Paleomagnetic Study on the Tertiary Rocks in Pohang Area

포항일원에 분포하는 제3기 암류에 대한 고지자기 연구

Paleomagnetic study of Tertiary rocks in Pohang area has been carried out to determine the characteristic directon of natural remanent magnetization, the position of paleomagnetic pole, the stratigraphic correlation, and the tectonic movement. A total of 196 specimens was collected from 5 sites in the Pohang Basin, 19 sites in the Janggi Basin, and 10 sites in the Eoil Basin, respectively. The mean declination and inclination of 4 sites (3 sites in the Yonil Group and 1 site in the Yonil Basalt) are $-3.2^{\circ}$ and $54.3^{\circ}$, and yield the paleomagnetic pole position $86.9^{\circ}N$ and $7.7^{\circ}E$. These are the characteristic direction and pole position of Miocene Epoch by comparison with contemporary Eurasian and Chinese data. The characteristic direction and pole position of remaining 30 sites are $47.6^{\circ}$ and $57.5^{\circ}$, and $52.3^{\circ}N$ and $201.5^{\circ}E$, respectively. These show clockwise rotation of $50.8^{\circ}$ with respect to the Miocene ones resulted by a tectonic movement before the deposition of the Hakjeon Formation of the Yonil Group about 15~16 Ma in the study area. The mechanism of the clockwise rotation is considered to be the dextral movement of the Yangsan Fault presumably caused by the opening of the East Sea. The Yonil Basalt is reclassified into pre- and post-deposition of the Yonil Group, i.e. the former is early Miocene and the latter late Miocene.