Ophiolites and the evolution of tectonic boundaries in the late proterozoic Arabian—Nubian shield of northeast Africa and Arabia

Abstract

The Arabian—Nubian shield is currently regarded as one of the best examples to demonstrate that processes of lateral crustal growth and modern-type obduction—accretion tectonics have operated since at least late Precambrian times.In Arabia a number of Pan-African volcano-sedimentary/plutonic belts have been identified that display internal evolutionary patterns suggesting a development from primitive intraoceanic arcs some 900–950 Ma ago to mature, andesite-dominated arcs some 640 Ma ago through processes of ocean-crust obduction, arc collision and magmatic crustal thickening. Several ophiolite-decorated sutures are preserved, but many early tectonic boundaries were obliterated during later overthrusting, faulting and shield-wide granitoid plutonism towards the end of Pan-African evolution and stabilization in the earliest Palaeozoic.In southeastern Egypt and in the Red Sea Hills of the Sudan early Pan-African clastic sediments suggest that a passive continental margin was probably separated from several evolving arcs to the east by marginal seas. These arc segments were later thrust over each other, from east to west, during widespread and considerable horizontal shortening and gave rise to spectacular nappe structures and extensive ophiolite mélanges.The apparent lack of well-defined accretionary thrust stacks, high-pressure metamorphic assemblages and widespread ophiolitic mélanges in Arabia indicates that accretion either did not occur along margins with deep ocean trenches but involved buoyant crust, or extensive overthrusting took place through which the forearc segments were overridden and are now concealed. This, together with the recognition of distinct tectonic belts and isolated fragments of possible ancient continental crust and oceanic plateaus, supports the contention that Arabia may represent a collage of previously independent exotic terranes that accreted by oblique convergence and strike-slip translation during shield evolution.It is suggested that the Arabian shield contains remnants of microcontinents with pre-Pan African (i.e., > 1000 Ma) crustal history and, perhaps, oceanic plateaus and that its evolution bears similarities with aspects of terrane accretion in the North American Cordillera and in the present western Pacific. The evolution in Egypt and in the Sudan, however, seems characterized by the transformation of a passive continental margin into a tectonically active belt along which ophiolites and arc volcanics were thrust over each other at approximately the same time when the exotic terranes and arcs of Arabia accreted farther east. Final stabilization of the shield occurred when the evolving Arabian plate “docked” with Nubia after marginal basin closure and cessation of arc magmatism some 600–640 Ma ago.