Control of Red Sea rift geometry by Precambrian structures

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Abstract
The location and geometry of a rift are controlled by some combination of the extensional stress field and preexisting lithospheric structures, unless the lithosphere is perfectly homogeneous. However, the nature and location of these structures may be difficult to identify because of subsidence and sedimentation associated with later rift stages. Two regional north trending Precambrian structures in northeast Africa have been identified that may have influenced the late Cretaceous and Tertiary evolution of the Red Sea. A large (300 km north‐south, 50 km east‐west) structure in northeastern Sudan (the Onib‐Hamisana zone) may be either a late Precambrian suture zone or a zone of transcurrent faulting. Highly deformed island arc and oceanic assemblages as well as associated ultramafic rocks occur throughout the zone. In one locality the arc assemblages are juxtaposed against possible continental margin metasedimentary rocks. To the southeast, on the Sudan‐Ethiopia border, a zone of similar size and orientation occurs and has been termed the Baraka suture zone. Offsets in the trend of the Red Sea coastline and axial trough coincide with both the location and spacing of the Onib‐Hamisana and Baraka zones. That part of the Red Sea immediately adjacent to the Baraka zone also coincides with a seismically active area. We present a model relating these preexisting lithospheric structures, which act as weak zones or stress guides, to the subsequent evolution and geometry of the rift. The rift propagation direction (northwest) is assumed to be perpendicular to regional extensional stress until the rift tip intersects a north trending stress guide at some arbitrary point. Rift propagation then proceeds northward until the zone ends or becomes poorly defined, whereupon northwest propagation resumes. In the resulting plate configuration, the guiding weak zones are observed only on the southwest plate.