Vergence boundaries: an extension of the vergence concept

The difficulty in distinguishing between vergence changes on adjacent limbs of tight primary folds and the similar effects produced by non-coaxial refolding of primary folds by larger-scale folds is outlined. It is suggested that a distinction should be made between typical and atypical vergence boundaries. Atypical vergence boundaries falsely suggest the presence of congruent major folds whereas major folds attributed to typical vergence boundaries are real. Practical rules are formulated that should be applied to vergence boundaries in non-coaxially refolded regions in order to recognize atypical vergence boundaries.