Petrological and isotope geochronological study on the uplift of the Hidaka Mountains, Hokkaido, Japan

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The Hidaka metamorphic belt(H. idaka Mountains), Hokkaido, Japan consist of easists -steeply dipping thrust sheet of island arc. The belt increases in metamorphic grade westward up to the granulite facies and thrust over the metamorphosed ophiolite complex on the west along the Hidaka Main Thrust. Sixteen and two K-Ar dates on biotites and hornblendes, respectively and five fission-track dates on zircons have been obtained from tonalites and gneisses and schists of various grades collected from localities with different altitudes on a traverse section through Mt. Kamuiekuutikaushi(1979m)in central part of the Hidaka metamorphic belt in order to examine the uplift(erosion)process and rate of the Mountains.The isotope geochronological data lead to the following conclusions :1. All of K-Ar biotite ages almost agree(17.9-16.3Ma)regardless of their metamorphic grade and altitudes. The fact means that the Present eastward-steeply dipping structure of the metamorphic belt had been already made up before the middle Miocene. Therefore, thrusting of the Hidaka island arc along the Hidaka Main Thrust had begun before that time.2. K-Ar dates on hornblendes and biotites from western granulite-facies rocks reveal the uplift(erosion)rate of the western part of the metamorphic belt reaches up to 6.7mm/a between l8Ma and 17Ma.3. Fission-track ages on zircons range from 15.7Ma Lo 12.4Ma with an exception of 10.9Ma which occurs near a thrust. These ages become younger with increase in altitude of sample locality. The tendency, which is the reverse of what observed generally, suggests that the uplift of the Mountains since middle Miocene age is not simple btit of rotational type.4. K-Ar and fission-track ages show uplift rates of the Mountains to be 3.5-0.8mm/a during the middle Miocene and 0.5-0.3mm/a during interval from the middle Miocene to the present. Thus, the uplift rate of the Hidaka Mountains decreases with time since the end of the early Miocene.