Gravity-driven continental overflow and Archaean tectonics

Whether modern tectonic processes differ substantially from those in Archaean times (>2,500 Myr ago) remains controversial. One view1 is that Archaean tectonic processes were some combination of modern ones, occurring faster or more shallowly because of the larger heat output of the early Earth, but others2 have proposed that significantly different processes operated. Here I argue that gravitational spreading of Archaean continents would have caused them continuously and pervasively to ‘overflow’ onto adjacent ocean basins, and that this process would have naturally ceased at the end of the 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 °C (ref. 3), it seems likely that such a ductile zone was universally present within the hotter Archaean continental crust. If the mean geothermal gradient of the continents had exceeded ∼25–30 °C km−1, then the resulting ductile zone would have caused continental overflow to occur, and such a process can account for many of the distinctive peculiarities observed in the Archaean geological record. The cessation of continental overflow corresponds naturally to the stabilizing ‘cratonization’ 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 depth of the ocean basins.