Viscous-plastic finite-element models of fault-bend folds

In finite-element models of fault-bend folds, viscous and plastic material properties are used to simulate pressure solution creep and cataclasis, respectively. Plastic deformation occurs primarily above ramp hinges. However, a band of high plastic strain is created in the lower hanging wall as material moves over the ramp hinges. Viscous deformation dominates above the ramp and flats, where plastic deformation is negligible. Hanging-wall material undergoes layer-parallel shortening on the lower flat and lower part of the ramp, layerparallel extension on the upper part of the ramp and upper flat, and layer-parallel shortening farther along the upper flat. The stress invariant √J′2 is highest above the ramp hinges and below the ramp, whereas stress invariant J1 is highest above and below the ramp. The high J1 below the ramp suppresses plastic deformation, so that footwall deformation is predominantly viscous. In multilayer models, strain is concentrated in the weak layers, especially on the limbs of the hanging-wall anticline. In models with bedding-parallel slip surfaces in the hanging wall, the strain on fold limbs is accommodated by interlayer slip, and layers between slip surfaces develop individual neutral surfaces. The models indicate that the relative and absolute amounts of deformation in fault-bend folds resulting from pressure solution creep and cataclasis vary as a function of structural position and history.