GRAIN-SCALE ELASTIC ENERGY DRIVES ROCK FABRIC DEVELOPMENT AND RHEOLOGICAL EVOLUTION (Invited Presentation)

During the deformation of polycrystalline rocks, individual minerals tend to change their shapes, positions 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 evolution and how it relates to rock rheology. Here we explore the idea that local gradients in elastic stresses and strains are ultimately responsible for much of the change, fueling the physical and chemical processes that drive fabric evolution. Important minerals involved in fabric development (e.g., quartz, feldspar, mica) have strongly anisotropic thermal expansion and elastic stiffness. When subjected to a macroscale thermal or mechanical load, 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, interactions of the different grains lead to very large peaks and troughs in the local elastic stresses. This inhomogeneous distribution in turn generates large gradients in parameters important for rheological considerations, such as normal stress, fluid pressure and chemical potential. In this presentation we speculate that elastic energy is a fundamental factor in rock fabric evolution, driving mineral redistribution, microfracture, reaction, and viscous flow. The resulting changes in rock fabric in turn feed back to the evolution of deformation mechanisms and rheology.