Two Stages of Thermal Evolution of the Continental Lithosphere

Аннотация

The evolution of the continental lithosphere, unlike the oceanic one, lasts more than 3.5 Ga. This evolution was largely determined by thermal factors, such as the removal of heat from the Earth’s surface, the interaction of the conductive layer of the lithosphere with mantle thermal convection, and the strong dependence of the viscosity of the upper layers of the Earth on temperature. The aim of this work was to simulate the long-term interaction of these factors over a period of several Ga on the basis of the whole-mantle thermal convection equations. In our evolutionary model, the nucleus of continental lithosphere with a thickness of 50 kilometers was inserted in the mantle, which then began to grow in depth due to the reorganization of convection under the continental lid with a conductive heat transfer mechanism. For the self-consistent modeling of the lithosphere thickness changes in time we set the condition of viscosity jump by three orders of magnitude with a decrease in local temperature below 1200°C. The results of successive calculations demonstrated that the initial period of growth of the continental lithosphere due to its cooling, which takes about one Ga, is subsequently replaced by its slow thinning due to the accumulation of heat under a thick heat-insulating lithospheric cover. The calculated maximum of the average thickness of the growing lithosphere, estimated by temperature and viscosity, is 162 kilometers, however, when estimated by the conventional point of intersection of the lithospheric geotherms with mantle adiabat, it is 100 kilometers more. The increase in the average mantle temperature from the heat-insulating effect is about 100 K/Ga and continues both at the stage of the lithosphere thickening and at the subsequent stage of its slow thinning. The mantle warming up due to the presence of continents is one of the competing factors in the global process of the Earth’s secular cooling. The conclusion about the presence of two stages in the evolution of the continental lithosphere clarifies existing ideas about the formation of the current state of the outer layers of the Earth.