Precambrian crustal evolution in the light of plate tectonics and the undation theory.

Abstract

The difference between Phanerozoic orogens and Precambrian mobile belts and granite/greenstone terrains suggests non-uniformitarian crustal evolution, probably related to changes in the asthenospheric convection system and lithospheric thickness as the earth cooled. Sea-floor spreading and modern-type plate tectonics did not operate during the Archaean and may only have begun in the upper Proterozoic as a result of increased rigidity of the crust following lithospheric thickening. Archaean and Proterozoic belts developed over primitive spreading centres where the heated lower crust was rendered ductile and reacted by laminar flow, thus leading to upper crustal grabens and deep ensialic basins. Only in rare cases did crustal rupture lead to small ocean basins. Many high-grade mobile belts were not produced during distinct orogenic periods, but represent sections of the lower continental crust which were brought to the surface through internal rotation of rigid blocks in large continents and/or through large-scale over- and underthrusting along straightening zones. Some features of the Precambrian evolution are in agreement with the undation theory, but the overall pattern seems to favour a systematic development characterized by increased rigidity and mobility of the earth's crust with time. Modern plate tectonics is the logical consequence of this evolution.