The influence of heterogeneous layering and boundary conditions on the structural development of fault-bend and fault-propagation folds has been investigated through petrographic study of nonscaled rock models. The models are deformed in a triaxial rock-deformation apparatus at room temperature and a 50-MPa confining pressure. The models consist of a single layer of sandstone containing a saw-cut ramp that is inclined 20° to the layering, and an overlying, intact, thinly layered unit that is composed of limestone interlayered with lead or mica.
Analysis of the fold-thrust structures generated in sequentially shortened models with different loading conditions and layer types suggests that the mode of fold-thrust interaction activated upon shortening will depend on fault zone drag, bending and shearing resistance of the hanging wall, shear strength of layer interfaces, and loading conditions. For the models, these parameters may be expressed as a strength ratio describing the resistance to foreland translation relative to the resistance to internal deformation of the thrust system. Low strength ratios favor fault-bend folding. High strength ratios favor internal shortening of the sheet; isotropic and thick (relative to ramp height) units above a propagating thrust tip will shorten primarily by faulting, whereas thinly layered, anisotropic units will shorten by fault-propagation folding.
During both modes of fold-thrust interaction, the dips of the fold limbs increase, interlimb angles decrease, and imbricate faults form in the hanging wall or footwall with shortening. In one model suite, the imbrication is associated with a transition from fault-bend folding to fault-propagation folding and produces a highly asymmetric ramp anticline similar to a second-mode fault-bend fold or to a transported fault-propagation fold. The model data suggest that fault-propagation folding in heterogeneously layered rock occurs by the discontinuous formation, growth, and linkage of faults below the growing fold. Amplification of the fault-propagation fold is affected by the amount of slip transferred out of the deforming region, imbrication, and buckling. The changes in the mode of fold-thrust interaction and modifications in the local geometry and strain distribution that occur during shortening result from slip hardening on faults, or from rotation- or strain-induced variations in the strength of the layers.