Self-induced craton compression: Potential implications for craton stability

Cratonic lithosphere underlies some of Earth's oldest crust, and some regions may be more than 3 billion years old. To explain such longevity, effectively neutral buoyancy and relatively large viscosities are typically invoked as factors that help resist the mantle's convective shearing. However, high lithospheric viscosity and thickness also leads to higher basal tractions due to mantle flow compared to other parts of the lithosphere. To determine if these elevated tractions tend to stabilize or destabilize cratons, we develop global, spherical numerical models of present-day thickened cratonic lithosphere embedded within mantle flow. Our results show that in the presence of thick and viscous cratonic roots, high tractions are produced at the margins of the craton. Such high tractions are absent in the models without cratons. The direction of tractions is usually directed inward, giving rise to compressive regions within the cratons. This is mainly due to the downward deflection of velocities along their margins, as can be shown by analysis of mantle flow velocity profiles and velocity gradients. Small increases in horizontal gradients of vertical velocity can significantly elevate traction magnitudes along the craton margins because craton viscosities may be 10-1000x greater than those of the surrounding mantle. Our results substantiate previous analyses showing that deflection of flow can lead to a strongly modified traction field and predominantly compressive stresses within cratons. Craton self compression could help stabilize cratons and may contribute to the development of a thicker lithospheric root.