Oblique rifts: structure, kinematics and infill. The example of the Main Ethiopian Rift

The Main Ethiopian Rift (MER) is an oblique continental rift separating Nubia from Somalia. Since decades, it is a natural laboratory for geologists and geophysicists who study continental rifting processes, tectono-magmatic interactions within an extending continental crust, incipient crustal break-up and seafloor spreading. In oblique rifts, such as the MER, the deformation pattern is characterized by two sets of anachronously formed faults: (i) border faults defining the outer boundaries of the rift and trending obliquely to plate displacement which are controlled by pre existing weaknesses and (ii) internal faults affecting the rift floor that trending orthogonally to the plate displacement and formed in response to the regional stress field. The amount of obliquity controls the strain migration from border faults to inner faults, increasing strain localization, enhancing both faster and higher thinning and triggering the emplacement of larger magmatic bodies.

In such oblique rifts, it is commonly expected that structures striking orthogonal to the extension direction show dip-slip kinematics whereas those being oblique display strike-slip kinematics. However, natural oblique rifts, among which the MER, show earthquake focal mechanisms and fault slip data contradicting this statement. With the aid of analog models, we investigate faults kinematics of oblique rifts. We show that there is a re-orientation of the extension direction, giving rise to pure dip-slip