COOL CRATONS AND THERMAL BLANKETS: HOW CONTINENTS AFFECT MANTLE CONVECTION

Convection models with moving continents show that continents profoundly affect the form of mantle convection. If the continents are wider than the wavelength of convection cells (~3000 km), they cause neighboring thermal upwelling zones to coalesce into a single focused upwelling. This focused upwelling zone will have a potential temperature anomaly of about 200 degrees C, much higher than the 100 degree C temperature anomaly of upwelling zones generated beneath typical oceanic lithosphere. This excess temperature anomaly persists for about 100 Ma after supercontinent breakup. In contrast, small continental blocks (<3000 km diameter) do not induce focused upwelling zones. Instead, small continental blocks are dragged to mantle downwelling zones, where they spend most of their time and will migrate laterally with the downwelling. As a result of sitting over relatively cold mantle (downwellings), small continental blocks develop thick, cold thermal boundary layers (cratonic roots). Thus, small continents are geologically favored to keep their cratonic roots and to have low geothermal gradients, as well as to affect the morphology of the upwelling and downwelling zones.