The role of initial perturbations in the development of folds in a rock-analogue

A series of experiments is described in which layered specimens were shorte ned parallel to the layering. The specimens comprise two salt (NaCl) layers sandwiched between three layers of salt-mica synthetic schist. All specime ns were prepared and deformed under the same conditions, except for the amo unt of shortening, which was varied. The resulting fold-shapes are variable , even where the amounts of shortening are the same. In one specimen, folds are believed to have developed essentially by buckling with very little co ncomitant bulk homogeneous shortening perpendicular to the axial-plane. The specimen lacks an axial-plane foliation. Other folds are believed to have experienced varying amounts of bulk homogeneous shortening before and durin g buckling, and all have axial-plane foliations that have developed by grai n-scale transposition of the original bedding-parallel mica foliation. The difference in the behaviour of the various specimens is explained in terms of initial perturbations. These irregularities take the form of initial def lections in the almost planar bedding, variation in the degree of preferred orientation of the mica grains, and local compositional variation within i ndividual salt-mica layers. (C) 2001 Elsevier Science Ltd. All rights reser ved.