MIOCENE PALEOMAGNETIC RESULTS FROM SOUTHEASTERN KOREA

Paleomagnetic samples were collected from 25 sites of Late Oligocene t o Middle Miocene age from southeastern South Korea. The mean direction obtained is D = 39.4-degrees, I = 46.2-degrees with alpha95 = 9.5-deg rees, which is statistically distinguishable from the present axial ge ocentric dipole field direction. When combined with the previous repor ted results of eight sites from this region, we get a mean direction ( D = 40.1-degrees, I = 47.4-degrees, alpha95 = 7.9) for 33 sites of Lat e Oligocene to Middle Miocene age. This direction yields an early Neog ene virtual geomagnetic pole of 56.5-degrees-N, 215.2-degrees-E with a n alpha95 of 7.9-degrees, discordant from the Cretaceous reference pol e for Korea. The paleomagnetic data suggest that portions of southeast ern Korea underwent a large clockwise rotation of about 40-degrees dur ing the Neogene. Two large NE-trending right-lateral faults (the Yangs an Fault and the Dongnae Fault, Fig. 1) cut the study areas. Both thes e faults show evidence for recent activity. In addition, minor faults with similar trends to these major faults occur. The observed deflecti ons in paleomagnetic declinations were probably caused by rigid block rotations of crustal blocks in a right lateral shear system during the Neogene. The paleomagnetic directions for Miocene aged rocks from sou theastern Korea and southwestern Japan are similar in direction and ma gnitude. This suggests that the cause of the rotations in both areas m ay be related to a similar tectonic event in this area during the Neog ene.