The determination of 'buckling rotation' and its application to theoretical and experimental models of buckle folds

The quantity 'buckling rotation' is defined, for buckle folds, as the total rotation of a fold limb minus the rotation that would occur due to pure shear if no competence contrast existed. Using existing models (theoretical and experimental) of buckle-fold development, the quantity 'buckling rotation' has been calculated for successive small increments of strain and plotted against strain or limb dip. The resulting curves are skewed and bell-shaped, indicating an initial sharp increase in buckling rotation early in fold development followed by a gentle, asymptotic decrease. The curve height and position are dependent on the competence contrast and, in multilayer systems, on the ratio of competent to incompetent layer thickness. The initial sharp increase in buckling rotation corresponds to the period of most active layer-parallel shortening during fold development.