Crustal underplating and evolution in the Red Sea Rift: Uplifted gabbro/gneiss crustal complexes on Zabargad and Brothers Islands
The nature of the crust of the Red Sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of Zabargad and The Brothers in the central and northern Red Sea. Zabargad exposes, in addition to mantle-derived peridotites, a crustal section consisting of metagabbros and metapyroxenites, associated with silicic gneisses and cut by doleritic dikes. Seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern Red Sea. The Zabargad peridotitesilicic gneiss-gabbroic association provides a sample of upper mantle-lower crust in a continent before the inception of a major rifting event. The petrology and mineral chemistry of the Zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure (>900–1000 MPa or >9–10 kbar), i.e., over 30 km deep. The gneisses have tonalitic-trondhjemitic affinity and were originally equilibrated in the garnet granulite facies, implying also high pressure and derivation from the lower continental crust. This suggests that after the subcrustal continental lithosphere along the proto-Red Sea rift had been consumed possibly by convective thinning, but before any significant thinning of the crust, basaltic melts intruded the base of the continental crust, underplating it with gabbroic complexes. Thus crustal rifting was probably preceded by a thermal anomaly in the mantle, which supports an “active rifting” hypothesis. The gabbro/pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing PT conditions, from granulite to amphibolite to greenschist facies implying decompression and uplift, probably related to the gradual thinning of the continental crust during rifting. During this stage, additional gabbroic intrusions and basaltic dikes, exemplified by the gabbros and dolerites from The Brothers Islands, were emplaced at decreasing depths beneath the thinned continental crust. The northern Red Sea crust is presently in this stage. It is neither oceanic nor continental: it lacks a continuous, thick, basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust. Subsequently, basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete, equidistant “hot points” (as observed presently in the central Red Sea), which by axial propagation evolved into nearly continuous zones of oceanic crust accretion (as observed presently in the southern Red Sea). Processes similar to those proposed here for the evolution of the Red Sea rift can be documented in passive margins of the Atlantic and in some Alpine ophiolite complexes.