Paleostress fields of the pyeongan supergroup distributed in south Korea

Paleostress fileds of the Pyeongan Supergroup which is distributed along the southeastern part of the Ogcheon belt in South Korea were clarified using the CSG (Calcite Strain Gauge) technique. Twin strain, average twin width, and average twin intensity range between 0.89-9.36%, 0.37-1.47 mum, and 32.30-64.40 twin/mm, respectively. Relationships between twin strain versus twin widths and twin strain versus twin intensity show that twin intensity and twin width generally increase with twin strain. The stress regimes are identical with strike-slip faulting in case of samples PA-3 (LDR, PEV), PA-5 (LDR, PEV, NEV) and PA-11 (LDR), and normal faulting in case of samples PA-3 (NEV) and PA-12 (LDR). This fact suggests that a change in stress regime of at least two times had been existed in the study area. The most dominant directions of maximum compressive principal stress are oriented to NEE-SWW and NNW-SSE.