Effects of burial history, rock ductility and recovery magnitude on inversion of normal faulted strata

Abstract

Inversion of normal faults at different burial depths is studied using physical models constructed with rock and deformed at confining pressure. Models consist of a 1 cm thick limestone layer above a fault dipping 70° in a rigid medium, and are subjected to a two-stage deformation path of layer-parallel extension followed by coaxial contraction. To investigate the effects of burial depth and relative ductility on kinematics of inversion, five model suites were run in which confining pressure and recovery magnitudes were varied. In all models, a normal fault forms in the limestone during extension. Subsequent inversion is accommodated in the limestone by reverse slip on the normal fault, creation and movement along new reverse faults, and distributed fracturing and folding. The relative contribution of these mechanisms depends on the relative ductility of the rock and magnitude of inversion. Reverse slip on the normal fault and distributed fracturing occur during early stages of inversion and new reverse faults form at intermediate stages. During late stage inversion, strata with low mean ductility shorten primarily by reverse slip on the pre-existing normal fault, whereas strata with high mean ductility shorten by continued slip on reverse faults. Evidence for inversion is provided by superposed fracture fabrics in the footwall at early stages (<35% recovery), fracture and fault fabric in the upthrusted wedge at intermediate stages (35% to 100% recovery), and extensive localized cataclasis and relatively thick fault zones with small net displacement at late stages of inversion (>100% recovery). This change in fracture fabric during inversion could lead to an overpressured footwall in natural inversion structures. Reverse reactivation of the normal faults may occur during coaxial contraction even though such faults are unfavorably oriented assuming typical rock friction behavior and a homogeneous stress state. Localized reverse slip on normal faults is favored when strata display low ductility and less favored when strata work-harden during extension, however, the models show that the final inversion geometry is dependent on the ductility during both phases of deformation. Even a fault that is work-hardened during extension can reactivate if the ductility during contraction is low enough.