RETHINKING THE VOCABULARY FOR CARBONATE FAULT ROCKS

The broadly used terminology for fault rocks is incomplete, omitting fabrics which are important for the interpretation of fault rock history. Sibson’s 1977 paper on fault rocks provides the most popular naming scheme in use. To distinguish rock types, he relied on grain size, presence or absence of a bimodal distribution, and cohesion. Cohesion is subjective and changes with weathering or post-fault cementation so is not useful for interpreting fault rock origins, and grain size does not always evolve monotonically with deformation. Sibson’s classification grouped all foliated rocks as mylonites; Chester et al. (1985) added foliated cataclasites. Other proposed modifications have not gained sufficient traction amongst field geologists to gain widespread use (e.g. Wise; Jébrak; Killick; Woodcock and Mort). These schemes emphasize reproducible or quantifiable observations of fault rocks but omit features that are essential for interpreting the history of deformation and post-deformational modification of fault rocks, which is the ultimate goal of looking at them in the first place.

Carbonate rocks pose additional challenges to observation, classification and interpretation. Carbonates form different structures than silicates when exposed to the same deformational conditions so generic schemes may not be applicable to carbonates. Carbonate rocks can display a convergence of deformational fabrics with primary or early diagenetic features (for example, ultramylonitic marble can be strikingly similar in outcrop to micritic limestone). The relationships between conditions of deformation and microstructure are often complex and non-unique. In particular, dissolution and precipitation are more influential in carbonate rocks than in silicates. We are building a classification system and associated workflow for carbonate fault rocks based on a global gallery that is descriptive, and can be applied in the field, and extended for microscopy work. We emphasize those attributes which are important for the interpretation of the formation and evolution of fault rocks. We hope to make the new classification scheme accessible for general use by the many disciplines who map, collect, or study carbonate rocks. We are eager for community feedback and advice to test and broaden the utility of our preliminary scheme.