Abstract--The Dongrae fault within the Yangsan fault system is considered one of the major faults in the southeastern part of Korea, extending over 150 km. The results of K-Ar radiogenic dating of fault gouges collected from six localities show a relatively wide range in age from 57.5 million years ago (Ma) to 40.3 Ma. Fault gouges are composed of newly formed minerals, including smectite, illite, zeolite, kaolinite, K-rich feldspar, apatite, and pyrite. The occurrence of abundant smectite and illite-lM d with lesser quantifies of zeolite suggests that the fault gouges experienced hydrothermal alteration at low temperatures. Smectite is probably unstable relative to other clay minerals, such as illite and zeolite. Considering that filiform mordenite is replacing the smectite, we suggest that mordenite formed by recrystallization involving a solid-state transformation. Under high fluid/rock ratios, smectite seems to have formed in the early stage of alteration. In contrast, zeolite minerals and authigenic K-rich feldspar progressively appeared with time as the fluid/rock ratio decreased with the changing chemistry of the hydrothermal fluids. The composition of clay minerals in the gouge materials probably was controlled by the chemistry and the amount of circulating fluids derived from adjacent granitic rocks.
SUMMARY
Early Jurassic sandstones and shales were collected at 14 sites from the Kimpo area in the western part of the Korean Peninsula for a palaeomagnetic study. A characteristic remanent magnetization component, which passes the fold test, was isolated from seven sites. This characteristic component is interpreted to be of primary Early Jurassic origin because the folding age is Middle–Late Jurassic. The primary directions, together with previously reported ones, provide a mean direction (D= 38.1°, I= 39.7°, α95= 3.8°, 11 sites) and a palaeomagnetic pole (54.1°N, 230.2°E, A95= 4.1°) for the Kimpo area. This palaeomagnetic pole shows good agreement with the coeval poles for the South China Block and is significantly removed from that for the North China Block, suggesting that the Kimpo area has been part of the South China Block since at least Early Jurassic times. The Kimpo area constitutes a part of the Imjingang Belt which is palaeontologically allocated to the South China Block in its origin, while all Precambrian tectonic blocks that compose the basement of the Korean Peninsula are interpreted to be of North China origin. We conclude that the Imjingang Belt, including the Kimpo area, is a tectonic extension of the South China Block on the Korean Peninsula. The South China elements were obducted onto the North China Block at the site of the present Korean Peninsula during the collision event of the North and South China Blocks, and hence the Korean Peninsula is composed of areas with both North and South China affinities.
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