In Switzerland, the Opalinus Clay (OPA) formation is favored to host a repository for nuclear waste. Thus, we must know its deformation behavior. In this study, we focused on the microstructure of gouge, a thin (< 2 cm), drastically strained clay layer at the so-called Main Fault in the Mont Terri rock laboratory. We suggest that in situ gouge deforms in a more viscous manner than undeformed OPA in laboratory conditions. Moreover, we speculate about the origin and evolution of the gouge layer.