Abstract
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 40-46 Ma. 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 (~20 Ma) 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 (29~36 Ma) 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 100ºC/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 20~30ºC/Myr estimated from thermochronology on the Hidaka metamorphics.
