Earth's evolving stress state and the past, present, and future stability of cratonic lithosphere

The vigour of convection in the Earth's mantle declines over time because of the decay of internal heat sources. Decaying heat sources alone would imply a gradual thermal change from the Archaean to the present. The larger impact for the Earth is due to the temperature dependence of mantle viscosity. As the mantle cools, viscosity exponentially increases. This is the dominant effect that leads to a decrease in the ratio of the forces that drive convection relative to those that resist it (i.e. to a decreasing mantle Rayleigh number). Provided that the mantle remains in the high Rayleigh number regime, as it is at present, increasing viscosity outweighs declining convective velocities in determining convection-generated stress, and mantle stress levels increase from the Archaean to the present. This is demonstrated by thermal history calculations and numerical simulations of mantle convection in a plate-tectonic regime. Thermal modelling studies further predict that bulk mantle viscosity will adjust to the Earth's cooling faster than deep continental lithosphere. This results in a greater coupling between the mantle and continental lithosphere over time. Collectively these arguments lead to the conclusion that sections of continental lithosphere that have remained stable since the Archaean and the Proterozoic are becoming progressively more prone to instability in the geologically modern era.