A deformation micromechanism refers to that working on lattice, grain, or crystal scale as opposed to a macromechanism beyond grains, crystals, and pores. For example, the nature of deformation on the grain scale in granular rocks is controlled by their textural properties. Typical granular rocks are made up of grains, pores, and cement. Deformation of such a medium involves a change of shape and size of these constitutive elements. Among these three elements, grains form the skeleton of a granular rock. The strength of this skeleton depends on the strength of constituting grains and the nature of cement supporting the grains. Grain mineral composition plays an important role in failure processes due to differences in shape, strength, susceptibility to pressure solution and cleavage, and chemical stability. Grains deform in three basic modes: (1) translation and rotation, (2) dissolution, and (3) internal strain dominated by elastic deformation or fracturing. Relative displacement and rotation of intact grains and fracturing of grains in porous sandstone are primarily controlled by the Hertzian grain-to-grain interaction (Zhang et al., 1990). Dissolution is generally dependent on the overburden thickness or the burial depth. If the stresses at grain contacts exceed the tensile strength of the grains or the fracture toughness in general, they break, leading to fragmentation and comminution, which are known as part of a cataclastic deformation.

Pores impact volumetric deformation and control the strength of rock (Dvorkin and Nur, 1996). They act as flaws and concentrate stresses that may lead to eventual failure by pore growth and coalescence (Berg, 1970; Antonellini et al., 1994a; Du Bernard et al., 2002) or pore collapse (Aydin, 1978; Curran and Corrall, 1979). In addition, pores store fluids and thus facilitate transmission of forces to the skeleton in high fluid pressure environments. It is well known, based on the effective stress law, that rocks under a high pore fluid pressure are considerably weaker (Hubbert and Rubey, 1959).

In summary, granular rocks under upper crustal conditions exhibit four fundamental micromechanical processes that characterize the failure modes and their structural products: Inter-grain sliding (grain translation and rotation also known as particulate flow), grain fracturing, pore volume change, and pressure solution. Grain fracturing results in grain size reduction and changes in pore volume which are the two quantities that can readily be identified and quantified. However, inter-grain movements are very difficult to detect in granular rock because of the lack of pre-existing markers.

In addition, all micromechanical processes are greatly influenced by diagenetic processes that affect the host rock properties. Cementation and dissolution control the amount, distribution, shape, and strength of cement and are therefore crucial in the failure processes in granular rocks (Bernabe et al., 1992; Dvorkin and Nur, 1996; and Bjorlykke, 2006). Cement exerts resistance to intergranular movement and distributes contact forces thereby reducing the stress concentration that is so critical for grain breakage (Shah and Wong, 1997; Wilson et al., 2003). With increasing burial depth and cementation, the deformation behavior of granular materials changes. A general trend is that failure structure changes from localized tabular mode to sharp discontinuity with increasing mechanical compaction and diagenetic porosity reduction. This corresponds to a switch from a contact-based Hertzian constitutive behavior to that of a distributed deformation controlled by the solid frame (Dvorkin and Nur, 1996). This trend in sediments and rocks can potentially be reversed by dissolution and removal of earlier pore cement leading to the creation of secondary porosity that may again instigate pore-controlled stress regimes and pore collapse mechanisms and a new phase of compactive deformation localization.