The transition between the Sheba Ridge and Owen Basin: rifting of old oceanic lithosphere

Abstract

Summary Magnetic quiet zones are present along the margins of the entire length of the Gulf of Aden to the Owen fracture zone. This includes the easternmost 300 km between the eastern edges of Arabia and Africa to the Owen fracture zone where old oceanic lithosphere was rifted to form the Sheba Ridge. Within this easternmost region the boundary between the quiet zone and the old oceanic lithosphere is marked by ridge complexes, the Sharbithat Ridge Complex to the north and the Error Ridge Complex to the south. These ridge complexes, which lack a magnetic signature, occupy a structural position similar to the hinge zones at the continental margins to the west. They appear to have formed early in the opening of the Gulf of Aden or perhaps to have been pre-existing features. The boundary between Sheba Ridge and the northern magnetic quiet zone is often marked by an abrupt end of the Sheba Ridge seafloor spreading magnetic anomaly pattern and a sharp basement deepening to the north. The boundary between the East Sheba Ridge and northern magnetic quiet zone becomes less distinct near the Owen fracture zone. This is also accompanied by changes in the East Sheba Ridge, specifically a decrease in magnetic anomaly amplitudes, increase in ridge flank depths and a loss of the ’cooling-curve’ ridge flank shape. This may be the result of lower mantle temperatures in the vicinity of the Owen fracture zone. The sediments within the magnetic quiet zone can be divided into a lower disturbed unit and an upper unit consisting of flat-lying reflectors. The disturbance of the lower sediments may have resulted from a period of diffuse extension in the magnetic quiet zone prior to the establishment of the Sheba Ridge spreading centre. The similarity of the easternmost quiet zone to quiet zones at rifted continental margins leads to the suggestion that these regions were formed by diffuse extension of old oceanic lithosphere. Using a two-layer lithospheric attenuation model and assuming extension during a period of 15 Myr followed by 10 Myr of cooling, the basement depths and heat flow measurements can be adequately matched. The modelling implies 45 per cent crustal extension in the quiet zone. The amount of extension calculated is compatible with documented motion between Arabia and Africa. The old oceanic lithosphere thus must have been substantially thinned to a thickness similar to 10-14 Myr old ocean before seafloor spreading was initiated at the Sheba Ridge.