造山帯の分類とその意義：古造構場復元の束縛条件

Abstract

The geotectonic division of the Japanese islands has been proposed by Isozaki and Maruyama (1991) such that the eastern extension of the Dabie-Sulu 240-220 Ma collisional UHP-HP belt (DSB) passes through the Korean peninsula to Japan. The corresponding belt in Japan is the Higo-Unazuki-Hitachi-Takanuki belt (HUHTB), because the protoliths are a unique A-type (platform sediments) and the metamorphic facies series belongs to the intermediate-pressure type, in addition to 240-220 Ma of metamorphic ages. Ishiwatari and Tsujimori (2012) claim that Maruyama et al. (2011) did not evaluate a new proposal by Ishiwatri and Tsujimori (2003), who proposed that the DSB extends not to the HUHTB but to the Sangun belt in a complex manner because of the promontory nature of the continental margin. This is a Q-A report requested by Ishiwatari and Tsujimori (2012). In this paper the author first introduces a classification of orogenic belts based on protoliths and its great significance for understanding the history of complex orogenic belts, following the original article by Maruyama et al. (1996), in addition to current topics on the role of tectonic erosion. Preceding the final formation of the collisional orogen, the Pacific-type orogen must have been present structurally above the collisional orogen against the hanging wall of the continent or arc. If not, it suggests the presence of tectonic erosion. Moreover, the size of any of regional metamorphic belt, arc, and TTG belt could be an excellent indicator of the scale of tectonic erosion. The Triassic DSB continues not to Sangun belt, but to the HUHTB, for four reasons: first, the protolith of those belts, second; the nature of regional metamorphism along intermediate-pressure type; third, structural units above and below the HUHTB; and finally, paleogeographic reconstruction of the Triassic North and South China cratons. The tectonic juxtapositions of the four so-called Sangun BS belts —450 Ma, 340 Ma, 250-210 Ma, and 170-180 Ma— against the HUHTB in a narrow zone as klippes can best be interpreted by extensive tectonic erosion.