Process of Folding to Unfolding of Competent Layer during Progressive Deformation

In progressive non-rotational deformation of a system consisting of incompetent rock and randomly oriented competent layers, the layers, which are inclined at low angles to the plane of the shortest and the intermediate principal axes (Z and Y) of the strain ellipsoid of mean longitudinal strain of the system, are folded and rotated towards either the longest principal axis (X) of the strain ellipsoid or the XY-plane. The folds appear to be generally generated with their axial surfaces normal or sub-normal to the layer surface. However, the axial surfaces of folds found on the layers immediately before pass through the surface of no infinitesimal longitudinal strain are oriented in all directions of between the normal to the layer surface and the XY-plane. At this stage, the axial surfaces of the folds which grew in fast rate tend to be oriented approximately parallel to the XY-plane, while those of the folds which grew in slow rate tend to remain in a direction normal or subnormal to the layer surface. After the layers passed through the surface of no infinitesimal longitudinal strain, the fold forms with the axial surfaces normal or subnormal to the layer surface yield to unfolding, while those with the axial surfaces approximately parallel to the XY-plane are not unfolded but yield further to closing. The latter yields to closing even after the layers passed through the surface of no finite longitudinal strain. It has been concluded that it may be frequently difficult to use TALBOT'S (1970) method for description of strain state.