Mechanics of extension and inversion in the hanging walls of listric normal faults

Fault shape, material properties and bedding anisotropy determine the style of deformation in the hanging walls of listric normal faults. We use numer ical models to study this deformation in both extension and inversion durin g displacement on a variety of master fault shapes. Elastic-plastic materia l properties in the models allow the development of shear bands, which simu late secondary faults within the hanging wall. If the master fault is compo sed of a planar ramp and planar flat separated by a sharp fault bend, a ser ies of antithetic normal shear bands develops in the hanging wall, propagat ing up from the fault bend. Each shear band is progressively abandoned as d isplacement on the master fault moves it away from the fault bend. Displace ments on the antithetic faults produce the limb of a hanging wall monocline and bound one side of a graben, the other side of which is bounded by the master fault. The antithetic shear bands are not rotated, and layering with in the graben remains subhorizontal. On the other hand, if the fault bend i s curved rather than sharp, symmetrical nested graben develop in the upper part of the hanging wall above the base of the ramp. Displacement on the ma ster fault moves these shear bands away from the fault bend, after which th ey are abandoned in favor of new shear bands. Early formed synthetic shear bands become shallower and concave upward because of the folding and rotati on of the hanging wall. With increasing radius of curvature of the master f ault, localization into shear bands decreases and, with a large radius of c urvature, shear bands do not develop. If weak bedding layers are included w ithin the hanging wall, they become sites of bedding-parallel shear bands t hat accommodate flexural slip folding and replace the synthetic shear bands that develop in homogeneous models. If extension is followed by shortening and inversion, normal shear bands that developed during extension are reac tivated as reverse shear bands but are also crosscut by new reverse shear b ands. The models produce results that are similar to both natural structure s and analogue models and provide explanations for many observations of def ormation in seismic profiles through extensional terranes.