Thermal effects of continental collisions: Thickening a variable viscosity lithosphere

Abstract

Based on data that indicate that the lithosphere and the crust are anomalously hot in some convergence zones, simple calculations of the thermal effect of crustal thickening are presented. The results indicate that post-tectonic melting of the lower crust requires high levels of mantle heat flux. It has been suggested that this high heat flux can be explained by the convective removal of thermal boundary layers thickened by convergence. To test this hypothesis a series of numerical experiments were carried out on the removal of thickened thermal boundary layers whose viscosity depends on temperature and pressure. It is found that the boundary layers can be removed quickly (10 m.y. in some cases). However, these layers are too thin to greatly alter the thickness of the lithosphere or the heat flux at the top of the mantle. A set of calculations was done with internal heat sources which give equilibrium lithospheric thicknesses half that of the thickened lithospheres. These indicate that the time scale for convective thinning by one half is extremely long (at least 100 m.y.) for initial lithospheric thicknesses of 100 km. Therefore, the lithosphere must be anomalously hot and thin before deformation for extensive crustal melting to occur after thickening in observed times (40 m.y. for Tibet). Finally, it is suggested that subduction-induced convection may raise the general mantle temperatures over a broad region behind a convergence zone. This effect can inhibit the cooling of the lithosphere for more than 50 m.y. after the subduction induced convection has ceased.