THE DISTRIBUTION OF STRESS WITH DEPTH IN THE LITHOSPHERE - THERMORHEOLOGICAL AND GEODYNAMIC CONSTRAINTS

Horizontal tectonic forces arising from the plate-tectonic process give rise to horizontal stress within the lithosphere whose depth distribution is controlled by plastic and brittle strength. A constant force model of lithosphere deformation incorporating the brittle-viscoelastic response of each infinitesimal component of lithosphere allows for the transient computation of lithosphere stress and overcomes the steady-state approximation of the constant strain-rate model. For the constant force model, creep in the lower crust and mantle leads to stress decay in these regions and to stress amplification in the upper lithosphere through stress redistribution. The lithosphere stress-depth relationship is primarily controlled by rheology which depends on geothermal gradient, crustal composition and thickness. For continental lithosphere, low-strength regions exist within, and at the base of, the crust due to vertical contrasts in composition and therefore rheology. Changes in tectonic force with time give reversals in the sign of stress with depth so that lithosphere has a 'memory' of tectonic force history. Continental lithosphere strength calculated by the constant force model is comparable with estimated levels of intraplate tectonic force. Horizontal stress within oceanic lithosphere arises due to plate-boundary forces and cooling of oceanic lithosphere. Application of the increasing ridge-push force with age to a cooling thermo-rheological model of oceanic lithosphere predicts compressive horizontal stresses whose magnitude increases with age and which decrease almost linearly with depth. A thermo-rheological model of thermal stress for cooling oceanic lithosphere, incorporating plastic and brittle deformation, predicts horizontal compression in the upper lithosphere above deeper tension. Cooling and ridge push combine to produce a compressive stress field in the ocean basins capable of producing brittle failure.