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

Research Project

Project/Area Number	02640590
Research Category	Grant-in-Aid for General Scientific Research (C)
Allocation Type	Single-year Grants
Research Field	地質学一般
Research Institution	Hokkaido University
Principal Investigator	ARITA Kazunori Fac. Sci., Hokkaido Univ. Lecturer, 理学部, 講師 (30091408)
Co-Investigator(Kenkyū-buntansha)	GANZAWA Yoshihiro Hakodate College, Hokkaido Univ. of Education Associate Prof., 函館分校, 助教授 (40161400) MAEDA Jin'ichiro Fac. Sci., Hokkaido Univ. Research associate, 理学部, 助手 (50165643)
Project Period (FY)	1990 – 1991
Project Status	Completed (Fiscal Year 1991)
Budget Amount *help	¥1,900,000 (Direct Cost: ¥1,900,000) Fiscal Year 1991: ¥500,000 (Direct Cost: ¥500,000) Fiscal Year 1990: ¥1,400,000 (Direct Cost: ¥1,400,000)
Keywords	Hidaka metamorphic belt / mountain uplift / Fission track dating / K-Ar isotope dating / Thrust movement / 日高山脈 / フィッショントラック年代 / グラニュライト相岩石
Research Abstract	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.