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

Hidaka山脉的隆升与沿Hidaka主逆冲作用有关，在该逆冲作用下，Hidaka变质带被逆冲到Poroshiri蛇绿岩带之上。为了阐明隆升过程，我们对Hidaka变质带南段的变质岩和花岗质岩石以及Hidaka山脉两侧中中新世至下上新世的花岗质砾石进行了黑云母K-Ar年龄测定。对变质带中部的锆石进行了裂变径迹年龄测定。变质带的K-Ar年龄在东侧30Ma左右，在西侧19-17Ma左右，被一条可能的断层明显地划分为东西两侧。这些早中新世年龄与样品所在地区的海拔高度呈负相关，即海拔越高，放射性年龄越年轻。这与喜马拉雅山和阿尔卑斯山的情况完全相反。此外，两侧花岗质砾石的K-Ar年龄也存在较大差异。东侧古近纪年龄为中始新世至早渐新世（45.9 ~ 33.1Ma），层位越高，古近纪年龄越老。另一方面，在西侧，年龄为早中新世（19-17Ma），无论地层层位如何，年龄几乎相同。Hidaka山中部锆石的FT年龄为16 ~ 12ma，且与样品所在地海拔高度呈负相关，尤其是西侧。辐射年龄揭示的特殊隆升过程被认为是由Hidaka主逆冲引起的旋转隆升引起的。

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.