The active East African Rift System traverses the east African plateau over 4500km from the Afar depression to the Zambezi river. It is divisible into the eastern and western rifts. The rift system consists of some graben structures arranged en echelon: each graben also consists of en echelon normal faults.
Geological phenomena in the areas of rift valleys suggest that the fracturing of the African continent had occurred under the stress field of tensional tectonics: the rifting area is apparently confined in the narrow belt along the rift valleys. We can consider the belt as a fractured region occurred between two rigid plates.
The geometrical analysis of a strain ellipse had led to a conclusion that the geometrical interrelationship of en echelon fracturing depends on the angle θ between the trend of the fractured zone and the direction of horizontal extention. We can find the latter direction by using a formula α=45°-θ/2, where α is the angle between an element of en echelon fractures and its row or array: α is defined as RE
-angle. The method was applied to the determination of the direction of the horizontal extension along the rift valley area (Fig. 1).
The eastern rift as well as the western one is considered as an element of en echelon structure and the region which involves both their elements is regarded as the first-order structural unit in Africa. It is supposed that some deep lineament in the lower part of the lithosphere might have controlled the trend and space of the structural unit since 2000m.y.