Field examples and numerical modelling of oversteps and bends along normal faults in cross-section

Abstract

Oversteps and bends along cross-sections through small-scale (< 2 m displacement) normal faults are described. Lithological variations, particularly the relative proportions of brittle to less brittle layers, have important effects on fault initiation, geometry and displacement. Faults usually initiate as extension fractures in more brittle layers, with shear fractures developing at lower angles in less brittle layers as displacement increases. When there is a greater thickness of brittle layers than less brittle layers, shear fractures occur in the brittle layers. Bends with overall contraction develop in less brittle layers, where faults have gentler dips. Pull-aparts (extensional bends) typically develop in brittle layers when there is a high proportion of less brittle layers in the rock sequence. Minor structures which can occur at oversteps and bends include pull-aparts, minor faults, brecciation, folds, compaction and pressure solution; these typically cause displacement minima at contractional bends. Numerical modelling (distinct element analysis) has been carried out to simulate the patterns of deformation at oversteps and bends. Area increase, pull-apart development and localised stress concentration occur at extensional oversteps and bends. Stress concentration and a tendency for compaction are typical of contractional oversteps and bends. Such results are compatible with the structures observed in the field. A sensitivity analysis of the modelling has been carried out to investigate the effects of variations in mechanical parameters on deformation. Although models with weaker properties are more deformable, the overall geometry of deformation varies little with variations in mechanical parameters.