Formation of normal faults along the axial zone of the Ethiopian Rift

The axial zone of the Ethiopian Rift is made up of Quaternary extensional fractures and normal faults. Field analysis was performed to study the mechanism of development of the normal faults. The collected data show that the normal faults (1) are subvertical at surface, (2) have dilation proportional to the throw, and (3) end laterally as extension fractures, that is, tension fractures which gradually decrease in dilation. The minimum measured opening of normal faults is 2 m and the maximum measured dilation of the extension fractures is 4 m. The minimum measured length of normal faults is 800 m and the maximum measured length of the extension fractures is 400 m. The collected data suggest that the normal faults nucleate from wider extension fractures in the axial zone. When the extension fractures reach critical dimensions (length = similar to800 m and dilation = 2-4 m, corresponding to a predicted depth of similar to700 m), the shear rupture behavior controls the further propagation of the fractures at depth. This mechanism has close similarities with models previously proposed for fault formation along the oceanic ridge of Iceland. This suggests common rifting processes along diverging plates, independently from the oceanic or continental nature of the lithosphere. (C) 2002 Elsevier Science Ltd. All rights reserved.