Bedding-plane slip in initial stages of fault-related folding

Frictional slip along bedding planes contributes to fault-related folding of layered rocks. We use numerical experiments to investigate the deformation of frictional bedding planes near dipping faults under layer-parallel contraction and extension. Within the numerical experiments, contraction boundary conditions produce asymmetric anticlines and extension produces asymmetric synclines. The fold shape may be used to infer dip of the underlying fault in situations where the fault may not be observable. Additionally, sense of slip along bedding planes may indicate proximity to the fault tip. Under uniform remote tectonic strain, fault slip induces deformation in both the hangingwall and the footwall. At depths as shallow as 1 km there is no significant difference between fold amplitudes in the hangingwall and the footwall; this result is contrary to many kinematic models currently in use. Kinematic models of fault-related folds commonly include the development of flat-ramp and flat-ramp-flat fault geometries which may be attributed to initial ramp thrusting and later fiat development. Our mechanical models show that fault flats may be produced from fault ramps due to slip along frictional bedding planes near the thrust fault tips. Our numerical experiments also evaluate joint initiation; joints perpendicular to bedding are promoted in extensional environments. We compare the results of the mechanical model to kinematic models of fault-propagation folds and conclude that mechanical models offer important insights to better understand the folding process. (C) 1997 Elsevier Science Ltd.