EVOLUTION OF THE DOBI BASIN, AFAR DEPRESSION, ETHIOPIA, FROM RADAR REMOTE SENSING AND FIELD STUDIES

Dobi graben in NE Ethiopia is situated in the intensely faulted East -Central part of the Afar depression that represents a rift-rift-rift triple junction. The northwestward propagating Manda Inakir and the southeastward propagating Manda Harare bound the Dobe basin in the northeast and southwest respectively. We present results of radar remote sensing (including digital and visual interpretation of Standard Beam RADARSAT data) and structural field studies (dominantly analysis of mesoscopic structures) contributing to the understanding of the evolution of the Dobi Basin within the framework of the overlapping rifts. Structures observed in the Dobi graben exhibit normal and strike-slip faults, down-dip layer-parallel gravitational slip, and cascading anti-clockwise rotation of dismembered blocks. Spaced axial cleavage, duplex stacking and frontal ramps (compress ional-like features in an extensional tectonic environment) are common mesoscopic structure. Stupendous border fault bound the Dobi basin on the northeastern and southwestern sides forming breakaway and hinge zone respectively. NE-trending synthetic and antithetic normal faults and dilatational fractures characterizes areas close to the boundary faults. Stretching lineation indicate local variation of tectonic transport directions, however down dip lineation on layer-parallel gravitation slip surface is dominant feature. Lateral imbrications of blocks, associated with gravitational collapse and concomitant sliding of individual blocks, are prominent features encountered as clusters along the western side of the Dobi basin. The accumulated slip of blocks gets progressively wider towards the basin center. The architect of the Dobi basin within the bounds of the boundary faults, particularly at southern and lower areas close to basin floor is shaped by the layer-parallel gravitational slippage along fissile hydrated trachytes tucked between successions of basaltic flows. The evolution of the Dobi basin started with normal and sinistral strike-slip faults and followed by gravitational collapse in response to NE-SW regional extension and clockwise rotation about a vertical axis.