ORIGIN OF INITIAL RIFTING IN THE RED SEA: WAS IT VERTICAL TECTONICS?

The evolution of the Red Sea has been explained as due to the separation of Arabia from Africa first (~22-10 Ma) in a NE-direction (rift orthogonal extension) due to rift push and second (~10-0 Ma) in a N-direction (rift-oblique extension) due to slab pull. This followed the extrusion of ~30 Ma Afar mantle plume volcanism. However, the Red Sea is characterized by a number of features that cannot easily be explained as associated with a rift basin that started with just the horizontal translation of Arabia from Africa: (1) The Red Sea is topographically asymmetrical. The Arabian side shows well-developed topographic escarpments that exceed ~1250 m whereas the escarpments in the African side are subdued and do not exceed 750 m. (2) Arabia shows a long wavelength eastward tilting away from the Red Sea. (3) The Red Sea is also magmatically asymmetrical. All ~30, ~22, and ~10 Ma magmatism occurred along the western margin of Arabia. (4) Correlation of Precambrian structures exposed on both sides of the Red Sea suggests that the basin can be closed near coast-to-coast indicating the lack of lithospheric attenuation before rupturing. (5) Fission-track studies suggest that the Red Sea opened at once along its entire length without lateral rift propagation following ~24 Ma uplift. (6) Geophysical data showed that the Moho rises sharply from ~40 km depth under the topographic escarpments of the Red Sea to ~10 km under its shorelines.Processing of seismic tomography data shows that the 0-100 km layer under the Red Sea and the western part of Arabia is characterized by a N-trending, ~600 km wide band with significantly slow shear wave velocity. This band is also observed under the Afar Depression and the Main Ethiopian Rift where it has a NE trend. The band disappears south of the Main Ethiopian Rift. We interpret the slow shear wave velocity band as a mantle flow rising from south of the Main Ethiopian Rift, flows in a NE-direction under the Main Ethiopian Rift and the Afar Depression, then flows in a N-direction under the Arabian side of the Red Sea. Further, we propose that this mantle flow might have started rising ~30 Ma under the Arabian side of the Red Sea, providing enough vertical lifting to initially open the basin at its entire length without lithospheric attenuation, tilt Arabia eastward, and provide ~30, ~22, and 10 Ma magmatic pulses on the Arabian side.