MODELING THE STABILITY OF CRATONS CONTAINING PRE-EXISTING WEAKNESSES

Cratons are the ancient, typically Archean in age, stable interior of continental plates. They are thicker and more stable than the surrounding lithosphere due to their depleted peridotite composition and limited interaction with convecting mantle. The inherent stability of the cratonic lithosphere should prevent it from undergoing major deformation. However, it has been proposed in some areas, including the Kaapvaal Craton and the North China Craton, there is evidence of reactivation. This reactivation could be due to pre-existing weak zones within the cratonic lithosphere. Using Underworld2, we model the stability of cratons with pre-existing weak zones that vary in size, density, and viscosity. Preliminary results show that the viscosity of the pre-existing weak zone plays a strong role the behavior of the craton within the simulations. For example, models with a low viscosity pre-existing weak zone experience delamination and removal of the weak zone. As viscosity of the weak zone increases, the weakened material experiences less deformation. We also varied the viscosity of the cratonic lithosphere ranging from the maximum viscosity of the weak zone to ten times greater than the maximum weak zone viscosity. Less viscous cratonic material is more likely to be deformed around the weak area. In some cases, this deformation resulted in entrainment and removal of some cratonic material. The size of the weakened area effects stability as well, with larger weak zones promoting more craton deformation. The density of the weak zone appears to not influence deformation as much as its viscosity. Other observations within the simulations demonstrate complex tectonic history for cratons with pre-existing weak zones. For example, in the models where delamination of the weak zone occurs, the cratonic lithosphere undergoes compression as the weak material is removed and subsequent extension at the base of the lithosphere once most material is gone. This extension could lead to rifting of the craton rifting, particularly in models that have low viscosity weakened areas.