THE IMPORTANCE OF STUDYING MESOSCOPIC STRUCTURES FOR UNDERSTANDING EXTENSIONAL TECTONIC REGIMES: EXAMPLES FROM THE AFAR DEPRESSION, ETHIOPIA

The Quaternary Dobi Graben in NE Ethiopia is dominated by flood basalts and is situated in the intensely faulted east-central block of the Afar Depression that represents a rift-rift-rift triple junction. We used mesoscopic structural analysis to understand the evolution of the graben within the regional framework of the Afar triple junction. Analysis of stretching lineation / fault plane geometrical relationship and kinematic indicators enabled us to identify early NW-trending high-angle normal faults, NW-sinistral strike-slip faults, and late layer-parallel gravitational slip planes arranged into extensional imbrication fans. Examination of folds and associated axial planer cleavage, frontal ramps and mesoscopic duplexes indicated that these extensional imbrication fan were formed by gravitational collapse induced by tilting of hanging-wall blocks toward the grabens’s center, resulting in the development of low- angle, layer- parallel slip planes. Anti-clockwise block rotation following the gravitational collapse is manifested by dextral strike-slip movement along steep planes that bound crescent-shaped blocks constituting the extensional imbrications fans. Evolution of the Dobi Graben can be summarized into three stages. (1) Development of the NW-trending, high-angle normal faults responding to an overall N-S extension. (2) Development of NW-trending sinistral strike-slip faults in response to clockwise rotation about a vertical axis. (3) Development of extensional imbrication fans due to progressive tilting of hanging-wall blocks towards the graben’s center. Mesoscopic structures often overlooked in structural studies of extensional tectonic regimes have proven useful in gaining insights into the dynamic processes leading to the tectonic evolution of the Dobi Graben.