Stress concentration in the upper lithosphere caused by underlying visco-elastic creep

The implications of subdividing the lithosphere into upper elastic and lower viscoelastic layers are investigated by finite-element analysis. Application of uniform horizontal boundary stresses at the ends of a lithospheric plate leads to amplification of the stress in the elastic layer by a factor about equal to the ratio of lithospheric to elastic layer thicknesses, and the visco-elastic layer becomes nearly stress-free except near its ends. The time constant for approach to equilibrium is proportional to viscosity, being for our model 0.21 My for 10²³ N s m⁻², and there is some accompanying flexure of the lithosphere. Local variation in the thickness of the elastic layer causes inverse variation in the stress, in part explaining the stability of shield regions and the tectonic activity of hot plateau uplift regions. It is shown that stress amplification also occurs where the stresses arise from body forces such as differential loading and isostatic compensation across continental margins.