Complex mantle flow around heterogeneous subducting oceanic plates

Abstract

The foundering of oceanic lithospheres controls the circulation patterns of the mantle around subducting slabs. Here, we investigate the sensitivity of the mantle flow to slab buoyancy variations along convergent margins using three-dimensional numerical models of subduction in a viscous mantle. The models illustrate that in a buoyancy-driven system varying subduction velocity arising from negative buoyancy variations effectively drives pressure gradients and confers a general flow component sub-parallel to the margin's strike, allowing for material transport over large distances around the slabs. The along-slab velocity component introduces widespread horizontal simple shear in the mantle flow which is maximized beneath the slab between ∼100 and ∼350km. Mantle flow complexities develop rapidly, although not instantaneously, upon subduction of heterogeneous plates. The resulting slab pull gradients are mostly accommodated by internal slab deformation, decoupling the mantle flow from motions at surface. Moderate slab pull variations have a minor impact on plate velocity, and might not result in trench motions, although effectively rearrange the flow. Slab buoyancy heterogeneities are firstly associated with age-dependent thickness variations, but also with slab break offs and windows, varying depth of subduction and entrainment of buoyant blocks. Because these are observed at all subduction zones, the process shown here should have a global relevance for the flow around slabs.