Tectonics of the Asela-Langano Margin, Main Ethiopian Rift (East Africa)

We provide insights into the tectonics of the Asela-Langano margin, at the eastern boundary of the central Main Ethiopian Rift, East Africa, by combining field structural data, remote sensing, analysis of the distribution of faults, new dating of faulted material, and analysis of seismicity. The area is characterized by the occurrence of two fault sets: boundary and axial (Wonji) faults, oriented NE-SW and NNE-SSW, respectively. Boundary faults show a complex arrangement in the lake Langano area where they display a rhomboidal pattern, due to the presence of NW-SE-trending structures, likely related to preexisting transverse fabrics (the so-called Langano Rhomboidal Fault System). Overall, our analysis supports a structure of the margin characterized by a monocline affected by a series of minor horsts and grabens; this confirms a marked asymmetry of the central Main Ethiopian Rift, with a master fault system on the opposite, western margin. Cumulative paleostress analysis indicates an overall N115 degrees E extension across the Asela-Langano margin; however, a local variation of the extension direction from axial (Wonji) faults (similar to N90 degrees E) to the boundary faults (similar to N115 degrees E) can be observed, pointing to a stress reorientation at rift margins. New(14)C dating of faulted material and seismicity analysis indicate Late Pleistocene-Holocene and current activity on several normal faults affecting the margin. This suggests that extensional deformation in the central Main Ethiopian Rift is accommodated at the rift margins, with subordinate activity on axial faults, supporting that this rift sector reflects an intermediate stage of rift evolution.