Structural Evolution of the Rift Zones of the Middle East—a Comment

Abstract

ROBERTS¹ has explained differences in the rate and direction of crustal spreading in the Red Sea and Gulf of Aden by postulating approximately east–west extension across the Ethiopian rift at a rate of 0.7 cm yr⁻¹ per limb. This rate would produce 140 km of widening of the Ethiopian rift during the last 10 m.y., an amount approximately twice the width of this rift² and twenty times the crustal extension that can be deduced from the observed structure. The eastern rift of Africa differs from the Red Sea and Gulf of Aden rifts in its small amount and rate of crustal extension, the importance of alkaline and silicic volcanism, and distinctive Bouguer gravity profiles³. These three branches of the rift system ought not to be regarded as three convergent spreading axes with comparable rates of spreading.