MICROSTRUCTURAL ANALYSIS OF OGCHEON METASEDIMENT ZONE, CHUNGJU DAM, S. KOREA

Ogcheon metasediment zone (OM), S. Korea, of NE-trend preserves local NW-trend around Chungju dam site, northern tip of the Middle to Upper Paleozoic Ogcheon Group. The NW-trend had been interpreted to a part of being pushed northwestward as a result of a series of southeast-heading NE-duplex thrust. Some other microtectonic studies suggested NW-SE shortening shaped the current NE-trend cleavage and fold of the OM. In this study, at Chungju dam site (ca. 1 km²), a comparative analysis between outcrop structure and microscopic examination has performed to understand NW-trend deformation process of the OM such as chlorite schist, chlorite-sericite schist and phyllite. From obtained 30 outcrops were measured 17 bedding (S₀), 313 cleavages (S₁), and 198 crenulated cleavages (S₂) along apparent bedding orientation and fold attitude. One horizontal and vertically radial 7~8 planes from each sample of the collected nine ones, crossing the study area, were cut for microstructural analysis under thin-section. It revealed that S₂ trends NE-SW after S₁ of NW-SE and that S₁/S₂ relationship from outcrop was coincident to microscopic examination. S₁, showing diverse trends from NW-SE to NE-SW under thin section, preserved on former NW-fold and had been rotated due to NE-trend shear (S₂). S₁-trend, measured from micro-fold hinge of thin-section, is well matched to NW-direction rather than that of micro-fold limb, where S₁ mixed with NE-trending S₂. Measured S₂ under the thin section uniformly appears similar pole orientation to NE-trending S₂ obtained from outcrop over the whole dam site. Deformation intensity of S₂ under microscope shows a strong tendency along NE-trending fold limb such as sinistral refolded-sigmoid. Contrarily at hinge part remains weak dextral-rotation, which is pre-sinistral, but clearer S₁ along NW-trending micro-hinge than that of micro-limb. The later sinistral rotation, mixed with S₁ and S₂, had been severely undergone elongation and s-shape sigmoid rotation along micro-limb.

All the data collected from this study support that the OM had experienced early NE-SW shortening, provided NW-trending S₁, and that S₁ had progressively rotated to S₂ direction, which newly developed to NE-trend due to late NW-SE shortening. S₁ of NW-SE is clearly preserved its original orientation in place, Chungju region.