Middle Tertiary rift volcanism in a continental-rift valley in the Arabian-Nubian Shield was the first surface expression of active mantle convection beneath an axis that was to become the Red Sea. Investigation of the coastal plain of southwestern Saudi Arabia suggests that the rift valley was filled with basaltic and felsic to rhyolitic volcanic rocks (Ad Darb and Damad formations), cherty tuffaceous siltstones (Baid formation), and subordinate Nubian-type quartz sandstone (Ayyanah sandstone) between about 30 and 20 Ma ago. These rocks are named herein the Jizan group. At the same time, alkali-olivine basalt was erupted on the stable Precambrian craton at locations 100 to 200 km east of the rift valley axis.
First-stage spreading of the Red Sea began about 20 Ma ago when diabasic dikes and gabbro and granophyre plutons (Tihamat Asir complex) of oceanic-tholeiitic parentage were intruded into the continental-rift deposits. The new oceanic crust was injected into a region of thinned continental crust. Because of thinning, the continental crust was extended and became fractured into a series of fault blocks. Crustal extension in the attenuated zone was accompanied by mafic and silicic volcanism producing a continental rift-valley tectonic environment. Fault blocks of the rift valley were initially rotated toward the Red Sea during the first-stage spreading episode. Inland from the continental rift, the thick continental crust was distended on long parallel fractures that were intruded by gabbroic to quartz syenitic magma to form continental dikes. This period of continental-margin extension was short lived, lasting perhaps only 1 or 2 Ma. The first-stage sea-floor spreading of the Red Sea continued until about 15 or 14 Ma ago at a half-spreading rate of about 2.2 cm/yr. Throughout early Tertiary time, the Arabian Shield erosion surface remained near sea level. First-stage uplift of the Red Sea Escarpment began during middle Miocene time, as evidenced by the coarse polymictic boulder conglomerate of the Bathan formation. Second-stage scarp uplift and second-stage sea-floor spreading followed during Pliocene, Pleistocene, and Holocene time.
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Middle Tertiary continental rift and evolution of the Red Sea in southwestern Saudi Arabia