Transition from buckling to subduction on strike-slip continental margins: Evidence from the East Sea (Japan Sea)

Gi-Bom Kim1, Seok-Hoon Yoon2, Seung-Sep Kim3, and Byung-Dal So4* 1Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, South Korea 2Department of Earth and Marine Sciences, Jeju National University, Jeju 63243, South Korea 3Department of Geology and Earth Environmental Sciences, Chungnam National University, Daejeon 34134, South Korea 4Division of Geology and Geophysics, Kangwon National University, Chuncheon 24341, South Korea

ABSTRACT Initiation of subduction is rarely encountered in modern tectonic environments due to its ephemeral and destructive nature. We report the geological and geophysical evidence indicat- ing a transitional phase from buckling to embryonic subduction along the eastern Korean margin. The transition appears to be caused by compressional reactivation of the strike-slip boundary between the continental (Korean Peninsula) and oceanic (Ulleung Basin) crusts since the Early Pliocene. Evidence for compressional reactivation includes (1) a west-dipping major thrust and coincident crustal buckling of the Ulleung Basin; (2) an east-west structural asymmetry inferred from the gravity anomaly and P-wave tomography; and (3) ongoing crustal uplift and high-angle faults along the eastern Korean margin. The juxtaposition of underthrusting and buckling of the crust in the Ulleung Basin, and its associated ubiquitous reverse faulting on the eastern Korean margin, imply the potential development of a new subduction system along the western margin of the East Sea (Japan Sea). We propose that the East Sea comprises two incipient subduction margins (i.e., the Korean and Japanese sides), which are now competing to reach a self-sustaining subduction stage.

Figure 1. Bathymetric map of study area show- INTRODUCTION and geophysical data on surface-level to crustal- ing location of seismic reflection data grids. The opening of an oceanic basin eventually level deformation should be integrated. Bathymetry is in meters. Blue triangles repre- ceases, and occasionally the basin switches to Our study reports a new example of the tran- sent locations of marine terraces (after Park the closing regime accompanied by subduction sitional phase from buckling to SI observed in et al., 2017). Focal mechanism solutions are initiation (SI). SI is a transient phase, before the western margin of the East Sea (Japan Sea). after Choi et al. (2012). Plate abbreviations: AM—Amur, OH—Okhotsk, YZ—Yangtze, PS— the onset of a mature subduction system, which Observations are based on multi-scale deforma- Philippine Sea, and PA—Pacific. can develop under tectonic conditions associ- tion structures constrained by seismic reflection, ated with preexisting mechanical heterogene- gravity, and P-wave velocity data. We focus on ities and/or external stress transmission (Stern the formative processes and mechanical rela- Though typical oceanic crust is only evident in and Gerya, 2017). However, evidence for SI is tionship between deep-crustal deformation fea- the northern region of the East Sea, the crust in rarely observed in either modern tectonic envi- tures, and discuss the geodynamic driving forces the south is also regarded as oceanic, which was ronments or geological records because SI itself for SI in this region. For the stratigraphic and thickened (~11 km) by magmatic underplating is a destructive process. Due to such limitations, structural analysis, we utilized ~10,700 km of (Sato et al., 2014). The western (i.e., Korean side) the understanding of SI mostly relies on infer- multi-channel seismic reflection data (Fig. 1). and eastern (i.e., Japanese side) boundaries of the ences regarding an embryonic phase preserved Digital SEG-Y (Society of Exploration Geo- sea, developed originally as strike-slip principal in some mature subduction systems (e.g., What- physicists standard) data were loaded into displacement zones of the pull-apart system, are tam and Stern, 2015) and on numerical simula- the PETREL software system (Schlumberger, currently under a transverse shortening regime tions (e.g., Gurnis, 1992). https://​www​.software​.slb​.com​/products​/petrel) (Jolivet et al., 1992). In particular, the eastern Many efforts have been made to find a link for stratigraphic correlation and mapping. The margin of the sea is regarded as an incipient sub- between hypothetical models of SI and known complete spherical isostatic gravity anomaly duction boundary based on compressional struc- geological examples, including seismic reflec- data of the World Gravity Map (WGM; Bonvalot tures and large-magnitude earthquakes (e.g., the tion analysis of structural components (Duarte et et al., 2012) was then used to identify the gravi- A.D. 1983 M7.7 Nihonkai-Chubu earthquake) al., 2013), focal mechanism and aftershock anal- tational manifestation of crustal-level deforma- hosted by an east-dipping reverse fault (No et al., ysis of seismicity along a master thrust fault (No tion imaged by the seismic reflection data. 2014, and references therein). The western mar- et al., 2014), and P-wave velocity tomography of gin of the sea, which exhibits a drastic transverse deep lithospheric structures (Eakin et al., 2015). NEOTECTONIC FEATURES OF THE transition from oceanic to continental crust (Cho However, no consensus has been achieved on EAST SEA MARGINS et al., 2004), also displays evidence of ongoing the criteria for defining ongoing SI in modern The East Sea, one of the major marginal basins compressional deformation, such as high-angle tectonic settings. Therefore, to identify SI and in the western circum-Pacific (inset in Fig. 1), reverse faults (Yoon and Chough, 1995), marine unravel its geodynamic mechanism, geological has experienced pull-apart–style backarc exten- terraces (Choi et al., 2008), and moderate-sized sion and subsequent multi-phase tectonic reacti- earthquakes (e.g., the 2004 M5.1 Uljin earth- *E-mail: [email protected] vations during the Cenozoic (Jolivet et al., 1992). quake; Kang and Baag, 2004).

GEOLOGY, July 2018; v. 46; no. 7; p. 603–606 | https://doi.org/10.1130/G40305.1 | Published online 7 June 2018 ©GEOLOGY 2018 The Authors.| Volume Gold 46 |Open Number Access: 7 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 603

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/46/7/603/4225330/603.pdf by guest on 25 September 2021 RESULTS with depth (occasionally overturned at the deep- topographic variation resulted from regional est level), and exhibit the shape of a tri-shear folding of the basin floor since ca. 3.8 Ma. Each Ulleung Neotectonic Boundary zone (Figs. 2D and 2E). The tri-shear zone can fold structure extends over 150 km along axis, The Ulleung Basin is filled with a >5-km- be interpreted as a fault-tip fold structure that with a wavelength of 60–70 km and an ampli- thick sedimentary succession deposited since accommodates differential mass movement in tude of 0.15–0.2 km. Isopach mapping reveals the Early Miocene (Kim and Yoon, 2017). The front of a fault tip propagating from deeper in two depocenters of the Pliocene-Quaternary entire succession is divided into syn-extensional the crust (e.g., Pei et al., 2014). The tri-shear sedimentary sequence (UPS) that overlap with (SS) and post-extensional (PS) sequences with zone is marked by a bottom-to-the-west reverse the two topographic lows on the UNB (Fig. 3B). respect to a sequence boundary that marks the sense and along-strike lateral continuity over termination of backarc extension at ca. 12.5 Ma 100 km (Fig. 3A). This west-dipping reverse Deep Crustal Anomalies (Fig. 2). The PS, comprising mostly submarine boundary fault and its downward extension are The isostatic gravity anomaly shows a NNW- mass-transported deposits, is divided into lower together referred to as the “major thrust” (MT). SSE–oriented, 25–40-km-wide zone with rela- (LPS) and upper (UPS) sequences with respect The tri-shear zone extends upward to the UNB tively low gravity anomalies (LAZ) (Fig. 3C). to the Ulleung Neotectonic Boundary (UNB). (Figs. 2D and 2E), which indicates that the major The eastern boundary of the LAZ follows the This UNB is a sequence boundary with an age thrust was initiated at ca. 3.8 Ma. Growth strata strike of the major thrust (Fig. 3C). Comparison constrained to ca. 3.8 Ma (Lee and Kim, 2002). over the UNB suggest that the major thrust is between a gravity profile (GP2) and equivalent In contrast to the concordantly stratified LPS, the still under development (Figs. 2F and 2G). The seismic tomographic model (modified after Cho UPS comprises growth strata that divergently fill second small-scale thrust farther to the east (Fig. et al., 2004) indicates that the LAZ correlates with the topography of the UNB with repetitive onlap 3A) implies a higher amount of compressional the zone underlain by a thickened, high P-wave stratal termination (Figs. 2F and 2G). The growth deformation in the south compared to the north. velocity anomaly (6.0–7.0 km/s) (Fig. 3D). Given strata suggest that, since ca. 3.8 Ma, submarine the west-dipping geometry of the major thrust, the sediments have gradually filled the topography Folding of the Basin Floor LAZ and thickened seismic anomaly appear to be concurrent with deformation of the basin floor. Interpretation of stratal architectures and associated with the major thrust. This further sug- isochronal mapping of the UNB reveal two gests that the crust has been thickened by thrusting Major Thrust elongate, regional-scale topographic highs (i.e., along the eastern Korean margin (Fig. 4). Similar In the western margin of the Ulleung Basin, central and western highs, CH and EH) and lows crustal asymmetry has been recognized from the the UNB and underlying strata display an east- (i.e., western and eastern lows, WL and EL) in Gagua Ridge in the western Philippine Sea, where down monoclinal fold (Fig. 2). The limbs of the the NNW-SSE direction (Figs. 2 and 3A). The a subduction zone failed during its incipient stage monoclinal strata become shorter and steeper growth strata above the UNB suggest that such (Eakin et al., 2015).

DISCUSSION

Tectonic Model for the East Sea Margins The neotectonic evolution of the eastern Korean margin is characterized by juxtaposed compressional deformation features at diverse crustal levels. These deformation features include a west-dipping major thrust, thickened crust beneath the thrust zone, regional-scale folding of the Ulleung Basin floor, and ubiquitous structural inversion in association with uplifting of the east- ern Korean margin. We argue that underthrusting of the Ulleung Basin beneath the eastern Korean Peninsula is the fundamental control on the jux- taposition of deformational features (Fig. 4). The structures identified in this study are identical to those recognized from modern incipient subduc- tion zones. Examples of these include a major thrust fault caused by crustal rupturing (e.g., Yelles et al., 2009), crustal downwarp and incipi- ent trench formation (e.g., Collot et al., 1995), and differential uplift caused by crustal shorten- ing transverse to the subduction boundary (e.g., Duarte et al., 2013). However, evidence for the sustaining stage of subduction, such as subduc- tion-related magmatism or an incipient Benioff zone, have yet to be reported. This implies that Figure 2. A–C: Selected west-east seismic reflection profiles across the western margin of subduction is most likely in its embryonic phase. Ulleung Basin (for locations, see Fig. 1). UNB—Ulleung neotectonic boundary, UTB—Ulleung tectonic boundary; SS—syn-extensional sequence, LPS—lower post-extensional sequence, Transient Regime from Buckling to UPS—upper post-extensional sequence. D,E: Cropped seismic sections showing the details of tri-shear zones. F,G: Cropped seismic section with stratigraphic interpretation. The ages Subduction Initiation of major stratigraphic boundaries are from Lee and Kim (2002) and Kim and Yoon (2017). The In the Ulleung Basin, there are two regional- seismic reflection data set was acquired by the Korea National Oil Corporation. scale folds, the axes of which appear parallel

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/46/7/603/4225330/603.pdf by guest on 25 September 2021 to the NNW-SSE strike of the major thrust (Fig. 3A). The folds infer buckling of the oceanic lithosphere accommodating east-west compres- sional stress in the Ulleung Basin. Previous stud- ies of focal mechanism solutions have revealed that the background compressional stress in the East Sea mainly originates from subduction of the Pacific Plate along the Japan trench (e.g., Choi et al., 2012). The horizontal force per unit length required to sustain a subduction system should be on the order of ~1012 N/m (Copley et al., 2010), which is comparable to the theoretical value for buckling of viscoelastoplastic oceanic lithosphere (Schmalholz and Mancktelow, 2016). We suspect that buckling of the oceanic lith- osphere along with shear zone (major thrust) formation is a strong indicator of future SI, as proven by several numerical studies (e.g., Thiel- mann and Kaus, 2012). The probability of SI becomes even greater considering the young oceanic crust (ca. 12 to ca. 20 Ma) of the Ulleung Basin (Kim and Yoon, 2017), which is similar to the estimated optimal age for SI (Cloetingh et al., 1989). Moreover, lithospheric weaken- ing, induced by stress from thick sediments (>5 km) on the western continent-ocean boundary of the basin (So et al., 2012), may lead to SI even without compressional background stress (Regenauer-Lieb et al., 2001). Figure 3. A: Isochronal (two-way traveltime) map showing the topographic relief on the Ulleung neotectonic boundary (UNB). Contours in milliseconds (ms). MT—major thrust, WL—west- We argue that the crustal asymmetry recog- ern low, CH—central high, EL—eastern low, EH—eastern high. B: Isopach map of the upper nized from P-wave tomography and a gravity post-extensional sequence (UPS). C: Isostatic gravity anomaly map showing locations of the anomaly (Fig. 3D) indicates regional uplift of gravity profiles (GP) and P-wave tomographic model. OBS—ocean bottom seismometer. D: the eastern Korean Peninsula (i.e., overriding Gravity profiles (GP1 and GP2) and a P-wave tomographic model equivalent to GP2. LAZ— plate) by underthrusting of the Ulleung Basin. zone with relatively low gravity anomalies. The P-wave tomographic model is modified after Cho et al. (2004). Age dating of Quaternary marine terraces on the eastern Korean margin (blue triangles in Fig. 1) support ongoing uplift of the hanging wall at a rate of 0.26–0.67 m/k.y. since at least 0.3 Ma (Choi et al., 2008). Similar localized uplift of the overriding plate by SI was reported in the Fiordland Region of New Zealand (House et al., 2002). We also suspect that folding of the oceanic lithosphere and Quaternary fault move- ments in the continental Korean Peninsula (Yoon and Chough, 1995) accommodated the horizon- tal compression. Evidence for both underthrust- ing and buckling are found simultaneously, sug- gesting that the western margin of the East Sea is under a transitional regime toward SI. Based on the observed geological evidence and mechanical plausibility, we propose that the Korean side of the East Sea is another candidate for SI, along with the well-known incipient sub- duction boundary on the Japanese side. Neither the Korean nor Japanese side of the East Sea appear to have reached a state of self-sustaining subduction. We suggest that the two incipient subduction zones are competing with each other to evolve into a self-sustaining system, as shown by the ancient case of the Gagua Ridge (i.e., Figure 4. Schematic tectonic model of the East Sea (Japan Sea) margins. For location of the model, see inset in Figure 1. Plate abbreviations as in Figure 1. Red bars in the inset map rep- failed SI) and the Manila subduction zone (i.e., resent direction of compressional background stress (after Choi et al., 2012). UNB—Ulleung successful SI) in the western Philippine Sea neotectonic boundary. Plate (Eakin et al., 2015). While both margins

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/46/7/603/4225330/603.pdf by guest on 25 September 2021 exhibit geological characteristics of SI, the seis- margin in the East Sea (Japan Sea): Geophysical revealed by seismic imaging: Earth and Planetary micity appears to favor the eastern margin as Research Letters, v. 31, https://​doi​.org​/10​.1029​ Science Letters, v. 400, p. 14–25, https://​doi​.org​ /2003GL019107. /10​.1016​/j​.epsl​.2014​.05​.026. a stronger candidate over the western margin Choi, S.J., Merritts, D.J., and Ota, Y., 2008, Elevations Park, C.-S., Kihm, Y.H., Nahm, W.-H., and Lee, G.- (No et al., 2014). However, such differences in and ages of marine terraces and late Quaternary R., 2017, Formative age of coastal terraces and seismicity may not be the only critical factor rock uplift in southeastern Korea: Journal of Geo- uplift rate in the east coast of South Korea: Jour- in determining an ongoing SI, because the pro- physical Research: Solid Earth, v. 113, B10403, nal of the Korean Geomorphological Association, https://​doi​.org​/10​.1029​/2007JB005260. v. 24, p. 43–55, https://​doi​.org​/10.16968​ ​/JKGA​ cess of SI has a time scale of ~5 m.y. (Toth and Choi, H., Hong, T.K., He, X., and Baag, C.E., 2012, .24​.4​.43. Gurnis, 1998). Instead, we speculate the eastern Seismic evidence for reverse activation of a pa- Pei, Y., Paton, D. A., and Knipe, R. J., 2014, Defining a and western margins of the East Sea are under leo-rifting system in the East Sea (Sea of Japan): 3-dimensional trishear parameter space to under- different stages of SI processes. Tectonophysics, v. 572, p. 123–133, https://​doi​ stand the temporal evolution of fault propagation .org​/10​.1016​/j​.tecto​.2011​.12​.023. folds: Journal of Structural Geology, v. 66, p. 284– Cloetingh, S., Wortel, R., and Vlaar, N.J., 1989, On 297, https://doi​ .org​ /10​ .1016​ /j​ .jsg​ .2014​ .05​ .018​ .​ CONCLUSIONS the initiation of subduction zones: Pure and Ap- Regenauer-Lieb, K., Yuen, D.A., and Branlund, J., The compressive neotectonic evolution of the plied Geophysics, v. 129, p. 7–25, https://​doi​.org​ 2001, The initiation of subduction: Critically by western strike-slip margin of the East Sea was /10​.1007​/BF00874622. addition of water?: Science, v. 294, p. 578–580, investigated using seismic reflection and gravity Collot, J.Y., Lamarche, G., Wood, R.A., Delteil, J., https://​doi​.org​/10​.1126​/science​.1063891​. Sosson, M., Lebrun, J.F., and Coffin, M.F., 1995, Schmalholz, S.M., and Mancktelow, N.S., 2016, Fold- anomaly data, along with a P-wave tomographic Morphostructure of an incipient subduction zone ing and necking across the scales: A review of model. Based on newly recognized multi-scale along a transform plate boundary: Puysegur Ridge theoretical and experimental results and their deformation structures, we have drawn the fol- and Trench: Geology, v. 23, p. 519–522, https://​ applications: Solid Earth, v. 7, p. 1417–1465, lowing conclusions: doi.org​ /10​ .1130​ /0091​ -7613​ (1995)023​ <0519:​ ​ https://​doi​.org​/10​.5194​/se​-7​-1417​-2016. (1) The tri-shear zone at the slope break of MOAISZ>2.3​ .CO;2.​ So, B.D., Yuen, D.A., Regenauer-Lieb, K., and Lee, Copley, A., Avouac, J.P., and Royer, J.Y., 2010, The S.M., 2012, Asymmetric lithospheric instability the eastern Korean continental margin suggests India-Asia collision and the Cenozoic slowdown facilitated by shear modulus contrast: Implica- strain localization across the continental-ocean of the Indian plate: Implications for the forces tions for shear zones: Geophysical Journal In- boundary in the manner of a west-dipping major driving plate motions: Journal of Geophysical ternational, v. 190, p. 23–36, https://​doi​.org​/10​ thrust. Research, v. 115, B03410, https://​doi​.org​/10​ .1111​/j​.1365​-246X​.2012​.05473​.x. .1029​/2009JB006634. Sato, T., No, T., Kodaira, S., Takahashi, N., and Kaneda, (2) The regional folding structure suggests Duarte, J.C., Rosas, F.M., Terrinha, P., Schellart, W.P., Y., 2014, Seismic constraints of the formation buckling of the oceanic lithosphere by ENE- Boutelier, D., Gutscher, M.A., and Ribeiro, A., process on the back‐arc basin in the southeast- WSW compressive horizontal stress, comparable 2013, Are subduction zones invading the Atlan- ern Japan Sea: Journal of Geophysical Research: to the general tectonic stress required to sustain tic? Evidence from the southwest Iberia margin: Solid Earth, v. 119, p. 1563–1579, https://doi​ .org​ ​ a subduction system. Geology, v. 41, p. 839–842, https://​doi​.org​/10​ /10.1002/2013JB010643.​ .1130​/G34100​.1. Stern, R.J., and Gerya, T., 2017, Subduction initiation (3) The isostatic gravity anomaly and P-wave Eakin, D.H., McIntosh, K.D., Van Avendonk, H.J.A., in nature and models: A review: Tectonophysics, tomographic model reveal that the crust around and Lavier, L., 2015, New geophysical con- https://​doi​.org​/10​.1016​/j​.tecto​.2017​.10​.014. the major thrust is thickened by underthrusting straints on a failed subduction initiation: The Thielmann, M., and Kaus, B.J.P., 2012, Shear heating of the Ulleung Basin crust beneath the eastern structure and potential evolution of the Gagua induced lithospheric-scale localization: Does it Ridge and Huatung Basin: Geochemistry Geo- result in subduction?: Earth and Planetary Sci- margin of the Korean Peninsula. physics Geosystems, v. 16, p. 380–400, https://​ ence Letters, v. 359–360, p. 1–13, https://doi​ .org​ ​ (4) Simultaneously, the thrust causes perva- doi​.org​/10​.1002​/2014GC005548. /10​.1016​/j​.epsl​.2012​.10​.002. sive uplift over the eastern Korean margin related Gurnis, M., 1992, Rapid continental subsidence fol- Toth, J., and Gurnis, M., 1998, Dynamics of subduc- to structural inversion since the Early Pliocene. lowing the initiation and evolution of subduction: tion initiation at preexisting fault zones: Journal Science, v. 255, p. 1556–1558, https://doi​ .org​ /10​ ​ of Geophysical Research: Solid Earth, v. 103, Therefore, we propose that these concurrent .1126​/science​.255​.5051​.1556. B8, p. 18,053–18,067, https://​doi​.org​/10​.1029​ geophysical signatures for buckling and SI in House, M.A., Gurnis, M., Kamp, P.J.J., and Suther- /98JB01076. the East Sea and the eastern Korean continen- land, R., 2002, Uplift in the Fiordland Region, Yelles, K., et al., 2009, Plio-Quaternary reactivation tal margin represent a transitional regime from New Zealand: Implications for incipient subduc- of the Neogene margin off NW Algiers, Algeria: buckling to the onset of SI. tion: Science, v. 297, p. 2038–2041, https://​doi​ The Khayr al Din bank: Tectonophysics, v. 475, .org​/10​.1126​/science​.1075328. p. 98–116, https://​doi​.org​/10​.1016​/j​.tecto​.2008​ Jolivet, L., Fournier, M., Huchon, P., Rozhdestvenskiy, .11​.030. ACKNOWLEDGEMENTS V.S., Sergeyev, K.F., and Oscorbin, L.S., 1992, Whattam, S., and Stern, R.J., 2015, Late Cretaceous We thank the editor, M. Quigley, and reviewers Cenozoic intracontinental dextral motion in the plume-induced subduction initiation along the L. Jolivet, S. Schmalholz, S. Whattam, and an anony- Okhotsk‐Japan Sea Region: Tectonics, v. 11, southern and eastern margins of the Caribbean: mous reviewer for their helpful reviews. This research p. 968–977, https://doi​ .org​ /10​ .1029​ /92TC00337.​ The first documented example with implications was supported by the National Research Foundation of Kang, T.S., and Baag, C.E., 2004, The 29 May 2004, for the onset of plate tectonics: Gondwana Re- Korea (NRF-2015R1C1A1A01055545, to G.B. Kim), Mw = 5.1, offshore Uljin earthquake, Korea: Geo- search, v. 27, p. 38–63, https://​doi​.org​/10​.1016​/j​ the KMA Research Development Program (KMIPA sciences Journal, v. 8, p. 115–123, https://​doi.org​ ​ .gr​.2014​.07​.011. 2015-3010-3, to Yoon), the KMA Research and Devel- /10​.1007​/BF02910189. Yoon, S.H., and Chough, S.K., 1995, Regional strike opment Program (KMI2018-02110, to S.S. Kim), and Kim, G.B., and Yoon, S.H., 2017, An insight into slip in the eastern continental margin of Korea the Earthquake Disaster Prevention Human Resource asymmetric back-arc extension: Tecto-magmatic and its tectonic implications for the evolution of Development project (from the Ministry of the Interior evidences from the Ulleung Basin, the East Sea Ulleung Basin, East Sea (Sea of Japan): Geologi- and Safety of Korea, to So). (Sea of Japan): Tectonophysics, v. 717, p. 182– cal Society of America Bulletin, v. 107, p. 83– 192, https://​doi​.org​/10​.1016​/j​.tecto​.2017​.07​.016. 97, https://doi​ .org​ /10​ .1130​ /0016​ -7606​ (1995)107​ ​ REFERENCES CITED Lee, G.H., and Kim, B., 2002, Infill history of the <0083:RSSITE>2​ .3​ .CO;2.​ Bonvalot, S., et al., 2012, World Gravity Map: Paris, Ulleung Basin, East Sea (Sea of Japan) and im- Bureau Gravimetrique International (BGI), plications on source rocks and hydrocarbons: Ma- CGMW-BGI-CNES-IRD, http://bgi​ .omp​ .obs​ -mip​ ​ rine and Petroleum Geology, v. 19, p. 829–845, Manuscript received 1 November 2017 .fr/activities​ /Projects​ /world_gravity_map_wgm.​ https://doi​ .org​ /10​ .1016​ /S0264​ -8172​ (02)00106​ -X.​ Revised manuscript received 15 May 2018 Cho, H.M., Kim, H.J., Jou, H.T., Hong, J.K., and No, T., Sato, T., Kodaira, S., Ishiyama, T., Sato, H., Manuscript accepted 16 May 2018 Baag, C.E., 2004, Transition from rifted conti- Takahashi, N., and Kaneda, Y., 2014, The source nental to oceanic crust at the southeastern Korean fault of the 1983 Nihonkai–Chubu earthquake Printed in USA

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