Sea-floor spreading initiation: constraints from geophysical data of the Thetis Deep, northern Red Sea

A major step in the "Wilson Cycle" is the splitting of a continent and the birth of a new ocean, with the consequent formation of passive plate margins. The transition from a continental to an oceanic rift can be observed today nowhere better than in the Red Sea/Gulf of Aden system. We have carried out during several years a number of expeditions in the axial portion of the Northern Red Sea, in the region where the northernmost nuclei of axial emplacement of oceanic crust can be observed. High resolution multibeam, magnetics, gravity and multichannel seismic reflection surveys from the Thetis Deep revealed rates and modes of initial pulses of sea floor spreading, velocity of S to N axial propagation of the oceanic rift, evolution of initial MORB-type crust and nature of the mantle thermal anomaly that caused the transition from a continental to an oceanic rift. The Thetis deep is made of three en echelon fault-bounded axial basins that are joined together with axial volcanic ridges and a large number of scattered small central volcanoes. The southern basin shows a strong linear magnetic anomaly corresponding to the axial neo-volcanic zone. Two negative symmetric anomalies identified as Matuyama are present in the southernmost part of this basin, suggesting that the emplacement of oceanic crust at this site started roughly 2.5 Ma, with an average half spreading rate of 6 mm/yr. The central sub-basin is also characterized by a strongly magnetic linear neo- volcanic zone that, however, is flanked only by a small, "vanishing" symmetrical negative anomaly suggesting emplacement of oceanic crust not earlier than about 1 Ma. The northern sub-basin does not show a clearly defined linear neo-volcanic zone although it displays a strong central magnetization suggesting initial emplacement of oceanic crust < 0.7 Ma. This pattern implies a south to north time progression of the initial emplacement of oceanic crust within the Thetis system, with a propagation rate of about 20 mm/yr. Gravity data inversions constrained by seismic data reveal that the oceanic crust extends from the axial neo-volcanic ridges toward the master faults of the axial depression with crustal thickness ranging from 4 to 6 km. The increasing thickness of basaltic crust toward the edges of the basin together with higher degree of melting, inferred by the geochemistry of the basaltic glasses, and higher central magnetization of the northernmost and youngest basin suggest a pulse of faster spreading rate at the onset of sea-floor spreading.