Transition from continental to oceanic rifting in the Gulf of Aden: Structural evidence from field mapping in Somalia and Yemen

The analysis of the spatial/temporal relationships between the tectonic structures of the continental margins bordering the Gulf of Aden and the adjacent oceanic structures provides a key for understanding the transition from continental to oceanic rifting. The Gulf of Aden is a young ocean in which generation of oceanic crust between the Tadjura Gulf and the Indian Ocean began 11 m.y. ago; west of the Tadjura Gulf spreading is more recent, as no oceanic crust older than 2 Ma has been found. Detailed field investigation along the continental margins of northeastern Somalia and southeastern Yemen (areas adjacent to the most prominent oceanic structures) has revealed a tectonic setting characterised by half-graben structures and tectonic depressions striking WNW-ESE. An Oligocene-Miocene age for this graben formation is documented by infilling of syn-rift sediment. Faults bordering contiguous half-graben systems sometimes dip in opposite directions, so that in the transition area between different systems a ''transfer zone'' or ''accommodation zone'' develops. The shelves of both margins are divided by NE-SW-oriented ridges and by WNW-ESE-oriented structural depressions. The former are always located on the projection into the continent of the oceanic fracture zone and correspond to the transfer zones present on the continental margins, while the tatter are located on the projection into the continent of the oceanic ridges and correspond to the Oligo-Miocene syn-rift basins. On the basis of structural and stratigraphical data from northeastern Somalia and southeastern Yemen it is possible to reconstruct the pre-Oligocene tectonic setting for the Gulf of Aden and its later rifting evolution. This study brings new evidence to demonstrate that: (1) During the Early-Middle Oligocene the Afro-Arabian Plate underwent a phase of intense faulting which led to the formation of small syn-rift basins elongated in a WNW-ESE direction, separated by transfer zones and structural highs. (2) Progressive crustal extension led to formation of spreading centers of oceanic crust in correspondence with the syn-rift basins. The fracture zones linking spreading centers formed in correspondence with continental transfer zones. (3) The progressive separation of the Somalian and Yemenite continental margins led to the formation of a complete oceanic basin.