Seafloor Spreading Initiation: Geophysical and Geochemical Constraints from the Thetis and Nereus Deeps, Central Red Sea

The rifting apart of continents involves interaction of tectonic and magmatic events that reflect the strain-rate and temperature-dependent processes of solid state deformation and decompression melting within the Earth. The spatial and temporal scales over which these mechanisms localize extensional strain, allowing continental rifts to evolve towards seafloor spreading, remain controversial. Here we show the role played by magmatism during the transition from a continental to an oceanic rift based on geophysical and geochemical data from the Thetis and Nereus Deeps, the two northernmost oceanic cells in the central Red Sea. The Thetis segment is made by coalescence of three sub-cells that become shallower, narrower and younger from south to north. Magnetic data reveal that the initial emplacement of oceanic crust is occurring today in the Thetis northern basin and in the southern tip of Nereus. The intertrough zones that separate the Thetis "oceanic" cell from the Nereus cell to the north, and the Hadarba cell to the south, contain thick sedimentary sequences and relicts of continental crust. A seismic reflection profile running across the central part of the southern Thetis basin shows a ~5 km wide reflector about 3.2 km below the axial neovolcanic zone, interpreted as marking the roof of a magma chamber or melt lens and as a last step in a sequence of basaltic melt intrusion from pre-oceanic continental rifting to oceanic spreading. The spatial evolution of mantle melting processes across Thetis and Nereus is evaluated from the chemical composition of 22 basaltic glasses sampled along 100 km of the rift axis. Trace and major element compositions corrected for crystallization show relationships with age of initial emplacement of the oceanic crust and preserve a clear signal of mantle melting depth variations. While Zr/Y and (Sm/Yb)n decrease, Na*/Ti* increases slightly from south to north. Na8 correlates positively with Fe8, and Zr/Y and (Sm/Yb)n with both Fe8 and Na8. This indicates that an increase in the degree of melting corresponds to a decrease in the mean pressure of melting, suggesting active mantle upwelling beneath Thetis and Nereus. The inferred sharp rift-to-drift transition marked by magmatic activity with typical MORB signature and a relatively high degree of mantle melting, with no contamination by continental lithosphere, suggests that lower crust and mantle lithosphere had already been replaced by active upwelling asthenosphere before separation of the Nubian and Arabian plates.