The interaction of plumes with the transition zone under continents and oceans

The dynamics of mantle flow with phase transitions can be influenced by the presence of a thick continental lithosphere, especially for the 400 km phase transition. We have studied the effects of the continental lithosphere on the mantle convection with the two major phase transitions at 400 km and 660 km. A 2-D model with an aspect-ratio 8 box was used in which the surface was divided into two domains: “oceanic” and “continental”. The “continental” lithosphere is modeled by a highly viscous, 200 km thick lid which is 100 times stiffer than the “oceanic” part. We also consider the consequences of plate reconfiguration by switching instantaneously the underlying rheological structure of the “oceanic” and “continental” domains. A depth-dependent viscosity with an additional jump at 660 km depth was employed. Other depth-dependent properties employed are the thermal expansivity and the thermal diffusivity. Both internal and basal heating configurations were studied. Our results show that there are distinct differences in the mode of interaction of plumes with the transition zone under continents and oceans. Much larger stationary plumes are developed in the lower mantle under continents, and erupt from time to time into the upper mantle, creating a very hot continental upper mantle. Plumes under oceans are weaker but produce fast horizontal jets, when the plumes impinge upon the surface. Very large plumes are formed in the internally heated cases because of the presence of depth-dependent properties in the lower mantle, creating a thick bottom boundary layer. This thick thermal boundary layer is swept upward under the continents by a large-scale flow in the lower mantle. These results can be important for the interpretation of seismic tomography under Africa and the central Pacific.