Intraplate lithosphere deformation and the strength of the lithosphere

Abstract

The intraplate deformation of continental lithosphere in response to applied stress has been investigated using a mathematical model which incorporates the elastic, ductile and brittle response of lithosphere material. Ductile deformation is assumed to be controlled in the crust by dislocation creep in quartz, and in the mantle by dislocation creep and plasticity in olivine. Brittle failure is predicted using modified Griffith theory. A fundamental feature of the model is the redistribution of stress within the lithosphere following stress release by both ductile and brittle deformation. This redistribution produces high levels of stress in the middle or lower crust immediately above the elastic‐ductile or brittle‐ductile transition. Lithosphere deformation is shown to be critically dependent on the temperature structure of which surface heat flow is a convenient indicator. For higher geothermal gradients, the release of stress in the lower lithosphere by ductile deformation is more rapid and complete and results in large stress levels in the upper lithosphere. For sufficiently large applied stresses or steep geothermal gradients, the stress levels in the upper and middle crust will cause complete fracture of the upper lithosphere. Whole lithosphere failure (WLF) then results, by continued brittle and ductile deformation, causing geologically significant strains. The critical value of applied stress required to give WLF has been calculated for both tensional and compressional deformation as a function of surface heat flow. The predicted lithosphere bulk strength is then compared with expected levels of intraplate stress arising from plate boundary forces and isostatically compensated loads, which are thought to give net stress levels in the continental lithosphere in the range +0.25 to ‐0.25 kB. Using these expected maximum stress levels, the model predicts significant extensional deformation in regions of moderate heat flow with q > c. 60mW m‐2 (e.g. Central Europe) as well as for areas of high heat flow like the Basin‐ and‐Range Province. Significant compressional deformation is predicted for areas of high heat flow with q > c. 75 mWm‐2 but only for restricted conditions of stress combination. These results are in good agreement with the observed rather widespread incidence of extensional intraplate deformation and the restricted occurrence of compressional deformation which may be confined to areas of unusually high heat flow or weak crust. The model may also be used to calculate the depth of the brittle‐ductile transition which is shown to become shallower with increase in heat flow, in good agreement with seismic evidence. Copyright © 1984, Wiley Blackwell. All rights reserved