Fault-related folding: A review of kinematic models and their application

Folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts, accretionary wedges, fold-and-thrust belts, and intra-plate settings in either strike-slip, compressional, or extensional regimes. Due to their widespread occurrence, knowledge about the development of these structures is important to a broad spectrum of geoscience sub-disciplines, such as structural geology, seismology, geomorphology, petroleum geology, and Quaternary geology. Fault-related folding has been analyzed intensively over the last 140 years. For the sake of this review, we divide the folds according to the way the faults and the folds form; that is into detachment, fault-bend, and fault-propagation folds. All fault-related folds are caused by changes in fault parameters. The simplest method to produce folds is to transport material along faults that have stepped, flat–ramp–flat geometries (fault-bend fold). Alternatively the slip can decrease along the length of the fault, and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy, either a detachment fold or a fault-propagation fold is formed, respectively. Detachment folding was first investigated in the early 20th Century, whereas the full significance of fault-propagation folds was recognized quite late in the 1980s. Seminal work on fault-related folding was carried out in the 1930s, but quantitative kinematic models have only been available in the last 30 years. These models are extremely valuable, because they allow a comprehensive understanding of the evolution of fault-related folds and lead to more accurate predictions of the sub-surface structure. From the mid-1990s onwards, numerical simulations have been used to identify how fault parameters (such as dip and fault-bend angle, propagation-to-slip ratio, and shape of the trishear zone) influence the geometry of the related folding. This is directly applicable to the analysis of the shape of anticlines produced. However, this does not mean that fold geometry is uniquely related to fault geometry; on the contrary, different kinematic approaches can lead to a similar fold shape.