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The Ulleung Basin, East Sea/Japan Sea, is a Neogene back‐arc basin and occupies a tectonically crucial zone under the influence of relative motions between Eurasian, Pacific and Philippine Sea plates. However, the link between tectonics and sedimentation remains poorly understood in the back‐arc Ulleung Basin, as it does in many other back‐arc basins as well, because of a paucity of seismic data and controversy over the tectonic history of the basin. This paper presents an integrated tectonostratigraphic and sedimentary evolution in the deepwater Ulleung Basin using 2D multichannel seismic reflection data. The sedimentary succession within the deepwater Ulleung Basin is divided into four second‐order seismic megasequences (MS1 to MS4). Detailed seismic stratigraphy interpretation of the four megasequences suggests the depositional history of the deepwater Ulleung Basin occurred in four stages, controlled by tectonic movement, volcanism, and sea‐level fluctuations. In Stage 1 (late Oligocene through early Miocene), syn‐rift sediment supplied to the basin was restricted to the southern base‐of‐slope, whereas the northern distal part of the basin was dominated by volcanic sills and lava flows derived from initial rifting‐related volcanism. In Stage 2 (late early Miocene through middle Miocene), volcanic extrusion occurred through post‐rift, chain volcanism in the earliest time, followed by hemipelagic and turbidite sedimentation in a quiescent open marine setting. In Stage 3 (late middle Miocene through late Miocene), compressional activity was predominant throughout the Ulleung Basin, resulting in regional uplift and sub‐aerial erosion/denudation of the southern shelf of the basin, which provided enormous volumes of sediment into the basin through mass transport processes. In Stage 4 (early Pliocene through present), although the degree of tectonic stress decreased significantly, mass movement was still generated by sea‐level fluctuations as well as compressional tectonic movement, resulting in stacked mass transport deposits along the southern basin margin. We propose a new depositional history model for the deepwater Ulleung Basin and provide a window into understanding how tectonic, volcanic and eustatic interactions control sedimentation in back‐arc basins.

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Multichannel seismic reflection profiles and well data were analyzed to unravel the origin and depositional history of stacked cut-and-fill structures (CF) in the southwestern margin of the Ulleung Basin, East Sea. The CFs in a syn-compressional megasequence are characterized by discontinuous, low-amplitude, chaotic, and transparent seismic reflections. They display both U-shaped and V-shaped morphologies along their lengths. The CFs are thickest and widest at the shelf edge and taper both landward and basinward. Based on the stratigraphic position, the CFs are found in three depositional sequences (DS8–DS10). Thirty CFs have been identified that range from 0.5 to 8.8 km in width and 58 to 453 m in thickness (assuming a 2000 m/s seismic velocity of the sediments). The larger and numerous CFs occurred in the middle depositional sequence. Seismic characteristics, spatial distribution, hundred-meter-scale incisions, and high gamma-ray responses indicate that the CFs were caused by submarine canyons. During the period of DS8, small CFs were formed locally on the shelf margin, and were little influenced by the deformation of the thrust-fold (Dolgorae Thrust Belt). Extensive and numerous CFs in DS9 developed in the oversteepened shelf margin, where uplift of both the thrust-fold and anticline (Gorae V structure) occurred simultaneously, and where a large volume of sediment was supplied. During the period of DS10, a general decreasing pattern in the occurrence and dimensions of the CFs resulted from waning tectonic activity of the thrust-fold that reduced sediment supply. Consequently, this study suggests that variation in contractional tectonic activity and sediment supply, associated with the back-arc closure of the East Sea, mainly controlled the evolution of the CFs rather than eustasy.

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