Earth Planets Space, Vol. 59 (No. 7), pp. 887-895, 2007
Hiroyuki Kato1, Nobuhiro Isezaki2, Chan Hong Park3, Chang Hwan Kim3, and Masao Nakanishi1
1Graduate School of Science and Technology, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
2Faculty of Science, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
3Marine Environment Research Department, Korea Ocean Research and Development Institute, Ansan, 425-600, Korea
(Received November 1, 2006; Revised May 28, 2007; Accepted May 30, 2007; Online published July 20, 2007)
Magnetic surveys by a Deep-tow Three-Component Magnetometer (DTCM) were conducted in the northeastern part of the Japan Basin and the central part of the Tsushima (Ulleung) Basin. Magnetic lineations are recognized clearly in the former area, whereas they were not recognized by previous studies in the latter area. The high-quality vector magnetic anomaly data obtained by DTCM enables the precise determination of the strikes of magnetic lineations and the positions of magnetic boundaries. Magnetic anomalies measured by DTCM show the characteristics of linear magnetic anomalies in both basins. The strikes of magnetic lineations are N47°E in the Japan Basin and N82°E in the Tsushima Basin. The estimated magnetization intensities of magnetic source models constructed from the amplitudes of analytic signal calculated from vector anomalies and the crustal structures determined by seismic studies are similar to those of typical extrusive basalt in both basins. The observed anomalies in the Japan Basin contain a short wavelength anomaly which cannot be explained by the model. Their ages may be chrons C5Cr (16.726-17.277 Ma), C5Dn (17.277-17.615 Ma), C5Dr (17.615-18.281 Ma), and subchron C5Dr.1n which was identified by a paleomagnetic study. The estimated half-spreading rate is 2.0 cm/yr, which is slower than that estimated by previous study. The observed anomalies in the Tsushima Basin show that there is a partial magnetization high. This may indicate that not all of the sources of magnetic lineations in the Tsushima Basin changed to low magnetization by the effect of thick sediment cover and the intrusions of a large amount of dikes after the formation.
Key words: Crustal magnetic structure, vector magnetic anomaly, Tsushima Basin, Japan Basin, Japan Sea.