Structural and provenance analyses of a newly defined major fault: The Homyeong Fault as a northern boundary fault of the Cretaceous Nagdong Trough, South Korea

摘要

The Homyeong Fault on the Yecheon Shear Zone is newly defined and described in the northwestern part of North Gyeongsang Province, South Korea. In the study area, the basement rocks consist, of the Precambrian rocks and Jurassic granitoid, and are dextrally deformed by the Jurassic Yecheon Shear Zone to the north of the Cretaceous Gyeongsang Supergroup in a ductile manner. These basement rocks are covered with unconformity by the sindong Group, the lowest part of the Gyeongsang Supergroup. The Homyeong Fault runs on the Yecheon Shear Zone, and is marked by cataclasite that contains mylonite fragments originating from the mylonitic rocks of the Yecheon Shear Zone and that shows sinistral sense of shear with normal-fault component dipping basinward, which indicates reactivation of the Yecheon Shear Zone. The conglomerates of the northern part of the Sindong Group usually contain the gravels of granite, quartzite, blackish quartzite, mylonite, ultramylonite, and mudstone. Petrological similarity between these gravels and the basement rocks suggests that most of the gravels were very likely derived from the Yecheon Shear Zone and its vicinity. Considering the Korean Cretaceous basins mostly formed by sinistral reactivation of the pre-existing shear zones, it can be judged that the reactivation of the Yecheon Shear Zone caused the basement rocks nearby the Homyeong Fault, or the Yecheon Shear Zone and its vicinity, to be deposited as the gravels to the Nagdong Trough where the Sindong Group was filled on the south of the fault. This gives new proposals that the Homyeong Fault had played a role as a northern boundary fault of the Nagdong Trough and also that the inversion tectonics from Jurassic dextral to Cretaceous sinistral motions, observed in the eastern margin of Asia, had an effect on the formation of the Nagdong Trough.