479 International Geology Review, Vol. 45, 2003, p. 479–496. Copyright © 2003 by V. H. Winston & Son, Inc. All rights reserved. 0020-6814/03/666/479-18 $10.00 Cold Cratonic Roots and Thermal Blankets: How Continents Affect Mantle Convection V ALERY P. T RUBITSYN,W ALTER D. MOONEY , 1 United States Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, California 94025 AND DALLAS H. ABBOTT Lamont-Doherty Earth Observatory, Palisades, New York 10964 Abstract Two-dimensional convection models with moving continents show that continents profoundly affect the pattern of mantle convection. If the continents are wider than the wavelength of the con- vection cells (~3000 km, the thickness of the mantle), they cause neighboring deep mantle thermal upwellings to coalesce into a single focused upwelling. This focused upwelling zone will have a potential temperature anomaly of about 200°C, much higher than the 100°C temperature anomaly of upwelling zones generated beneath typical oceanic lithosphere. Extensive high-temperature melts (including flood basalts and late potassic granites) will be produced, and the excess temperature anomaly will induce continental uplift (as revealed in sea level changes) and the eventual breakup of the supercontinent. The mantle thermal anomaly will persist for several hundred million years after such a breakup. In contrast, small continental blocks (<1000 km diameter) do not induce focused mantle upwelling zones. Instead, small continental blocks are dragged to mantle down- welling 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 are favored to keep their cratonic roots. This may explain the long-term survival of small cratonic blocks (e.g., the Yilgarn and Pilbara cratons of western Australia, and the West African craton). The optimum size for long-term stability of a continental block is <3000 km. These results show that continents pro- foundly affect the pattern of mantle convection. These effects are illustrated in terms of the timing and history of supercontinent breakup, the production of high-temperature melts, and sea level changes. Such two-dimensional calculations can be further refined and tested by three-dimensional numerical simulations of mantle convection with moving continental and oceanic plates. Introduction ARCHEAN CRATONS have thick lithospheric roots and low heat flow (Grand and Helmberger, 1984; Pollack et al., 1993; Artemevia and Mooney, 2001). The preservation of Archean diamonds within these roots, as known from kimberlite eruptions, implies that they have had low heat flow and thick lithos- phere since they formed ( Richardson et al., 1984; Boyd et al., 1985; Nisbet, 1987). Indeed, the oldest model ages of cratonic roots are indistinguishable from the oldest ages of the overlying crust (Pearson et al., 1995), implying that Archean continents have experienced low heat flow since they formed. In contradiction to these observations, some investigators have proposed that continents act as thermal blankets, i.e., low heat conduction through lithospheric roots causes the underlying asthenos- phere to heat up (e.g., Anderson, 1989). This latter proposal is consistent with the development of superplumes that eventually break up superconti- nents (Condie, 1998; White and McKenzie, 1995). Late-potassic granites within cratons also imply strong basal heating of the continental lithosphere (Campbell and Hill, 1988; Hill et al., 1992). Furthermore, if we consider Africa today, it is sta- tionary with respect to the hot spot reference frame, and has an anomalously high topography relative to the other continents (Cogley, 1985). This anomalous elevation is consistent with the thermal blanket model: the African continental lithosphere is both heated and uplifted by a deep-seated mantle plume. Thus, we seem to have two sets of contradictory observations. One set implies long-term cold conti- nental roots and a second set implies hot continental roots. In this paper, we examine these apparently 1 Corresponding author; email: mooney@usgs.gov