Crustal and uppermost mantle structure beneath the Afro-Arabian rift system

Abstract

This contribution presents data and results from long-range seismic refraction-wide-angle reflection lines in the Afro-Arabian rift system. From the derived crustal structure an evolutionary model is proposed in terms of the amount of crustal thinning. In this model, the Gulf of Aden and the axial trough of the southern Red Sea, where the continental crust was thinned so much as to allow thin, oceanic crust to form, are representative of one end of the spectrum. Progressing through the northern part of the Red Sea main trough on the Egyptian side, with a crustal thickness of 11–14 km, to the northern and southern parts of the Red Sea main trough on the Arabian side and the northern Afar depression with 15–20 km thick crusts, followed by the southern Afar depression with a crustal thickness of 22–26 km, the members of the other end of the spectrum are reached. These include the Jordan-Dead Sea and East African rifts which, each with a crustal thickness of 30–36 km, represent the least thinned, closest to “normal” thickness continental crust. In determining the amount and nature of opening of the Red Sea there remain, however, many problems argely due to the non-unique correspondence of seismic velocity to rock type. This may lead to uncertainty in Moho depths especially in areas close to the mid-ocean ridge system e.g., the southeastern part of the Afar depression in the region around the Gulf of Tadjura, where velocities as low as 6.8–6.9 km/s have been interpreted as representirg hot, anomalous mantle. It also causes uncertainty as to what percentage of the present crust is stretched and thinned original continental crust and what percentage is new crust, whether it be truly oceanic or basic intrusiors into continental crust. Thus it is felt that there is still an urgent need for geoscientific experiments in the Afro-Arabian rift to resolve the details of continental rifting and subsequent oceanic crust emplacement.