South of 21 degrees N, the main trough is bisected by a deep axial trough which has formed by sea-floor spreading during the past 4 m.y. and is associated with large-amplitude magnetic anomalies and high heat flow. North of 25 degrees N, an axial trough is not present and the main trough has smooth low-amplitude magnetic anomalies. Between these regions, the axial trough is discontinuous. The different regions represent successive phases in the rifting of a continent and the development of a continental margin. The main trough formed during the Miocene by the same processes of diffuse extension that are still active in the northern Red Sea. The initial pre-sea-floor spreading stage results in considerable extension (160 km) at 25 degrees N, can last for several tens of millions of years, and is an important factor in the development of the continental margin.--Modified journal abstract.