THE THERMAL STRUCTURE OF STABLE CONTINENTAL LITHOSPHERE WITHIN A DYNAMIC MANTLE

Abstract

The thermal structure of stable continental lithosphere is determined by (1) the concentration and distribution of heat sources within the crust and (2) the amount of heat input from the convecting mantle. The self-consistent coupling of these two factors has not been included in thermal models of stable lithosphere to date. We conducted two suites of numerical simulations (one with variable crustal heat production and the other with a chemically distinct cratonic root) to explore the thermal coupling between stable continental lithosphere and the convecting mantle. The distribution of heat producing elements within the crustal column was found to play a significant role in determining the local thermal structure of the continental lithosphere. Concentrating heat producing elements in the lower crust lead to a thinner thermal lithosphere. Mantle heat flux into the base of stable continents was low relative to surface heat flux and did not vary significantly within the simulations regardless of the presence or absence of a thick cratonic root. A suite of simulations with variable root thickness indicated that although cratonic roots have a weak effect on surface heat flow patterns, relative to crustal heat source variations, they do have a pronounced effect on deeper thermal structure. Roots stabilized temporal variations of deep continental geotherms and were required to generate a thick thermal lithosphere. The ratio of thermal to chemical lithospheric thickness was found to decrease toward unity with increasing root thickness and thick cratonic roots limited small-scale mantle convection beneath themselves.