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Rocks older than 3 billion years are rare on Earth. These ancient rocks are exclusively found on continental cratons, which are remarkably stable, some surviving for over 3 billion years. Understanding the long-term stability of cratons has been considered as one of the grand challenges of geodynamics. In a tectonically active planet, this stability was originally attributed to inherent buoyancy contrasts and a higher viscosity of cratons.
Here, I seek to reanalyze the concept of “stability” by evaluating numerical models for traction and strain-rate patterns underneath cratons as well as the evolution of their viscosity with time. The magnitude of mantle tractions increases with lithospheric thickness and viscosity. However, the thick and viscous cratonic lithosphere experiences the least deformation, thereby maintaining the tectonic stability of cratons over billions of years. The elevated tractions tend to converge towards the center of highly viscous cratons, establishing a self-compressive stress regime in almost all cratons, with the exception of South Africa. This compressive regime arises due to cratons’ capability to divert mantle flow around them, mostly in a downward direction. Consequently, I attribute the "stability" of cratons to the combined effects of viscosity, thickness, and neutral buoyancy.
Considering cases where cratons may have in fact been destroyed, the original stability has to be affected by some process. The “stability” of cratons can be affected due to weakening, which can be induced by the thermal impact of mantle plumes (e.g. the Indian craton affected by the Reunion plume) or metasomatism by slab-released fluids (e.g. North China craton and paleo-Pacific slab). Weakened cratons may experience gradual erosion due to mantle shearing, as they no longer possess the strength required to withstand convective forces. Some partially destroyed cratons (e.g. Slave craton) seems to have regained their thickness with time. Currently, there are limited hypotheses to explain the phenomenon of recratonization. However, I speculate that the convective self-compression mechanism may play a role in maintaining craton thickness over billions of years. |