Thrust tectonics of Hidaka Collion Zone and formation of Hidaka Mountains, Hokkaido, Japan

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Uplift of the Hidaka Mountains has been related to thrusting along the Hidaka Main Thrust by which the Hidaka Metamorphic Belt is thrust over the Poroshiri Ophiolite Belt. In order to educidate the uplift process we decided biotite K-Ar ages on the metamorphic and granitic rocks in the souther Hidaka Metamorphic Belt and on granitic gravels in the Middle Miocene to Lower Pliocene molassses distributed in both sides of the Hidaka Mountains. Fission-track ages also were measured on zircons from central part of the metamorphic belt.K-Ar ages of the metamorphic belt are clearly separeted into eastern and western sides by a possible fault which is situated arond the crest line, that is, around 30Ma in the east and 19-17Ma in the west. These Early Miocne ages have an inverse correlation with the altitude of the sample localities, that is, the higher the localities are, the younger the radiometric ages are. This is completely contrary to the cases in the Himalaya and Alps. In addition K-Ar ages of granitic gravels from the mollases also present quite differnt ages in both sides. In eastern side the ages show the Middle Eocene to Early Oligocene (45.9 to 33.1Ma) , and the higher the stratigraphic horizon of the mollase samples are, the older these Paleogene ages are. On the other hand in the western side the ages are Early Miocene (19-17Ma) , and almost same regardless of the stratigraphic horizon. Zircons from central Hidaka Mountains yield FT ages of 16-12Ma, and also show the inverse correlation with the altitudes of sample localities especially in western side. The peculiar uplift process revealed by the radiometric ages is considered to be caused by rotational uplift due to thrusting of the Hidaka Main Thrust.