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(2) "colder segments" which present a rough axial morphology with a deep, wide and well-defined rift valley, a low AMBA and a small length. For "hotter segments" the formation of the abyssal hills is mainly due to a magmato-tectonic cycle over periods of 0.3 to 1 Myr, whereas on "colder segments" the axial morphology is mainly controlled by a tectonic rift valley formation. We propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years.

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A geophysical survey in the eastern Gulf of Aden, between the Alula–Fartak (52°E) and the Socotra (55°E) transform faults, was carried out during the Encens–Sheba cruise. The conjugate margins of the Gulf are steep, narrow and asymmetric. Asymmetry of the rifting process is highlighted by the conjugate margins (horst and graben in the north and deep basin in the south). Two transfer fault zones separate the margins into three segments, whereas the present‐day Sheba Ridge is divided into two segments by a transform discontinuity. Therefore segmentation of the Sheba Ridge and that of the conjugate margins did coincide during the early stages of oceanic spreading. Extensive magma production is evidenced in the central part of the western segment. Anomaly 5d was identified in the northern and southern parts of the oceanic basin, thus confirming that seafloor spreading in this part of Gulf of Aden started at least 17.6 Ma ago.