Structure and Physical Properties of the Mid-west Crust of Korea (III): Paleomagnetism of the Cretaceous Volcanic Rocks in the Southern Choogaryong Rift Zone

The extension of the so-called Choogaryong Rift Zone to NNW direction from Seoul and the geotectonic characteristics of the Zone were investigated using paleomagnetic techniques. Eighty six paleomagnetic samples from 19 sites were collectd from the Cretaceous volcanic rocks distributed from Seoul to Daechon. Thermal or alternating field demgnetizations, aided by principal component analysis, yields 13 sites with stable both normal and reversed directions, and 6 sites indicating transitional fields. Combining this result with the previous paleomagnetic and K-Ar age data, it is inferred that the volcanic rocks of this study were extruded or intruded during normal-transitional-reversed fields which prevailed in and around 83 Ma. This inferrence leads to the conclusion that the Choogaryong Rift Zone could be extended to Daechon, Chungnam Province. Both normal and reversed directions in the Rift Zone show NNW and SSE declinations. Mean directions of the normal and inverted reversed sites has NW declination (D/I=327.7°/45.5°, =7.6°). Comparison with the expected direction in Korea indicates about 40° anticlockwise rotation of the Rift Zone, which was resulted from left-lateral fault movement since the formation of the Rift Zone.