From Rifts to Fast-Spreading Ridges

Abstract

The expansion of continental rifts is of the fissural type: a fracturing in the lithosphere causes a wedge of hot partially molten asthenosphere to ascend from the mantle. Basaltic liquid drawn from this wedge fills the fissures in the crust, creating a swarm of dykes. This episodic activity progressively heats the lithosphere, making it weaker and allowing it to respond by tectonic stretching, the second type of expansion. In this way we proceed to oceanic rifts like the Red Sea framed between two stretched continental margins. When the so far rather low spreading rate increases to more than 2 cm/a, the continuous creation of oceanic lithosphere will lead to the relatively regular structure of fast-spreading ridges (Chap. 7) characterized by the harzburgitic ophiolites. Let us sum up the concepts inherent in this expansion: rapid rise of asthenosphere, little loss of heat, strong melting, highly depleted nature of the residual mantle (harzburgitic), copious and constant supply of basaltic liquid to the crust, development of magma chambers, crustal thickness 6 km. When, in contrast, the rate of oceanic expansion is low, formation of crust may become discontinuous, as suggested by recent studies along the Atlantic Ridge and ophiolites of the Iherzolite-type form. There is a succession of episodes of “normal” basaltic supply, possibly caused by the uprise of a mantle diapir with intervening periods, during which tectonic stretching may lay bare the mantle on the ridge. On a global scale, the slow or discontinuous ascent of asthenosphere diapirs leads to a lower degree of melting (mantle residue Iherzolitic) and thus to a lower rate of basalt formations (thinner crust, episodic presence of magma chambers). The spreading rate thus appears to be the key factor controlling the type of mantle accretion below the ridges as well as their segmentation. Differences in the temperature of the rising asthenosphere should also be considered.