The extensional strength of the continental lithosphere: its dependence on geothermal gradient, and crustal composition and thickness

Abstract

Summary The response of lithosphere to an applied tectonic tensile force and the resulting stress distribution with depth has been investigated using a mathematical model incorporating the elastic, plastic and brittle behaviour of lithospheric material. Lithospheric strength is shown to be primarily controlled by lithospheric rheology and as a consequence is critically dependent on geothermal gradient and lithospheric composition. The rheologies of the upper crust, lower crust and mantle are assumed to be controlled by dislocation creep in quartz, plagioclase and olivine respectively. The critical level of tensional tectonic force required to generate geologically significant strains has been calculated as a function of surface heat flow, and the predicted lithosphere strength compared with available levels of tensile tectonic force arising from subduction plate boundaries and isostatically compensated plateau uplift loads. The model predicts significant extensional deformation in regions with surface heat flow >65 mWm −2 subjected to a tensile tectonic force of 3 × 10 12 N m −1 and is in good agreement with observed examples of intraplate extension. Lithosphere strength is critically controlled by the crustal thickness since the quartzofeldspathic rheology of the crust is weaker than the olivine rheology of the mantle. A decrease in crustal thickness thus increases the strength of the lithosphere. However, lithospheric extension also increases the geothermal gradient serving to weaken the lithosphere. The rate of extension is critical in determining which of these processes predominates. Fast strain rates (> 5 × 10 −15 sec −1 ) produce a weakening of the lithosphere (i.e. strain softening) while slower strain rates lead to strengthening of the lithosphere (strain hardening). Extensional style is consequently controlled by the lithospheric extension rate; fast extension producing, through strain softening, intense localized lithospheric extension with high (potentially infinite) β values, and slow extension, through strain hardening, giving broader regions of lithosphere extension with finite β values of the order of 1.5. For intermediate geothermal gradients ( q = 55−70 mWm −2 ) the model predicts a low stress-low strength region at the base of the crust due to the contrast between plagioclase and olivine rheology at the Moho. Other low-strength regions are predicted within the crust at major compositional (and rheological) boundaries. These low-strength zones are expected to control the location of detachment horizons by which crustal extension occurs particularly at slower strain rates. High geothermal gradients favour the shallower detachment horizons at the expense of the deeper horizons. The reverse is true for low geothermal gradients.