AAPG Middle East Region Geoscience Technology Workshop

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Rift, Drift and Salt Tectonics in the Red Sea: The State of the Divergence

Abstract

It was previously thought and accepted that sea-floor spreading started around 6 Ma ago in the southern and central parts of the Red Sea, splitting apart the continental basement and its overburden of salt and other sediments. Although this model is of elegant simplicity, it conflicts with geological and geophysical data indicating that seafloor spreading started ~12 Ma and that oceanic crust covers approximately 2/3 of the basin floor. The past decade saw a quiet revolution in understanding the geologic evolution of the Red sea, driven by better data and distinction of structural and sedimentation styles resulting from rifting, sea-floor spreading, transforms, and salt tectonics. A new yet unpublished model has emerged, which includes the following stages: 1. Continental rift valley (30-22 Ma) stage A narrow rift valley formed, similar to the East African Rift, composed of linked half grabens filled with basaltic volcanics (Jizan Group) from Jeddah to Yemen, and alluvial red beds (Al Wajh Fm.) north of Jeddah. The volcanics are heavily intruded by sheeted dikes, layered gabbros, as well as granite stocks and batholiths along the eastern hinge zone of the rift. 2. Open marine rift (22-16 Ma) stage During northward rift propagation the basin was flooded with seawater from the Tethys Ocean, creating a linear gulf aligned with the Gulf of Suez (GOS). The rift filled with marine turbidites and pelagic sediments of the Rudais (Barqan) Fm.The continental and marine rift sediments reach thicknesses of 6 kms in half graben basins. Rifting ended when the normal faults rotated to lower dips over mid-crustal detachments. 3. Restricted marine post-rift trough (16-12 Ma) stage Deposition of evaporites ensued unconformably covering the syn-rift section. The evaporites include the massive Middle Miocene salt (South Gharib/Mansiyah Fm.) whose initial thickness was estimated at 1-2 kms. The evaporites alternate with siliciclastic sediments and carbonates and they were initially deposited at at the transition from extension to sea-floor spreading. 4. Oceanic passive margins (12-0 Ma) Breakup and propagation of an oblique spreading ridge progressed north towards the Sinai triple junction, stepping to the right across two major transform zones (Jeddah and Zabargad). The transforms include pull-apart structures such as the Conrad deep and Gulf of Aqaba, as well as positive flower structures that probably exhumed the mantle at Zabargad Island. Evaporitic conditions continued until the Messinian salinity crisis (5.4 Ma). Then open marine conditions resumed following a Zanclean flood from the Gulf of Aden. The Middle Miocene salt decouples the rift structures below from thin-skinned salt structures above. The salt is overlain by a syn-kinematic overburden of Late Miocene layered clastics and evaporites, and Plio-Pleistocene sediments and reefs of the Lisan Group. This overburden is regionally detached above the salt and deformed into extensional raft structures along the margins and contractional structures offshore, similar to those of the Atlantic passive margins. The margins exhibit characteristic structures of thin-skinned extension above a salt detachment, including sediment rollovers, salt rollers, and split carbonate rafts. Away from the margins, the salt accumulated under the shelves in a variety of autochthonous and allochthonous structures, including seaward-verging recumbent folds and thrusts. The salt also flowed over large tracts of younger oceanic crust and exhumed mantle (?). In contrast to previous split-salt models, the axial troughs and deeps are simply areas that were not covered by the advancing salt. In the northern Red Sea, the absence of a mid-ocean ridge and the ultraslow spreading has almost completely smothered the oceanic crust by salt.