Continental growth and strike-slip tectonics

Despite the increased precision of radiometric dating and the steady accumulation of ages when segments of continental crust first formed, two nagging oddities refuse to go away.  There seem to have been spurts in continental growth, rather than a steady build up over time.  Odder still, some areas have more crust of a certain age range than seems feasible.  The problem is a fundamental one, because the Earth generates radiogenic heat continually, though the amount has declined as the heat-producing isotopes of uranium, thorium and potassium decay.  Earth scientists assume that most geothermal energy exits to space through the process of sea-floor spreading.  Hot, new oceanic crust is invaded by seawater, thereby losing heat through hydrothermal activity.  The dominantly felsic magmas that build continental crust originate through partial melting processes where old, cold ocean floor descends at subduction zones.  Although some heat escapes through volcanism associated with mantle plumes, most researchers reckon that it is unlikely that this loss has ever come close to the quiet cooling at mid-ocean ridges, except possibly during the Archaean.  Averaged out, subduction and continent formation ought to keep pace with sea-floor spreading, though slowly declining over time.  There are those who focus on massive mantle turnovers in the form of superplumes that build large volcanic plateaux on land and on the sea floor, suggesting that their subduction generates greater volumes of crust than usual.  The main problem is that such plateaux are unlikely to be subducted.

The evidence for periods of accelerated continental growth comes from restricted regions, albeit very large.  Examples are 1900 to 1650 Ma crust in North America, Greenland and Europe, and 800 to 550 Ma crust in NE Africa, whose volumes are equivalent to between 1 and 10 times the present global rate of crust production at volcanic arcs.  Jonathan Patchett and Clement Chase of the University of Arizona offer a solution to the conundrums (Patchett, P.J. and Chase, C.G. 2002.  Role of transform continental margins in major crustal growth episodes.  Geology, v. 30, p. 39-42).  They show that strike-slip movement at modern subduction zones gives a 16% probability of more than 400 km transport of new continental crust parallel to the margins of existing continents.  Such motions are likely to concentrate continental growth where such terranes become docked together.  Such relative plate motions stem from particular configurations of spreading axes and the margins of old continents, and can therefore vary – some periods may have been dominated by head-on subduction, others by a greater amount of oblique relative movements.  By bundling together new continental material generated in magmatic arcs, the second would give the appearance of extraordinary rates of crust formation in some areas.  If the transform faults that channelled such lateral movements became obscure – and early strike-slip motions in ancient terranes are not easy to find or to quantify – the special natures of  terrane dockyards could go unnoticed.  Patchett and Chase note that the seeming pandemonium of 800-550 Ma crustal growth in NE Africa and Arabia has a counterpart in an age gap in the record of the northern continents, and cite several other examples.

While variations in strike-slip motions of terranes helps to resolve the apparent episodicity of continental growth, there is another line of approach.  Not all modern subduction zones generate voluminous magmas, even where plate motions are head to head.  The Andes has two huge segments where active subduction is unaccompanied by volcanism, and the angle of subduction is unusually shallow.  Low-angled subduction is likely where warmer than usual oceanic lithosphere enters a subduction zone, which is what might happen to segments blanketed by young ocean-plateau lavas formed by mantle plumes.  Constant sea-floor spreading need not necessarily result in constant rates of magmagenesis at destructive plate margins.

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