Oceanization starts from below during continental rupturing in the Red Sea

The role of magmatism in continental rupturing and in the birth of a new ocean are not well understood. Continental rupture can take place with intense and voluminous volcanism, as in the Southern Red Sea/Afar Rift or in a relatively amagmatic mode, as in the Northern Red Sea rift. Within the Red Sea system, magnetic anomalies show a south to north time progression of the initial emplacement of oceanic crust. Mantle upwelling and melting may be affected by the south-north decreasing opening rate of the Red Sea and by the influence of the Afar plume, also decreasing from south to north. The tholeiitic basalts of the Red Sea spreading system contrast with the extensive Cenozoic basaltic lava fields occurring in the western part of the Arabian peninsula and forming one of the largest alkali basalt provinces in the world. In order to establish possible relationship between the Red Sea rift evolution and the widespread intraplate alkali volcanism of the western Saudi Arabia, field work was carried out on lava fields lying along the 25°N parallel, for over 150 km far from the shoreline. Samples from the Lunayyir, Ishara, al Kura and Khaybar volcanic fields, covering the full range of chemical diversity (olivine basalt, hawaiite, mugearite, benmoreite and trachyte) and spanning over a 20 Ma interval, were selected for chemical analyses. We attempt a comparison of the geochemistry of igneous rocks from western Arabia dykes and volcanic fields with those from the Red Sea axis and from the islands of Zabargad and Brothers in the northern Red Sea, that represent basaltic melts injected into the thinned continental crust before continental rupturing and initiation of seafloor spreading. The origin of the western Arabia igneous rocks and their relationship with rifting processes in the Red Sea have been assessed. Gabbros from the Brothers and Zabargad islands suggest that continental break up in the northern Red Sea, a relatively non-volcanic rift, is preceded by intrusion of oceanic-type basaltic melts that crystallize at progressively shallower crustal depths as rifting progresses towards continental break-up. A seismic reflection profile running across the central part of the southern Thetis basin shows a ~5 km wide reflector below the axial neovolcanic zone that marks the roof of a magma chamber or melt lens located ~3.5 km below the seafloor. The presence of a few kilometers deep subrift magma chamber soon after the initiation of oceanic spreading implies the crystallization of lower oceanic crust intrusives as a last step in a sequence of basaltic melt intrusion from pre-oceanic continental rifting to oceanic spreading. Thus oceanic crust accretion in the Red Sea rift starts at depth before continental break up, emplacement of oceanic basalt at the sea floor, and development of Vine- Matthews magnetic anomalies, pointing to a rift model, where the lower continental lithosphere has been replaced by upwelling asthenosphere before continental rupturing.