Reconstruction of chert-clastic sequence of the Ashio terrane in the Kuzu area, central Japan

Abstract

Sedimentary complex, widley distributed in the Kuzu area of the Ahio Mountains, central Japan, is characterized by tectonic repetition of coherent stratigraphic sequence of pelagic to terrigenous sedimentary rocks with huge thrust bodies of limestone and greenstone. This complex is subdivided into three tectonostratigraphic units. Units 1 and 3 are composed of the Triassic to early Late Jurassic chert-clastic sequence which pirmarily consists of siliceous claystone, bedded chert, siliceous shale, and coarse clastic rocks in ascending order. The primary succession of the chert-clastic sequence was reconstructed by interrelationship of each type of rock and by radiolarian dating. Unit 2 is made up of middle Permian greenstone and limestone which are attributed to part of seamounts capped by calcareous rocks. Reconstruction of the chert-clastic sequence contributed to recognition of imbricate structure which was formed by tectonic repetition of the sequences. Each slice does not include commonly all of the reconstructed seccession and a part of the succession is excluded. The basal part of the slices comonly consists of siliceous claystone. It is suggested that the siliceous claystone functioned as tectonic discontinuous zone in formative process of the imbricate structure.