Scaly fabric and slip within fault zones

Scaly fabric was first described early in the development of Geology, and has experienced a recent renaissance with the realization that it may play a key role in the shallow coseismic deformation of plate boundary faults, such as the M9 Tohoku-Oki megathrust event. The fabric itself is an anastomosing network of polished surfaces. It is often present in clay-rich sediments deformed by pure and/or simple shear. The faults where it is found typically have multiple modes of slip, from creep to tremor to seismic slip. It is a macroscopic, multi-scale structure that has, to date, been impossible to create under laboratory conditions. In this paper I review the state-of-the-art of our understanding of this fabric, its mechanical modes of deformation, and its capacity for both aseismic and seismic slip, and also summarize the challenges we face to understand its full importance in the evolution of plate boundary faults. The conclusion is that scaly fabric is linked to frictional-viscous shear zones where bulk viscous deformation is accompanied by localized shear: a condition typical of frictional transition zones in faults.