北海道中軸部,日高帯・中の川層群において認められる3帯のpetroprovinceとその意義  [in Japanese] Three petroprovinces identified in the Nakanogawa Group, Hidaka Belt, central Hokkaido, Japan, and their geotectonic significance  [in Japanese]

北海道中軸部に広域に分く分布する最末期白亜紀〜古第三紀前期の日高累層群は, 2つの島弧海溝系, すなわちユーラシア東縁を構成していた古東北日本弧と現在の千島弧のプロトタイプである古干島弧に関連して形成された地域であるとされている。筆者は日高帯の中の川層群において砂岩のモード組成の検討を行い, 以下の点を明らかにした。(1)中の川層群・I帯の砂岩組成は同時代の古千島弧の前弧海盆地域の組成(根室層群・佐呂間層群)に類似する。(2)中の川層群・III帯の砂岩組成は, 同時代の北部地域の日高累層群(たとえば, 二ペンツ層および上興部層)および古東北日本弧の前弧海盆地域の組成(函淵層群)と類似する。(3)I帯とIII帯の間には中間要素を持つII帯が存在する。

The existence of two arc-trench systems, one resulting from westward subduction beneath the eastern side of Eurasia (Paleo-Japan arc-trench system), and the other from northward subduction beneath the southern side of the Okhotsk terrain (Paleo-Kuril arc-trench system), are recognized in central Hokkaido during the late Cretaceous to early Paleogne. The Hidaka Belt, related to the above mentioned two systems, is mainly composed of late Cretaceous to Paleocene turbidite facies. Such facies must have accumulated near the trench area and later contributed to the respective accretionary bodies. However, previous to this study the provenance of the terr igenous material and the sedimentary framework have not been made clear. The Nakanogawa Group in the Hidaka Belt is now divided into three petroprovinces: zone I, zone II and zone III (south to north) according to sandstone petrology and paleocurrent directions. The modal component of rocks from zone I indicates they are characteristically rich in volcanic rock fragments of intermediate to basic composition, clinopyroxene and hornblende, and poor in quartz, sedimentary and plutonic rock fragments. The characteristics of zone I indicate that turbidites derived from an undissected and primitive arc region (Paleo-Kurilarc region) closely correlate to those of the Nemuro and Saroma Groups which accumulated near the Paleo-Kuril arc region. The modal component of rocks from zone III consists predominantly of monocrystalline quartz, K-feldspar and is characteristically rich in acidic volcanic and tectonite fragments. The characteristics of zone III indicate that turbidites are derived from a dissected mature arc region or active continental margin (Paleo-Japan arc region). The characteristics of rocks from zone II suggest they are intermediate between zone I and zone III.