Dynamic controls on subduction kinematics: the role of the downgoing lithosphere and passive mantle flow

It is much debated whether the forces associated with the downgoing plate, the overriding plate, passive or active mantle flow are dominant in controlling the paths of plates into the mantle. We investigate the dynamics and energetics for a free subduction system, driven solely by downgoing plate buoyancy, using a finite-element model of a viscoelastic plate with a free surface, sinking into a passive unbounded mantle represented by drag forces. Parameters are varied to study effects of an asthenosphere, ridge push, and a passive overriding plate, for a range of subducting plate viscosities and densities. Such a single, free plate achieves subduction mainly through trench retreat. Most of the energy dissipation occurs in driving the passive mantle response. As a result, the slab's sinking velocity is its Stokes velocity, determined by lithospheric buoyancy and mantle viscosity. The total subduction velocity and dip adjust to minimize bending dissipation in the lithosphere, and are affected by slab rheology as well as buoyancy. A low viscosity asthenosphere and ridge push facilitate plate advance, increasing plate dips and lowering subduction velocity, while suction and buoyancy of a work-free passive overriding plate decreases plate dips, thus increasing subduction and rollbackvelocities. However, the geometrical relation between the different parameters is the same in all model cases, because the slabs sink according to their Stokes velocity. The free subduction models thus provide a reference that can be used to distinguish the signature of downgoing plate buoyancy from that of other driving forces in global compilations of subduction parameters.