Kinematics of experimental forced folds and their relevance to cross-section balancing

We report the results of a series of laboratory experiments in which packages of layers (consisting of rock and lead) are deformed under 50 MPa confining pressure into forced folds by the uplift and rotation of pre-machined steel forcing blocks. The models are not fully scaled, but the geometries resulting form the deformation are remarkably similar to many natural forced folds observed in the U.S. Rocky Mountains foreland. During the folding, detachment and quantifiable layer-parallel translation occur between the layered sequence and the forcing assembly, confirming limited observations from earlier model studies. Away from the fold, there is a pattern of movement in which the layered sequence first moves away from the uplift, but, with greater structural relief, those motions reverse their sense to become layer-parallel translations towards the uplift. The very ductile lead unit at the base of the layered sequence flows laterally, especially across the crest of the uplift to the downthrown block, in response to pressure gradients which are inherent to asymmetric uplifts. The flow of this ductile unit causes layer-parallel translation of the rock layers, thereby transporting material into the fold. If the models were to be treated as proposed cross sections, and if they were subjected to the usual techniques of cross-section balancing, incorrect interpretations would result; this is because there are no suitable sites for either pin lines or no-flow boundaries.