Thermally-activated creep and flexure of the oceanic lithosphere

Abstract

Thermal models of the oceanic lithosphere^1–4 explain ocean floor bathymetry, heat flux and gravity and geoid⁵ anomalies by the cooling and thickening of the lithosphere as it ages away from ocean ridges. Surface wave studies^6,7 tend to confirm that thickness increases with thermal age. Thickness estimates based on the lithosphere's flexural response, parameterized in terms of an elastic model floating on a inviscid substratum, are, however, factors of two to three times less than those from the thermal and seismic models^8,9. We show here that a unification of estimates can be achieved by a different, yet simple, parameterization of the bending response of the lithosphere. The lithosphere is treated as a non-convective layer that undergoes thermally-activated creep according to its temperature at the time of loading. The solutions to this problem for loads emplaced on lithosphere of increasing age are then used as a basis set to construct the solution to the problem of stress relaxation in a thermally ageing lithosphere. We show that the predicted deformation passes through a series of apparently weaker quasi-elastic states, which can be equated with those of the elastic thin-plate flexural models, but the process is controlled by the thermal lithosphere and its associated thickness. The controlling parameters are the basal viscosity of the lithosphere and the activation energy for creep in mantle rocks.