STRESS PERCOALTION AND THE DEVELOPMENT OF PATTERNS IN METAMORPHIC ROCKS
IN METAMORPHIC ROCKS
One of the hallmark features of deformed rocks is the presence of patterns; for example patterns created by shear localization, patterns associated with preferential dissolution like stylolites and slatey cleavage or patterns such as compositional banding that are created by mineral segregation. Even rocks that appear initially homogeneous like shale or randomly patterned like granites develop patterned features when deformed in the Earth’s interior. The role of stress in governing the orientation of features has long been recognized, but less is understood about what governs their spacing and what causes them to arise from an un-patterned material. Much research has been done on each type of feature separately but there is no overarching theory for pattern formation in deformed rocks. Using a 2D plane-strain finite element model of a large ensemble of grains in a hypothetical elastic-plastic polycrystalline material, I show that local variations in stress and strain participate in large-scale patterns. These patterns are a function of the heterogeneity and statistical distribution of elastic and plastic properties across the population of mechanical components (grains and grain boundaries) in the material and are likely produced by stress percolation. The principle components of the stress tensor each have their own pattern that evolves with deformation. Initially, the principle compressive stress forms a strong pattern with a fabric parallel to compression. Once ductile deformation initiates, the least compressive stress evolves into a fabric perpendicular to compression, while the intensity of the compressive stress pattern diminishes. The density and intensity of the stress modulations in the principle compressive stress pattern dictates the development and spacing of shear localization. The density and intensity of the stress modulations in the least compressive stress pattern may serve as a template for mass transport of the most mobile chemical components, leading to phase separation and foliation development.