Patterns of Clay Mineral Transformations in Fault Gouges

Neoformed clay minerals in fault rocks in the brittle crust are increasingly recognized as being key to the sealing behaviour of faults . Academic literature has recognized the importance of neoformation of clay in fault gouge for a number of years, but the concept has not reached most industry seal analysis workflows. Clay-rich gouges that form as a consequence of new clay mineral growth are distinct from clay smears or cataclastic fault rocks that form as a result of mechanical incorporation of wall-rock phyllosilicates, in that they form by chemical and not physical processes. We report a comprehensive field study of clay mineralogy on fault rocks from sedimentary basins and low-angle normal faults in the American Cordillera. We then synthesize the field study with a literature survey to identify controlling conditions for neo-formed clay in fault gouge. Neoformed mineral in gouges are illite, illite/smectite, smectite, and chlorite/smectite phases. Chlorite and kaolinite do not form as neoformed clays in fault gouges. Controlling conditions are wallrock chemistry, temperatures of ~60-180 C and fluid availability.