CRATON STABILITY: WHAT ABOUT SHAPE?

Cratons remain amongst the most enigmatic regions on the Earth. They are ancient, tectonically quiescent areas that have been going with the flow, moving across the surface of the Earth during multiple supercontinent cycles with little to no tectonic consequences. Cratons are often assumed to be permanent, unchanging features of Earth’s history. Yet several cratons (in particular, the Wyoming and North China Craton) have shown signs of destruction. Many studies suggest they were destroyed in catastrophic events. Craton stability has been connected to rheology, chemical composition/buoyancy, thickness, proximity to weaker material. But what about the shape of the craton? Does that contribute to stability? Does the shape of a craton reflect the conditions of stability within a craton, itself help drive stability, or a combination of the two? We explore these questions by simulating how different shaped craton margins are impacted by mantle flow directed by a subducting plate. We present a series of three-dimensional instantaneous flow simulations that demonstrate distinct differences in response to slab driven mantle flow depending on the shape of the craton margin. Margin shapes with progressive thickening toward the interior (such as diagonally or tiered shaped) demonstrate less resistance to deformation than those with sharp transitions in thickness (a vertical margin). We also observe these similar trends on seismic anisotropy patterns in different cartons with varying margin shape. In other words, thinner cratonic lithosphere is more susceptible to deformation. In addition, thinner cratonic lithosphere can introduce margin shapes that will channelize mantle flow. These results suggests a novel, and perhaps more subtle, mechanism of craton destruction – the progressive removal of cratonic lithosphere along its margin. This could lead to a runaway process which could result in the ultimate demise of a once, previously stable craton. Our results offer an additional, previously unconsidered, component to craton stability - the shape.