Mantle circulation and the lateral migration of subducted slabs

The irregular motions of plates and the irregular distribution of plate consumption and generation require that subduction zones and descending lithospheric slabs migrate laterally. Absolute plate motions indicate that slab migration is generally retrograde; that is, it is opposite to the direction of motion of the plates to which the slabs are attached, at rates of 10–25 mm yr−1. As a result, the downward motions of slabs are generally steeper than their dips, probably because of their negative buoyancy relative to the surrounding mantle. An important consequence of lateral slab migration is the displacement of material away from one side of the slab and inward flow of an equal volume toward the other side. This generates a mass flux in the mantle that is comparable in magnitude with the flux involved in the overturn of the oceanic lithosphere. The flow induced by slab migration is, therefore, an important part of the large‐scale mantle circulation associated with plate motions. Two‐dimensional numerical models with retrograde slab migration explicitly prescribed as a boundary condition show that migrating slabs, in contrast with nonmigrating ones, are at an angle to streamlines in the surrounding mantle. Accordingly, the parallelism of Benioff‐Wadati zones and streamlines cannot be used to discriminate among mantle flow models. Laterally migrating slabs do not separate streamlines that turn in different directions at depth, and these slabs do not turn backward beneath the plates to which they are attached. Instead, these slabs become parts of the circulations beneath the overriding plates, and even at depth they continue to move away from their oceanic ridge sources. Migrating slabs do not separate mantle convection cells. The numerical simulations also show that slab migration slows the flow under the attached plate by diverting part of it to the circulation beneath the overriding plate. The enhanced flow under the overriding plate pulls it more forcefully toward the subduction zone by increasing both the magnitude of the basal drag and the length of the overriding plate subject to a trench‐directed drag. This drag on the overriding plate is the source of the "trench suction" force that appears in models of plate‐driving mechanisms. Retrograde slab migration increases trench suction and may, thereby, play an important role in initiating and maintaining back arc spreading. Over long periods of time, migrating slabs sweep out large volumes of the mantle, altering flow patterns and dispersing material to great distances. By preventing the formation of regular closed cells, slab migration helps to mix the mantle and make it more chemically homogeneous. Slab migration is an important factor that causes mantle flow to be geometrically complex and time dependent.