Structure and tectonics of the northern Red Sea: catching a continental margin between rifting and drifting

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The northern Red Sea is a continental rift that has developed nearly to the point where seafloor spreading initiates. We present new geophysical data from the northern Red Sea which bear on how the transition from continental rifting to seafloor spreading takes place. Two distinct provinces within the “main trough” of the northern Red Sea in the area north of 26° N latitude have been identified which are referred to here as the “marginal areas” and the “axial depression”. The marginal areas form a series of bathymetric terraces that step down from the eastern and western shelves toward the central part of the Red Sea. Geophysical data indicate that the marginal areas are underlain by faulted and tilted basement blocks with structural characteristics similar to those observed at “sediment starved” passive margins. Seismic reflection profiles show that recent deformation in the overlaying sediments of the marginal areas has been relatively widely spaced, episodic in places and, although the deformation continues to the present, it is generally decreasing with time. The axial depression forms a continuous narrow zone of deeper water which is located near the center of the Red Sea except in the northernmost part where it shifts to the west in alignment with the entrance to the Gulf of Suez. Sediments in the axial depression exhibit a more intense and closely spaced deformation which appears to continue in activity to the present. Seismic activity in the northern Red Sea south of the Gulf of Suez area is concentrated in the axial depression and heat flow profiles across the main trough show maximum values which occur consistently over the axial depression. These observations indicate that the extension and tectonic activity which had previously been distributed across the rift, are becoming localized in the axial depression. Three major cross-trending “accommodation zones” partition the rift into fault block provinces along strike and serve to take up differential motion between blocks. Small “deeps” occur at various locations in the northern Red Sea. In particular, a series of these is associated with large isolated dipolar magnetic anomalies which appear to result from large intrusions. These intrusion-associated deeps occur at evenly spaced intervals in the axial depression, midway between sets of accommodation zones. This suggests that the accommodation zones may structurally control the emplacement of these intrusions and the formation of the associated deeps. These deeps appear to represent precursors to localized seafloor spreading, such as occurs in the large “transition zone” deeps farther south in the central Red Sea. Geophysical and morphologic evidence suggests that the large transition zone deeps of the central Red Sea represent isolated propagating seafloor spreading cells which eventually may coalesce to form a continuous seafloor spreading axis. The seafloor spreading axis of the southern Red Sea may have formed by this process.