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 geomet ry, fault friction, material properties and anisotropy on the initiation an d reactivation of faults in the hanging wall of a thrust-fault ramp. The mo dels use an elastic-plastic, frictional, dilatant, cohesion-softening mater ial, in which deformation may localize as shear bands. With a sharp lower f ault bend, backthrust shear bands propagate up from the concentration of di fferential stress at the fault bend. With a rounded lower fault bend, beddi ng-parallel shear bands develop above the fault bend in the center of the l ayer, where differential stress is highest. Friction on the ramp enhances t he development of backthrusts, and causes them to initiate at shallower dip s. Increased overburden decreases the amount of localization within shear b ands and the spacing between them. Shear bands that develop at the lower fa ult bend are weaker than the surrounding material and are thus potential si tes of reactivation during subsequent deformation. Because of this potentia l reactivation, the style of deformation at the upper fault bend depends on the deformation that accumulates at the lower fault bend. At the upper fau lt 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 she ar. Low-angle extension faults are listric into the reactivated bedding-par allel shear bands, producing hinterland-verging extensional duplexes above the upper Rat. (C) 2001 Elsevier Science Ltd. All rights reserved.