Viscous mantle flow under moving lithospheric plates and under subduction zones

Abstract

Summary. Some simple solutions are presented for slow steady viscous flow in fluid layers or half-spaces with moving boundary segments. The results suggest the following conclusions concerning flow in the Earth's mantle. (a) Viscous flow entrained by the larger lithospheric plates would penetrate the whole mantle if its properties are fairly uniform. (b) Secondary circulation cells are generated under stationary plates by the flow entrained by an adjacent moving plate. The secondary cells are of opposite sense to the adjacent cells, and of very low amplitude when there is no heat input into the fluid. Heating of the fluid from within or below would amplify them. (c) Flow under a convergence boundary between a stationary and a moving plate tends to dip obliquely under the stationary plate. A maximum dip of the flow of about 30 degrees occurs vertically under the convergence boundary if the effect of the subducted plate is ignored. This suggests a straightforward explanation for the observed oblique dip of the Benioff seismic zones: the sinking plates are deflected from the vertical by the mantle flow entrained by (or associated with) the moving plates to which they are attached.