The Gulf of Suez and Red Sea rifts are currently bordered by large asymmetric uplifts that are undergoing erosion. We find that the amount and timing of erosion vary systematically along the strike of the margin and have been controlled by variations in the prerift stratigraphy. The prerift strata are composed of cliff-forming Eocene-Cretaceous carbonates overlying the easily eroded Cretacous-Cambrian ''Nubian'' sandstone. This lithologic succession promotes scarp retreat of the sedimentary section, followed by dissection of the underlying basement. The prerift section thins from over 2000 m at the northern end of the rift to less than 400 m at its junction with the Red Sea. Thus, at the northern part of the Gulf of Suez, the Nubian sandstone is minimallyexposed, and the carbonates form a scarp at the rift border fault. Farther south, undercutting of the carbonates by erosion of the sandstone has resulted in scarp retreat. The escarpment cuts diagonally away from the border fault and is over 100 lan inland from the border fault at the southernmost Gulf of Suez. The amount of retreat varies inversely with die sediment thickness. Exposure and erosion of basement are initiated by the retreat of the escarpment, and the depth of erosion, as indicated by fission track ages, increases with distance from the escarpment. These observations are explained by a model in which erosion along the Gulf of Suez is initiated as rift flank uplift becomes sufficiently large to expose the friable sandstones. Undercutting of the escarpment and exhumation of basement has been propagating northward and westward for at least 20 m.y. The average rate of scarp retreat has been 6 km/m.y. and the along-strike propagation of the erosion has been 12 km/m.y. The diachronous erosion of the the rift flanks at the Gulf of Suez highlights the importance of distinguishing between the timing of uplift and of erosion. Both thermochronometric and stratigraphic data primarily indicate the timing of erosion, which may differ significantly from the timing of the uplift that initiates it. They must be interpreted carefully to avoid erroneous conclusions about rift tectonics.
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