Influence of initial fabric on fractures developed in shear zones in clay

We examine the influence of initial fabric on the development of fractures in experiments in which clay cakes are sheared over two horizontal base plates that move past each other in wrench-fault motion. An anisotropic fabric in the kaolinite was produced during the preparation of the clay cakes for these experiments. To produce the fabric, the clay was either screeded perpendicular to the underlying fault, screeded parallel to the underlying fault, or molded to develop a cake-parallel fabric that is isotropic in the horizontal plane. The influence of the initial fabric is striking. With cake-parallel fabric, both Riedel (R) and conjugate Riedel (R′) shears develop, the former being much more pronounced than the latter. With a fault-parallel fabric, only R shears developed, while with a cake- and fault-perpendicular fabric, only closely spaced high-angle shears developed initially, with more widely spaced R shears forming later. There is a difference in the orientation of the R shears in the three types of fabric, with the angle being smallest for a fault-parallel fabric (mean of initial fractures of 15°) and largest for a fault-perpendicular fabric (mean of initial fractures of 25°). The high-angle shears have an initial average angle of 80° for cake-parallel fabric and 100° for fault-perpendicular fabric. Only in the case of the high-angle fractures for a fault-perpendicular fabric is there significant rotation after formation. It is likely that the difference in fracture orientation attributed to anisotropy of the initial fabric of the clay also has its counterpart in nature and is not confined to fractures formed in shear zones.