Tectonic evolution of the northwestern Red Sea margin as related to basement structure

Abstract At the northwestern, Egyptian margin of the Red Sea a crystalline basement of Pan-African, Late Proterozoic age is exposed beneath a Phanerozoic sedimentary cover. This tectonic situation provided an opportunity to study the younger, Red Sea tectonic evolution in relation to features that are inherited from or reactivated in an underlying basement. Two structural maps compiled from published sources and our data, covering the eastern Egyptian basement from Hurghada to the south, show the most important structural elements: the major, central and southern parts of the basement in eastern Egypt (CED and SED) are dominated by low angle thrusts. Other fractures (N-S, E-W, NW-SE) are distinctly less frequent and of local importance. NW-SE oriented strike-slip faults (Najd System) overprinted the low-angle thrusts of the northern CED. These fault are related to NE-SW trending normal faults, which characterize the northern part of the basement (NED). Before discussing general relationships between basement structure and Red Sea evolution a detailed example of the post-Pan-African, Phanerozoic evolution of the low-angle thrust dominated domains of the CED and the SED from a traverse south of 25° N is presented. There, detailed structural mapping showed Phanerozoic normal faults close to the Red Sea (trending NNW-SSE) and at the margin of the Phanerozoic cover east of the Nile (trending NNW-SSE and NW-SE), leading to a horst-like position of the basement. Within this horst deformation is restricted to a local sinistral strike-slip fault trending N-S. The normal faults frequently used and reactivated fractures parallel or normal to the axial surface of earlier (Pan-African) folds. Igneous activity is confined to the western part of the basement, where mafic dykes, mafic to trachytic volcanics, the alkaline Gabal Abu Khrug ring complex and trachyte plugs (in that time sequence, from 104 to 80 Ma) are exposed. Mafic dykes are dependent on host rock properties, distributed mainly in relatively brittle granitoid rocks. Trachyte intrusions are generally not related to particular host rocks but may follow pre-existing structures. Crosscutting relationships with the igneous rocks imply a Late Cretaceous age for the faults in the west of the traverse. However, in the east, at the Red Sea coast, faults cut Middle Miocene sediments and are thus younger. Tectonic activity in the traverse area is therefore confined to two distinct episodes: a Late Cretaceous event of alkaline magmatism and normal faulting at the western margin of the basement and a Miocene or younger event of normal faulting at the present Red Sea coast. In contrast to earlier views neither transform faults nor large-scale Riedel shears related to Red Sea tectonics have been met in the area. This holds also true for the CED and SED as a whole, where only local faults remain for potential reactivation during Red Sea tectonics (except, perhaps, the Duwi Shear zone at Quseir). The prevailing N-S, E-W or NW-SE directions of these faults preclude an interpretation as possible transform faults normal to the spreading axis of the Red Sea. Therefore, it is no longer possible to assume a continuation of Red Sea transform faults into the continental crust.