Stratigraphy and geologic structure of the Sambagawa metamorphic belt in the Oboke area, central Shikoku, Japan

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  • 四国中央部大歩危地域の三波川帯の層序と地質構造
  • シコク チュウオウブ オオボケ チイキ ノ サンバガワタイ ノ ソウジョ ト

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Abstract

The Oboke area has been the type locality of the Sambagawa metamorphic rocks in Shikoku. We reexamined the stratigraphy and the geologic structure in this area, and the previous stratigraphy had been considerably revised as follows. The Sambagawa schists in this area are divided lithologically into three formations, i. e., the Kawaguchi, Koboke, and Minawa Formations. The Kawaguchi Formation is composed mainly of pelitic schist with subordinate amounts of basic, siliceous, calcareous and psammitic schists, kieslager, and metagabbros. The Koboke Formation is composed mainly of psammitic schist with small amounts of pelitic, conglomeratic and siliceous schists. The Minawa Formation consists mostly of pelitic schist with subordinate amounts of basic, siliceous and psammitic schists, and kieslager. The Kawaguchi Formation is interfingering with the lower part of the Koboke Formation. The Minawa Formation overlies the Koboke Formation. By the previous workers (KOJIMA, 1951; KOJIMA and MITSUNO, 1966), the schists in this area were divided into the "Oboke," "Kawaguchi," "Koboke" and "Minawa" Formations in ascending order. However, detailed field survey has revealed that the "Kawaguchi" Formation is interfingering with the "Koboke" Formation in the northern part of this area, and the "Koboke" Formation is traceable to the "Oboke" Formation. Therefore, the "Koboke" Formation is almost equivalent with the "Oboke" Formation, and they are rear-ranged into one formation, that is, the Koboke Formation. The schists in the southern part of this area form an E-W-trending anticline plunging eastward at low angle, which is the Oboke anticline (Ogawa, 1902). The axis of the Oboke anticline is cut by an ENE-WSW normal fault named the Oboke Fault near Hibihara, eastern part of this area.

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