The Plate-Tectonics Interpretations of the Japanese Paired Metamorphic Belts-An Armchair Appraisal

Abstract

The Japanese Akiyoshi and Sakawa paired metamorphic belts recognized and documented by MIYASHIRO appear most likely apparent pairs consisting of high pressure belts survived from Palaeozoic origin and low pressure belts created due to Mesozoic convergence. In general, the age of regional metamorphism is younger than that of the original sedimentaries and igneous rocks. And in plate tectonics considerations, both members of the metamorphic pair are coeval but their age of formation can't be younger than that of their non-metamorphic equivalent sedimentaries which were formed as an arc-trench-gap or a convergence-relaxation/cessation deposit derived from the same continent. Among the six Japanese stratigraphic-structural zones (Table 1) originated from the Chichibu geosyncline, the non-metamorphic Yamaguchi in the Sangun-Hida pair and Chichibu in the Sambagawa-Ryoke combination are considered as such ;the Sangun and Sambagawa high pressure belts are therefore regarded as being formed in the trench-subduction zones not younger than Permo-Carboniferous. Such an original age of origin for the paired metamorphic belts is also supported from the stratigraphic evidence as well as from the radiometric dates. The apparent Mesozoic ages are believed to be resulted from overprinting by thermal events due to Akiyoshi-Sakawa plate convergence with concurrent uplifting and subsidence.