Flexural isostasy and loading sequence of the Dokdo seamounts on the Ulleung Basin in the East Sea (Sea of Japan)

摘要

The isostatic compensation and loading sequence of the Dokdo seamounts on the Ulleung Basin in the East Sea are studied by a flexure model using gravity and bathymetric data. The Dokdo seamounts comprise Dokdo (Dok Island), lying as a cluster of emerged small islets surmounting a large submerged volcanic edifice, and two voluminous tablemounts, Simheungtaek and Isabu. In spite of their similarity in topographic expression, a large difference of about 50 mGal between the observed gravity anomaly peaks over Dokdo and the Isabu Tablemount implies different amounts of compensation. A comparison between the observed and calculated gravity anomalies of the seamounts shows that the flexural compensation investigated in this study predicts well the observed data. The flexure model suggests that the Dokdo seamounts might be composed of high-density volcanic rocks. Modeling results show that the flexural rigidity or effective elastic thickness of the lithosphere under Dokdo is stronger or thicker than that under the Isabu Tablemount. It implies that the age of lithosphere at the time of loading for Isabu was younger than that for Dokdo. The effective elastic thicknesses associated with the Dokdo seamounts approximately follow 200-400 ℃ isotherms in the cooling plate model. The results of this study and the former age dating of Dokdo suggest that the Isabu Tablemount was formed first, followed by Dokdo after the opening of the East Sea ceased.