Experiments on clay smear formation along faults

A ring shear apparatus was used to investigate the development of clay smea r along faults in sand-clay sequences. Experiments were performed, using si x different clay types, different stress conditions (sigma (n)=6-500 kPa) a nd different amounts of clay (4 and 12.5%). The development of clay smear s eems to depend on the competence contrast between the clay and the surround ing sand. Clay, when it is less competent than sand, behaves in a ductile m anner, resulting in the development of clay smears along the fault. Clay wh ich is more competent than sand behaves in a brittle manner, resulting in t he formation of angular fragments. Whether the clay is more or less compete nt than the sand is dependent on the stress conditions, the initial porosit y of the sand and the mechanical properties of the clay. The results sugges t that stress conditions allowing the sample to contract will result in the formation of fluid flow barriers, whereas dilation results in the formatio n of conduits.