AAPG Middle East Region Geoscience Technology Workshop, Rift Basin Evolution and Exploration

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The Gulf of Aden Margins System: Evolution from Magma-Poor to Magma-Rich Margins

Abstract

The Gulf of Aden margins system is strongly influenced by plates boundaries and volume forces and structural inheritance. During the stretching phase (Oligocene), major fracture zones individualize margin segments with specific structures and evolutions. Distributed deformation is accommodated along faults (N110°E) that are orthogonal to the extension (N20°E) under the effect of stresses at the plate boundaries. These faults reactivate N110°E oriented structures inherited from the Mesozoic and are active until the formation of the Ocean Continent Transition(OCT) in Burdigalian. During the thinning phase (Upper Oligocene - Lower Miocene), the western margins near the Afar mantle plume undergo a weakening of the lithosphere. The prevalence of buoyancy forces then causes the abandonment of the structural inheritance and the development of oblique structures at the extension (N70°E) until the formation of OCT. In this part of Gulf of Aden, crustal attenuation is associated with a salt tectonics of the Yemeni margin with the gravitational sliding of syn-rift units in the OCT during its formation. We propose that crustal thinning in the asymmetric margins of the eastern Gulf of Aden is essentially accommodated by the flow of the lower crust. During the thinning phase (Chattien - Burdigalien), the deformation which is until then symmetrical becomes asymmetrical. In the eastern part of the Gulf (Socotra-Sharbithat segment), crustal break-up occurs during the development of antithetical faults that cut across the entire continental crust and root in the mantle. They are at the origin of the formation of a hyperstretched continental domain (of thickness < 10 km) in the distal margins. Crustal attenuation leads to the asymmetric ascent of the asthenosphere towards the southern margin. This episode is followed by the migration of the mantle exhumation channel and the deformation to the north. In all the segments studied, crustal thinning ends with the development of exhumation faults that dip northward and complete the formation of very asymmetric margins (shorter in the South (50 - 160 km) than in the North (60 - 200 km)). Moreover, conjugate margins of the Gulf of Aden are characterized by a diachronous volcanism of intensity decreasing towards the East. These characteristics are probably related to a decreasing influence of the Afar plume toward the east. Thus, the margins of the central Gulf of Aden are called hybrid. They are distinguished by SDRs set up from the Lower Miocene in the West. Further east, volcanism appears during and/or shortly after the subcontinental mantle exhumed in the OCT (Burdigalian) during the activity of multiple detachment systems. Volcanoes are observed in the magma-poor distal margins of the Gulf of Aden in the eastern Gulf of Aden by partial melting of the mantle during the exhumation phase (~21-18 Ma). Mantle exhumation is driven by detachment faults that migrate with the relocation of deformation to the exhumed domain. However, no major volcanism was observed before the emplacement of oceanic proto-crust in Burdigalian (~18 - 17 Ma). The establishment of a steady state oceanic crust occurs around 17 Ma.