CONTINENTAL BREAK-UP
Introduction
Continental break-up initiates the Wilson cycle. It is at the origin of the formation of new continental margins and that of new oceanic plates.
From an economic point of view, it is the process that leads to the accumution of sediments that trap organic matters in regions of enhanced geothermal gradient, the prerequisits to the formation and maturation of coal, oil, and gas resources that power the world economy.
From an environmental point of view, regions where the continental crust breaks pose unique safety issues to local populations. First, the availability of water in low-standing region, plus the occurence of rich basaltic soil drive population towards zone of continental extension (the East African rift for instance). However, volcanic and hydrothermal activities make these regions particularly dangerous. The sudden release of carbonic gases dissolved into lakes, triggerred by the rapid overturn of the surficial and deep water, is known to have caused the dead of many thousand of inhabitants.
In this section we examine the dynamic (the forces at the origin of continental extension), the geometry, and the kinematic aspects of continental break-up.
Dynamic aspects of continental breakup: Why do continents break ?
Our intuition tells us that things break because tensional forces are acting on them. We know that continental plates are subjected to plate boundary forces such as slab-pull and ridge-push. Because the ridge-push is directed from mid-oceanic ridges toward continents it can only lead toward compressional stresses and therefore contractional deformation in the continent. If a continent is attached to a subducting oceanic lithosphere then the slap-pull can lead to tensional stresses and therefore to extensional deformation and continental rifting. During rifting decompression melting of the mantle leads to volcanism. This dynamic model is called "passive rifting" because mantle upwelling is the passive results of extensional tectonics.
The African plate is surrounded by mid-oceanic ridges on its west, south and east boundaries, and to the north by a convergent boundary with the European plate. Yet, the East African rift system tells us that the African continent is under extension. Clearly, passive rifting is a dynamic model which is not applicable to the East African extensional tectonics, so what is the alternative?