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Mica separates from 44 metamorphic rocks (mostly pelitic) from the southern area of Kamuikotan Gorge, central block of the Kamuikotan zone were dated using the K-Ar method. The ages of white micas, separated from pelitic schists, range from 51 to 74 Ma. The Palaeogene K-Ar ages are supported by 40Ar-39Ar dating. Judging from mineral paragenesis and X-ray diffraction studies (RM values) of white micas, most of the pelitic schists belong to the intermediate pressure type. In the Orowen River district, the northern part of the study area, biotite from metavolcanic rocks of a lower pressure assemblage yields the youngest K-Ar ages (45-50 Ma). Although the Kamuikotan zone was considered to have been a subduction-related metamorphic belt along a Mesozoic consuming margin, the time span of the metamorphism and the lowering of metamorphic pressure in younger rocks suggest that: (1) exhumation of the younger Kamuikotan rocks accompanied the erosion of forearc basin sediments (the Yezo Supergroup); (2) the increase in the thermal gradient was probably related to the approach of the Kula-Pacific ridge to the subduction zone; and (3) the main stage of exhumation of the Kamuikotan rocks ceased before westward movement of the Pacific plate, which induced the collision of the Hidaka-Nemuro belts (palaeo-Kurile arc-trench system) with the Kamuikotan zone.

Abstract:Rivers in northern Japan are subjected to ice formation each winter. They are typically very steep with rapid changes in channel slope. As a result, ice covers are usually discontinuous with open water sections. Winter discharges in Japanese rivers are usually very small. Water temperature and ice production in these streams are very sensitive to the change in air temperature. The open water sections enable the formation of frazil and anchor ice during the winter. Owing to the relatively stable winter weather and heavy snow cover, premature break-up and ice jams rarely occur, even though the channel geometry of these rivers is favourable for their occurrence. In this paper, hydrometeorological factors related to ice-cover formation, frazil and anchor-ice development, and ice-jam formation, as well as measurements of the undercover discharge in rivers in northern Japan are discussed.

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Frazil ice is an ice crystal formed in turbulent water.