Plumes and Plateaus


MANTLE PLUMES: from PLATE TECTONICS to PLATEAU TECTONICS

The important question arises: how far back in time did plate tectonic processes operate? Is it just a modern phenomena? Or can we interpret global geology in terms of plate tectonic concepts right back to the early Archaean?

The common markers of plate tectonics are:

(1) Ophiolite complexes, that textbooks usually argue represent obducted fragments of ocean floor (though most are probably back-arc basin or young forearc). But these appear to be absent before the latest Proterozoic (ca. 700 Ma)

(2) Blueschists (with the Na-amphibole glaucophane) are indicative of a high-P, low-T hydrous environment that is only achieved in a subduction zone. Again, these appear to be absent before the latest Proterozoic (ca. 700 Ma).

(3) Island Arcs (like the Marianas), formed where oceanic plates subduct beneath one another. They are common at the present day, but are much less easy to recognise back in time, particularly in the Precambrian.

 

So for a majority of geological time (4.0 b.y. to 0.6 b.y.) some of the common markers of plate teconics are absent. Was the mantle regime too hot to allow preservation of blueschists? Because arcs and back-arcs require a subduction-flip or major change of plate direction to initiate them, perhaps these flips or plate direction changes did not occur, and there was a more regular, small scale pattern of mantle convection in the Precambrian? It has been suggested quite a long time ago (e.g. Fyfe, 1978) that "hot-spot teconics" may have been more important in the past than "plate tectonics". However this does not mean than one excludes the other, because hotspots (e.g. Iceland, Hawaii) occur at the present day, and it may just mean than one was dominant over the other.Plume