RELAY ZONES: AN EXPLANATION FOR THE JOGS ALONG THE EASTERN MAIN ETHIOPIAN RIFT ESCARPMENT

The border faults along the eastern margin of the Main Ethiopian Rift (MER) are segmented and display en echelon arrangements. The three prominent en echelon border fault pairs identified from Enhanced Thematic Mapper Plus imagery and ASTER digital elevation model data, from north to south, are Arobye-Sire, Huruta-Asela and Langano-Munesa. The en echelon pair of rift bounding faults are linked by relay zones that are breached. The Arboye-Sire border faults strike N45ºE and are connected by a ~30 km wide relay zone that is breached by a north dipping, ESE striking (96º) normal fault. The Huruta-Asela border faults strike N35ºE and are connected by a ~12 km wide relay zone. The relay zone breaching fault has an overall ESE strike (120º) and dips to the north. The Langano-Munesa border faults display different strikes, N30ºE and N10ºE respectively, and are linked by a relay zone that is ~6 km wide. The relay zone breaching fault strikes ESE (130º) and dips to the south. These relay zone breaching normal faults, which at times have been interpreted as strike slip faults, are in fact extensional deformation features that link the interacting en echelon pair of rift bounding normal faults. The rift bounding faults are believed to have developed at the initial stage of rift formation and are inactive. The presence of relay zones between the border faults signifies that these en echelon pair of rift bounding faults must have formed contemporaneously. This finding questions the notion that the different segments of the Main Ethiopian Rift are initiated at different times, particularly between the Northern and Central segments whose eastern boundary is believed to lie along a breached relay zone.