Paleostress of the Joseon and Pyeongan Supergroups in South Korea using the New Calcite Strain Gauge (NCSG)

초록

Limestone bodies under the tectonic environment have experienced various tectonic processes, and also changedthe stress state. In this study, calcite twins found in limestones of the Joseon Supergroup and Pyeongan Supergroupin the northeastern part of the Ogcheon Belt, South Korea were measured, then the paleostress (i.e., the maximumshortening axis) was reconstructed using the new calcite strain gauge (NCSG) technique. The average twin thicknessand average twin intensity increase as the total twin strain increases. We utilize the appearance of twins, theaverage twin thickness and average twin intensity, and the total twin strain to estimate that the observed calcitetwins were produced at temperatures of < 200oC in the Joseon Supergroup and 170oC in the Pyeongan Supergroup. In the Joseon Supergroup, the dominant direction of the maximum shortening axis WNW-ESE to NW-SE; NE-SWshortening is also observed. The maximum shortening axes in the Pyeongan Supergroup are oriented NW-SE andNE-SW. The NE-SW direction of maximum shortening is associated with the occurrence of the Songrim orogenyof the Paleozoic to Early Jurassic, and the NW-SE direction of maximum shortening correlates to the Daebo orogenyof the Early Jurassic to Late Jurassic. It is thus concluded that the paleostress across the study area changedfrom NE-SW to NW-SE during the Mesozoic.

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