A numerical study of the dynamics of subduction

Abstract

The mechanics and dynamics of subduction have been studied using 2-D finite element analysis. Two finite element formulations have been employed; one formulation for Newtonian viscous flow and one formulation for linear elasticity and viscoelasticity. Quadratic isoparametric quadrilateral and triangular elements are used for both formulations. Models of flow in the mantle driven by oblique subduction produce an asymmetric depression of the surface above the slab. The width and depth of this depression are dependent on the value of the viscosity of the lower mantle, the length and mechanical strength of the slab. Analysis of the flow patterns suggests that the viscosity contrast at the 670 km sesmic discontinuity is likely to be of the order x 10. The stress regime at an island arc margin with a subducting slab dipping at 45 has been modelled using an elastic-viscoelastic rheology. The body forces of the slab produce an asymmetric depression of the surface above the slab which generates horizontal deviatoric compression in the plates. Unlocking the thrust zone between the subducting and overriding plates eliminates the shear stress in the fault plane resulting in regional horizontal tension in both plates, uplift of the leading edge of the overriding plate and depression of the subducting plate. The regional tension is interpreted as the source of the plate driving forces of slab pull and trench suction. Local horizontal compression in the arc-forearc region produced by the surface depression exceeds the regional tension and this may be the source of lateral variation in stress that is ob- served across the strike of convergent margins. It may also be the source of backarc compression for low angle slabs at Chilean type margins. Depression of the surface provides partial compensation of the slab body forces. Thus the downdip force is reduced and the resulting stress regime in the slab is controlled by the isostatic upthrust at the trench and the viscosity contrast at 670 km depth. A low pressure zone above and high pressure zone below the slab may act against the body forces which rotate the slab towards vertical subduction. Anomalous pressures in the mantle are created and sustained by continuous subduction and rollback, and may behave in a self-regulating mechanism. A low viscosity zone in the mantle wedge above the slab leads to the development of double seismic zones as suggested by Sleep (1979).