Creep constitutive laws for rocks with evolving structure

Abstract

Abstract Zones of localized strain are common geological features at all spatial scales, are persistent in time and have important implications for rock strength. Technical advances now allow mechanical tests to be carried out to high strain, and, thus, there is both need and the opportunity to formulate constitutive laws for rocks as their structure (i.e. fabric, texture and mineralogy) changes. This paper reviews some attempts to describe quantitatively the mechanical behaviour of calcite rocks deformed to large strains. To include the effect of evolving structure on strength, three or more interlinked laws are needed: an evolution equation describing the rate of change for each structural variable; a kinetic equation relating mechanical and thermodynamic loading and the structural variables to the rate of strain; and a kinematic equation involving a time integral of inelastic strain rate. Structure variables may be explicitly identified or implicitly determined without identification. Appropriate explicit state variables might include aspects of the dislocation microstructure or the grain size, as are common in studies of plasticity in metals. But because natural tectonic situations are more complex, a much broader class of state variables will be needed. Among these additional variables might be crystal lattice preferred orientation, progress variables for metamorphic reactions, solid-solution chemistry, porosity and pore fluid fugacity.