When do folds unfold during progressive shear?

Folded layers in rocks can be stretched again, potentially unfolding the folds back to straight layers. Little is known, however, about how to recognize partly or even entirely unfolded layers. When folded layers can unfold, what determines their mechanical behavior, and how can we recognize them in the field? In order to address these questions, we present a series of numerical simulations of the stretching of previously folded single layers and multilayers in simple shear. Layers do not completely unfold when they undergo softening before or during the stretching process, or when adjacent competent layers prevent them from unfolding. Intrafolial folds and cusp-like folds adjacent to straight layers as well as variations in fold amplitudes and limb lengths of irregular folds are indicative of stretching of a fold train. © 2013 Geological Society of America.