KINEMATICS OF INVERSION OF NORMAL-FAULTED STRATA AS A FUNCTION OF BURIAL DEPTH AND ROCK DUCTILITY

We investigate inversion of normal faults at different burial depths using physical rock models deformed under confining pressure. Samples, which consist of a 1-cm thick limestone layer above a 70° dipping fault in a rigid medium, 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 and reverse-reactivation of normal faults, we ran five experiment suites in which confining pressure and reactivation magnitudes were varied. In all tests, a normal fault forms in the limestone during extension. Subsequent inversion is accommodated in the limestone by reverse slip on the normal fault, creation of 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 occurs 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 the overprinted fracture fabric in the footwall at early stages, fracture and fault fabric in an upthrusted wedge at intermediate stages, and extensive localized cataclasis and relatively thick fault zones with small net displacement at late stages of inversion. Reverse reactivation of the normal faults may occur during coaxial contraction even though such faults are "unfavorably" oriented if standard frictional models and a homogeneous stress state are assumed. Locally occurring reverse slip on normal faults is favored when strata display low ductility and less favored when strata work-harden during extension, regardless of conditions imposed during subsequent contraction.