• Economic and Environmental Geology
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• v.40 no.2 s.183
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• pp.191-207
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• 2007

A paleomagnetic investigation for the Cretaceous rocks in the Buyeo and Hampyeong Basins, located out of the Gyeongsang Basin, was carried out in order to elucidate the paleomagnetic directions in conjunction with the formation of the basins. Typical stepwise thermal demagnetization and measurement methods were used to determine the directions of characteristic remanent magnetizations (ChRMs). The mean direction of the sedimentary rocks from the Buyeo Basin after bedding correction $(D/I=356.5^{\circ}/61.5^{\circ},\;k=39.3\;\alpha_{95}=7.4^{\circ})$, is more dispersed than that before bedding correction $(D/I=356.5^{\circ}/61.5^{\circ},\;k=39.3\;\alpha_{95}=7.4^{\circ})$, which suggests that the rocks in the Buyeo Basin were remagnetized. However, the statistics and dispersion of the ChRM directions after bedding correction are still acceptable and the paleomagnetic pole position after tilt correction $(Lat./Long.=69.3^{\circ}N/186.7^{\circ}E,\;K=11.6\;A_{95}=14.0^{\circ})$ is closer to that of the Late Cretaceous pole of the Korean Peninsula. More detailed study is needed to confirm the nature of the remagnetization in the Buyeo Basin. On the other hand, the paleomagnetic pole before bedding correction $(Lat./Long.=81.6^{\circ}N/106.9^{\circ}E,\;K=25.1\;A_{95}=9.3^{\circ})$ is positioned near the paleogene pole of the Eurasian APWP. The mean ChRM direction of the sedimentary rocks from the Hampyeong Basin after bedding correction is $D/I=32.5^{\circ}/55.4^{\circ},\;(k=35.6,\;\alpha_{95}=8.7^{\circ})$. It is more clustered than that before bedding correction $D/I=18.3^{\circ}/62.5^{\circ},\;k=14.1,\;\alpha_{95}=14.2^{\circ})$, indicating that the ChRM was acquired before tilting of the strata. The paleomagnetic pole position of the Cretaceous sedimentary rocks in the Hampyeong Basin, averaged out of site pole positions calculated from the tilt-corrected ChRMs, is $Lat./Long.=63.9^{\circ}N/202.7^{\circ}E,\;(K=21.3,\;A_{95}=7.6^{\circ})$, similar to the Late Cretaceous paleomagnetic pole of the Korean Peninsula $(Lat./Long.=70.9^{\circ}N/215.4^{\circ}E,\;A_{95}=5.3^{\circ})$, suggesting that the Hampyeong Basin has been stable since the Late Cretaceous period. One normal and two reversed ChRM directions are revealed through the measurements of the volcanic rocks from the Hampyeong Basin. Although these normal and reversed directions are not exactly antipodal, it is interpreted that the normal direction is the representative primary direction of the volcanic rocks of the Hampyeong Basin and the mixed polarity is the records of geomagnetic field at the time of the formation of the volcanic rocks. Paleomagnetic poles are at $Lat./Long.=70.2^{\circ}N/199.5^{\circ}E,\;(K=18.1,\;A_{95}=9.6^{\circ})$ for the normal direction, and $Lat./Long.=65.5^{\circ}S/251.3^{\circ}E,\;(K=7.1,\;A_{95}=20.7^{\circ})$ for the reversed direction. Compared with the representative pole positions of the Cretaceous period of the Korean Peninsula, it is concluded that the age of the volcanic rocks in the Hampyeong Basin is of the Late Cretaceous.

• Economic and Environmental Geology
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• v.37 no.4
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• pp.413-424
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• 2004

A paleomagnetic investigation of the Mesozoic Daedong Supergroup and the Precambrian Seosan Group in the Kyeonggi massif is carried out to elucidate the tectonic evolution of Korea under the effect of the collision between Korea and the North/South China Blocks. For the Daedong Supergroup, the characteristic direction of D/I=74.5$^{\circ}$/36.7$^{\circ}$(k=60.7, $\alpha$=5.1$^{\circ}$) after tilt correction is better clustered than that before tilt correction (D/I=61.9$^{\circ}$/52.8$^{\circ}$, k=4.4,$$\alpha$_{95}$=21.5$^{\circ}$), indi-cating that it is a primary magnetization acquired during the formation of the rock. Paleomagnetic pole position of the formation locates at 208.0$^{\circ}$E, 24.5$^{\circ}$N (n=14, K=67.5, $A_{95}$=4.9$^{\circ}$), statistically similar to those of Middle Triassic period of the SCB, revealing that the two had occupied the same tectonic unit during this period. It is observed that only 6 out of 33 sites of the Seosan Group yield remagnetized paleomagnetic direction. The rest of the sampling sites reveals severe dispersion of magnetic directions presumably due to the consequences of the collision between Korea and the North/South China Blocks. The characteristic direction of the Seosan Group is D/I=45.7$^{\circ}$/60.1$^{\circ}$(k=41.2,$$\alpha$_{95}$=10.6$^{\circ}$) and the corresponding pole is at 195.0$^{\circ}$E, 51.6$^{\circ}$N (n=6, K=20.8, $A_{95}$=12.4$^{\circ}$). Although the pole position is close to those of Jurassic period of the Kyeonggi massif and Early Cretaceous of the Kyeongsang basin. it is interpreted that the Seosan Group was remagnetized by the influence of the emplacement of the Jurassic Daebo Granite after or at the closing stage of the orogenic episode rather than under the direct effect of deformation and/or metamorphism caused by the collision.