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

日本北海道东达卡变质带(H.idaka Mountain)由东向陡峭的岛弧逆冲席组成。该带由变质级别向西增加至麻粒岩相，并沿东高主逆冲推覆至西侧变质蛇绿岩杂岩之上。对采自不同海拔地区的黑云母、片麻岩和片岩分别进行了16和2个K-Ar年龄测定，获得了5个锆石裂变径迹年龄。通过对日达卡变质带中段Kamuiekutikaushi(1979m)的K-Ar同位素年代学研究，得出以下结论：1.不同变质程度和高度的黑云母K-Ar年龄基本一致(17.9-16.3 Ma)。这意味着变质带现今的东倾构造在中中新世以前就已经形成了。因此，东高岛弧沿着东高主逆冲推覆在此之前就已经开始了。来自西部麻粒岩相岩的角闪岩和黑云母的K-Ar年龄显示，在18 Ma至17 Ma之间，变质带西段的抬升(剥蚀)速率高达6.7 mm/a。锆石的裂变径迹年龄范围为15.7 Ma~12.4 Ma，但10.9 Ma出现在逆冲带附近。这些年龄随着样地海拔的升高而变得年轻。这一趋势与一般观察到的相反，表明中中新世以来的山脉抬升不是简单的旋转式隆升。K-Ar和裂变径迹年龄显示，中新世中期的隆升速率为3.5-0.8 mm/a，中新世中期至今的隆升速率为0.5-0.3 mm/a。因此，自早中新世结束以来，日达卡山脉的抬升速率随着时间的推移而减小。

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.