Segmentation and along-strike asymmetry of the passive margin in Socotra, eastern Gulf of Aden: Are they controlled by detachment faults?

On the island of Socotra, the southern passive margin of the Gulf of Aden displays along its strike two different types of asymmetric structures. Western Socotra is made up of a series of southward tilted blocks bounded by consistently northward dipping normal faults. Eastern Socotra consists of a broad asymmetric anticline with a steep northern limb and a gently dipping southern limb. A zone of NE-SW striking strike-slip and normal faults separates the two areas. The overall structure is interpreted as representing two rift segments separated by a transfer zone. The along-strike juxtaposition of crustal-scale asymmetric structures on the southern margin of the Gulf of Aden is complemented by the asymmetry of the conjugate margins on either side of the gulf. Whereas the western Socotra margin is narrow and characterized by oceanward dipping normal faults, the conjugate Oman margin is broader and dominated by horsts and graben. Considering that asymmetric structures in the upper crust are often associated with synthetic shear zones at deeper ductile levels, we propose that the western and eastern Socotra margin segments were controlled at depth by two detachment faults with opposite dips and senses of shear. The normal faults of western Socotra would sole out into a top-to-the-north ductile shear zone, whereas the asymmetric anticline of eastern Socotra would be associated with a top-to-the-south detachment fault.