The strength of intraplate lithosphere

Abstract

The deformation of intraplate lithosphere in response to laterally applied stress has been investigated using a mathematical model of the elastic, brittle and viscous response of the lithosphere. Fundamental to the model is the conservation of the horizontal force associated with the applied stress and the redistribution of stress within the lithosphere after stress release by ductile or brittle failure. Ductile deformation is assumed to be controlled by dislocation creep in quartz in the crust and by dislocation and Dorn law creep in olivine in the mantle. Brittle failure is predicted using Griffith theory. The redistribution of stress after brittle and ductile deformation results in large levels of stress in the middle and lower crust immediately above the elastic-ductile or brittle-ductile transition. Lithosphere deformation is controlled critically by the lithosphere thermal structure of which surface heat flow, q, is taken as a convenient indicator. For hotter lithosphere, stress release by ductile deformation in the middle and lower lithosphere is more extensive and rapidly leads to larger levels of stress in the upper lithosphere. For sufficiently large levels of applied stress, or sufficiently hot lithosphere, complete failure ( Whole Lithosphere Failure of the lithosphere occurs by ductile and brittle deformation resulting in geologically significant strains. Critical levels of applied stress required to produce Whole Lithosphere Failure have been computed as a function of the lithosphere surface heat flow. The predicted strength of the lithosphere has been compared with expected levels of intraplate stress arising from plate boundary forces and isostatically compensated loads. The model predicts significant extensional intraplate deformation in regions of moderate heat flow with q > 60 m W m −2. For compressional deformation, however, a hotter lithosphere is required with q > c. 75 m W m −2. The model predictions are in general agreement with the comparatively widespread occurrence of extensional intraplate deformation and the restricted occurrence of compressional deformation. The model has also been used to investigate the relationship between the depth of the brittle-ductile transition and lithosphere temperature structure. The brittle-ductile transition depth is shown to become shallower with increasing heat flow, in good agreement with seismic evidence.