CRATON FORMATION DOES NOT REQUIRE TOTAL REMOVAL OF UPPER CRUST

In some recent models, craton formation was ascribed to excessive crustal thickening and subsequent removal of the upper crust by a diverse set of mechanisms. We have compiled data from the world’s cratons to show that in by far the most the greenschist-grade metamorphics and even supracrustal sedimentary rocks are preserved and with few exceptions surfaces of cratons that formed during the cratonization do not expose high-grade rocks everywhere, most not even in the largest part of their surfaces. Low grade metamorphic and non-metamorphic rocks dominate the large parts of the strongest craton in the world, namely the Laurentian. They are not even distributed in only one part of the craton but occur in widely dispersed areas from north to south and east to west. In the Singhbhum and Raj Mahal cratonic pieces in India, in North China, in the Baltic, Tanzanian, Congo, North African and Arabian, Guyana, São Francisco, Pilbara and Yilgarn cratons greenschist-facies to non-metamorphic rocks were present at the surface when these cratons were stabilised. None of these cratons show an uniform Tibet-like environment for their formation. Instead, it seems that they formed by the amalgamation of magmatic arcs and associated subduction-accretion complexes that were subsequently involved in a final continental collision, but that their surfaces were never very much higher than the prevailing sea-level as they do not seem to be deeply eroded. We think the eclogitisation of mafic roots of arcs may have been responsible for weighing them down. A more recent example of a craton forming event is seen in the Palaeozoic Altaid superorogenic system.