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

Abstract

Mesozoic maximum horizontal stress (σH 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°W, while N10°W and N80°E are well developed as secondary and the tertiary peaks, respectively. N30°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°W, while the secondary peak is at N5°E. 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∼149 Ma, 176∼160 Ma in the Inje granite, 116∼88 Ma in the Wolaksan granite and 92∼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∼84 Ma and 92∼84 Ma, respectively. This means that the rotation of principal stress axes occurred at 100∼95 Ma. The σHmax 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∼Early Cretaceous with the σH max direction of NW-SE mainly having an effect on the Jecheon, the Inje and the Wolaksan granite, and the Middle∼Late Cretaceous Bulguksa tectonic event with the σHmax direction of N-S mainly having an effect on the Sokrisan granite.