Letters to Nature

Nature 398, 413-415 (1 April 1999) | doi:10.1038/18866; Received 17 November 1998; Accepted 10 February 1999

Gravity-driven continental overflow and Archaean tectonics

R. C. Bailey1

  1. Geology and Physics Departments, University of Toronto , 60 St George Street, Toronto, Ontario M5S 1A7, Canada

Correspondence to: R. C. Bailey1 Correspondence and requests for materials should be addressed to the author (e-mail: Email: bailey@geophy.physics.utoronto.ca).

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 approx25–30 °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.