Geotherms, evolution of the lithosphere and plate tectonics

Abstract

For the purpose of calculating Precambrian geotherms, modern “average” geotherms need to be defined. Curves calculated from estimated plate sections are close to those of Clark and Ringwood (1964). Thicknesses of lithospheric plates at any time in the past can be estimated from “fossil” geotherms. Because subduction of oceanic plates depends upon their density, average densities of Precambrian plates should place important constraints on the beginning of subduction. This method, however, only indicates that in all probability subduction did not begin before 1000 Ma ago. Geological data suggest a date of about 800 or 900 Ma ago. Formation of Archean continents would have proceeded from partial melting of basaltic crust, and the end of that phase of earth's evolution would be a thermally controlled feedback mechanism lowering geothermal gradients in sub-continental mantle. The beginning of subduction was also thermally controlled, and probably accompanied the increasing stability of eclogite in the upper mantle.