African roots

Africa to a large degree exerts a control over modern plate tectonics, because it barely moves at all.  The base of its lithosphere connects in several places with the solid mantle, so that asthenosphere is not universally present beneath the continent.  These roots slow down Africa’s motion.  One name applied to them is “tectosphere”, and they are partly governed by the low heat production in the lithosphere and underlying mantle, as a result of U, Th and K having been extracted from depth by processes that led to separation of continental crust.  These processes reach completion beneath the most ancient segments of continental crust, and result in them eventually becoming geologically inert; they become cratons. 

Studies based on samples brought from deep below cratons by volcanism, particularly that which formed the kimberlite plugs of Africa, suggest that their roots date back almost as far as the age of continental material above them.  But that natural sampling is haphazard, and relationships cannot be found.  Where large extraterrestrial bodies have excavated material to great depths, tectosphere material might well have reached the surface en masse by rebound following impact.  Such a deep section formed around the Vredfort Dome in the Kaapvaal  Craton of southern Africa after a major impact about 2 billion years ago.  It exposes the crust-mantle boundary. 

A programme of dating the Vredfort materials (Moser, D.E. et al.  2001.  Birth of the Kaapvaal tectosphere 3.08 billion years ago.  Science, v. 291, p. 465-468) shows that welding of crust to mantle in Archaean times, and formation of both craton and tectosphere, took place about 3.1 billion years ago, more than a hundred million years after crustal material itself coalesced.  Tectospheres seem not to begin forming at the same time as large masses of continental crust.  Instead they accrete to the base of the crust through later processes that probably involve subduction.  Other workers have suggested that the Kaapvaal tectosphere accumulated from masses of oceanic lithosphere that failed to descend completely into the mantle.  Curiously, the fragments in kimberlite pipes from which those conclusions were drawn are very dense eclogites.  Such material should descend easily into the deep mantle because their density exceeds that of peridotite.  That poses the question of why they came to stay close to the surface so long ago.  Perhaps their eclogite mineralogy stabilized long after they accreted beneath Kaapvaal, and they are “stuck” in the inert tectosphere that they form, out of gravitational equilibrium.  Should such high-density roots eventually become detached from their overlying materials, then the surface would pop up to become eroded dow to great depths.  The fact that most of the worlds cratons (the continental “shields”) preserve great volumes of material that crystallized at quite shallow depths, suggests that such “delamination” does not commonly happen beneath them.


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