Crustal structure at the southwestern margin of the Ulleung Basin, East Sea (Japan Sea), from wide-angle seismic data

Abstract

The Ulleung Basin is a major back-arc basin in the East Sea (Japan Sea) in the western Pacific. Unlike the multichannel seismic reflection profiles collected over the years, only a few wide-angle seismic experiments using ocean bottom seismometers have previously been conducted at the margins of the Ulleung Basin. The results show regional differences in crustal structure between the southwestern and southeastern margins. Clarifying the difference in crustal structure and investigating its origin are important for understanding the evolution of the basin. To better understand the crustal structure at the southwestern margin of the basin, we analyzed multichannel and wide-angle seismic data acquired by the Korea Institute of Geoscience and Mineral Resources (KIGAM) in 2007. The overall crustal thickness along a track line across the basin margin varies from 21 to 10 km. The crust is divided into upper and lower layers. The upper crust, with P wave velocities of 4.5–6.3 km/s, has nearly uniform thickness below the continental slope (4–6 km); the lower crust, with P wave velocities of 6.3–7.0 km/s, thins towards the basin (8–15 km). The resulting velocity-depth profiles of the crust at the southwestern margin of the basin show that the velocity gradients are higher than that of the Korean Peninsula and lower than that at the center of the Ulleung Basin. This contrast suggests that the crust at the southwestern margin is a transition zone from unextended to extended continental crust. Our study shows two remarkable features of the lower crust at the southwestern margin of the basin: (1) a high P wave velocity zone (7.0–7.3 km/s) above the Moho discontinuity resulting from magma underplating by voluminous igneous activity during basin opening and (2) localized crustal thickening attributed to compressional tectonic forces after basin closure. These crustal features were also observed in a previous study at the southwestern margin of the basin, suggesting that they are distributed linearly along the southwestern margin. Therefore, our study implies that the crustal features at the southwestern margin originated from tectonic movements controlled the evolution of the basin. A gravity model provides an additional constraint on the resulting velocity structure and indicates that considering crustal features in the lower crust plausibly explain the observed gravity anomalies.