Tectonics of the westernmost Gulf of Aden and the Gulf of Tadjoura from submersible observations

Abstract

The Gulf of Tadjoura, separating the Danakil and Somalia plates¹, represents the western extension of the Sheba Ridge between Arabia and Somalia in the Gulf of Aden (Fig. 1). It has been assumed that a simple plate-tectonic scheme could be applied to the area with NE–SW transform faults offsetting roughly east–west zones of accretion represented by the major bathymetrie troughs^1–8 (Fig. 1c). More recently this boundary has been interpreted as a westward-propagating rift with its present-day tip on land in the region of the Asal Lake^9,10. Here we present data from dives with the submersible Cyana which show that there have been two episodes of extensional faulting, an older inactive phase parallel to the general ENE–WSW direction of the gulf and a younger fault set along discontinuous zones striking 140°. There is no surface evidence for any transform fault zone in the area covered and the incipient spreading segments whose geometry implies extension on 050° do not define a plate boundary. The faulting observed is at odds with that predicted by orthodox plate tectonics.