Late Cenozoic Tectono-Sedimentary History in the Oki-Dogo Island, and its Implication for the Origin of Topographic and Structural Relief in the Southwestern Japan Sea
Sea of Japan
The Japan Sea is a marginal basin formed through backarc basin opening during the early to middle Miocene. It is composed of topographic depressions (basin) and highs (ridge, sea mount and rise) and shows obvious large topographic irregularities. The crust of marginal basin is understood to be a mosaic consisting of fragmental continental crust, rifted continental crust, extended continental crust, and newborn oceanic crust. The crust of Oki-Dogo Island, which in one of the topographic high in the southwestern Japan Sea, is considered to be a fragmental continental crust without marked disturbances during the opening process of the backarc basin.
In the Oki-Dogo Island, a thick series of Tertiary and Quaternary strata have developed covering unconformably the crystalline basements. They are classified into the following five groups: the upper Oligocene to lowermost Lower Miocene Tokibariyama Formation, the Lower to Middle Miocene Dogo Group, the uppermost Upper Miocene Oki Group, the Pliocene Omine Group, and the Pleistocene Saigo Group and Holocene Nijiyama Gravel Beds. They form structural association of dome and graben structures.
The Tokibariyama Formation, mostly composed of talc-alkaline volcanics, was deposited in relation to the formation of Tokibariyama cauldron located on the Oligocene volcanic front. The Dogo Group was deposited in and around the Dogo graben which is closely connected to the Japan Sea rifting. It is made up of alkali basalts (shoshonite) and acid pyroclastics in the lower part, and of the clastic and biosiliceous sediments deposited in the paralic and shallow and deep-marine environments, in the upper part. The Oki Group closely related to the formation of the Tsuzuraoyama dome structure is represented by thick piles of subaerial felsic alkaline volcanic rocks with small amount of shallow marine elastics in the lowermost part. The Omine Group forms monogenetic volcanoes which mostly consist of alkali basalts with mafic and ultramafic xenoliths erupted on the Fuse tilted block, tilting gently toward northwest in the northeastern part of the island. The Saigo Group is composed of alkali basalts and the Nijiyama Gravel Beds consist of terrace deposits. They are distributed in the limited area of the souththern part of the island.
According to the tectono-sedimentary history reconstructed by the author, the Oki-Dogo area has experienced the continental-arc type volacanism of the latest Oligocene to earliest Miocene age, and the rifting with bimodal volcanism and subsequent subsidence, down to middle bathyal zone of early Early to Middle Miocene age. These processes are characteristic of the rifted and the extended continental crusts in tha Japan Sea. Since the Late Miocene, the three-step successive tectono-magmatic uplifting processes have transformed the Oki-Dogo area into an island which projects above the sea surface.
In conclusion, the Oki-Dogo Island is not a simple piece of continental crust that fragmented through the opening process of the Japan Sea, but the results of a complex evolutionary process, which involves the earlier rifting and deep subsidence followed later on by tectono-magmatic uplifting. Such multi-phased and superimposed formative porcesses of the Oki-Dogo Island may imply that various structural highs in the Japan Sea also have their respective evolutional histories, and that tectono-sedimentary histories should be individually reconstructed to elucidate the origins of topographic and structural highs. Correlations between the variation in the character of volcanism and tectonic events can be due to changes in the tectonic status of the crust.