Effects of interlayer slip in model forced folds

Forced-fold models, constructed from multiple sheets of rock layers and other materials, are deformed, under confining pressure, by the translations and rotations of a steel-block forcing assembly. The first-order response of these models is like that observed in earlier sets of models: an asymmetric fold pair develops over the uplifted edge of the rigid, primary forcing block; the layers translate into the folding area without being pushed; and bending deformation imparts spatially variable strains onto the layered package. The second-order response of the new experiments is quite different from earlier work — the new multilayered models develop temporally variable bending strains caused by the progressive development of interlayer slip on some of the layer-layer interfaces. The presence of the layer-parallel slip surfaces reduces the bending resistance, and it causes the resulting folds to be more localized and to have different shapes than in similar experiments where layering is less effective. In the multilayer models, faulting in the layered package is much reduced compared to similar models without multiple layering, and material strains are everywhere smaller in the layers. Layer contractions and elongations of the layering are produced in these forced-fold models as a consequence of the flexure process, without regard to farfield causative loads. The models reveal a progression of deformation consequences that is related to the mechanical effectiveness of the layering. If the understanding of process gained from these models is extended to natural folds, it becomes possible to explain both the observed differences of fold shape and some of the variations in subsidiary deformations.