State of stress in the oceanic lithosphere in response to loading

Summary. Flexure studies of the oceanic lithosphere constrained by bathymetry and gravity data suggest that the lithosphere behaves elastically over geological time-scales. For loads to be supported, however, large bending stresses (approaching 10 kb in some cases) are required at the top and bottom of the elastic plate. These stress-differences can be significantly reduced by introducing more complex rheologies: we propose a model of layered lithosphere, consisting of a purely elastic upper layer, a transition zone with viscosity varying with depth and a perfectly plastic lower layer. The transition layer is grossly centred at the bottom of the elastic plate. Such a model results in a noticeable reduction of stress differences; reaching 60 per cent for flow laws representing creep mechanisms in olivine. When applied to a number of seamount loads, this model leads to maximum stress-differences which do not exceed 1–2kb. The approach used in this study allows us to follow stress relaxation over time. Taking account of the thermal cooling of the lithosphere, we show that the elastic thickness of the lithosphere is stabilized after a given time, while the time required for stabilization is found to be of the order 5—6 per cent of the age of the lithosphere at the date of loading.