Formation mechanism of listric normal faults and calcite veins within the shale-dominant strata of the upper Jinju formation in the cretaceous Gyeongsang Basin, Korea

This paper focuses on the occurrence and formation mechanism of normal faults and calcite veins within the shale-dominant strata of the upper Jinju Formation, Cretaceous Gyeongsang Basin. Normal faults observed in two cut slopes (Jinju Service Station and Chukdong-myeon sites) offset shale strata, but do not cut the upper and lower sandstones. They are characterized by various strikes, normal and transtensional slip senses, vertical offsets less than 2 m, as well as radial tensional paleo-stress field calculated from slip data of faults and shear fractures. Fibrous calcite veins observed mostly in the shale strata are grouped into bedding-parallel and fault-parallel (or fault-filling) veins. Bedding-parallel veins (beef), which are less than 3 cm in thickness and a few to tens of meters in length, often show black median lines including host rock particles. The orientations of calcite fibers are perpendicular or oblique to the bedding. Fault-parallel and -filling veins less than 1 cm in thickness and of a few meter in length are thinner and shorter than the beef veins, and often do not show median line. Relative chronologies of the faults, shear fractures, and veins indicate that the faulting and the injection of calcite veins were repeatedly occurred during the geologically same time period. In addition, both the fault-parallel and beef veins are concentratedly observed in more deformed hanging-wall adjacent to the fault surfaces, indicating that the normal faulting and associated structures played an important role in the migration of carbonate-rich fluids. These results also imply that the normal faults and calcite veins were formed as a result of fluid overpressure caused by differential compaction of incompetent shale strata in the deep part of basin, rather than regional tectonic stress.