The role of low-angle normal faulting and isostatic response in the evolution of the Suez rift, Egypt

Abstract

A new geometric model for the Suez rift suggests that isostatic uplift of an underlying low-angle normal fault system accompanying early block faulting localized subsequent deep-seated extension. This, in turn, created a superimposed late-stage graben system offset to the east from an earlier locus of extension. Initial extension in the Oligocene and earliest Miocene drove minor rotation on wide tilt blocks over an eastward-dipping low-angle normal fault system and formed a series of wide half-grabens. With increasing extension, more closely spaced faults formed smaller rotating tilt blocks, fragmenting the larger, earlier half grabens and creating numerous sub-basins. In response to this brittle thinning of the upper crust, isostatic uplift of the lower crust arched the underlying low-angle fault surface, limiting gravity-driven tilt-block movement in the west by flattening the low-angle fault surface and creating a broad gulf-parallel high in the fault surface and Moho under the eastern portions of the rift. This process diminished faulting in the west and localized continuing deep-seated extension under the crest of the arch, possibly through induced mantle convection. Through-going faults thus initiated cut the old low-angle fault system and bound a system of down-dropped grabens offset to the east of the most extended portions of the older tilt-block array. In the Red Sea, immediately to the south, this process has continued, with the increased separation allowing the formation of mantle-derived oceanic crust. This model suggests that early movement over low-angle detachments and accompanying isostatic response may play an important role in localizing deformation within rifts, and are probably critical elements in the early formation of passive margins.