Normal fault inversion by orthogonal compression: Sandbox experiments with weak faults

Linear frictional failure criteria predict that normal faults form dipping around 60°, and reverse faults around 30°, depending on dry rock properties. Therefore, it is unlikely that normal faults be reactivated as reverse faults, unless the stress conditions are favourable, or the intrinsic properties of the intact rock or of the precursor fault (PF) are modified. In the present study, we focused on friction (strength) of the PF. We used sandbox experiments with an initially embedded weak PF dipping 60° or 70°, filled with a thin film of silicone putty that lubricated the (weak) fault, to investigate inversion of high angle PF by orthogonal compression. The results show that: (1) the PF can be inverted if it is weak during compression, even if the angle of dip is as great as 70°; (2) after inversion initiation, the reverse movement on the PF can last for as much as 30% model shortening, leading to great amounts of reverse displacement along the PF before the formation of a thrust ahead; (3) in models with more than one PF, the weakness of the reactivated fault closer to piston was not enough to prevent reactivation of the PF further ahead, after an amount of shortening that depended on distance between PFs. scous material used to weaken the fault is scalable to salt in nature. However, the great decrease in friction due to lubrication with a viscous material can simulate many other weakening mechanisms observed in nature.