On Afro-Arabian graben tectonics

This study of the Afro-Arabian gräben, which concentrates on the region between Kenya and the Levant reviews their evolution that begun not earlier than in the latest Oligocene or in the lower Miocene. The gräben continue to sink in several major downfaulting phases, especially during the transitional times of Miocene to Pliocene, of Pliocene to Pleistocene and in the Mid-Pleistocene. We have no proof of erstwhile major graben structures in the crystalline Precambrian basement or in the pre-Oligocene sedimentary cover revived during the young Cenozoics that could have redeemed the formation of the Afro-Arabian megagräben. Significant faults in the pre-Oligocene sedimentary cover are seldom observed. Where they occur, their tectonic direction scarcely runs parallel to the principal trend of the Cenozoic graben. At best these ancient faults may have had an impact on the development of cross — and diagonal faulting which played also a role in the disposition of the subgräben, grabensplays and other offshoots of the main gräben. Equally rarely has parallelism of trend between the basement metamorphics as well as their intrusive dykes and that of the main graben lineations been established. The Afro-Arabian gräben originated in a lower Tertiary peneplain of regional dimension which had expanded over large part of Gondwana. The graben dissection thus took place regardless of former tectonic highs and lows such as swells and basins — a phenomenon that negates the theory of key-stone dropping as a result of preceding upwarping. The occasional impression of vaulting with the graben as its axis is purely of morphological significance; it is due to “isostatic” uplifting and tilting of the main horsts attached to the main graben. Besides, most megahorsts are not “halfhorsts” but huge blocks mountains (Levant, Etbai, Ethiopia, Danakil, Yemen) bordered on both flanks by major tensional faults with throws of thousands of meters. These major uplifts, which are especially noticed in the Plio-Pleistocene, caused, moreover, the removal of the sedimentary cover by erosion and denudation in the elevated parts of the graben shoulders and its hinterland. The few known outcrops of Cretaceous-Eocene in the coastal area of the Red Sea, believed to be indications of ancient marine gulfs and thus forerunners of the Rift valley are in all likelihood the relics of such coverbeds that were saved from erosion in the down dropped blocks of the graben. The surveyed marginal faults are normal dip-slips of average 70° dip. These remained in the ensuing taphrogenic episodes the “localized” fixed graben frames. In the interior of the graben they are followed by a Vorberge zone consisting of tilted blocks that implicate also subsidiary horsts and gräben. In the superdimensional rifts of Afar and Red Sea, the subsidiary gräben and horsts of the Vorberge zone may approach a width comparable to that of the East Levant rift valleys. In the axial part of the Afar, Red Sea and Gregory Rift valley appears as youngest, that is, Pleistocene structural element a distinct graben-in-graben or rift-in-rift. This young structure is much less developed in the graben of the Gulf of Aden. The Aden graben differs also in other aspects from the pattern of the Red Sea, particularly in its submarine axial topography and in the peculiar transversal fault offsets, both morphotectonic features reminiscent of the Indian mid-oceanic ridges. A comparison of geophysical, especially of gravity measurements, carried out in the various main gräben reveals very conflicting results. While higher positive Bouguer values led to the conjecture of heavy magmatic material in the bottom of the rift-in-rift of the Red Sea, in most of the Afar fault-funnel and in the entire Gregory Rift valley there prevails an unusual large gravity deficiency that would point to an extremely thick crust. This is all the more remarkable as both rift valleys and horsts of Ethiopia and of Kenya are dominated by huge basaltic trap lavas and by other basic volcanic material. The hypothesis of anticlockwise rotation and drifting of the Arabian peninsula (including its supposed impact on the orogenic origin of the Zagros fold belt) has many pitfalls.

1. As the taphrogenic destruction is not restricted to the intercontinental Afro-Arabian gräben, but extended far into the Mediterranen and into the Indian Ocean, any anticlockwise rotation of Arabia must exceed the 7° postulated movement.
2. Yemen and Ethiopia, that is Arabia and Africa, were in the Miocene still a continuous continent. When they became separated in the Pliocene by the Bab el Mandeb splay, northern Sinai and northernmost Egypt had been transformed into a landlocked isthmus that has remained a continental bridge to this day. In fact at the end of the Miocene the Red Sea graben had already been turned — though for a short period — into an enclosed evaporitic basin.
3. As regards the axial graben-in-graben of the Red Sea that especially prompted the assumption of a drift between Africa and Arabia, its Pleistocene age would demand a drifting rate of half a meter per year. There are, however, at the coastal area of the Red Sea no prehistoric, historic or recent witnesses of lateral stretching at the Red Sea and certainly not of such excess. The deduced annual drift of half a meter there, exceed even the most optimistic calculations and images hitherto made by the adherents of such large drifting as has been postulated to have taken place between Africa and South America.
4. Finally, in the studies on drifts — and this also applies to the question of horizontal fault displacement — there is wanting a proper discussion on the mechanical deformations to which the rigid crustal parts must have been subjected in the course of such intensive tearing apart motions.