Lateral variations of strain in experimental forced folds

Packages of rock layers, deformed under confining pressure by the uplift and rotation of a steel-block forcing assembly, translate both towards, and away from, the margin of the principal uplifted block. The resulting asymmetric forced folds, and especially their long, planar limbs, exhibit along-layer variations in strain. Alternations of layer elongation and contraction occur along profiles extending away from the antiform/synform couplet. Layer-normal strains are mostly nil, so the longitudinal strains largely equate to volume strains in these plane-strain models. Spaced anomalies in outcrops, indicating either increased cementation, or erosional weakness, may suggest that similar processes operate in nature to produce variations in damage caused during the flexural-slip process. Two, non-exclusive explanations are offered to account for the patterns of strain observed in the experiments: (1) they may be caused by decaying wavetrains of small-scale flexural deflections (and their local strain patterns) related to the bending of the major forced folds; or (2) they may be caused by a 'patchy' development of layer-parallel slip, and the consequent spatial variability in displacements.