Gravity-driven continental overflow and Archaean tectonics

Whether modern tectonic processes differ substantially from those in Archae an times (>2,500 Myr ago) remains controversial, One view(1) is that Archae an tectonic processes were some combination of modern ones, occurring faste r or more shallowly because of the larger heat output of the early Earth, b ut others(2) have proposed that significantly different processes operated. Here I argue that gravitational spreading of Archaean continents would hav e caused them continuously and pervasively to 'overflow' onto adjacent ocea n basins, and that this process would have naturally ceased at the end of t he Archaean era. Because modern continental crust is believed to be ductile rather than brittle below a depth corresponding to a temperature of about 350-400 degrees C (ref. 3), it seems likely that such a ductile zone was un iversally present within the hotter Archaean continental crust. If the mean geothermal gradient of the continents had exceeded similar to 25-30 degree s C km(-1), then the resulting ductile zone would have caused continental o verflow to occur, and such a process can account for many of the distinctiv e peculiarities observed in the Archaean geological record. The cessation o f continental overflow corresponds naturally to the stabilizing 'cratonizat ion' which marked the end of the Archaean era, with its timing dependent on the evolution of both the geothermal gradient in the continents and the de pth of the ocean basins.