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SUBSIDENCE HISTORY OF THE CENTRAL SUB-BASIN IN THE NORTHERN SOUTH YELLOW SEA BASIN,

OFFSHORE KOREA

Eun Young LEE University of Science & Technology, Dept. of Petroleum Resources Technology, South Korea

queenily2@gmail.com

ABSTRACT

The Northern South Yellow Sea Basin (NSYSB) is located between East China and West Korea, and is one of the basins in East Asia, with its initial rifting from Late Mesozoic times onwards, caused by large-scale interaction between the Pacific, Eurasian, and Indian plates. To analyze the details of the tectonic evolution of the NSYSB, this study focuses on the subsidence history of the Central Sub-basin which covers the southern and northern regions of the NSYSB. Subsidence curves analyzed from artificial wells can be divided into 5 segments on the Northern half-graben structure and 6 segments on the Southern sag structure. The differing movement is caused by a different history of both structures where the southern block is of Late Cretaceous age. The subsidence history of the Central Sub-basin can be divided into 3 phases. These are: (1) Main subsidence phase (Late Cretaceous – Oligocene): this phase covers more than 90 % of the total and tectonic subsidence in the Central Sub-basin, (2) Uplift and Erosion phase (Early Miocene): the denuded thickness is about 500 m, (3) Secondary subsidence phase (Middle Miocene – Present).

1. INTRODUCTION The South Yellow Sea Basin is located between East China and West Korea, and is subdivided into the Northern and Southern South Yellow Sea Basins by a central uplifted area (Zhang et al., 1989) (Fig. 1). The Northern South Yellow Sea Basin (NSYSB) is one of a number of Mesozoic-Cenozoic, non-marine, back-arc, transtensional rift or pull-apart basins that are distributed along a general NE-SW trend in China and the Yellow Sea. The NSYSB is filled with mainly Late Cretaceous and Cenozoic non-marine (fluvial–alluvial and lacustrine) clastic sediments of considerable thickness (several thousand meters) (Yi et al., 2003). During the last few decades, comprehensive research programs have been carried out in the NSYSB. But the kinematic processes and geodynamic evolution of this basin are far from being understood. This basin has been interpreted as a transtensional structure (Marathon, 1987) or pull-apart basin (Baag and Baag, 1994). But recently, Ren et al. (2002) suggested similarities with intracontinental rift basins and the SYSB. To analyze the detail of the tectonic evolution of the NSYSB, this paper focuses on the subsidence history of the Central Sub-basin which covers the southern and northern

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region of the NSYSB. Backstripped subsidence curves are useful in investigating the basin-forming mechanisms (Allen and Allen, 2002). Understanding the subsidence history of a basin is very important to quantitatively subdivide its tectonic evolution (Cao et al., 2007).

Fig. 1. Geological setting of the South Yellow Sea Basin, separated into Northern and Southern South Yellow Sea Basins by the Central High. Locations of 5 exploratory wells (diamonds) and 2 artificial well sites (black circles) analyzed for subsidence analysis (modified from PEDECO, 1997).

2. GEOLOGIC SETTING East Asia comprises a mosaic of distinct continental fragments separated by fold belts. These fold belts are suture zones resulting from the accretion of various fragments formerly separated by intervening areas of oceanic crust (Watson et al., 1987). The Yellow Sea lies on the Yangtze (or South China) Platform and the Sino–Korean (or North China) Platform with the Qinling–Dabie–Sulu collisional belt sutured during the early Mesozoic (Hsu, 1989; Gilder and Courtillot, 1997; Kim et al., 2000; Wu, 2002). Since the Late Mesozoic time, hundreds of extensional or transtensional basins developed in East China. The Northern South Yellow Sea Basin is one of the basins where rifting started in the late Mesozoic time. The cause of the rifting in eastern Eurasia is large-scale interaction between Pacific, Eurasian, and Indian plates (Ren et al., 2002). During the Early Late Cretaceous, the Pacific Plate moved to the edge of eastern China and changed its subduction direction from NNW to WNW, at a steep angle, at 65 Ma (Uyeda and Kanamori,

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1979; Engebretson et al., 1985; Maruyama et al., 1997). The change in subduction velocity and angle of the Pacific Plate led to upwelling of asthenosphere and thinning of the lithosphere in eastern China (Yu et al., 2008, Zhou and Li, 2000). It assisted the rifting process in eastern Eurasia, and resulted in Late Cretaceous extensional tectonics. The eastern NSYSB is divided into three sub-basins by basement highs and faults within the basin; Central, South-West and North-East sub-basins (Yi et al., 2003; KIGAM, 2006) (Fig. 1). The Central Sub-basin lies NW-SE trending from southern to northern regions of the basin in between basement highs, and its Northern and Southern regions have different structural types. Its Southern structure is a sag type with having gently sloping boundaries. The Northern structure is defined as half-graben structure with its rotated hanging wall relatively down-thrown at the bounding normal faults. The South-West Sub-basin has graben and half-graben structures which were developed along E-W trending southern bounding faults (Shin et al., 2005). Due to difficulties to place its exact border to the northeast, the North-East Sub-basin is not easy to characterize, here a few NW-SE trending normal faults are recognized on seismic profiles.

3. DATA AND METHODS

3.1 DATA In order to determine total and tectonic subsidences of a sedimentary basin from well-log information or from calibrated seismic reflection data, the effects of compaction, sedimentary loading, water depth, and changing sea-level during deposition must be considered carefully (Allen and Allen, 2002, Sclater & Christie, 1980; Steckler & Watts, 1978). The data on which this work is based were collected by the Korea National Oil Company (KNOC) and Korea Institute of Geoscience and Mineral (KIGAM). The geological ages of the key unconformity surfaces; acoustic basement, top of the Late Cretaceous (Maastrichtian), top of the Paleocene, top of the Eocene, bottom of the Middle Miocene, recognized on seismic data were taken directly from KIGAM (2006) that are based on exploration wells (IIC-1X, IIH-1Xa, Haema, Inga, and Kachi) in the area.

3.2 METHODS Present-day stratigraphic thicknesses are a product of cumulative compaction through time. A quantitative analysis of subsidence rates through time, sometimes called geohistory analysis, relies primarily on the decompaction of stratigraphic units to their thickness at the time of interest. The decompaction of stratigraphic units requires the variation of porosity with depth to be known (Allen and Allen, 2002). The variation of porosity is taken to be a simple exponential decrease with increasing depth, z, Ø= Øoexp(-cz) (Øo: the subsurface porosity, c: the porosity-depth coefficient) (Sclater & Christie, 1980). But in this area, porosity information from wells is not good to use. Thus, values for the

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subsurface porosity and the porosity-depth coefficient were taken from Tab. 1. The technique whereby the effects of the sediment load are removed from the total subsidence to obtain the tectonic contribution is called as backstripping (Allen and Allen, 2002). Following Steckler & Watts (1978), water-loaded basement subsidence, Y, is given by Y = S[(ρm–ρs)/(ρm-ρw)] + Wd – ΔSL[ρm /(ρm-ρw)] (S: sediment layer thickness corrected for compaction, SL: sea-level, Wd: water depth, at burial time. ρm, ρs, and ρw: the mantle, mean sediment, and water densities). Because the NSYSB was a typical nonmarine facies basin, the paleobathymetry was shallow and ΔSL changed little. So, ΔSL and Wd were not considered in this study.

Tab. 1. Exponents of Surface porosity (Øo), Porosity-depth coefficient (c), Sediment grain density (ρsg) for different lithologies (from Sclater & Christie, 1980).

3.3 RESTORATION OF DUNUDATION The method which this paper applies is to analyze the stratum thickness trend and denuded tilted fault block on the seismic section, based on the typical tectonic and stratigraphic evolution. As a result, the denudation thickness was estimated to about 500 m. When reconstructing the denudation amount by uplift and erosion during the Early Miocene, results are not good enough mainly because a lack of well data.

4. MODELING OF THE SUBSIDENCE HISTORY To model the subsidence history, two “artificial wells” were selected from typical seismic profiles of the two different types of structures in the Central Sub-basin. One artificial well is located on the half-graben structure of northern region, and the other is on the sag structure in southern region of the Central Sub-basin (Fig. 1). 4.1. SUBSIDENCE MODELING IN NORTHERN STRUCTURE OF THE

CENTRAL SUB-BASIN The subsidence curves of this site can be divided into 5 segments (Fig. 2); 1) Paleocene (65.5–55.8 Ma), 2) Eocene (55.8–33.9 Ma), 3) Oligocene (33.9-23.03 Ma), 4) Early Miocene (23.03-15.97 Ma), 5) Middle Miocene – Present (15.97 Ma–Present). The

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subsidence curves during the Paleocene are steep with a subsidence rate of about 169 m/Ma in total subsidence and about 67 m/Ma in tectonic subsidence. In Oligocene times, subsidence decreased steadily to a rate of about 41 m/Ma in total subsidence and about 8 m/Ma in tectonic subsidence. This decrease is very prominent on the tectonic subsidence curves (Fig. 2). In the Early Miocene, the curves rise and show uplift and erosion. Again from the Middle Miocene onwards, this structure subsides with rate of about 48 m/Ma in total subsidence and about 13 m/Ma in tectonic subsidence.

Fig. 2. Subsidence history of the northern structure in the Central Sub-basin. 4.2. SUBSIDENCE MODELING IN SOUTHERN STRUCTURE OF THE

CETNRAL SUB-BASIN The subsidence curves of this site can be divided into 6 segments (Fig. 3); 1) Late Cretaceous (Maastrichtian) (70.6–65.5 Ma), 2) Paleocene (65.5–55.8 Ma), 3) Eocene (55.8–33.9 Ma), 4) Oligocene (33.9-23.03 Ma), 5) Early Miocene (23.03-15.97 Ma), 6) Middle Miocene – Present (15.97 Ma–Present). Compared with the curves in the north, the main difference in the subsidence curves in the south is the existence of the Late Cretaceous segment. The subsidence curves during the Late Cretaceous are very steep with subsidence rate of about 558 m/Ma in total subsidence and about 187 m/Ma in tectonic subsidence. Paleocene subsidence curves are also steep with subsidence rates of about 182 m/Ma in total subsidence and about 45 m/Ma in tectonic subsidence. On the whole, subsidence rates decrease gradually to about 45 m/Ma in total subsidence and about 9 m/Ma in tectonic

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subsidence from the Late Cretaceous to Oligocene. Similar to the northern part of the basin, uplift and erosion starts in the Early Miocene. From the Middle Miocene onwards, subsidence starts again with rate of about 30 m/Ma in total subsidence and about 7 m/Ma in tectonic subsidence.

Fig. 3. Subsidence history of the southern structure in the Central Sub-basin.

5. SUBSIDENCE HISTORY OF THE CENTRAL SUB-BASIN

On the whole, the two structures have a different subsidence history up to the Paleocene. The main difference is that the southern sag structure started to subside in the Late Cretaceous (Maastrichtian), and the amount of subsidence of this period is about 43 % of total subsidence and about 51% of tectonic subsidence. In the northern half-graben structure, subsidence started in the Paleocene. From the start of subsidence, the subsidence curves of both structures grew less steep towards the Oligocene, and prove a steady decrease of subsidence rates from the start (the Late Cretaceous in the Northern and the Paleocene in the Southern). The subsidence up to Oligocene times covers more than 90 % of the total and tectonic subsidence occurred in the Central Sub-basin. Therefore this period is defined as the main subsidence phase in the Central Sub-basin. Both areas were uplifted after the subsidence of the Oligocene, and this uplift and erosion phase completed the main subsidence phase. The Central Sub-basin was denuded during the Early Miocene, and it was analyzed as about 500 m.

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Since the Middle Miocene, the Central Sub-basin has subsided again, and it is defined as the secondary subsidence phase. Its amount is about 760 m in the northern region and about 470 m in the southern region.

Fig. 4. Summary of basin evolution studied from the Northern and Southern South Yellow Sea Basin; The Central Sub-basin (from this study) and the SW Sub-basin (from Park et al., 2005) of the eastern Northern South Yellow Sea Basin, the Northern South Yellow Sea Basin (NSYSB) (from Yi et al., 2003), the Jiangsu Basin (the onshore part of the Southern South Yellow Sea Basin) (from Pang et al., 2003).

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6. SUMMARY OF BASIN EVOLUTION STUDIED IN THE NORTHERN AND SOUTHERN SOUTH YELLOW SEA BASINS

Fig. 4 shows a summary of the basin evolution studied from the Northern and Southern South Yellow Sea Basins. In the SW Sub-basin of the NSYSB, it is divided to three megasequences by study with seismic stratigraphy; MSQ I (Late Cretaceous-Paleocene), MSQ II (Eocene) and MSQ III (Middle Miocene-Present) (Park et al. 2005) (Fig. 4). MSQ I and MSQ II is interpreted as synrift sediments and postrift sediments, respectively. The Oligocene - Early Miocene period is non-deposited due to uplift. The basin evolution of the NSYSB by Yi et al. (2003) is studied by palynological analysis of two wells (Kachi-1 and Haema-1), which are located in the SW Sub-basin and northern structure of the Central Sub-basin. According to Yi et al. (2003), following stages in the development of the NSYSB can be separated: (1) initial stage of rifting or pull-apart basin formation during the Late Jurassic(?) - Cretaceous; (2) subsidence from the Paleocene to Middle Eocene; (3) alternation of uplift and subsidence in the Late Eocene; (4) synrift inversion and erosion through the Oligocene; (5) uplift during the Early Miocene; and (6) widespread subsidence from the Middle Miocene onwards apart from during the Early Pliocene when the region was subjected to uplift once more (Yi et al., 2003). The Jiangsu Basin is the onshore part of the Southern South Yellow Sea Basin (Zili et al., 1997). Corresponding to the local orogenic event, development of the Jiangsu Basin is 1st tectonic layer of the Wobao movement (Middle Late Cretaceous - Middle Eocene), 2nd tectonic layer of the Sanduo movement (Middle Eocene - Oligocene) and 3rd tectonic layer (Miocene - Present) (Pang et al., 2003) (Fig. 4).

7. DISCUSSION

The subsidence histories calculated from two structures in the Central Sub-basin have similar patterns with the exception of the Late Cretaceous period (Fig. 2, 3). The southern sag structure has the Late Cretaceous (Maastrichtian) formation overlying acoustic basement, and it is the first segment in its subsidence curves. In the northern half-graben structure, the lowest formation is the Paleocene over acoustic basement which is granite generated in the Late Cretaceous (KIGAM, 1997). This means that in the south, rifting started earlier than the north in the Central Sub-basin. A fact is also supported by a deeper acoustic basement (max. 4.5 s TWT) in the southern region than in the northern region, which consists of basement highs and relatively shallow depressed structures (KIGAM, 2006). A probable reason for the different initial rifting age and magnitude of subsidence is the different lithologic characteristics of the acoustic basement. In accordance to well data, the acoustic basement of the southern region is mainly composed of sedimentary rocks, while the basement of northern region is formed by granite. It is verified very evidently on the magnetic anomaly map (GSJ, 2002) (Fig. 5).

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Fig. 5. Magnetic anomaly map of the Yellow Sea and adjacent region (modified from GSJ, 2002). Outline (black line) of the Northern South Yellow Sea Basin is adopted from PEDECO (1997) (See Figure 1). In this study, the subsidence of the Central Sub-basin is grouped into the main subsidence phase of the Late Cretaceous – Oligocene, the uplift and erosion phase of the Early Miocene and the secondary subsidence phase of the Middle Miocene – Present (Fig. 4). Compared with other studies of the Northern and Southern South Yellow Sea Basins (Fig. 4), the main subsidence phase is related with the MSQ I, II of the SW Sub-basin, the rifting stage of the NSYSB, and the Wobao - Sanduo movement of the Jiangsu Basin. But, ages of each stage are different depending on study methods and regions (Fig. 4). In this study, the geologic ages of the key unconformity surfaces recognized in the Central Sub-basin were taken from KIGAM (2006), which state that the Late Cretaceous corresponds to the Maastrichtian. The MSQ I is interpreted widely stretching over the entire period of the Late Cretaceous. Yi et al. (2003) analyzed that previous and early Cretaceous sedimentary rocks are in-filled formations of the basin, but this study and Park et al. (2005) interpret the sedimentary rocks as acoustic basement on seismic data. In the Central Sub-basin, the uplift and erosion phase occurred in the early Miocene (this study). Park et al., (2005) defined this phase as Oligocene – Early Miocene in the SW Sub-basin and Yi et al., (2003) as Late Eocene – Early Miocene in the NSYSB. According to Pang et al. (2003), the Jiangsu Basin was uplifted after the deposition of the Sanduo

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Formation, thus large parts of the Oligocene sediments were not preserved. It is difficult to confirm the period of uplift and erosion in the Jiangsu Basin. But, all studies of the NSYSB show that the uplift period finished in the Early Miocene. Difference of starting age in this phase depends on the age of formations under the regional unconformity at the bottom of the middle Miocene. While the Oligocene and partly early Miocene formations are found below the regional unconformity in the Central Sub-basin, Eocene formations are found below the unconformity in the SW Sub-basin. The secondary subsidence phase is equivalent to MSQ III of the SW Sub-basin, the regional Subsidence of the NSYSB and 3rd tectonic layer of the Jiangsu Basin. Its duration is the same with MSQ III and the regional Subsidence stage of the Middle Miocene – Present, and similar with 3rd tectonic layer of the Miocene – Present. In this phase, few faults are developed, and the seismic reflections within strata are consistent, parallel and continuous. Thus, it is interpreted that this phase subsided in stable sedimentation environment by relatively weak tectonic activity.

8. CONCLUSIONS AND SUMMARY 1) In the Northern South Yellow Sea Basin, the Central Sub-basin covers the southern and northern region, and includes two type structures; northern half-graben and southern sag structures. Analysis of subsidence history confirmed 5 subsidence segments in the northern structure and 6 subsidence segments in the Southern structure. 2) In the Subsidence histories of two structures in the Central Sub-basin, the biggest difference is the existence of the Late Cretaceous segment. The southern sag structure has the segment with the period of largest subsidence, but the northern half-graben structure does not have such a period. This study suggests that different lithologies of the acoustic basement caused a lag of initial rifting and differences in the magnitude of depending between regions. 3) The subsidence history of the Central Sub-basin consists of 3 phases; the main subsidence phase (Late Cretaceous - Oligocene), the uplift and erosion phase (Early Miocene), the secondary subsidence phase (Middle Miocene - Present). 4) The main subsidence phase is related with the MSQ I (Late Cretaceous - Paleocene), II (Eocene) of the SW Sub-basin, the rifting stage (Early Cretaceous - Late Eocene) of the NSYSB, and the Wobao (Middle Upper Cretaceous - Middle Eocene) – Sanduo (Middle Eocene - Oligocene) movements of the Jiangsu Basin. For this subsidence stage, studied ages of each region are not same. A reason for this is the lack of combined research between biostratigraphic and seismostratigraphic analysis. 5) The uplift and erosion phase is during the Oligocene – Early Miocene in the SW Sub-basin and during the Late Eocene – Early Miocene in the NSYSB. In comparison, the Jiangsu Basin was uplifted after the deposition of the Sanduo Formation in the Oligocene. Even though all these phases ended in the early Miocene, the stating age differs remarkably between the Late Eocene – Early Miocene. It depends on age of formations below the regional unconformity in each region. 6) The secondary subsidence phase is equivalent to MSQ III (Middle Miocene –

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Present) of the SW Sub-basin, the regional subsidence (Middle Miocene – Present) of the NSYSB and 3rd tectonic layer (Miocene – Present) of the Jiangsu Basin. The temporal extent of the last phases is very similar. ACKNOWLEDGEMENTS The author wishes to express my thanks to professors and colleagues in Dept. Petroleum Resources Technology, University of Science and Technology, South Korea, and to doctors and colleagues in Dept. Geodynamics and Sedimentology, University Vienna, Austria, for their discussion and advice to improve this manuscript. And, thanks to IGS-6 and referees for helping publication. REFERENCES Allen, P.A., Allen, J.R., 2005. Basin Analysis: Principles and Applications. Blackwell Scientific Publications, pp.549. Baag, C., Baag, C.-E., 1998. Aeromagnetic interpretation of southwestern continental shelf of Korea. In: Gibson, R.I., Millegan, P.S. (Eds.), Geologic Applications of Gravity and Magnetics: Case Histories. Society of Exploration Geophysicists Geophysical References Series 8, pp. 63–68. Cao, D.-Y., Wang, X.-G., Zhan, W.-F., Li, W.-Y., 2007. Subsidence History of the Eastern Depression in the North Yellow Sea Basin. Journal of China University of Mining & Technology, 17 (1), 90-95. Engebretson, D.C., Cox, A., Gordon, R.G., 1985. Relative Motions Between Oceanic and Continental Plates in the Pacific Basin. The Geological Society of America, Special Paper, 206, 1–59. Geological Survey of Japan(GSJ) and Coordinating Committee for Coastal and Offshore Geoscience Programmes in East and Southeast Asia (CCOP), 2002. Magnetic anomaly map of East Asia 1:4,000,000 CD-ROM Version(2nd Edition), Digital Geoscience Map 2 (P-1). Gilder, S., Courtillot, V., 1997. Timing of the North-South China collision from the middle to late Mesozoic paleomagnetic data from the North China Block. Journal of Geophysical Research 102 (B8), 17713–17727. Hsu, K.J., 1989. Origin of sedimentary basins of China. In: Zhu, X. (Ed.), Chinese Sedimentary Basins. Sedimentary Basins of the World 1. Elsevier, New York, pp. 208–227. KIGAM, 1997. Comprehensive Assessment for the Domestic Continental Shelf (Yellow Sea Basin I). Report of Korean Institute of Geology, Mining and Materials. Korea Petroleum Development Corporation, 208 pp. (in Korean). KIGAM, 2006. Joint Study on Sedimentary Basins between Korea and China. Korea Institute

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of Geoscience And Mineral Report GAA2003002-2006(4) (in Korean, with English abstract). Kim, J.-N., Ree, J.-H., Kwon, S.-T., Park, Y., Choi, S.-J., Cheong, C.-S., 2000. The Kyonggi shear zone of the central Korean Peninsula: late orogenic imprint of the North and South China collision. The Journal of Geology, 108, 469–478. Marathon, 1987. The geology and petroleum potential of Block II, volume I-IV. Maruyama, S., Isozaki, Y., Kimura, G., et al., 1997. Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present. Island Arc, 6, 121–142. Pang, X., Li, M., Li, S., Jin, Z., Xu, Z., Chen, A., 2003. Origin of crude oils in the Jinhu Depression of North Jiansu-South Yellow Sea Basin, eastern China. Organic Geochemistry, 34, 553-573. Park, K.S., Kang, D.H., Shinn, Y.J., Shin, J.B., 2005. Tectonic evolution of the western Kunsan Basin, Yellow Sea, offshore Korea. Journal of the Geological Society of Korea, 41, 141–155 (in Korean, with English abstract). PEDCO, 1997. ’96 Report of Geophysical Interpretation on South-West Sub-basin, Yellow Sea, Korea. Korea Petroleum Development Corporation, 50 pp. (in Korean). Ren, J., Tamaki, K., Li, S., and Junxia, Z., 2002. Late Mesozoic and Cenozoic rifting and its dynamic setting in Eastern China and adjacent areas. Tectonophysics, 344, 175-205. Sclater, J. G., and Christie P. A. F., 1980. Continental stretching: An explanation of the post- mid-Cretaceous subsidence of the central North Sea Basin. J. Geophysics. Res., 85, 3711-3739. Shin, J.-B., Park, K.-S., Shinn, Y.-J., Kang, D.-H., 2005. Analysis of Geologic Structure for the Seismic Data in the Block II of Kunsan Basin, Offshore Korea. KIGAM Bulletin, Vol.9, No.1, pp.72-83 (in Korean, with English abstract). Steckler, M. S., and Watts A. B., 1978. Subsidence of the Atlantic-type continental margin of New York. Earth Planet. Sci. Lett., 41, 1-13. Uyeda, S., Kanamori, H., 1979. Backarc opening and the mode of subduction. Journal of Geophysical Research, 84, 1049–1061. Watson, M.P., Hayward, A.B., Parkinson, D.N., Zhang, Zh.M., 1987. Plate tectonic history, basin development and petroleum source rock deposition onshore China. Marine and Petroleum Geology, 4, 205-225. Wu, S., 2002. Mesozoic-Cenozoic rifting and origins of the North Yellow Sea Basin. Continent–Ocean Interactions within the East Asian Marginal Seas. American Geophysical Union Chapman Meeting Abstracts, San Diego, CA, 10–14 November 2002, p. 42. Yi, S., Yi, S., Batten, D.J., Yun, H., Park, S.-J., 2003. Cretaceous and Cenozoic on-marine deposits of the Northern South Yellow Sea Basin, offshore western Korea: palynostratigraphy and palaeoenvironments. Palaeogeography, Palaeoclimatology, Palaeoecology, 191, 15–44. Yu, Z., Wu, S., Zou, D., Feng, D., Zhao, H., 2008. Seismic profiles across the middle Tan-Lu fault zone in Laizhou Bay, Bohai Sea, eastern China. Journal of Asian Earth Sciences, 33, 383-394. Zhang, Y., Wei, Z., Xu, W., Tao, R., Chen, R., 1989. The North Jiangsu-South Yellow Sea

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Basin. In: Zhu, X. (Ed.), Chinese Sedimentary Basins. Elsevier, Amsterdam, pp. 107- 123. Zhou, X.M., Li, X.W., 2000. Origin of late Mesozoic igneous rocks in southeastern China: implications for lithosphere subduction and underplating of mafic magmas. Tectonophysics, 326, 269–287. Zili, W., Liging, Z., 1997. The Regularity of Oil and Gas Abundant Accumulation in North Jiangsu Basin, China. In: Zhaocai, S., Tinbin, W., Deliao, Y., Guojun, S.,(Eds.), Geology of Fossil Fuels-Oil and Gas. Proceedings of the 30th International Geological Congress, Vol.18, pp.313-331