Geophysical study of the Gulf of Aden continental margins : geodynamic implications for the development of the Afro-Arabian Rift System

• Main Author: Sahota, G.
• Published: Swansea University 1991
• Subjects: 550
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638740

This thesis contains gravity, magnetic and seismic data from two surveys over the Gulf of Aden. It examines the structure of the continental margins, the seafloor spreading history of the Gulf and the geodynamic implications for the development of the Afro-Arabian Rift System. Total opening has occurred by crustal extension and seafloor spreading. The extent of seafloor spreading opening in the Gulf of Aden (and Red Sea) is greatest farthest from the Eulerian opening pole; crustal extension increases as the angular distance to the opening pole decreases. In the eastern Gulf, average B is 1.4 across the margins and the transition from continent to ocean occurs over 40 km. In the central and western Gulf, average B is 1.7 and the continent-ocean transition zone is 50-80 km wide. The transition zones are magnetically quiet. The extension mechanism changes from pure shear extension in the east to pure shear extension with dyke injection in the west. Seafloor spreading models of the Gulf show that oceanic crust has formed at Sheba Ridge axis in a single continuous episode. Oldest oceanic crust in the Gulf is about 20 Myr (anomaly 5E) and not 10 Myr (anomaly 5) as widely cited in the literature. This work does not support the 2-stage seafloor spreading models proposed for the Gulf of Aden and Red Sea. The location of rift boundary faults, continental margins, fracture zones, magnetic quiet zones and oceanic crust in the Gulf can be explained by a rift tip migrating through the lithosphere toward the Arabia-Somalia opening pole. The propagating rift focused the extension along the weakest part of the lithosphere until it split apart to form a seafloor spreading axis. The seaward magnetic quiet zone boundary is not an isochron and the age of the oldest isochron decreases poleward. Up to 10 Myr differences in the oldest isochron occur across major fractures, where oceanic crust juxtaposes magnetic quiet zone crust. Occasionally, rift tip propagation slowed across zones resistant to fracturing or 'soft zones'. Across these, plate divergence ahead of the rift tip occurred by crustal extension forming magnetic quiet zones while behind, new crust was emplaced at the oceanic rift. Major fractures temporarily separated crustal extension zones from seafloor spreading zones. Eventually the crust split apart and the rift tip continued its poleward migration; it presently lies near Lake Asal, at the western end of the Tadjurah Gulf. The evolution of the Gulf constrains the timing of plate motions in the Gulf of Aden-Red Sea area to continuous development since Late Oligocene-Early Miocene. Only the narrow axial zone of the southern Red Sea is underlain by oceanic crust formed in the last 5 Myr. The remaining opening across the flanking wide main troughs and also across the entire northern Red Sea has occurred by pure shear extension with dyke injection over the last 20 Myr. Total opening in the Gulf of Aden and plate kinematics suggest 125% extension across the northern Red Sea and Suez Gulf, a figure supported by existing field data.