Thickening of cratonic lithosphere: Implications for craton growth and kimberlite eruption trends

Cratons are thought to be the stable cores of continental lithosphere that have survived for 3000 Myr. Such long term survival is often attributed to the excess thickness and elevated viscosity of the cratonic lithosphere. Yet, the evolution of craton thickness during these 3000 Myr has remained highly debated. Several studies have explored three possible scenarios.  First, cratons may have been thicker in the past and thinned to ~200 km in the present day. Second, cratons may have thickened slowly or third, they have maintained their current thickness since their origin. In this study we explore the evolution of craton thickness in the past 3000 Myr using 2-D thermo-mechanical numerical models. We initiate each model with a thick and compositionally lighter (1.5% less dense) craton of 200 km in a hot convecting mantle and let it run for 3000 Myr. We impose periodic compression and extension on the craton to mimic supercontinental cycles.  We run a total of 24 models exploring a range of initial thicknesses, density contrasts, radioactive heating, and mantle cooling parameters, in order to test multiple evolutionary scenarios. The main results suggest that due to its lower density, the craton is initially flattened. As the craton cools, thermal density overcomes the compositional density, and the craton thickness increases. Viscosity increases concurrently and the mantle flow is diverted along the cratonic edges to self-compress the craton gradually. Due to periodic compression and extension in the model, craton topography varies within a few hundred meters, consistent with observations suggesting basin opening and erosion during and after the assembly and break up of supercontinents. However, the continental lithosphere remains stable. After 1500 Myr, the craton becomes thicker than 160 km depth, a crucial depth for generating kimberlites. Kimberlites are volatile-rich ultramafic rocks that are generated within a depth range of 160-250 km, and are only found above thick continental cratons. Importantly, most kimberlite ages cluster within the last 300 Myr, and available databases suggest that kimberlites were scarce between 3000 and 2000 Myr. Eruptions began occurring more continuously after ~1500 Ma, and accelerated after ~1100 Ma. This pattern is consistent with our models of a slowly growing craton thickness. We find that before 1500 Ma cratons were mostly thinner than the critical depth for kimberlite generation. After 1500 Myr, their thickness increased, allowing them to host more kimberlites. Although previous hypotheses emphasize mantle temperature and carbon availability as primary controls on kimberlite eruptions in the later part of Earth’s history, our results suggest that craton thickness also exerts a strong control on the eruption of kimberlite magmas.