Evidence from the northern Red Sea on the transition from continental to oceanic rifting

Abstract

The northern Red Sea is an active rift in the last stages of continental rifting which is beginning to undergo the transition to oceanic seafloor spreading. The rift can be divided into two distinct regions. The “marginal areas” consist of a series of bathymetric terraces, and as indicated by geophysical data, are underlain by a series of rotated fault blocks. The “axial depression”, which forms the axis of deep water, is a continuous region 10 to 25 km wide near the center of the sea which is characterized by high heat flow, a large gravity minimum and very deformed sediments. Both regions are divided along strike into segments roughly 60 km long by cross-trending transfer zones. The presently active tectonics are characterized by the concentration of extension and deformation in the axial depression and by the emplacement of a number of large intrusions. The intrusions occur at distinct sites located at the intersection of transfer zones with the faults bounding the rotated blocks in the marginal areas, and at deeps located in the axial depression halfway between transfer zones. Although extension is becoming concentrated in the axial depression, geophysical data imply that at least the upper few kilometers of the crust in that region consists of non-magnetic “continental type” material and that this ca'apace has been ruptured by large intrusions only at the deeps. A wide range of crustal seismic velocities has been observed in the northern Red Sea, including velocities normally associated with oceanic crust. The velocity variations appear spatially related to corresponding large variations in basement rock type observed on shore in the Eastern Desert of Egypt. The northern Red Sea is in an intermediate stage between a continental rift valley and a young ocean basin and delineation of its structure and tectonics allows a model to be developed for the evolution of a rifted continental margin. Rifting starts in a series of linked half grabens 50–60 km long and formed over detachment surfaces which probably penetrate no deeper than the mid or lower crust. This style of rifting can accommodate at least the amcunt of extension represented by the Gulf of Suez (35 km). At some greater amount of extension, the rift acquires a more symmetric shape, probably through the development of antithetically faulted blocks in the hanging wall. Extension in this stage is still distributed throughout the rift and is accommodated in the upper crust by rotation of fault blocks. At the stage represented by the northern Red Sea, extension becomes concentrated in an axial depression and begins to be partially accommodated by large intrusions which form deeps. The position of the deeps is controlled by the transfer zones established in the earliest stages of rifting. With further extension, the deeps serve as nucleation points that develop into small, discrete cells of seafloor spreading. These cells grow and coalesce to form a continuous mid-ocean ridge spreading center such as is found in the southern Red Sea.