Patterns of Clay Mineral Transformations in Fault Gouges

Abstract

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.