Ductile shear zones in the northern Red Sea Hills, Sudan and their implication for crustal collision

Abstract

In the northern Red Sea Hills, of Sudan, deformation is largely concentrated into a network of shear zones. Several of these zones strike north–south, but the important Nakasib and Sol Hamid zones are northeast-trending, and there are also minor shear zones which strike east–west. We distinguish between massive shear zones, such as that of Nakasib, and more diffuse braided shear zones, typified by the Oko zone. The massive type owes its character to an origin as a reactivated oceanic suture, whereas the braided type was characterized by strike-slip shearing from its inception. Most of the shear zone rocks are mylonites formed under greenschist facies conditions, but early shearing along the Oko zone took place at higher temperature and resulted in gneissose mylonites with amphibolite facies mineralogy. The northeast-trending, Nakasib shearing appears to have preceded a phase of batholithic intrusion, whereas north–south shearing in the Oko and Abirkitib zones is younger than the batholithic intrusions and is in turn post-dated by emplacement of bimodal granite–gabbro complexes. These events cannot be dated precisely as yet, but took place between 99 Ma and 700 Ma, and probably towards the end of that interval. The pattern of shear and suture zones in northeast Sudan suggests that the northeast-trending Hijaz and Asir arcs, recognized in Saudi Arabia, terminate to the west against a north–south suture which was later rejuvenated to form the Abirkitib shear zone. West of this suture was an immature volcanic arc which may have lain along the margin of the African continent. The rejuvenation which caused northeast and north–south strike-slip shear was probably a consequence of soft collisions in East Arabia.