GRAIN-SCALE ELASTIC ENERGY, ROCK FABRIC DEVELOPMENT AND RHEOLOGICAL EVOLUTION

During the deformation of polycrystalline rocks, individual minerals tend to change their shapes and orientations resulting in planar and linear fabrics. It has long been known that these fabrics can be related to the bulk deformation kinematics, but much less attention has been given to the underlying drivers of fabric formation and evolution. Here we explore the idea that elastic stresses are ultimately responsible for much of the change, driving a variety of physical and chemical processes that work to reduce the local and bulk elastic strain energy density. In rocks, the important minerals involved in fabric development (e.g., quartz, feldspar, micas) possess anisotropic properties such as fracture toughness, thermal expansion, thermal conductivity, and elastic stiffness. When subjected to a macroscale load (e.g., thermal, fluid, topographic, tectonic), each grain in the polycrystalline volume responds elastically by changing its dimensions by different amounts in different directions. Owing to the GPa-level stiffness-tensor components of these minerals, the interactions of the different grains lead to very large peaks and troughs in the local elastic stresses and strains. An important point about the elastic stresses is that they are instantaneous, and while their grain-scale magnitudes can be reduced by brittle, viscous, or chemical processes, they can never be fully dissipated owing to the interactions of the individual anisotropic grains. In this presentation we explore how changes in mineralogy, grain shape, and shape/crystallographic preferred orientations affect the magnitudes and distributions of the local elastic stresses and strain energy densities, and bulk elastic properties of the rock. We speculate that this is the fundamental cause of rock fabric development, driving mineral distributions, microfractures, reactions and viscous flow.