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

Abstract

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.

Nature Versus Nurture: Preservation and Destruction of Archean Cratons

“…In other words, cratons must not only stabilize shortly after formation, but also remain stable over the range of their lifetime. Yet, convective stresses at depth, and those that drive surface tectonics, do not remain constant as the Earth ages, but increase as the Earth continues to cool (Sandu et al, 2011). Thus, a craton originating with strong lithospheric material will be best able to withstand the progressive increase in stress and potential for deformation (Beall et al, 2018;Cooper et al, 2006).…”

“…Doing so allows for comparison across differing dynamic settings as well as providing a proxy for time; the Rayleigh number scales with the internal temperature of the mantle. As the Earth cools from hotter temperatures in the past, the Rayleigh number decreases with time (e.g., Cooper et al, 2006;Sandu et al, 2011). Exploring the connection to the Rayleigh number/time thus introduces the concept of longevity into the stability regimes.…”

The structural evolution of the deep continental lithosphere

“…For example, convective stresses increase as the Earth cools. This is due to the dependency of the mantle's viscosity on temperature -as the Earth cools, the average mantle temperature decreases and the viscosity increases which nets in increased convective stresses (e.g., Cooper & Conrad, 2009;Sandu et al, 2011). In addition, the condition of isopycnicity, which explains the present day neutral buoyancy of cratonic lithosphere, may not have been met in the past when the mantle was hotter and the average thermal boundary was thinner (Eaton & Perry, 2013).…”

Dynamic cause of marginal lithospheric thinning and implications for craton destruction: a comparison of the North China, Superior, and Yilgarn cratons

“…When oceanic lithosphere subducts beneath cratons, water is carried into the mantle via hydrous mineral phases in the slab and overlying sediments, and the most important dehydration reactions occur at the facies transition from blueschist to eclogite, which releases abundant water to the overlying mantle (Peacock 1993). This hydrates and thereby weakens the upper mantle and therefore lowers the melting temperature (solidus), and decreases the mantle viscosity (Komiya and Maruyama 2006;Sandu et al 2011;Kusky et al 2014b). Water would preferentially be removed with the melt in the partial melting process of mantle peridotite and pyroxenite, leading to the formation of refractory lithospheric mantle (Karato 1986;Hirth and Kohlstedt 1996).…”