Mesozoic and Precambrian rocks are exposed in the Danakil Alps of coastal Eritrea, but the Danakil Depression between the Alps and the Ethiopian plateau is covered by Tertiary-Quaternary rocks. The physiography, structural geology, egional stratigraphy and evaporite lithofacies distribution of this area all suggest that it is underlain by an asymmetrically subsided block of old sialic crust. The western edge of this block has subsided deeply along the Ethiopian rift and is covered, in the Danakil Depression, by an evaporite-basalt veneer, but its eastern edge has been uplifted as the Danakil Alps. These are bounded on the east by a rift escarpment facing the Red Sea. Although geologic data here is sparse compared to the Danakil region, certain features suggest that a similar asymmetrically subsided block of older sialic rocks, with an evaporite-basalt veneer, may also lie beneath much of the Red Sea. This tectonic evolution apparently commenced in Miocene time with rifting near the centre of an earlier Mesozoic-Paleogene sedimentary basin. Uplift along this central rift caused tensional failure along a secondarily induced rift to the west, and east-side-down subsidence along this structure formed the asymmetrically subsided block. There were apparently two successive cycles of this tectonic activity. The earlier, of Miocene age, formed the easterly (Red Sea) block with a thicker veneer of older evaporite-basalt, and the later, of Plio-Pleistocene age formed the westerly (Danakil) block with a thinner veneer of younger evaporite-basalt. The separation of Arabia from Ethiopia across the southern Red Sea would thus be relatively minor, presumably represented by the width of the Red Sea's axial trough plus a few kilometres across each of the Danakil Alp and Ethiopian rifts. Similar tectonic developments may accompany initial rifting and separation in the development of ocean basins by seafloor spreading, and might explain why oceans like the Atlantic, that have apparently developed in this manner, are fringed by shallow continental shelves with thick evaporite sequences and steep walled submarine canyons.