Abstract

The sedimentation and evolutionary history of the Gyeongsang Basin, and their implications for plate tectonics are reviewed based mainly on sedimentological work on the Gyeongsang strata, and the regional geology of the Japanese Islands. During the Cretaceous, E Asia was characterized by an Andean-type continental margin formed by subduction of the Kula plate and Kula–Pacific ridge. The marginal zone was subjected to precursory extensional tectonism of the magmatic arc so that fault-bounded continental depressions developed, including the proto-Gyeongsang Basin.

The Gyeongsang strata (about 9,000 m thick) consist in ascending order of the Sindong, the Hayang with volcanic' stics, and volcanic Yuchon Groups. During the Sindong period extensional tectonism prevailed a the proto-Gyeongsang Basin was a graben occupied by alluvial fan, fluvial plain and lake environments from the margins to the centre of the basin. Streams flowed from the upthrown lip areas to the centre. Alluvial fans expanded periodically with rejuvenation of fault activity, while the fluvial plaid/lake boundary shifted in response to climatic changes.

During the Hayang period the faults were no longer active and consequently the basin extended beyond them (probably by back faulting). The water level of the lake(s) at the basin centre fluctuated with changes of the evaporation/precipitation ratio. During parts of the period, the marginal fluvial plain was drained by ephemeral streams which disappeared before reaching the lake(s), depositing fluvial sequences lacking channel sandstones. Localized volcanism formed volcanic terrains and supplied pyroclastics: this volcanism was climactic during the Yuchon period and terminated the sedimentation. Granitic plutons intruded all the Gyeongsang strata at about the end of the cretaceous.

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