The Eurasian continent consists of accreted microplates. Major amalgamation of the continental blocks in the East Asia occurred during the collision of the South China and the North China Blocks. The Korean peninsula is located in the eastern margin of the East Asia. Okchon belt in the central part ...
The Eurasian continent consists of accreted microplates. Major amalgamation of the continental blocks in the East Asia occurred during the collision of the South China and the North China Blocks. The Korean peninsula is located in the eastern margin of the East Asia. Okchon belt in the central part of southern Korea is a boundary between the Archaean to middle Proterozoic Kyonggi and the Ryongnam Blocks. Some reseachers postulated that the Kyonggi Block and the early Paleozoic Okchon sequence west of the Honam Shear Zone belong to the South China Block, whereas the Ryongnam Block to the North China Block, so that the NE-trending right-lateral strike-slip fault of the Homan shear zone may be the a boundary between the South China Block and the North China Block. Palaeomagnetic studies have been conducted for the Paleozoic to Early Mesozoic sedimentary rocks in the Okchon belt. A total of 2390 indenpendently oriented samples were drilled from 230 sites of the 10 areas, and their remanence components were analyzed. The most effective way to isolate constituent remanence components was thermal demagnetization. The ChRM of Carboniferous to Early Mesozoic rocks are considered to be the pre-folding magnetization, as they passed the fold test and/or reversals test. As the formation-mean directions become more dispersed after bedding-tilt correction of all the formations of Ordovician limestone, it is concluded that the ChRM was acquired after the tilting of the strata. The declination of remanent magnetization of Carboniferous rocks changes from WNW through NNW to NW toward the upper sequences. Apparently scattered declination directions become more clustered if they are grouped seperately into the upper and lower beds. The scattering of declination in a single outcrop without know uncomformities or deformation structures cannot be explained simply by vertical-axis block-rotation, but might be explained by the flollowing scenario. That is, the Carboniferous beds continued to deposit in a continent containing the Korean peninsula while it wandered. Thus, the Carboniferous beds had come to be deposited in the changing longitudes and latitudes with time as the continent drifted. Based on the magnetic polarities, a more precise age of the so-called Carboniferous (Hongjom Formation, Manhang Formation), early Permian and late Permian rocks has been assigned to be early or early middle Carboniferous (Serpukhovian or Bashikirin), early Permian, late Pemian (Tatarian), respectively. The stable remanent directions of the Triassic rocks from the Chungnam area clustered with northeasterly declination and shallow inclination. In contrast, those of the Carboniferous and Triassic rocks of the Okchon belt (Yongwol, Yemi, Taebaek, and Hwasun area) clustered with northwesterly declination and shallow inclination. Thus, palaeomagnetic directions of the above two groups are completely different. The palaeomagnetic poles of the northeasterly ChRMs are correlated with the contemporary South China poles, while those of the northwesterly ChRMs are correlated with the contemporary North China poles. These results support the indentation (Yin and Nie, 1993) or crustal-detachment (mega-nappe) model (Li, 1994) of the South and North China collision. However, it must be noticed that the palaeomagnetic data from Hwasun and Yongwol area suggest that the boundary between the South and North China Blocks in Korea lies somewhat more westerly than the previously proposed. Inclination value of the formation-mean primary magnetizations indicates that the study area was located in the equatorial latitudes during the Carboniferous and then drifted northward to the latitude of about 15˚N by the Triassic. This conclusion is compatible with tropical to subtropical palaeoclimates suggested by the paleoflora of the study area.
The Eurasian continent consists of accreted microplates. Major amalgamation of the continental blocks in the East Asia occurred during the collision of the South China and the North China Blocks. The Korean peninsula is located in the eastern margin of the East Asia. Okchon belt in the central part of southern Korea is a boundary between the Archaean to middle Proterozoic Kyonggi and the Ryongnam Blocks. Some reseachers postulated that the Kyonggi Block and the early Paleozoic Okchon sequence west of the Honam Shear Zone belong to the South China Block, whereas the Ryongnam Block to the North China Block, so that the NE-trending right-lateral strike-slip fault of the Homan shear zone may be the a boundary between the South China Block and the North China Block. Palaeomagnetic studies have been conducted for the Paleozoic to Early Mesozoic sedimentary rocks in the Okchon belt. A total of 2390 indenpendently oriented samples were drilled from 230 sites of the 10 areas, and their remanence components were analyzed. The most effective way to isolate constituent remanence components was thermal demagnetization. The ChRM of Carboniferous to Early Mesozoic rocks are considered to be the pre-folding magnetization, as they passed the fold test and/or reversals test. As the formation-mean directions become more dispersed after bedding-tilt correction of all the formations of Ordovician limestone, it is concluded that the ChRM was acquired after the tilting of the strata. The declination of remanent magnetization of Carboniferous rocks changes from WNW through NNW to NW toward the upper sequences. Apparently scattered declination directions become more clustered if they are grouped seperately into the upper and lower beds. The scattering of declination in a single outcrop without know uncomformities or deformation structures cannot be explained simply by vertical-axis block-rotation, but might be explained by the flollowing scenario. That is, the Carboniferous beds continued to deposit in a continent containing the Korean peninsula while it wandered. Thus, the Carboniferous beds had come to be deposited in the changing longitudes and latitudes with time as the continent drifted. Based on the magnetic polarities, a more precise age of the so-called Carboniferous (Hongjom Formation, Manhang Formation), early Permian and late Permian rocks has been assigned to be early or early middle Carboniferous (Serpukhovian or Bashikirin), early Permian, late Pemian (Tatarian), respectively. The stable remanent directions of the Triassic rocks from the Chungnam area clustered with northeasterly declination and shallow inclination. In contrast, those of the Carboniferous and Triassic rocks of the Okchon belt (Yongwol, Yemi, Taebaek, and Hwasun area) clustered with northwesterly declination and shallow inclination. Thus, palaeomagnetic directions of the above two groups are completely different. The palaeomagnetic poles of the northeasterly ChRMs are correlated with the contemporary South China poles, while those of the northwesterly ChRMs are correlated with the contemporary North China poles. These results support the indentation (Yin and Nie, 1993) or crustal-detachment (mega-nappe) model (Li, 1994) of the South and North China collision. However, it must be noticed that the palaeomagnetic data from Hwasun and Yongwol area suggest that the boundary between the South and North China Blocks in Korea lies somewhat more westerly than the previously proposed. Inclination value of the formation-mean primary magnetizations indicates that the study area was located in the equatorial latitudes during the Carboniferous and then drifted northward to the latitude of about 15˚N by the Triassic. This conclusion is compatible with tropical to subtropical palaeoclimates suggested by the paleoflora of the study area.
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