows of eruptive centers—collectively referred to as extension structures—are used in the paper to infer the direction of plate motion in the Ethiopian rift. Northwest to north-northwest extension of the rift is deduced by assuming that the direction of opening of extension structures is also the orientation of the minimum principal stress σ3. Fault-slip inversion on faults that cut post-Miocene volcanic rocks confirms this interpretation.