| Subduction is driven by gravitational forces and resisted primarily by viscous forces. Three independent types of mantle flow combine to create viscous tractions on the surface of a subducting slab: (1) flow induced by plate convergence and plate subduction, (2) flow caused by trench migration, and (3) "regional" upper mantle flow unrelated to convergence or trench migration. The equilibrium dip of a slab reflects a balance of viscous tractions with (age-dependent) gravitational forces and slab stiffness. This study presents results from a two-dimensional finite-difference model of upper mantle flow for different slab geometries where mantle viscosity increases exponentially with depth and the slab, terminating in the upper mantle, is assumed to be infinitely viscous. The subducting slab creates a strong eddy in the overlying mantle wedge; the eddy is largely unaffected by trench migration or regional flow. Regional flow has the greatest potential impact on viscous forces, trench migration has an intermediate effect, and convergence/subduction the least effect. By balancing gravitational and viscous torquing and dip-parallel forces on the slab, constraints on mantle viscosity and on regional flow in specific subduction zones may be inferred from observed subduction parameters, such as slab dip. A viscosity of <2 x 1020 Pa s at the base of the lithosphere best fits this model. The intrinsic asymmetry of the subduction process induces a regional flow in the direction of subduction, which helps to balance slab forces. For slabs approaching the 670 km discontinuity, calculated pressures reach unrealistically high values and viscous resistance greatly exceeds gravitational pull, suggesting that below deep subducting slabs there must be flow from the upper to lower mantle. |
