LITHOSPHERE RHEOLOGY - A NOTE OF CAUTION

It has become common practice to use laboratory-determined, low-temperature frictional sliding data, together with power-law equations for 'steady-state' creep of rocks at high temperatures, to construct inferred profiles of rock strength and mode of failure with depth in the lithosphere. In some cases this may involve unwarranted extrapolation of rock mechanics behaviour beyond the region of its validity. The form of transition from frictional behaviour to intracrystalline plastic flow in the Earth is much more complex than is suggested by the above model. Within the regime of plastic flow, substantial changes in flow-strength and mode of failure (whether flow becomes localized into shear zones) may accompany microstructural changes developed over large strains. When deformation is accompanied by metamorphic changes, existing flow laws for rocks are likely to be wholly inapplicable. Until a clearer understanding has emerged of the full range of expression of the rheological behaviour of rocks, existing models of lithosphere rheology should be treated with caution.