Integrated outcrop, 3D seismic, and geochronologic interpretation of Red Sea dike-related deformation in the Western Desert, Egypt – The role of the 23 Ma Cairo “mini-plume”

Abstract

Most of the northern Red Sea and Gulf of Suez formed as a non-volcanic rift. This is in stark contrast to the Afar province far to the south where flood basalts several kilometers thick cover an area of >600,000km2. The Afar volcanism erupted coevally with the onset of Gulf of Aden rifting and the two events are generally thought to be genetically linked. The lack of northern Red Sea exposed volcanic rocks suggests that this segment of the rift system initiated through a mechanism very different from Afar-Aden. However, a large late Cenozoic basalt field covering >15,000km2 does exist in northern Egypt centered on the city of Cairo. Large-scale basaltic dikes, monogenetic volcanoes, and coeval extensional faults and grabens are associated with these alkali basalts and physiographically and structurally link this province to the Gulf of Suez. Compositionally the northern Egypt basalts are very similar to the initial 31–30Ma flood basalts of Afar. New 40Ar/39Ar dating indicates that this widespread and voluminous Egyptian volcanism occurred over a short time interval of less than 2Ma at the Oligocene–Miocene boundary (23Ma). In the subsurface west of Cairo, high-quality 3D reflection seismic data indicate that narrow, linear grabens formed above the intruding dikes and show that the structural effects of Red Sea rifting extended far into the Western Desert. Circular depressions within the grabens probably formed first as conduits to overlying cinder cones and then underwent collapse when magma was evacuated from below. Some outliers of basalt flows were erupted at the cores of Late Cretaceous Syrian arc anticlines as at Bahariya oasis.The northern Egypt 40Ar/39Ar ages are indistinguishable from those of early syn-rift Red Sea-parallel dikes in western Arabia, Sinai and the Eastern Desert of Egypt. This suggests that the Red Sea propagated through Arabia/Sudan and into Egypt as a single, very rapid pulse and only stopped at the interface with stronger Neotethyan oceanic crust near the coast of the modern Mediterranean Sea. Extension almost immediately thereafter became focused in the Gulf of Suez where it evolved into well-developed rift basins, and deformation in the Western Desert and vicinity of Cairo ceased. Prior to extrusion of the northern Egypt basalts there was no identifiable surface extension and the lithosphere was not yet thinned. The relatively constant thickness of the basalts suggests that there was little pre-rift topography or uplift in northern Egypt. The volcanism of northern Egypt was volumetrically smaller than that of Afar some 7–8 My earlier, but we suggest that it played a similar role as a trigger for a large-scale rift event. We propose that this volcanic center be referred to as the Cairo mini-plume. Similar modest, focused magmatic events may have played equivalent roles in other older “non-volcanic” rift systems.