The Afar Triple Junction and sea-floor spreading

Abstract

The Red Sea, Gulf of Aden, and African rift systems meet at the Ethiopian Afar. The Afar depression is generally agreed to represent oceanic crust, part of the Red Sea–Gulf of Aden floor produced by drift of Arabia from Africa. This simple thesis faces several difficulties: the fact that most of Afar lies anomalously above sea level; the existence of two large sialic horsts separating Afar from the Red Sea and the Gulf of Aden; the parallel development of spreading axes in the Red Sea and northern Afar; the complex and intense fault-belt virgations of the Afar floor, and the absence of ridge-transform structures; the concentration of seismic-energy release along the western margin of Afar, and not along the axis of the depression; and the contrasting gravity situation of Afar with that of the Red Sea. From the known data on Afar, a new analysis in terms of plate tectonics shows how the sialic horsts have played a fundamental role in deter-mining the existence of Afar. A concept of bifurcated spreading zones is introduced that does not require the existence of transform faults and leads to rotational structures and longitudinal shear within the Red Sea basin. The influence of the Gulf of Aden spreading on Afar has become significant only in the Quaternary, and the dynamic evolution of the pattern of spreading at the triple-rift junction is emphasized, contrary to a static picture inferred from D. P. McKenzie and W. J. Morgan.