Thermal localization as a potential mechanism to rift cratons

Cratons are cold regions of continents that have remained stable since at least the Precambrian. The longevity of cratons is often attributed to chemical buoyancy and/or high viscosity of cratonic root material. Yet examples of destructed cratons (such as the North China Craton) suggest that there are conditions under which chemical buoyancy and/or high viscosity are insufficient to keep cratons stable. The formation of continental rifts, as weak zones that reduce the total stresses of cratons, may be a mechanism that increases the longevity of cratonic lithosphere. Since continental rifts result from localized deformation, understanding the mechanism of shear localization is thus important for understanding the stability or breakup of cratons. Here, we perform 2-D numerical models for a cratonic lithosphere under extension to understand the initiation of shear localization, for visco-elasto-plastic rheologies. Results reveal that three modes of deformation exist: no localization, symmetric localization and asymmetric localization. To further understand the underlying physics, we develop a 1-D semi-analytical method that predicts the onset of localization as well as whether rifting will be symmetric or asymmetric. Applications of the semi-analytical method to geological settings show that fast deformation, cold thermal state or strong mantle rheology may result in localized deformation and stabilize the remaining adjacent cratons. Our results successfully interpret the major features of the North China Craton.