Some kinematical patterns leading to the formation of similar folds

Abstract

A new mechanism, named “heterogeneous rotation shear”, is defined to explain the development of similar folds. It is based on the heterogeneous distribution of a type of deformation in which a line rotates without length change and another maintains constant its direction and length (pure rotation shear) or undergo a constant stretching through the rock (rotation shear with area correction). With these constraints, a kinematical numerical model of similar folds is developed. The mechanism produces a divergent pattern of the major axis directions of the strain ellipses throughout the fold, which does not agree with the usual axial plane cleavage of natural similar folds. The operation of layer parallel shortening is necessary to solve this problem; this shortening agrees with the incompetent character of the rocks where similar folds develop. Eventually fold flattening can modify the fold shape whilst maintaining the similar geometry. The model is applied to explain the development of a natural similar fold, indicating that this could be formed by a combination of layer parallel shortening and heterogeneous rotation shear. Finally, three types of similar or sub-similar folds have been distinguished: a) sub-similar folds formed by intense flattening of parallel folds; b) sub-similar folds formed by simple shear acting on previous perturbations, and c) similar folds produce by layer-parallel shortening and heterogeneous rotation shear.