Initiation and reactivation of faults during movement over a thrust-fault ramp: numerical mechanical models

With numerical continuum models, we investigate the effects of fault geometry, fault friction, material properties and anisotropy on the initiation and reactivation of faults in the hanging wall of a thrust-fault ramp. The models use an elastic–plastic, frictional, dilatant, cohesion-softening material, in which deformation may localize as shear bands. With a sharp lower fault bend, backthrust shear bands propagate up from the concentration of differential stress at the fault bend. With a rounded lower fault bend, bedding-parallel shear bands develop above the fault bend in the center of the layer, where differential stress is highest. Friction on the ramp enhances the development of backthrusts, and causes them to initiate at shallower dips. Increased overburden decreases the amount of localization within shear bands and the spacing between them. Shear bands that develop at the lower fault bend are weaker than the surrounding material and are thus potential sites of reactivation during subsequent deformation. Because of this potential reactivation, the style of deformation at the upper fault bend depends on the deformation that accumulates at the lower fault bend. At the upper fault bend, backthrusts are reactivated as extension faults, and are crosscut by more steeply dipping extension faults. Bedding-parallel shear bands are reactivated at the upper fault bend with top-to-the-hinterland sense of shear. Low-angle extension faults are listric into the reactivated bedding-parallel shear bands, producing hinterland-verging extensional duplexes above the upper flat.