The Influence of Mesozoic-Cenozoic Plate Motions on Thermochemical Piles

The large low-velocity seismic anomalies under the Pacific and Africa are often interpreted as being piles of more-dense material. We have performed numerical modeling of thermochemical convection in a three-dimensional spherical geometry in order to determine whether the presence of a dense chemical component can lead to the formation of two large antipodal piles in the Earth's lower mantle. We find that without imposing plate motions on the surface of the model, the dense material forms a network of linear ridges which are passively swept around by downwellings and that thermochemical structures are generally controlled by the geometry of the downwelling system. If plate motion history from 119 Ma to the present was imposed, we find that large thermochemical piles form in the lower mantle under Africa and the Pacific. Furthermore, the general shape of these structures are such that a ridge-like pile forms under Africa and a more rounded pile develops under the Pacific, consistant with seismic tomography. These general features of our models hold for a wide choice of density contrast and initial layer thickness.