Median valley tectonics: Air photographs of the Ghoubbet-Asal rift, Afar

The Ghoubbet-Asal rift in Afar is the western continuation of the Gulf of Aden seafloor spreading centre onto land. Air photographs were used to examine its small-scale tectonic structures. A central, relatively flat, zone about 5 km across is the main centre of volcanism. This is bounded by large-throw normal faults spaced at 1–2 km with scarps that face the spreading axis, which are responsible for the gross morphology of the lower walls of the median valley. Short, small-throw, antithetic faults are situated near the base of the main fault scarps in places. These are formed as the main fault blocks ascend the median valley walls, and make only a minor contribution to the morphology of the median valley. Rotation of the main fault blocks away from the spreading centre, as the crust ascends the valley walls, is the main process giving rise to the morphology of the transition from the median valley to the crestal mountains. Antithetic faults close to the bases of major synthetic faults seen on the air photographic data are uncommon observations on Gloria side-scan sonar data for marine slow-spreading centres. If present in greater numbers, however, the scarps of such features would lie in acoustic shadows cast by the major synthetic faults and thus be hidden from Gloria. Other interesting features on the air photographic data include a subsidiary rift that intersects a large-throw normal fault associated with the main rift, and a linear volcanic ridge which constitutes the main centre of volcanism within the spreading centre.