Experimentally produced slickenside lineations in pyrophyllitic clay

During shearing of pyrophyllitic clay, fault planes bearing slickenlines were generated on previously initiated C- and C'-planes. These slickenlines have excess lengths with respect to the measured slip displacement on the fault plane and are of the ridge-in-groove type with the hangingwall nesting into the footwall, and vice versa. These features cannot be explained in terms of either frictional asperity ploughing or dissolution/ reprecipitation but are produced during the development of a complex ductile shear zone that contains the fault plane. This initial shear zone nucleates from localized shear bands that link up to form a throughgoing shear plane. The nucleation and propagation of the fault post-dates the formation of the shear plane and depends on the development of a zone of highly oriented layer silicates within the pyrophyllitic clay. The fault is confined to this zone. The lineations observed on the fault surfaces are produced while the material behaves plastically prior to the brittle faulting. With increasing shear strain the brittle—ductile transition is crossed and sliding continues under brittle conditions along the former shear zone.