Tectonic, sedimentary, and volcanic evolution of a back-arc basin in the East Sea (Sea of Japan) S.H. Yoon a , Y.K. Sohn b , S.K. Chough c, ⁎ a Department of Earth and Marine Sciences, Jeju National University, Jeju 690-756, South Korea b Department of Earth and Environmental Sciences and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, South Korea c School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, South Korea abstract article info Article history: Received 2 May 2013 Received in revised form 25 February 2014 Accepted 3 March 2014 Available online 13 March 2014 Keywords: East Sea Ulleung Basin back-arc basin tectonics continental rifting pull-apart This paper focuses on the tectonic, sedimentary, and volcanic evolution of a unique back-arc basin (Ulleung Basin) in the southwestern part of the East Sea (Sea of Japan). The basin consists of thick extended continental (or transitional) crust and an overlying sedimentary succession (4–8 km thick), with interlayered volcanic flows and sills, defining a number of seismic units of variable reflection characteristics. The northern margin is bounded by faulted continental blocks (South Korea Plateau) with isolated basement lows and sub-basins with intruded and extruded volcanics, whereas the southern margin is underlain by deep-seated basement with a thick (N 8 km thick) sedimentary succession. The western margin is bounded by a series of strike-slip and normal faults produced by NNW–SSE-directed dextral movements. These structural features suggest a south–southeastward drift of the southwestern Japanese Arc away from the South Korea Plateau during the Early to Middle Miocene, involving large-scale right-lateral strike-slip deformation along the western margin, akin to pull-apart basin formation. During the back-arc opening, the thinned continental crust was largely mod- ified by intrusive volcanics under tensional stress regime. The volcanics also extruded across the axis of extension as well as along the boundary faults on the west. In the Middle to Late Miocene, major faults in the southern and western margins were inverted, forming partial closure of the basin especially in the southern part, although the plateau in the northern margin experienced continuous subsidence. The Ulleung Basin thus provides an example of an immature back-arc basin which experienced a rather brief episode of rifting and extension followed by clo- sure due to tectonic inversion. © 2014 Elsevier B.V. All rights reserved. 1. Introduction A back-arc basin forms when the tectonic stress is tensional in the region adjacent to a subduction-related volcanic arc above a subducting plate. In the northwest Pacific, back-arc basins formed in relatively short time, from the Late Oligocene to the Middle Miocene, and are generally shallow in water depth (Taylor and Karner, 1983). The relatively deep (N 3000 m) back-arc basins are commonly floored by oceanic crust and characterized by prominent magnetic anomalies indicative of sea- floor spreading, such as the Kuril and Japan basins (Jolivet et al., 1994). On the other hand, the relatively shallow (b 3000 m) back-arc basins, such as the Yamato and Ulleung basins, are underlain by an anomalously thick transitional (or extended continental) crust, charac- terized by an obscure magnetic anomaly pattern and the lack of basaltic seafloor (Fig. 1)(Tamaki et al., 1992; Jolivet et al., 1994; Kurashimo et al., 1996). The transitional crust is largely modified by sediments and volcanics derived from the adjacent continent and arc. For these reasons, it has been difficult to decipher the mode of back-arc rifting and extensional processes of the shallow back-arc basins. A back-arc basin in the southwestern corner of the East Sea, the Ulleung Basin, comprises an extended continental crust without a prominent magnetic anomaly pattern, but with high heat flow (Watanabe et al., 1977; Isezaki, 1986). The Ulleung Basin is connected to the deeper Japan Basin in the north by an interplain gap, but separat- ed from the Yamato Basin by the Oki Bank and Yamato ridges in the east and northeast, respectively (Fig. 1). The three basins are bounded by the Japanese arc on the east and south, and retain their own characteristics in basin configuration, crustal structure, and subsidence history. These variations are largely due to the changes in stress regime during the ini- tial rifting and subsequent spreading and crustal extension. Paleomag- netic analyses of adjacent continents (Otofuji et al., 1985; Faure and Lalevee, 1987; Otofuji and Matsuda, 1987; Hayashida and Fukui, 1991; Lee et al., 1999; Otofuji et al., 2003) have generally favored fan-shaped Marine Geology 352 (2014) 70–88 ⁎ Corresponding author at: School of Earth and Environmental Sciences, College of Natural Sciences, Seoul National University, Seoul 151-747, South Korea. E-mail addresses: shyoon@jejunu.ac.kr (S.H. Yoon), yksohn@gnu.ac.kr (Y.K. Sohn), sedlab@snu.ac.kr (S.K. Chough). http://dx.doi.org/10.1016/j.margeo.2014.03.004 0025-3227/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo