RIFT OUTER HANGING WALL EXTENSIONAL STYLES: AN EXAMPLE FROM THE AFAR DEPRESSION, ETHIOPIA

During rifting, extension is dominantly accommodated within border faults and subsequently will be transferred to the rift floor either because of the injection of magamatic bodies or because of rift obliquity. However, extension within border faults might not be strongly localized. Rather, extension can be distributed with wider zones and these are referred to here as rift outer hanging wall. This work examines such extensional style by focusing on the southern margin of the Afar Depression around the city of Dire Dawa. Field study and remote sensing analysis of Landsat Thematic Mapper (TM) spectral data and Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) are used. Extension within the outer hanging wall in the southern margin of the Afar Depression is distributed within a ~15 km wide E-W trending zone. This zone is characterized by the presence of 2-3 km wide E-W elongated blocks that are tilted to the south away from the Afar Depression. These blocks are separated by N-dipping normal faults resulting in the drop of the elevation from ~2100 m to ~1300 m. Within this zone pre-rift formations represented by the Precambrian crystalline rocks of the Arabian-Nubian Shield and Mesozoic sedimentary rocks are exposed. The Precambrian crystalline rocks are dominantly overlain by a Mesozoic limestone unit, but in places thin possibly Paleozoic-Mesozoic sandstone unit is encountered below the limestone unit. The outer hanging wall is separated from the Southeastern Ethiopian Plateau (representing the footwall of the Afar Depression) by an E-trending normal fault showing a southward displacement of ~200 m. To the south of this fault the bedding planes of the Mesozoic sedimentary rocks and the layering planes of the overlying Cenozoic Trap basalts are horizontal. Unlike the outer hanging wall, a thicker and continuous Mesozoic sandstone unit is present. North of the outer hanging wall, rift-related formations of the Afar Depression (represented by the dominantly Quaternary basalts and lacustrine sedimentary rocks) are encountered at or below ~1300 m elevation. It is concluded here that deformation that extension within the outer hanging wall in the southern margin of the Afar Depression was accommodated through northward displacement on N-dipping normal faults combined with southward block rotation.