A model for the structure and development of fault zones

Cross-sections of fault zones commonly show two sub-parallel slip surfaces which bound more or less deformed rock. This characteristic geometry can be explained by a combination of 'tip-line' and 'asperity' bifurcation processes. Fault tip-line bifurcation is due to irregular tip-line propagation resulting in leading edge segmentation, fault overlap and eventual duplex formation. Asperity bifurcation removes a fault surface irregularity by the generation of a new slip surface which by-passes the original slip surface. Repetitions of either or both bifurcation processes give rise to a fault zone with a complex and unpredictable internal geometry. These processes are described mainly on the basis of observation of normal faults but can be applied to other fault types. Fault zone thickening by bifurcation is intermittent rather than progressive and localised fault zone thinning may occur.