Secular cooling and thermal structure of continental lithosphere
Résumé
Temperatures in thick continental lithosphere do not adjust rapidly to secular changes of mantle temperature and in-situ radioactive decay. In the past, enhanced heat production may have led to geotherms that turn over above the base of the lithosphere, such that the lower lithosphere was hotter than, and was losing heat to, the underlying convecting mantle. Lithosphere with a turning geotherm would be unstable, undergoing delamination due to convective shear stresses imparted by the underlying mantle and in-situ partial melting if the lithospheric mantle contains small amounts of water and carbon. Both processes act to stabilize continental roots through reductions of lithospheric thickness and in-situ heat production, and hence may be responsible for the present-day characteristics of those roots that have survived until today. According to these arguments, a stable thermal structure requires that the average heat production rate in the lithospheric mantle does not exceed a critical value which depends on lithosphere thickness. The threshold value of heat production is 0.025 μW m− 3 in lithosphere that is thicker than 300 km. Cratonic roots that grow by underplating of oceanic lithosphere in a subduction environment undergo an initial heating phase which may lead to partial melting and to the formation of near-solidus mantle melts without any external heating event involved.