Mesozoic paleostress from healed microcracks and fluid inclusions in some granites of South Korea

초록

Mesozoic maximum horizontal stress (set/max) directions are determined from healed microcracks and the secondary fluid inclusions within healed microcracks in granites from the Inje, Jecheon, Wolaksan and Sokrisan areas. The strike of healed microcracks in granite from the Inje area shows a strong peak at N50 degrees W, while N10 degrees W and N80 degrees E are well developed as secondary and the tertiary peaks, respectively. N30 degrees W is the most dominant strike direction of healed microcracks in granites from the Jecheon area, while E-W is the second most dominant strike direction. Orientations of healed microcracks in granite from the Wolaksan area show a strong peak trend of N55 degrees W, while the secondary peak is at N5E. The most dominant strike direction of healed microcracks in granites from the Sokrisan area is N-S. The age for the formation of healed microcracks is determined from homogenization temperatures of fluid inclusion and the cooling history of granites. The age of healed microcrack formations in the Jecheon granite might be 207 similar to 149 Ma, 176 similar to 160 Ma in the Inje granite, 116 similar to 88 Ma in the Wolaksan granite and 92 similar to 84 Ma in the Sokrisan granite. In addition, the age of the NW-SE healed microcrack in the Wolaksan granite is 95 Ma, and that of the N-S healed microcracks in the Wolaksan and Sokrisan granites is 100 similar to 84 Ma and 92 similar to 84 Ma, respectively. This means that the rotation of principal stress axes occurred at 100 similar to 95 Ma. The sigma(H max) direction in the Korean peninsula was NW-SE in the Late Triassic to Early Cretaceous times, but changed to N-S in the Late Cretaceous. The directions and ages for the formation of healed microcracks in the study area are well correlated with Mesozoic tectonism in South Korea, that is, the Daebo orogeny in the Jurassic similar to Early Cretaceous with the sigma(H max) direction of NW-SE mainly having an effect on the Jecheon, the lnje and the Wolaksan granite, and the Middle similar to Late Cretaceous Bulguksa tectonic event with the sigma(H max) direction of N-S mainly having an effect on the Sokrisan granite.

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