T. Kanamatsu

Sedimentary petrology and paleotectonic analysis of the arc—arc junction: the Paleocene Nakanogawa Group in the Hidaka Belt, central Hokkaido, Japan

Abstract Two arc-trench systems have been recognized by using paleomagnetic data in the Hokkaido Central Belt, northeast Japan, during Late Cretaceous to Early Paleogene: the Paleo-Japan and the Paleo-Kuril arc-trench systems. The Hidaka Belt is composed mainly of Paleocene turbidite facies, with a small amount of hemipelagic sediment and melange facies. These sediments accumulated near the trench area, later composed accretionary bodies in the two arc-trench systems. The Nakanogawa Group is typically exposed on the southern side of the Hidaka Belt. This group is divided into three petroprovinces; zones I–III, from south to north on the basis of sedimentary, petrological and paleocurrent analyses. The modal component of sandstones from zone I indicate that they are characteristically rich in volcanic rock fragments (intermediate to basic composition), clinopyroxene, hornblende and poor in quartz. The characteristics of zone I are similar to those of forearc to slope basin sediments in the Paleo-Kuril arc-trench system. The modal component of sandstones from zone III consists predominantly of monocrystalline quartz, K-feldspar, acidic volcanic fragments, radiolarian chert and tectonite fragments, similar to those of forearc basin sediments of the Paleo-Japan arc-trench system. The modal component of rocks from zone II are regarded as being intermediate between zones I and III. However, clinopyroxene and chromian spinel chemical data suggest that the sandstones of zones I and II were derived from the Paleo-Kuril arc region. The sediments of the Hidaka Belt were thus derived from two different arc-trench systems and deposited in the arc—arc junction area during the Paleocene.