A geophysical survey of the westernmost Gulf of Aden

The Gulf of Aden has the features of a miniature Atlantic Ocean, namely a central rough zone, main trough and continental margins. It has probably evolved within the last 45 Ma, i.e. it is approximately one third the age of the Atlantic. Being youthful, it is a good place for studying the early stages of continental drift, sea floor spreading and evolution of continental margins. Sixteen precision depth, gravity and total intensity magnetic profiles have been obtained in the westernmost Gulf of Aden along the direction N 32/212°, estimated to be the direction of sea floor spreading from the computer fit of Arabia and Somalia. In addition, a continuous seismic reflexion profile was obtained over the northern part of one of the profiles from the axial rift zone to the Arabian continental margin. The reflexion profile reveals that the basement (surface of oceanic layer 2) has at least three distinct slopes. Changes in the characters of the gravity and magnetic anomalies are noticed corresponding to the changes in slopes of the basement. In accord with recent ideas on the formation and cooling of oceanic lithosphere, it seems unlikely that the Gulf of Aden has evolved by continuous sea floor spreading and more likely it has evolved in at least three distinct phases. The earliest of these is difficult to date from the magnetic anomalies and three possible models are presented. The most likely indicates sea floor spreading from 0 to 4.5 Ma (Plio-Pleistocene), 16 to 23.5 Ma (latest Oligocene to lower Miocene) and 35.5 to 43 Ma (upper Eocene to lower Oligocene). The most surprising result is that the seismic reflexion and gravity data require the ocean-continent boundary to be between the 100 fathom contour and the coast. This implies that the continental margins are underlain by early oceanic crust and should more accurately be called oceanic margins rather than continental margins. Other discoveries include three previously unmapped transform faults and a jump in the spreading axis in the eastern part of the survey area. These, together with the locations of the recent spreading axes and a possible triple junction, are shown on a new tectonic map of the area.