Implications of discrete strain compatibility in multilayer folding

Abstract

Theoretical kinematic analysis of idealised folded multilayers shows that, throughout the fold, axial planar stretch of repeated points within the fold must be homogeneous. Layers are effectively pinned at fold limbs such that the rotation of competent layer limbs produces an axial planar stretch. To be strain compatible, this stretch (measured between discrete points) must be accommodated by an equivalent stretch in the fold hinges. Differing modes of accommodation of stretch in the hinge result in fold styles including saddle reef folds, Class 1b/3, Class 1c/3 and Class 2 multilayers. The process of accommodating stretch induced by limb rotation is termed compatible folding. Strain analysis of folds using the compatible folding model predicts significantly less shortening than for other fold flattening models. The model also predicts that Class 2 folds formed by compatible folding will redistribute a pre-existing lineation within a plane. Compatible folding is, therefore, one alternative to the shear folding model for explaining great-circle redistribution patterns of pre-existing linear elements.