Rock rheology and sharpness of folds in single layers

Many folds in rocks display angular profiles with sharp hinges and straight limbs. Previous studies by many authors have demonstrated that such fold shapes in multilayers result from an intrinsic anisotropy possessed by layered or foliated rocks. Our results of two-dimensional finite-element modeling show that folds with sharp hinges and straight limbs may also develop in isolated competent layers under suitable rheological conditions. Two basic material properties that affect fold shape are non-linearity and anisotropy. Viscous-plastic flow, power-law flow (strain-rate softening), and strain softening are three types of non-linear behavior. All lead to folds with sharp hinges and straight limbs. Anisotropy has a similar influence on fold geometry. Angularity of folds in isolated competent layers increases with increasing non-linearity of the layer or increasing anisotropy of the layer, and a quantitative relationship between fold angularity and degree of non-linearity or anisotropy may be established. Virtually identical angular fold shapes may be produced by either non-linear or anisotropic layer behavior. The strain distribution associated with these shapes is very different, however. For non-linear behavior, strain is focused in the fold hinges and minimal on the limbs, and for anisotropic behavior the reverse is the case, with shear in the limbs being dominant. These differences suggest that it should be possible to distinguish non-linear from anisotropic rheological behavior using layer shape and strain pattern.