Paleostress from healed microcracks and fluid inclusions in granites of the Ogcheon folded belt, South Korea

초록

In the northeastern and southwestern areas of the Ogcheon folded belt where Mesozoic granites are distributed, healed microcracks and fluid inclusions in the granite bodies were measured to elucidate the paleostress field that acted on these areas. The predominant orientations of healed microcracks in the northeastern and southwestern areas of the Ogcheon folded belt are the N50 degrees W for the Inje granite, N30 degrees W for the Jecheon granite, N55 degrees W for the Wolaksan granite, N5 degrees W for the Sokrisan granite, N30 degrees W for the Daebo granite, and N70 degrees W for the foliated granite. The homogenization and trapping temperatures of fluid inclusions in the Jecheon granite are 170-290 degrees C and 260-390 degrees C, respectively, and the formation period of healed microcracks is estimated to be 209-149 Ma. In the Inje granite, the homogenization and trapping temperatures of fluid inclusions are 165-290 degrees C and 250-400 degrees C, respectively, and the formation period of healed microcracks is estimated to be 176-160 Ma. In the Wolaksan granite, the homogenization and trapping temperatures of fluid inclusions are 165-375 degrees C and 230-570 degrees C, respectively, and the formation period of healed microcracks is estimated to be 116- 88 Ma. In the Sokrisan granite, the homogenization and trapping temperatures of fluid inclusions are 155-280 degrees C and 210-410 degrees C, respectively, and the formation period of healed microcracks is estimated to be 92-84 Ma. In the foliated granite, the homogenization and trapping temperatures of fluid inclusions are 150-310 degrees C and 380-550 degrees C, respectively, and the formation period of healed microcracks is estimated to be 120-166 Ma. In the Daebo granite, the homogenization and trapping temperatures of fluid inclusions are 150-320 degrees C and 380-440 degrees C, respectively, and the formation period of healed microcracks is estimated to be 183-166 Ma. Consequently, during the Mesozoic, the predominant maximum horizontal stress direction in the Korean Peninsula was NW-SE, with secondary influences from N-S and E-W directions. Moreover, the direction of these maximum horizontal stresses is thought to have rotated from NW-SE to N-S around the mid Cretaceous period. The directions and formation periods of healed microcracks in the study area coincide well with the Mesozoic orogenies of the Korean Peninsula. The Daebo orogeny of the Jurassic to early Cretaceous mainly influenced the Jecheon granite, Inje granite, Wolaksan granite, foliated granite, and Daebo granite in the NW-SE direction, while the Bulguksa orogeny of the mid to late Cretaceous mainly influenced the Sokrisan granite in the N-S direction.

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