The modification of parallel folds by progressive shearing parallel to the axial plane

Abstract

Using numerical simulations, parallel folds are progressively sheared along axial surfaces such that a simple shear couple acts parallel to the axial surfaces of the initial folds. The original folds change to resemble flattened-parallel and similar folds. During the deformation, successively developed folds show progressive increase in the degree of asymmetry, arc length, hinge thickness, interlimb angle and ratio of long to short limb lengths. The hinges of outer and inner arcs migrate differentially with respect to angular shear so that the axial traces and hinges of the adjacent folds approach each other. The net effect of these changes is reflected by a reduction in the number of cycles in the fold train. This paper demonstrates the development of flattened-parallel fold-like geometries by progressive, non-coaxial deformation of parallel folds. Other structures such as unfolded folds, boudinaged folds and transposed foliations can also be produced by axial plane shearing of parallel folds.