西中国地域における三郡期・領家期の重複変形  [in Japanese] Foldings in the ages of the Sangun and Ryoke metamorphisms and their superposition in West Chugoku, Japan  [in Japanese]

In the Sangun-Yamaguchi zone in the eastern Yamaguchi Prefecture, folds in the microscopic scale ( microfolds), which are considered to have been formed at the time of the Sangun metamorphism, are flow fold, fold with crenulation cleavages, or fold with slaty cleavages in folding style. The axes of the microfolds have not a uniform trend. Folds in the mappable scale, which have E-W trendig axes, are flexure-slip ones in style. In the Ryoke zone, microfolds, the axes of which trend in a E-W direction, change their style from south to north, from the style of flow fold to that of fold with slaty cleavages through that of fold with crenulation cleavages. This change of folding style is considered to be due to a northward temperature decrease at the stage of the Ryoke metamorphism. Microfolds are presumed to change into flexure-slip ones in style further northward. Folds in the mappable scale have the axes running from east to west, which agree in the direction with those of the macroscopic folds in the Sangun-Yamaguchi zone as well as those of the microfolds in the Ryoke. Folds in the mappable scale both in the Ryoke and in the Sangun-Yamaguchi zone appear to be harmonious with each othering cologic profiles and their axial trend. The folded strata in the zones include Upper or Middle Jurassic ones and are covered unconformably by the Upper Cretaceous sedimentary or volcanic rocks or intruded by dykes of the same age. On the basis of these data, it is concluded that the folding in both zones were of the same age. It took place in the time ranging from the latest Jurassic to the Early Cretaceous, which is in agreement with the age of the Ryoke metamorphism, when the microfolds in the Ryoke zone were also formed. The macroscopic folds with the style of flexure-slipones in the Sangun-Yamaguchi zone are considered to have been formed in this age under the lower temperature condition than in the Ryoke metamorphic rocks proper, superposing upon the microfolds which had been formed in the age of the Sangun metamorphism.