Transversal structures in the Main Ethiopian Rift and implications for rifting, volcanism and geothermal systems

Although systems of extension-orthogonal normal faults are the typical expression of continental rifting, structures oblique or even sub-parallel to the plate motion vector may at places develop. These transversal structures, which may be related to reactivation of pre-existing structures, have been suggested to have a control on rift-related volcanism and caldera systems (for instance controlling their extension-parallel elongation) and on the circulation of fluids in geothermal fields associated to these volcanic features. Here we present data from the central sector of the Main Ethiopian Rift showing the existence of a major NW-SE trending system, which likely resulted from the linkage of major segments of the Main Ethiopian Rift exploiting pre-existing structures. This transversal structure, characterised by both a normal and right lateral slip component, influenced the morphology, hydrography, seismicity and volcanism in the area. Specifically, this NW-SE structure, and the interaction with the NE-SW faults delimiting the rift, controlled the location and alignment of silicic volcanism in the Bora-Bericha and Tulu Moye systems. Seismicity analysis indicates that this transverse structure is still active, reaching at least mid-crustal depths, and indicates that this structure is involved in the fluid circulation at upper crustal levels in the associated geothermal system. Therefore, transverse structures may have important influence on rift evolution and architecture and on the pathways of magma and on the flow pattern of crustal fluids, with important implications for geothermal activity related to continental rifting, as suggested for other important geothermal fields in the Main Ethiopian Rift.