Numerical modelling of the stress regime at subduction zones

Abstract

The stress regime at subduction zones has been modelled using a visco-elastic, quadratic isoparametric finite element model. An isoparametric model is used because it performs more accurately than constant strain triangular elements (CST) and also allows curved sided elements to be introduced.A method for modelling the frictional sliding on Isoparametric fault elements has been developed by extending Mithen's (1980) CST model. The resulting method is suitable for modelling the deformation on both plane and listric, normal and thrust faults. Graben widths predicted by normal fault models agree with analytic solutions and this implies than Mithen's CST models failed to do so because they were too stiff. Application of this model to subduction zones demonstrates that the slab pull force induces tension in the subducting plate and compression in the overlying plate. Part of the lateral variation in stress which is observed at all subduction zones is therefore inferred to arise from the slab pull force. Differences in the magnitude of these stresses at different subduction zones may therefore be accounted for by local variations in the magnitude or dip of the slab pull force, and also by the extent of the coupling across the plate boundary. Various forces account for the stress regime in back arc regions. Tensional stress is generated by lateral density variations, and the heating and shearing caused by slab induced convection. Compressive stress, arising from the slab pull force, is superimposed upon this. The magnitude of the compression, however, is dependent upon the dip and size of the slab pull force and also the degree of mechanical coupling between the plates at the subduction zone fault. Local variations in the magnitude of the compressive stress may therefore explain why the stress regime is observed to be so variable in back arc regions, and is more commonly tension than compression.