Quaternary volcano-tectonic activity in the Soddo region, western margin of the Southern Main Ethiopian Rift

We present an analysis of the distribution, timing, and characteristics of the volcano-tectonic activity on the western margin of the Southern Main Ethiopian Rift in the Soddo area (latitudes between similar to 7 degrees 10'N and similar to 6 degrees 30'N). The margin is characterized by the presence of numerous normal faults, with limited vertical offset and often sigmoidal in shape, which accommodate a gentle transition from the rift floor to the Ethiopian plateau. New radiocarbon dating indicates post-30ka fault activity, pointing to a significant Late Pleistocene-Holocene tectonic activity of the Soddo margin. Comparison of the fault architecture with analog models suggests that deformation has been controlled by a sub-E-W (roughly N100 degrees E) extension direction, resulting in an oblique extension with respect to the roughly NE-SW-trending rift. This well accords with inversion of fault slip data collected on faults with Pleistocene-Holocene activity and is also in good agreement with recent GPS data from the Southern Main Ethiopian Rift. Our data support a close correlation between the recent volcanic activity and deformation in the study area, with eruptive vents located along the recent border faults; the axial tectono-magmatic activity is subordinate in the area. These findings support a transition from axial tectono-magmatic deformation in the Northern Main Ethiopian Rift to marginal deformation in the Central and Southern Main Ethiopian Rift, in turn indicating an along-axis, north to south decrease in rift maturity.