Local Strain Reorientation Explains Deformation of Rift-oblique Tectonic Lineaments Along the Main Ethiopian Rift

The interaction between the NE-SW striking Main Ethiopian Rift (MER) and the E-W oriented Yerer-Tullu Wellel Volcano-tectonic lineament (YTVL) represents one of the least understood tectonic problems in the East African Rift System. Despite the numerous studies that have been conducted in the region, the following questions still remain to be answered: did the MER and YTVL evolve simultaneously? Was there a change in plate motion direction to allow a diachronous evolution of the rift and the lineament? How does the E-W oriented YTVL remain active under ca. E-W oriented regional extension? Previous studies propose a two-phase tectonic evolution, involving a change in the direction of plate motion at around 6 Ma causing deformation to focus in the MER. However, this interpretation contradicts plate-tectonic reconstruction data suggesting a constant plate divergence direction since ca. 16 Ma. We use analogue models to study how deformation may occur along the YTVL. We find that the activation of lineaments oriented (near-)parallel to the plate divergence direction is in fact possible, if (1) the lineament sufficiently reduces the strength of the crust and (2) the main rift trend is sufficiently oblique to the overall plate divergence direction. We interpret this to be the result of a local reorientation of the regional extensional strain caused by the obliquity of the main rift, which allows for deformation along the otherwise unfavourably oriented lineament. As such, no multiphase scenario is required to explain the development of the YTVL, and a single-phase scenario that is in line with plate tectonic reconstructions can be adopted instead. Moreover, our model results suggest that other lineaments associated with the MER could also be active in the current tectonic regime. These insights may be of relevance to the interpretation of deformation along similar oblique structures associated with the MER, and with other past and present rift systems as well.