The magnetic quiet zone in the eastern Gulf of Aden: implications for the early development of the continental margin

Abstract

Summary. The ‘magnetic quiet zone’ in the eastern Gulf of Aden is located between the oceanic crust of Sheba Ridge and the continental crust of Arabia and Somalia, and is separated from both by important structural boundaries. The seaward boundary is marked by the end of the seafloor spreading magnetic anomaly sequence and by a basement depth discontinuity. The landward boundary is marked by escarpments made up of a series of normal faults. These escarpments extend from 2–3 km below sea-level to 1500 m above sea-level and are equivalent of the ‘hinge zone’ found at mature continental margins. The magnetic field in the quiet zone is flat in some areas and in others is characterized by anomalies of up to several hundred gammas which are correlatable over distances of up to about 20 km and which appear related to basement topography. The basement lacks the topographic slope characteristic of mid-ocean ridge flanks and is characterized by moderately rough relief. The crustal structure appears quite heterogeneous and where the crustal thicknesses have been determined, they are slightly greater than those of oceanic crust. New heat flow measurements show high values (95.7–123.3 mW m−2) in the quiet zone with values decreasing from Sheba Ridge toward the coast. The unusual structure of the quiet zone and the observations that more opening appears to have occurred between Arabia and Somalia than can be accounted for by the oceanic crust of Sheba Ridge leads to the suggestion that the magnetic quiet zone was generated by diffuse extension of continental crust through a combination of rotational (listric) faulting and dyke injection. This possibility is investigated using both a ‘stretching’ or ‘lithospheric attenuation’ model and a model in which a portion of the extension occurs through dyke injection. It is found that these models can adequately match the observed heat flow and basement depths although very large amounts of extension (β= 4–6) are required in the deep seaward portion of the quiet zone. This results in more extension than is compatible with the documented motion between Arabia and Africa. However, formation of the magnetic quiet zone occurred over a period of 10–15 Myr rather than instantaneously as assumed in the simple models. When the effects of a finite length rifting episode are considered, less extension is required and the observed geophysical data are consistent with a diffuse extension origin for the magnetic quiet zone.