Interaction of plume and thermal convection in the continental upper mantle

Numerical modeling of interaction of a thermal convective flow and a plume head risen to the base of the continental lithosphere is discussed. The results of the modeling are compared with the peculiarities of the Cenozoic plume magmatism in the east of Australia. Correct modeling of the interaction process requires a model for thermal convection in the continental upper mantle and a model for gravitational spreading of plume head in the base of the continental lithosphere in the absence of convection. The thermal-convection model described is based on considering the continental lithosphere a highly stable conductive layer chemically distinct from the upper mantle. The ascending convective flow is typical of the zones with ancient thick cratonic lithosphere, and the descending flow takes place in the zones with younger thin lithosphere. Modeling of gravitational spreading of plume head in the absence of thermal convection shows that the plume spreads in the base of the continental lithosphere with a velocity of 3 cm/year over 15 Ma and then dissipates through cooling. Thermal convection acts on the plume through an increase in the velocity of plume spreading up to 6-7 cm/year and a decrease in the plume lifetime to 10 Ma. In the zone of abnormally thin (<100 km) lithosphere the plume head shows an unusual behavior. Here, in contrast to the thinspot model, the plume does not rise to the base of the thin lithosphere because of its interaction with convective flows. However, some rising of the plume occurs, and it is sufficient for decompression melting and formation of magmatic chamber.