熱年代学データに基づく日高山脈の上昇史(<特集>日高衝突帯研究の最近の進歩(1)-その深部過程と上昇過程)  [in Japanese] Uplift history of the Hidaka Mountains, Hokkaido, Japan : A thermochronologic view(<SPECIAL ISSUE>Recent Advances in Research of the Hidaka Collision Zone, Hokkaido, Northern Japan: Deep-Seated Processes and Tectonic Uplift (Part 1))  [in Japanese]

既存の年代資料と,新たに得た日高帯の花こう岩体およびそれに由来する砕屑物の熱年代値から,日高山脈の上昇-削剥過程を考察した.地殻浅部構成岩である日高帯の花こう岩と日高変成帯東縁の低度変成岩は始新世の冷却年代を示し,暁新世の変成作用ピーク後の広域的な温度降下を記録する.一方,高度変成岩が示す中新世の冷却年代は,衝突テクトニクスによるものである.前者に由来する花こう岩礫は中部中新統に含まれ,後者から供給されたトーナル岩や変成岩礫は上部中新統に出現する.このことは日高地殻の削剥が中新世を通じて深部へ進んだことを示す.一方,日高山脈南東域には前期漸新世頃の冷却年代が知られ,漸新世の地殻規模の水平すべり運動との関連を示唆する.一部の花こう岩礫が示す中新世のFT年代と堆積年代から,中期中新世初頭において最大100℃/Myrの冷却速度が概算される.これは日高変成岩類から見積もられる値20-30℃/Myrと比べかなり大きい.

Compilation of published and newly obtained thermochronologic data for the granitoid plutons in the Hidaka belt of Hokkaido and granitoid clasts in the Middle Miocene foreland basin-fill provides new insights on the uplift and exhumation processes of the Hidaka Mountains. The granitoid plutons in the Hidaka belt and low-grade metamorphic rocks in the Hidaka metamorphic belt yield biotite K-Ar ages of 40-46Ma. They constitute shallow crustal sequence and record the Middle Eocene regional cooling that postdates the Paleocene metamorphic peak of the Hidaka metamorphics. Meanwhile, many age determinations yielding Miocene cooling ages (〜20Ma) of the high-grade Hidaka metamorphic rocks indicate that the deep-seated rocks were exhumed during the Miocene. Simultaneous deposition of thick and coarse-grained foreland basin-fill suggests a vigorous erosional denudation. The clasts of monzogranite-granodiorite rocks yielding Eocene cooling ages are dominant in the Middle Miocene turbidites. In contrast, the tonalitic and metamorphic rock clasts yielding Miocene cooling ages occur within the Late Miocene fan delta deposits. The temporal change of the detritus show sequential unroofing of the ancestral Hidaka Mountains through Miocene time. Besides the above two clusters of the cooling ages, another cluster of Early Oligocene ages (29〜36Ma) is known for the mid-crustal rocks in the southeastern part of the Hidaka Mountains. The cluster indicates local exhumation of the mid-crustal rocks during the Oligocene most probably caused by sub-horizontal southward crustal stacking. Zircon and apatite fission-track ages of some granitoid clasts and their depositional ages suggest that cooling rates of initially eroded shallow crustal rocks are up to 100℃/Myr in the early Middle Miocene. The rates probably reflect thrusting under the condition of increasing thermal gradient due to synchronous magmatic activities. They are much greater than those of 20〜30℃/Myr estimated from thermochronology on the Hidaka metamorphics.