Late Cretaceous-Tertiary Petrotectonic Evolution of the Gyeongsang Basin, SE Korea

Petrogenesis of Late Cretaceous-early Tertiary granitic rocks in the Gyeongsang Basin indicates a change in tectonic environment from compression to tension based on the geochemical variation from I-type to A-type quartzofeldsparthic plutons. New SHRIMP-RG zircon dating documents two age groups; 75-65 Ma for I-type magma mixing, and 56-53 Ma closely related to A-type magmatism. This magmatic pause at 65-56 Ma may reflect a tectonic transition in the environment of SE Korea. The granites were cut by two distinctly different strike-slip fault systems: the WNW-trending sinistral Gaum fault system in northwestern part of the basin and the NNE-trending dextral Yangsan fault system in the southeastern part of the basin. Based on a new geotectonic model involving wrench tectonics and oroclinal bending of the basin after granite emplacement, these fault sets could have been generated as conjugate faults accompanying block rotation by NE-SW shortening derived from NNE-trending right-lateral shear (wrench force). Secondary wrench movement of the Yangsan fault may have produced the oroclinal bending in the eastern part of the fault. This oroclinal deformation may not only be related to beginning of Tertiary basins in SE Korea, but also to the East Sea opening, especially the clockwise rotation of SW Japan.