State of stress in slabs as a function of large-scale plate kinematics

The state of stress of slabs subducting worldwide, as revealed by seismicity, is extremely variable both with depth and between different subduction zones. Although in principle, slab pull should enhance down-dip extension in the slab, the reconstructed stress fields for intermediate depths (between 100 and 300 km) range from downdip compression to downdip extension. Using 2-D viscoelastic plane strain models we investigate the dependency of the stress field of slabs on geometry (dip of the slab) and kinematics (velocity of convergence between upper and lower plates and their absolute velocity with respect to the underlying mantle) of subduction zones. We conclude that although the state of stress in slabs is also controlled by other processes, downdip compression in the subducting slab is enhanced by mantle flow opposing the direction of the dip of the slab, whereas downdip extension is favored by mantle flow in the same direction of the slab dip (i.e., sustaining it). These predictions are in agreement with available geophysical observations, although exceptions to this simple pattern are observed worldwide. In addition, if the slab is decoupled from the upper plate, convergence between upper and lower plates induces a downdip compressional component of stress within the slab, decreasing the magnitude of extension in models characterized by mantle flow sustaining the slab and increasing compression in models with mantle flow opposing subduction. However, these are second-order variations when compared to the control exerted by absolute plate kinematics and by the magnitude of slab pull. Sensitivity analysis of rheological parameters allows us to conclude that these results are generally consistent, although low values of viscosity of the lithospheric mantle render this prediction less stable.