동해 울릉분지 북동부의 독도해산들의 탄성판 지각 변화

Abstract

The isostasy compensation and the loading process of Dok seamounts located at the northeastern part of the Ulleung Basin in the East Sea(the Sea of Japan) were studied by using gravity data and bathymetry data. Comparison between observed gravity anomalies and calculated gravity anomalies from isostasy models suggests compensation degrees and elastic thicknesses associated with Dok seamounts. Dok seamounts are composed of three seamounts which have guyot summits. There are the highest and the lowest free-air gravity anomaly peak in the 1st Dok seamount and the 3rd Dok seamount, respectively. In spite of similar size, the difference between free-air gravity anomaly peaks of two seamounts is about 50 mGal, suggesting different compensation degrees. The magnetic anomaly amplitude of the 3rd Dok seamount is much lower than that of the 1st Dok seamount. Comparison between the calculated and the observed gravity anomalies of Dok seamounts suggests that the flexure model is the most applicable model. Assuming the effective elastic thickness along seamounts to be 5 km for the 1st Dok seamount, 3 km for the 2nd Dok seamount, and 2 km for the 3rd Dok seamount, the modeling result shows that the flexural rigidity of lithosphere of the 1st Dok seamount is stronger than the 3rd Dok seamount, indicating that the age of lithosphere at the time of loading of the 3rd Dok seamount is younger than that of the 1st Dok seamount. Effective elastic thicknesses of Dok seamounts range from 2 to 5 km, which is similar to the case of seamounts near ridges in the Pacific. This suggests that Dok seamounts erupted on oceanic crust near the spreading axis of the Ulleung Basin. Effective elastic thicknesses of Dok seamounts approximate 200°C - 400°C isotherms in the cooling plate model. Above results and the age dating of the Dok island over sea level suggest that the 3rd Dok seamount was formed first and followed by the 1st Dok seamount. The low magnetic anomaly in the 3rd Dok seamount is suppo