The nature and origin of cratons constrained by their surface geology

Cratons, defined by their resistance to deformation, are guardians of crustal and litho spheric material over billion-year time scales. Archean and Proterozoic rocks can be found in many places on earth, but not all of them represent cratonic areas. Some of these old terrains, inappropriately termed "cratons" by some, have been parts of mobile belts and have experienced widespread deformations in response to mantle-plume-generated thermal weakening, uplift and consequent extension and/or various plate boundary deformations well into the Phanerozoic. It is a common misconception that cratons consist only of metamorphosed crystalline rocks at their surface, as shown by the indiscriminate designation of them by many as "shields." Our compilation shows that this conviction is not completely true. Some recent models argue that craton formation results from crustal thickening caused by shortening and subsequent removal of the upper crust by erosion. This process would expose a high-grade metamorphic crust at the surface, but greenschist-grade metamorphic rocks and even unmetamorphosed supracrustal sedimentary rocks are widespread on some cratonic surfaces today, showing that craton formation does not require total removal of the upper crust. Instead, the granulitization of the roots of arcs may have been responsible for weighing down the collided and thickened pieces and keeping their top surfaces usually near sea level. In this study, we review the nature and origin of cratons on four well-studied examples. The Superior Province (the Canadian Shield), the Barberton Mountain (Kaapvaal province, South Africa), and the Yilgarn province (Western Australia) show the diversity of rocks with different origin and metamorphic degree at their surface. These fairly extensive examples are chosen because they are typical. It would have been impractical to review the entire extant cra-tonic surfaces on earth today. We chose the inappropriately named North China "Cra-ton" to discuss the requirements to be clas-sified as a craton.