日高山脈のテクトニクスと上昇過程ー熱放射年代学からの検討

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The Hidaka Collision Zone in the southern part of Central Hokkaido including the Hidaka Mountains has undergone two types of collision since the Early Tertiary. The Hidaka Mountains consist of the Hidaka Metamorphic Belt in the eastern main part and the Poroshiri Ophiolite Belt in the western marginal part. The former is composed of a steeply eastward-tilted crustal succession of the paleo-Hidaka magmatic arc, and is separated from the latter by the Hidaka Main Thrust. Recent vibroseismic reflection profiling across the collision zone imaged delamination-wedge tectonics of the lower crust beneath the Hidaka Mountains. We performed radiometric dating on biotite and hornblende using the K-Ar method, and on zircon using the Fission-Track method to clarify the process and the mechanism of uplift of the Hidaka Mountains. The minerals dated were obtained from metamorphic and tonalitic rocks of the southern Hidaka Metamorphic Belt, and also from metamorphic and tonalitic cobbles constituting the middle to late Miocene molasse occurring in the western foreland and eastern hinterland basins of the southern Hidaka Mountains. The cobbles in these formations were derived from the proto-Hidaka Mountains. The conclusions are as follows: 1. K-Ar ages of biotites from cobbles in the molasse, the sedimentary ages of which are 12-7 Ma, differ according to origin in the hinterland or foreland basins. The ages from the hinterland basin vary successively from lower (45.9 Ma) to upper (33.1 Ma) stratigraphic levels. The ages from the foreland basin, however, vary between 19.3 Ma and 16.4 Ma, regardless of their stratigraphic positions. These differences indicate that denudation (uplift) was slow and steady in the hinterland basin, but rapid and complex in the foreland basin during the middle to late Miocene. 2. Such a difference in the biotite K-Ar ages between the two sides is caused by the rotational uplift of the mountain due to thrusting along the Hidaka Main Thrust in the west and normal faulting in the east. The rotational uplift resulted in the rather fast denudation rates (0.9-1.4 mm/yr) in the foreland basin during the middle to late Miocene. 3. Denudation of the mountains estimated from K-Ar ages for biotites and hornblendes, and FT ages for zircons from the Hidaka granitic and metamorphic rocks remained fast (5-6 mm/yr) during the late Early Miocene, after which it slowed to 0.4-0.6 mm/yr on average. 4. The fast denudation (uplift) rates (up to 6 mm/yr) of the Hidaka Mountains during Early Miocene age was due to thrusting in the late stage of dextral transpression between Eurasia and Pacific plates. The rotational uplift of the mountains occurred in the Middle Miocene, and resulted in different styles of uplift between the sides of the mountains. From the late Miocene onward, the westward movement caused by the westward migration of the Kuril forearc shifted to the west, forming a foreland fold-and-thrust belt in the west.

日高山脈のテクトニクスと上昇過程ー熱放射年代学からの検討

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