ABSTRACT
The structures within continental shields potentially influence the manner in which the shields are rifted and ultimately the development of oceanic seafloor spreading cells. Lying within a young oceanic basin, the Red Sea margins have thermally subsided less than those of mature ocean basins and thus could reveal these influences better. Here, we investigate velocity structures from three seismic refraction lines off Sudan. Seismic velocities are converted to densities and used to predict free-air gravity anomalies to compare with satellite-derived anomalies. The density structures are also tested in an isostatic sense by computing their column weights at discrete locations. The predicted anomalies are found to be ~40 mGal smaller than those observed at two locations close to inferred ocean-continent boundaries on their oceanic sides. This suggests that the crust does not thin as rapidly seaward as originally inferred. In contrast, at two further locations over the continental crust, observed anomalies are up to 60 mGal larger than those predicted. The refraction experiment therefore did not detect bodies of high velocity and/or over-estimated crustal thickness in these places. They lie offshore from suture zones within the Nubian shield; high densities could be due to mantle or other high-density bodies incorporated within the crust when the original collision events created the suture zones. If correct, the Red Sea contains clues to how segmented gravity structures, which are common at continental margins, could be due to old shield structures.
