Fault rocks and fault mechanisms

TLDR

Physical factors such as friction, temperature, effective normal stress, and differential stress influence fault rock genesis, energy budgets, fault velocity modes, and fault types (thrust, wrench, normal). A conceptual model of a fault zone in crystalline crust shows an elastico‑frictional (EF) layer producing random‑fabric fault rocks overlying a quasi‑plastic (QP) ductile shear zone that forms mylonite, with peak shear resistance at the EF/QP transition around 10–15 km depth, where seismic faulting can generate but often obliterate random‑fabric rocks. Fault rocks formed by transient seismic faulting can be distinguished from those produced by slow aseismic shear in some cases.

Abstract

Physical factors likely to affect the genesis of the various fault rocks—frictional properties, temperature, effective stress normal to the fault and differential stress—are examined in relation to the energy budget of fault zones, the main velocity modes of faulting and the type of faulting, whether thrust, wrench, or normal. In a conceptual model of a major fault zone cutting crystalline quartzo-feldspathic crust, a zone of elastico-frictional (EF) behaviour generating random-fabric fault rocks (gouge—breccia—cataclasite series—pseudotachylyte) overlies a region where quasi-plastic (QP) processes of rock deformation operate in ductile shear zones with the production of mylonite series rocks possessing strong tectonite fabrics. In some cases, fault rocks developed by transient seismic faulting can be distinguished from those generated by slow aseismic shear. Random-fabric fault rocks may form as a result of seismic faulting within the ductile shear zones from time to time, but tend to be obliterated by continued shearing. Resistance to shear within the fault zone reaches a peak value (greatest for thrusts and least for normal faults) around the EF/QP transition level, which for normal geothermal gradients and an adequate supply of water, occurs at depths of 10–15 km.