BEDDING-PLANE SLIP IN INITIAL-STAGES OF FAULT-RELATED FOLDING

Frictional slip along bedding planes contributes to fault-related fold ing of layered rocks. We use numerical experiments to investigate the deformation of frictional bedding planes near dipping faults under lay er-parallel contraction and extension. Within the numerical experiment s, contraction boundary conditions produce asymmetric anticlines and e xtension produces asymmetric synclines. The fold shape may be used to infer dip of the underlying fault in situations where the fault may no t be observable. Additionally, sense of slip along bedding planes may indicate proximity to the fault tip. Under uniform remote tectonic str ain, fault slip induces deformation in both the hangingwall and the fo otwall. At depths as shallow as 1 km there is no significant differenc e between fold amplitudes in the hangingwall and the footwall; this re sult 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 i nitial ramp thrusting and later fiat development. Our mechanical model s show that fault flats may be produced from fault ramps due to slip a long frictional bedding planes near the thrust fault tips. Our numeric al experiments also evaluate joint initiation; joints perpendicular to bedding are promoted in extensional environments. We compare the resu lts of the mechanical model to kinematic models of fault-propagation f olds and conclude that mechanical models offer important insights to b etter understand the folding process. (C) 1997 Elsevier Science Ltd.