On the state of sublithospheric upper mantle beneath a supercontinent

S U M M A R Y An assumption of isothermal and static asthenosphere as the normal state of sublithospheric mantle, as commonly employed in studies of terrestrial magmatism, may be physically implausible for a system cooled from above. The growth of an unstable thermal boundary layer at the base of the lithosphere can lead to small-scale convection, so asthenosphere is expected to be usually convecting. A first-order estimate based on the energetics of convection suggests that mantle upwelling rate associated with such small-scale convection is on the order of 10 cm yr−1 for asthenospheric viscosity of 1018–1019 Pa s. To investigate the potential role of such sublithospheric convection in anomalous magmatism associated with rifting of the supercontinent Pangaea, a simple upper-mantle convection problem with thick cratonic lithosphere is considered through finite element modelling. Strong three dimensionality is exhibited in our solutions, and the planform of convection is largely affected by the bottom topography of continental lithosphere. Our model also suggests that differential cooling, imposed by a variation in lithospheric lid thickness, may lead to large-scale convection, which brings uncooled, ‘hot’ material from the base of thick lithosphere to shallow asthenosphere. The potential importance of such sublithospheric convection in magmatism during continental rifting is discussed.