Nature and evolution of the Cretaceous basins in the eastern margin of Eurasia: A case study of the Gyeongsang Basin, SE Korea

Abstract

The Gyeongsang Basin is the largest among the non-marine Cretaceous sedimentary basins in Korea and Japan, formed by sinistral strike-slip movement induced by the oblique subduction of the paleo-Pacific plate under the eastern Asian continental margin, and has an evolutionary history of about sixty million years. SHRIMP and LA-MC-ICPMS U-Pb analyses were performed on the detrital zircons separated from the constituent formations of the Gyeongsang Basin. We determined the maximum depositional ages of the four formations based on their ages of the youngest groups as follows; 118.0 ± 2.6 Ma (n = 4) for the Hasandong Formation, 108.7 ± 0.8 Ma (n = 11) for the Chilgok Formation, 106.0 ± 0.2 Ma (n = 63) for the Silla Conglomerate, 105.4 ± 0.4 Ma (n = 14) for the Haman Formation, and 99.9 ± 0.7 Ma (n = 5) for the Jindong Formation, belonging to the period from Aptian to Cenomanian. The pre-Cretaceous detrital zircons of the lower to middle Gyeongsang Supergroup indicate their derivation from the Yeongnam Massif and the Okcheon Metamorphic Belt surrounding the Gyeongsang Basin. At around 115 Ma, as reflected as enhanced proportion of such ages in the detrital zircons and occurrence of farther inland igneous activities, the tectonic environment of Gyeongsang Basin changed to intra-arc due to the more active reestablishment of subduction-related igneous activity. However, the upper Gyeongsang Supergroup has a new age component of the mid-Permian, along with changes in the direction of paleocurrent flowing from the northeast and east, indicating sediment supply from SW Japan with rocks of that age. The changes in the direction of the paleocurrent flow seem to indicate that the center of igneous activity has moved in the direction of the subduction zone to the east. As the intensity of igneous activity continued to increase, the Gyeongsang Basin was closed by the filling of the basin by volcanic activity. The Cretaceous basins of the Korean Peninsula and southwest Japan show similar evolutionary histories, suggesting a type model of the creation and the evolution of basin around the continental margin subduction zone; creation of sedimentary basin by pull-apart or rifting in the backarc region, evolution to intra-arc basin by the landward progress of subduction-related igneous activity, and finally the closure of the basin due to the advent of highly enhanced igneous activity.