DEVELOPMENT OF KINKBANDS AND CHEVRON FOLDS IN ANISOTROPIC MATERIALS: A NUMERICAL MODELING STUDY

Kinkbands and chevron folds are common in well-layered rocks but their origin and development are poorly understood. Rheological anisotropy has been suggested to play a significant role. We have been using finite difference modeling with the commercial code FLAC to address the problem. FLAC built-in ubiquitous joint model (a Mohr-Coulomb solid with an embedded penetrative weak plane representing anisotropy) is used to simulate anisotropic plastic materials and our own newly developed anisotropic Maxwell model is used to simulate anisotropic viscous materials. For anisotropic plastic materials, we define the strength of anisotropy, m, as the ratio of the internal friction of the material to the friction of the ubiquitous joint (the penetrative weak plane). When such a material is subjected to anisotropy-parallel shortening, a transition in deformation mode is observed as m increases. At low m, strain localization is the deformation mode and one set or two conjugate sets of simple kinkbands develop. Kinkbands widen and rotate slightly as deformation advances. As m increases, strain localization into kinkbands is still the deformation mode but kinkbands get more complex – there are kink (chevron) folds within individual kinkbands, and there may be kink folds outside kinkbands as well. As m gets still higher, strain localization completely gives way to pervasive kinking and only chevron folds develop. The scale of the folds is determined by the grid size. No strain localization is observed in anisotropic Maxwell viscous materials subjected to anisotropy-parallel shortening. Instead non-periodical chevron folds develop. When a stack of anisotropic viscous layers of varying parameters or one of mixed anisotropic and isotropic layers is shortened along the plane of anisotropy, the resulting folds have multi-scale characteristics. Our work provides the preliminary basis for using geometrical characteristics of natural folds to constrain the rheological behavior of rocks at the time of folding. Occurrence of kinkbands appear to suggest uniquely that the dominant rheology was plastic, and non-periodical chevron folds are indicative of viscous behavior.