A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - Southern Arabia and northern Somalia: Comparative geology

Abstract

Detailed geological compilation maps for the two sides of the Gulf of Aden from various published and unpublished sources were originally prepared on 1:500 000 scale and later reduced to 1:1 000 000 scale (figures 3 and 4, in accompanying wallet). These were reconstructed to postulated pre-separation positions along the 100- and 500-fathom (183 and 914 m) bathymetric contours to give acceptably close fits for most of the area west of Alula-Ras Fartaq, leaving water gaps of 25 40 and 50-80 km respectively between the two shores. The 500-fathom fit shows several striking similarities and apparent continuity of geological features between the two sides, but detailed correlation reveals a number of offsets and mismatches. The 100- fathom fit gives a considerably improved overall correlation with continuity of structural and facies belts across the intervening water, though detailed comparisons do not bring out many correlations that give direct and positive geological support for separation and these are mainly structural rather than stratigraphic. This paucity in positive correlation may be partly accounted for by the width of the remaining water gap and by the differing geomorphology and degree of erosion between the eastern portion of the two sides. Although the reconstructions reveal appreciable circumstantial support for separation and no major geological evidence against it, an overlap results in the extreme west of the map area (appreciably greater for the 100-fathom fit) where Basement and Mesozoic rocks on either side are superimposed on one another; a considerably greater overlap occurs outside the map area to the west between similar rocks of Yemen and the Danakil alps of Ethiopia. If Arabia moved as a single block in a northeast direction and with counter clockwise rotation away from the Somali (and Nubian) block, reconstructing it to its preseparation position requires the satisfactory resolution of this overlap before it can be acceptable; a separate northeast movement of the Socotra shelf is moreover required to accommodate a fit with the reconstructed Dhufar-Kuria portion of the Arabian block.