저작권법에따른이용자의권리는위의내용에의하여영향을받지않습니다.

Transcription

1 저작자표시 - 비영리 - 변경금지 2.0 대한민국 이용자는아래의조건을따르는경우에한하여자유롭게 이저작물을복제, 배포, 전송, 전시, 공연및방송할수있습니다. 다음과같은조건을따라야합니다 : 저작자표시. 귀하는원저작자를표시하여야합니다. 비영리. 귀하는이저작물을영리목적으로이용할수없습니다. 변경금지. 귀하는이저작물을개작, 변형또는가공할수없습니다. 귀하는, 이저작물의재이용이나배포의경우, 이저작물에적용된이용허락조건을명확하게나타내어야합니다. 저작권자로부터별도의허가를받으면이러한조건들은적용되지않습니다. 저작권법에따른이용자의권리는위의내용에의하여영향을받지않습니다. 이것은이용허락규약 (Legal Code) 을이해하기쉽게요약한것입니다. Disclaimer

2 이학박사학위논문 Mesozoic tectonics and thermal history of the Korean Peninsula and provenance of the southeastern Yellow Sea sediments 한반도의중생대지구조및열사와 황해남동부퇴적물의기원지연구 2012 년 8 월 서울대학교대학원 지구환경과학부 최태진

3 ABSTRACT Mesozoic tectonics and thermal history of the Korean Peninsula and provenance of the southeastern Yellow Sea sediments Taejin Choi School of Earth and Environmental Sciences The Graduate School Seoul National University This study deals with four topics of Mesozoic tectonics and thermal histories of the Korean peninsula and provenance of the southeastern Yellow Sea sediments, using single-grain age dating analyses on detrital apatite and zircon grains collected from the Cretaceous sedimentary rocks and modern river/seafloor sediments. In Chapter 1, the temporal and spatial distribution of Mesozoic arc magmatism related to the subduction of the paleo-pacific plates are inferred by U-Pb ages of detrital zircon grains collected from the sediments of seven major South Korean rivers. Mesozoic detrital zircon U-Pb ages indicate continuous arc magmatic activity in the Korean Peninsula throughout the Mesozoic era with NW-SE spatial migration of the arc magmatic centers, and changes in subduction modes of the paleo-pacific plates in the Mesozoic East Asian continental i

4 margin. In Chapter 2, apatite fission track (FT) and illite crystallinity (IC) analyses are carried out to reconstruct thermal history of the Jinan Basin. Then, those of other Cretaceous basins are summarized to understand response of the East Asian continental margin due to subduction of the paleo-pacific plate. Comparison of thermal history of Korean Cretaceous basins with those of other area in the East Asian continental margin reveals that the Upper Cretaceous regional exhumation of East Asian continental margin including the Korean Peninsula during ca Ma was facilitated by the subduction of the Izanagi-Pacific ridge. Chapter 3 deals with FT ages of detrital apatite grains in three major river sediments of the southern Korean Peninsula to estimate age distribution of apatite FT ages of the rocks in their drainage areas and investigate relationship between the apatite FT age components and tectonic events of the Korean Peninsula. Comparison of the analytical results with the previously reported apatite FT age distributions of rocks in the river drainage areas indicates that the each age component corresponds to the regional exhumation event of the river drainage areas. Provenance of the sandy sediments in the southeastern Yellow Sea is investigated in Chapter 4, with U-Pb analyses on detrital zircon grains collected from the sandy sediments. The zircon age spectra showing gradual E-W direction change indicate temporal and spatial changes in sediment supply from South China and the Korean Peninsula. Keywords: detrital thermochronology, detrital geochronology, fission track, U-Pb dating, apatite, zircon, Mesozoic tectonics Student Number: ii

5 CONTENTS ABSTRACT i PREFACE MESOZOIC DETRITAL ZIRCON U-PB AGES OF MODERN FLUVIAL SEDIMENTS IN KOREA INTRODUCTION ANALYTICAL METHODS RESULTS AND INTERPRETATION DISCUSSION TECTONIC IMPLICATIONS CONCLUSIONS THERMAL HISTORIES OF CRETACEOUS BASINS IN KOREA INTRODUCTION JINAN BASIN GEOLOGICAL SETTING OF THE JINAN BASIN EXPERIMENTAL METHODS RESULTS INTERPRETATION THERMAL HISTORY OF EAST ASIAN CONTINENTAL MARGIN THERMAL HISTORY OF THE OTHER CRETACEOUS BASINS IN KOREA iii

6 2.3.2 UPLIFT OF THE CRETACEOUS BASINS IN KOREA AND SUBDUCTION DIRECTION CHANGE OF THE OCEANIC PLATE UPPER CRETACEOUS RIDGE SUBDUCTION AND REGIONAL UPLIFT OF THE EAST ASIAN CONTINENTAL MARGIN CONCLUSIONS DETRITAL APATITE THERMOCHRONOLOGY OF SOUTH KOREAN RIVER SEDIMENTS INTRODUCTION GEOLOGICAL SETTING EXPERIMENTAL METHODS RESULTS & INTERPRETATION DISCUSSION PROVENANCE OF APATITE FT AGE COMPONENTS THE PALEOGENE EXHUMATION OF THE KOREAN PENINSULA CONCLUSIONS PROVENANCE OF THE SOUTHEASTERN YELLOW SEA SEDIMENTS INTRODUCTION GEOLOGICAL SETTING OF THE YELLOW SEA EXPERIMENTAL METHODS RESULTS INTERPRETATION & DISCUSSION iv

7 4.5.1 ZIRCON AGE POPULATIONS OF SOUTHEASTERN YELLOW SEA SEDIMENTS PROVENANCE OF THE SOUTHEASTERN YELLOW SEA SEDIMENTS THE CAUSE OF E-W VARIATION OF DETRITAL ZIRCON AGE SPECTRA CONCLUSIONS SUMMARY REFERENCES ABSTRACT IN KOREAN v

8 LIST OF TABLES Table 0-1 Characteristics of apatite fission track dating and zircon U-Pb dating Table 1-1 U-Pb isotopic data for detrital zircon grains of the South Korean river sediments determined by LA-ICP-MS Table 1-2 K-S test P values of detrital zircon age distributions of the fluvial sediments in South Korea Table 2-1 Apatite fission track analytical results of the Jinan Basin Table 3-1 Apatite fission track analytical results of Korean river sediments Table 3-2 Age peaks and their percentages in the deconvoluted apatite FT age components of Korean river sediments Table 3-3 Rb/Sr, K/Ar, and apatite FT ages of Mesozoic granites and their cooling rates during Ma Table 4-1 U-Pb isotopic data for detrital zircon grains of the southeastern Yellow Sea sediments determined by LA-ICP-MS Table 4-2 P values for K-S test of age distributions of detrital zircon U-Pb ages of southeastern Yellow Sea sediments vi

9 LIST OF FIGURES Figure 1-1 A) Mesozoic igneous rock distribution in East Asia. B) Drainage basins of major rivers in South Korea Figure 1-2 Probability density plots presenting detrital zircon U-Pb ages of seven major river sediments in South Korea Figure 1-3 Representative SEM-CL images of analyzed Mesozoic detrital zircon grains from South Korean river sediments Figure 1-4 Landward limits of the Mesozoic magmatic front of the paleo-pacific plates in the East Asian continental margin during A) the Triassic period, B) the Jurassic period, and C) the Cretaceous period Figure 2-1 The distribution of the Cretaceous basins in the southern Korean Peninsula Figure 2-2 Geological map of the Jinan Basin, Korea Figure 2-3 Representative X-ray diffraction patterns for <2µm, air-dried samples from the Jinan Basin Figure 2-4 Radial plots and track length distributions of apatite FT analytical results of the Jinan Basin Figure 2-5 Representative thermal history of the Jinan Basin calculated by apatite FT length modeling results of the Jinan Basin Figure 2-6 Thermal histories and apatite FT ages of the Cretaceous basins and Palgongsan Granite in the Korean Peninsula vii

10 Figure 2-7 a) Paleogeographic map of the East Asian margin at ca. 90 Ma. b) Thermal histories of the Cretaceous Gyeongsang and Jinan basins, Yanji granitoids, and cooling histories of the Ryoke, Sambagawa, and Shimanto accretionary belts in Japan Figure 3-1 a) The distribution of the mountain ranges in the southern Korean Peninsula. B) Drainage basins of the Han, the Geum, and the Nakdong rivers in South Korea Figure 3-2 Probability-density plots presenting detrital apatite FT ages of a) the Han, b) the Geum, c) the Nakdong, and d) the total river sediments Figure 3-3 Deconvoluted probability-density plots of detrital apatite FT ages of a) the Han, b) the Geum, c) the Nakdong river sediments Figure 3-4 Representative thermal histories of the Jurassic and Cretaceous granitoids in the southern Korean Peninsula, calculated by apatite FT length modeling using the data of Jin et al. (1987) Figure 4-1 (a) Map of rivers feeding the Yellow Sea. (b) Bathymetric chart and grain size distribution of the Yellow Sea Figure 4-2 Probability-density plots of the detrital zircon U-Pb ages in the southeastern Yellow Sea sediments Figure 4-3 Proportions of detrital zircon age groups in the southeastern Yellow Sea sediments Figure 4-4 Possible provenance discrimination of southeastern Yellow Sea sandy viii

11 sediments based on their P values in Table Figure 4-5 Paleo-coastlines of the Yellow Sea during the Holocene successive transgressive stages, assuming that its topography has not been changed sea level since the last glacial stage ix

12 PREFACE The East Asian continental margin, including the Korean Peninsula, has been an active margin since the Late Permian (Li et al., 2006). Subduction of the paleo-pacific plates beneath the Asian continent has caused major tectonic events in the continental margin during the Mesozoic era such as arc magmatism, orogeny, strike-slip and/or extensional movements, etc (Kim, 1996; Maruyama et al., 1997). Mesozoic tectonism in Korea is characterized by strike-slip movement and arc magmatism throughout the Mesozoic and compressive and extensional phases during the Early and the Late Mesozoic, respectively (Chang, 1995; Kim, 1996). Single-grain age dating can determine ratios between radiogenic elements and their daughter products in individual mineral grains to yield unmixed age spectra of the minerals. It is a powerful tool for sediment/sedimentary rock research and can be applied to studies on formation age of crystalline rocks, timing of cooling or exhumation, maximum depositional age of fossil-barren strata, sediment provenance, etc. This study is based on two single-grain age dating methods of U-bearing detrital minerals: apatite fission track (FT) dating and zircon U-Pb dating using Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS). Characteristics of two methods are summarized in Table 0-1 and their detailed principles are referred to Wagner and van den Haute (1992) for apatite FT dating and Dickin (2005) for zircon U- Pb dating. Although both methods use U-bearing minerals, FT dating is based on nuclear fission of 238 U whereas U-Pb dating is based on decay of 238 U and 235 U, respectively. This differs in their daughter products as fission damage trail (FT dating) 1

13 and 206 Pb and 207 Pb (U-Pb dating), resulting in different characteristics such as measurement of isotopic ratios, stability of daughter products, closure temperature, and so on (Table 0-1). Consequently, zircon U-Pb dating generally provides information about formation age of the zircon grain and its source rock, whereas apatite FT dating is suitable for studies on cooling age and pattern of the apatite grain and its source rock. This study comprises four topics about the Mesozoic tectonics and thermal histories of the Korean peninsula and provenance of southeastern Yellow Sea sediments. Detrital zircon U-Pb age dating was applied to Mesozoic detrital zircon age study of modern fluvial sediments in Korea (Chapter 1) and provenance study on the southeastern Yellow Sea sediments (Chapter 4), which need formation ages of sediment grains. Detrital apatite FT dating was applied to studies on thermal histories of the Cretaceous basins in Korea (Chapter 2) and detrital apatite thermochronology of South Korean river sediments (Chapter 3), which need cooling ages and cooling pattern of sediments. In Chapter 1, detrital zircon ages of South Korean river sediments were analyzed to investigate temporal and spatial distribution of Mesozoic arc magmatism related to the subduction of the paleo-pacific plates. Chapter 2 includes thermal history of the Cretaceous Jinan Basin inferred by apatite FT and illite crystallinity (IC) analyses and previous studies on those of other Cretaceous basins, to understand response of the East Asian continental margin due to subduction of the paleo-pacific plate. Chapter 3 deals with FT ages of detrital apatite grains in Korean river sediments to estimate age distribution of apatite FT ages of the rocks in their drainage areas and investigate relationship between the apatite FT age components and tectonic events of the Korean 2

14 Table 0-1. Characteristics of apatite fission track dating and zircon U-Pb dating. apatite fission track zircon U-Pb mother element 238 U 238 U, 235 U daughter product fission damage trail 206 Pb, 207 Pb number of tracks (age) measurement concentration of U, Th, and Pb track length closure temperature ( C) 100 (60-110)* 900** information formation age formation age cooling timing (age) deformation timing cooling pattern (track length) *Donelick (1991); **Cherniak and Watson (2000) 3

15 Peninsula. In Chapter 4, U-Pb analyses were carried out on detrital zircon grains of the southeastern Yellow Sea sediments to investigate changes in sediment supply from their source rocks. 4

16 1. MESOZOIC DETRITAL ZIRCON U-PB AGES OF MODERN FLUVIAL SEDIMENTS IN KOREA 1.1 INTRODUCTION The East Asian continental margin has been an active margin since the Late Permian (Li et al., 2006), and its tectonism has been mainly related to subduction of the paleo- Pacific plates, resulting in Andean-type arc magmatism. The Mesozoic granitoid belt is distributed over a ca. 500-km width in the East Asian continental margin, from South China to Japan (Fig. 1-1A; Sagong et al., 2005). In the Korean Peninsula, it consists mainly of Late Triassic to Early Jurassic and Late Cretaceous to Early Tertiary igneous rocks generally showing arc affinity (Kee et al., 2010; Kim et al., 2011b; Sagong et al., 2005 and references therein), except for several Triassic transitional granites in central western South Korea (Cho et al., 2008; Lee and Cho, 2003). The location of arc magmatism is determined by where the vertical depth from the surface to the Benioff zone becomes about 110 km regardless of the subducting dip angle of the oceanic plate (Tatsumi, 1989). Thus, subduction dip changes in the oceanic plate are considered a major cause for the migration of the magmatic front in several arcs (Winter, 2002) and the switching of tectonic modes in several active margins (Lister and Forster, 2009 and references therin), which can result in a broad magmatic belt (i.e., Coney and Reynolds, 1977; Li and Li, 2007). U-Pb detrital zircon geochronology of river sediments is a powerful tool for determining the age distribution of igneous rocks located in the catchment area. 5

17 6 Fig A) Mesozoic igneous rock distribution in East Asia (modified after Kee et al., 2010). The boxed area is shown in Fig. 1B. B) Drainage basins of major rivers in South Korea. Sample locations are marked with circles. The shaded area in the southeast represents the Cretaceous Gyeongsang Basin.

18 Compared to ages obtained from granitoid outcrops in the catchment, detrital zircon dating of river sediments may provide their representative age population with additional ages not represented in granitoid outcrops due to removal of granitoids by erosion, cover by young rocks, and human bias. Considering that the Mesozoic granitoids in South Korea generally show arc affinity, U-Pb age analyses on detrital zircon grains of Korean river sediments can detect timing of arc magmatism in the southern Korean Peninsula during the Mesozoic. In this study, detrital zircon U-Pb ages from modern river sediments in South Korea were analyzed by a LA-ICP-MS and were compared to investigate spatial and temporal changes in the Mesozoic arc magmatism and their relationship with the time-varying subduction dip angle of the paleo-pacific plates beneath the Korean Peninsula. 1.2 ANALYTICAL METHODS Zircons were separated from medium- to coarse-grained sands collected near mouths of seven major rivers draining South Korea (the Hantan, Han, Osipcheon, Geum, Seomjin, Nam, and Nakdong rivers; Fig. 1-1B). Detrital zircon grains collected were dated using a laser-ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). The LA- ICP-MS used in this study is a Thermo Elemental PlasmaQuad3 housed at the Earthquake Research Institute, the University of Tokyo. The details of LA-ICP-MS technique and analytical procedure are described in Orihashi et al. (2008) and Lee et al. (2010). To determine whether there is a statistically significant difference between two age 7

19 distributions, the K-S (Kolmogorov-Smirnoff) test (Press et al., 1986) was used, which has been used successfully by Dickinson and Gehrels (2008). A P value is determined by the K-S test and where P > 0.05, one is unable to reject the null hypothesis, with 95% confidence, that two age populations were selected randomly from the same parent population. P values of detrital zircon age distributions of the fluvial sediments in South Korea were obtained. 1.3 RESULTS AND INTERPRETATION Among 700 detrital zircons analyzed, 178 grains yielded concordant or slightly discordant (<15%) Mesozoic U-Pb ages (Fig. 1-2 and Table 1-1). Zircon ages range from 72 ± 3 Ma to 249 ± 8 Ma, with a gap between 120 and 162 Ma, to which only two grain ages belong. All Mesozoic zircons have Th/U ratios larger than 0.2, which indicates that they are of igneous origin (Hoskin and Black, 2000; Fig. 1-3). Probability-density plots of Mesozoic detrital zircon ages of the South Korean river sediments show NW-SE age trends (Fig. 1-2). Triassic and Cretaceous detrital zircon ages become younger towards the southeast, while Jurassic ages become younger toward the northwest. Mesozoic zircon ages of the North Korean river sediments are restricted to the age interval between the Middle Triassic to earliest Jurassic periods and to the mid-cretaceous period, but they do not contribute to the NW-SE temporal trends observed in the South Korean river sediments. The observed NW-SE age trends indicate that the arc magmatism had changed its location, spatially, throughout the Mesozoic era in South Korea. 8

20 9 Fig Probability density plots presenting detrital zircon U-Pb ages of seven major river sediments in South Korea. Probability density plots of Mesozoic detrital zircon ages of North Korean river sediments from Wu et al. (2007b) are also shown for comparison (the top black line).

21 10 Fig Representative SEM-CL images of analyzed Mesozoic detrital zircon grains from South Korean river sediments. The zircon grains generally show euhedral and oscillatory zoning indicating their igneous origin and some grains have apparent inherited cores with dark luminescence. Red circles mark locations of laser ablation and their diameters are 30µm.

22 Table 1-1. U-Pb isotopic data for detrital zircon grains of the South Korean river sediments determined by LA-ICP-MS Sample Th/U 206 Pbc* 207 Pb/ 206 Pb Error 206 Pb/ 238 U Error 207 Pb/ 235 U Error Disc** 238 U- 206 Pb age Error 235 U- 207 Pb age Error Name (%) 2σ 2σ 2σ (%) (Ma) 2σ (Ma) 2σ Hantan River HTG n.d ± ± ± ± ± 14 HTG ± ± ± ± ± 13 HTG ± ± ± ± ± 13 HTG ± ± ± ± ± 8.2 HTG ± ± ± ± ± 13 HTG n.d ± ± ± ± ± 13 HTG n.d ± ± ± ± ± 10 HTG ± ± ± ± ± 6.8 HTG n.d ± ± ± ± ± 25 HTG ± ± ± ± ± 10 HTG n.d ± ± ± ± ± 13 HTG ± ± ± ± ± 5.1 HTG n.d ± ± ± ± ± 9.9 HTG ± ± ± ± ± 10 HTG ± ± ± ± ± 11 HTG ± ± ± ± ± 5.9 HTG n.d ± ± ± ± ± 11 HTG n.d ± ± ± ± ± 28 HTG n.d ± ± ± ± ± 13 HTG ± ± ± ± ± 8.0 HTG n.d ± ± ± ± ± 9.5 HTG ± ± ± ± ± 12 HTG n.d ± ± ± ± ± 6.2 HTG n.d ± ± ± ± ± 16 HTG n.d ± ± ± ± ± 18 HTG n.d ± ± ± ± ± 12 HTG n.d ± ± ± ± ± 16 HTG n.d ± ± ± ± ± 10 HTG n.d ± ± ± ± ± 16 HTG ± ± ± ± ± 15 HTG ± ± ± ± ± 9.2 HTG n.d ± ± ± ± ± 10 HTG n.d ± ± ± ± ± 11 Han River GP n.d ± ± ± ± ± 8.8 GP n.d ± ± ± ± ± 10 GP ± ± ± ± ± 7.4 GP n.d ± ± ± ± ± 26 GP ± ± ± ± ± 12 GP n.d ± ± ± ± ± 12 GP ± ± ± ± ± 8.4 GP ± ± ± ± ± 5.5 GP n.d ± ± ± ± ± 15 GP n.d ± ± ± ± ± 12 GP n.d ± ± ± ± ± 9.0 GP n.d ± ± ± ± ± 8.3 Geum River n.d ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± 15 Osipcheon River OSC n.d ± ± ± ± ± 6.9 OSC ± ± ± ± ± 3.8 OSC ± ± ± ± ± 5.2 OSC ± ± ± ± ± 11 11

23 OSC n.d ± ± ± ± ± 6.4 OSC n.d ± ± ± ± ± 9.6 Seomjin River n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± 6.0 Nam River ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± n.d ± ± ± ± ± ± ± ± ± ± 12 Nakdong River ESD n.d ± ± ± ± ± 12 ESD n.d ± ± ± ± ± 13 ESD n.d ± ± ± ± ± 21 ESD n.d ± ± ± ± ± 8.0 ESD n.d ± ± ± ± ± 7.8 ESD n.d ± ± ± ± ± 9.8 ESD ± ± ± ± ± 17 ESD n.d ± ± ± ± ± 10 ESD ± ± ± ± ± 13 ESD ± ± ± ± ± 7.9 ESD n.d ± ± ± ± ± 13 ESD ± ± ± ± ± 12 ESD n.d ± ± ± ± ± 11 ESD n.d ± ± ± ± ± 6.6 ESD ± ± ± ± ± 5.8 ESD n.d ± ± ± ± ± 8.6 ESD ± ± ± ± ± 8.7 ESD n.d ± ± ± ± ± 21 12

24 ESD n.d ± ± ± ± ± 8.9 ESD ± ± ± ± ± 7.3 ESD ± ± ± ± ± 6.3 ESD ± ± ± ± ± 6.0 ESD n.d ± ± ± ± ± 20 ESD ± ± ± ± ± 6.5 ESD ± ± ± ± ± 6.7 ESD n.d ± ± ± ± ± 9.5 ESD n.d ± ± ± ± ± 13 ESD ± ± ± ± ± 14 ESD n.d ± ± ± ± ± 34 ESD ± ± ± ± ± 11 ESD n.d ± ± ± ± ± 7.6 ESD n.d ± ± ± ± ± 7.8 ESD n.d ± ± ± ± ± 15 ESD ± ± ± ± ± 5.7 ESD n.d ± ± ± ± ± 5.6 * Percentage of 206 Pb contributed by common Pb on the basis of 204 Pb. Value of common Pb was assumed by Stacey and Kramers (1975) model; n.d. : no detection of 204 Pb. ** Degree of discordance (%); negative numbers and blanks show normal discordant and concordanct within 2σ of the analytical error, respectively. 13

25 Table 1-2. K-S test P values of detrital zircon age distributions of the fluvial sediments in South Korea. P values in bold and yellow background are larger than 0.05, which denies a statiscally significant differences between two distribution (Guynn, 2006). Hantan River Han River Geum River Seomjin River Nam River Nakdong River Osipcheon River Hantan River Han River Geum River Seomjin River Nam River Nakdong River Osipcheon River

26 Based on the P values of the Korean river sediments, as determined by the Kolmogorov-Smirnoff (K-S) test, the detrital zircon age distributions of the studied river sediments can be subdivided into three different groups of rivers; 1) the Hantan, Han and Geum rivers, 2) the Seomjin, Nam, and Nakdong rivers, and 3) the Osipcheon River (Fig. 1-1B and Table 1-2). Major differences in the zircon age populations between rivers of groups 1 and 2 are associated with the presence of Early Cretaceous age population and a magmatic gap from Late Triassic to Early Jurassic times in group 1 rivers (Fig. 1-2). Such grouping indicates spatial difference in the Mesozoic igneous rock distribution, considering that drainage areas of the rivers in groups 1 and 2 occupy northwestern and southeastern parts of the southern Korean Peninsula, respectively. 1.4 DISCUSSION The Nam and Nakdong rivers flow through the nonmarine Cretaceous sedimentary basin, the Gyeongsang Basin, in the middle to lower reaches (Fig. 1-1B), which can supply recycled Mesozoic zircon grains to these rivers. The Gyeongsang Basin sediments were derived from Precambrian metamorphic rocks, Paleozoic sedimentary rocks, and Mesozoic intrusive rocks distributed in the west and northwest of the basin (Koh, 1986). Thus the Triassic and Jurassic zircons in the Nam and Nakdong river sediments may provide information on the Mesozoic basement rocks adjacent to the Gyeongsang Basin. Detrital zircon ages, corresponding to the Triassic transitional granitoids having ages of Ma (Cho et al., 2008; Kim et al., 2011a; Kim et al., 2011b) are observed in the Han, Geum and Seomjin river sediments (Fig. 1-2). These 15

27 Triassic granitoids, distributed in the catchment areas of the Han, Geum and Seomjin rivers, do not belong to arc magmatic rocks, and thus their ages are not dealt with in the discussion. The southeastward younging trend of the Triassic zircon ages ( Ma) in the southern Korean Peninsula suggests that the arc magmatism migrated trenchward during this time interval, whereas the Triassic magmatism in North Korea occurred at Ma. The tectonic affinity of Triassic granites in North Korea is not known, but their tectonic setting can be inferred from similar-aged Triassic granitoids of extensional origin in adjacent northeastern China (Yang et al., 2005). Assuming that the Triassic granitoids in North Korea were also formed in the extensional regime, the spatial distribution of Triassic detrital zircons in the southern Korean Peninsula river sediments suggests a rollback of the paleo-pacific plate during the Triassic period, which resulted in the trenchward arc migration from the central part of the Korean Peninsula (Fig. 1-4A), to which the drainage area of the Hantan River belongs, towards the southeast and probable back-arc magmatism in central western South Korea (transitional granitoids), North Korea, and northeastern China. The Jurassic northwestward-younging trend ( Ma) of the detrital zircon ages in South Korea suggests inlandward migration of arc magmatism, probably caused by subduction-dip shallowing of the paleo-pacific plate with time. The age distribution of Jurassic granitoids in South Korea has been similarly interpreted as a result of low-angle subduction with a high convergence rate (Sagong et al., 2005). Detrital zircons of North Korean river sediments do not have Jurassic ages, suggesting that Jurassic arc 16

28 magmatism did not migrate into North Korea. Upper Mesozoic igneous rocks, including ca Ma Jurassic granites, are distributed in the Shandong Peninsula of eastern China, west of central Korea (Fig. 1-1A; Hou et al., 2007; Wang et al., 1998). Such distribution of upper Jurassic granites in the Shandong Peninsula may indicate that the Shandong area is the place where the most inland-directed arc magmatism occurred in the Jurassic East Asian continental margin (Fig. 1-4B). The southeastward younging trend in the Cretaceous detrital zircon-age distribution of the South Korean river sediments ( Ma) suggests trenchward arc migration (Fig. 1-4C). Cretaceous igneous rocks in the East Asian continental margin, including South Korea, eastern China, and southwestern Japan, show a southeastward younging trend in their emplacement/extrusion ages, which is interpreted to have been caused by slab rollback or trench retreating (Sagong et al., 2005; Uyeda and Miyashiro, 1974; Wu et al., 2007a). The timing of arc retreating in South Korea is consistent with that of a reduced convergence rate of the Izanagi plate due to its highly oblique subduction (Maruyama et al., 1997). In the Shandong Peninsula, the distributed Lower Cretaceous igneous rocks have peak magmatic ages of ca Ma, which are interpreted as having been formed by the subduction of the Izanagi and Pacific plates (Qiu et al., 2002; Wang et al., 1998). From the Liaodong Peninsula, northeastern China, granites Ma in age were reported, but their emplacement occurred under the extensional tectonic regime (Wu et al., 2005). Extension in the continental inland area and trenchward arc migration in the continental margin support the interpretation of rollback of the subducting oceanic plate during the Cretaceous period. 17

29 1.5 TECTONIC IMPLICATIONS The Mesozoic arc migration trends in the southern Korean Peninsula suggest changes of paleo-pacific plate subduction; slab rollback in the Triassic and Cretaceous periods and slab-dip shallowing in the Jurassic period. Migration of arc magmatism during the Mesozoic era has also been reported in South China: Triassic inland-directed arc migration of ca km by flat-slab subduction and subsequent trenchward arc retreating by slab foundering and rollback in the Jurassic period (Li and Li, 2007). Oceanic plateau subduction was a suggested cause for the Triassic flat-slab subduction associated with magmatic belt development in South China, by Li and Li (2007). According to Maruyama et al. (1997) and Kinoshita (1995), however, the Farallon- Izanagi plate ridge was subducted beneath South China ca. 250 Ma. Thus, the Triassic inlandward arc migration in South China might have occurred by either subduction of the Farallon-Izanagi plate ridge or subduction of an oceanic plateau. In contrast, Triassic zircon age distributions of this study imply a possible slab rollback in South Korea in this period. Thus, when the flat-slab subduction occurred in South China, the cold-slab subduction of the Farallon Plate might take place beneath the Korean Peninsula, resulting in the subduction angle increase and resultant trenchward retreating by the slab rollback (Fig. 1-4A). The Jurassic inland-directed arc migration observed in South Korea also seems to have been caused by the subduction of either a ridge or an oceanic plateau. The Farallon- Izanagi plate ridge subduction began at ca. 250 Ma at the South China margin and its 18

30 19 Fig Landward limits of the Mesozoic magmatic front of the paleo-pacific plates in the East Asian continental margin during A) the Triassic period, B) the Jurassic period, and C) the Cretaceous period (Li and Li, 2007 for arc migration in South China). The arrows represent the migration direction of the arc magmatism front.

31 subduction location moved northeastward along the trench (Kinoshita, 1995; Maruyama et al., 1997). Kinoshita (1995) proposed that the location of the ridge subduction was beneath the Korean Peninsula at ca. 180 Ma, while Maruyama et al. (1997) suggested that the ridge subduction was located beneath the Korean peninsula before the Jurassic period. If Kinoshita (1995) s interpretation is correct, the Jurassic arc magmatism in South Korea might have migrated in an inland direction due to the subduction of the Farallon-Izanagi ridge and associated young and hot oceanic plates with relatively shallow subduction angles. As the ridge subduction location migrated northeastward, away from the South China continental margin, the subduction angle of the Izanagi Plate became large in South China since ca. 180 Ma (Li and Li, 2007; Zhou and Li, 2000), resulting in trenchward arc migration (Fig. 1-4B). However, the Jurassic ridge subduction beneath the Korean Peninsula cannot explain contemporaneous accretion of the Paleozoic greenstone and limestone in Southwest Japan (Kimura, 2000), which has been located close to the Korean Peninsula since the Jurassic period (Lee, 2008). Thus, the Farallon-Izanagi plate ridge subduction might have passed from the Korean Peninsula before the Jurassic period, as Maruyama et al. (1997) suggested, and may not be a cause of the flattening of the slab-dip in South Korea during the Jurassic period. Previous studies have revealed that several Jurassic oceanic plateaus on the Izanagi Plate were subducted beneath the East Asian continental margin after the Farallon- Izanagi ridge had passed northeastwards (Koizumi and Ishiwatari, 2006). The Jurassic accretionary complex (Tamba terrane) distributed in southwestern Japan includes oceanic plateaus accreted during the Early to Late Jurassic period (Koizumi and 20

32 Ishiwatari, 2006), which is consistent with the time period of inland-directed arc migration in South Korea. Thus, subduction of oceanic plateau seems to be a more probable cause for the subduction angle shallowing of the Izanagi Plate in South Korea and the Jurassic inland-directed arc advance. In the Cretaceous period, another paleo-pacific plate ridge had subducted in the East Asian continental margin; the Izanagi-Pacific plate ridge subduction occurred in southwestern Japan at ca Ma (Maruyama et al., 1997). Although such ridge subduction caused extensive magmatism during this period in the southeastern Korean Peninsula and southwestern Japan (Kinoshita, 1995; Uyeda and Miyashiro, 1974), the subducting slab was rolled back continuously during the Cretaceous period in the East Asian continental margin (Chang et al., 2000; Wu et al., 2007a; Fig. 1-4C). It is likely that the subducting ridge was segmented by many transform faults, which might have resulted in a short period of ridge subduction in a certain region and the subsequent northeastward shifting of the ridge subduction location with time. Thus, the Izanagi- Pacific ridge subduction might have had little effect on the slab rollback of the Izanagi and Pacific plates. In summary, the Mesozoic arc migration in the East Asian continental margin was caused by changes in the subducting conditions of the paleo-pacific plates, between slab rollback during the cold-slab subduction and shallow-dip subduction during subduction of the plate ridge or oceanic plateau. Along the trench, it is noted that temporal and spatial changes in subduction angles of the oceanic plates took place due to their buoyancy changes, which resulted from the approach of a ridge and associated hot 21

33 plates or an oceanic plateau, and from shifting subduction locations of these buoyant materials. 1.6 CONCLUSIONS U-Pb geochronological analyses of detrital zircon grains from the major South Korean river sediments yielded Mesozoic magmatic ages ranging from 72 ± 3 Ma to 249 ± 8 Ma. The relationship between the age distributions of detrital zircons in the rivers sediments and their drainage areas shows temporal arc migration trends in the Korean Peninsula during the Mesozoic era. Triassic trenchward arc migration ( Ma) in South Korea and coeval magmatism in extensional settings in North Korea and northeastern China are indicative of the rollback of the oceanic plate. This trenchward migration gradually changed to an inland-directed advance of arc magmatism in the Jurassic period ( Ma) as a result of shallow angle subduction of the oceanic plates to the central part of the Korean Peninsula and the Shandong Peninsula in northeastern China. The Cretaceous cold-slab subduction of the Izanagi Plate caused a slab rollback and the retreat of the arc front in a trenchward direction, between Ma. Subduction modes of the paleo-pacific plates might have been changed by buoyancy differences associated with the subduction of the Farallon-Izanagi plate ridge or oceanic plateaus, the shifting of their subducting location with time during the Triassic and Jurassic periods, and subduction of the cold Izanagi and Pacific plates during the Cretaceous period. 22

34 2. THERMAL HISTORIES OF CRETACEOUS BASINS IN KOREA 2.1 INTRODUCTION Subduction of paleo-pacific plates beneath the East Asian continent has caused major tectonic events in the continental margin setting since the Mesozoic. There were arc magmatism, orogeny, and formation of strike-slip faults and their related basins during the Mesozoic by the change of the subduction direction or dip of the paleo- Pacific plate (Maruyama et al., 1997). Tectonics of the Korean Peninsula, which has been located at the eastern margin of the Asian continent since the Mesozoic, can be characterized by orogenies due to the orthogonal subduction of the paleo-pacific plate during the Triassic Late Jurassic and by extensional/transtensional regime due to the oblique subduction of the oceanic plate during the Late Jurassic Early Cretaceous (Chang, 1995). During the Early Cretaceous, the northward oblique subduction of the paleo-pacific (Izanagi) plate has induced sinistral shearing in the overriding continental plate, resulting in the creation of many coeval NE-SW trending strike-slip basins at the East Asian continental margin including the Korean Peninsula (Lee, 1999a; Okada and Sakai, 1993). In Korea, the Gyeongsang Basin, the largest Cretaceous basin in Korea, was formed occupying its southeastern part, whereas smaller strike-slip basins (Pungam, Eumseong, Gongju, Buyeo, Yeongdong, Jinan, Neungju and Haenam basins) were located along both the northwestern and southeastern boundaries of the NE-SW trending Okcheon Fold Belt (Fig. 2-1). The formation mechanism and evolution of the Gyeongsang Basin are not yet fully understood, whereas those of the other Cretaceous 23

35 Fig The distribution of the Cretaceous basins in the southern Korean Peninsula (modified from Lee 1999). 1: Gyeongsang Basin, 2: Pungam Basin, 3: Eumseong Basin, 4: Gongju Basin, 5: Buyeo Basin, 6: Yeongdong Basin, 7: Jinan Basin, 8: Neungju Basin, 9: Haenam Basin. Most of them (except for 1 & 2) have pull-apart origin. 24

36 basins are relatively well recognized (Chun and Chough, 1992; Lee, 1999a). In the Late Cretaceous, the subduction direction of the Izanagi Plate changed from north to northwest (Lithgow-Bertelloni and Richards, 1998), resulting in the basin inversion due to transpression. Except for the Gyeongsang Basin and the transpressional Pungam Basin, the Cretaceous basins experienced similar evolution: 1) formation as a pull-apart basin, 2) sediment accumulation with intermittent volcanic activities, and 3) compression by transpression (Lee, 1990). The purposes of this paper are to characterize the thermal history of Cretaceous sedimentary basins in Korea and to discuss the tectonic evolution of the active East Asian continental margin during the Late Cretaceous. Among the Cretaceous sedimentary basins, thermal histories of the Gyeongsang Basin (Lim and Lee, 2005; Lim et al., 2003) and the Pungam and Yeongdong basins (Choi and Lee, 2006) were studied by fission track (FT) analysis. In this contribution, the thermal history of the Jinan Basin (Fig. 2-1) was investigated by illite crystallinity (IC) and FT analyses. Then, the analytical results of the Jinan Basin are compared with those of the other Cretaceous basins in Korea, of granitoids in northeastern China in the inland side, and of the Japanese accretionary complexes in the ocean side to understand how the East Asian continental margin responded to the subduction of the Izanagi Plate. 2.2 JINAN BASIN GEOLOGICAL SETTING OF THE JINAN BASIN 25

37 The Jinan Basin is located in the southwestern Korean Peninsula (Fig. 2-1) along the southern boundary (Yeongdong-Gwangju fault system) of the NE-trending Okcheon Belt which was presumably formed by closing of a Late Precambrian failed continental rift during Permo-Triassic time (Cluzel, 1992). The Jinan Basin is ca. 18 km wide and 32 km long in the NE-SW direction, and occupies an area of ca. 580 km 2 (Lee, 1992). The basinfill of the Jinan Basin, the Jinan Group, is underlain by Precambrian gneiss and Jurassic granite, and was intruded by Upper Cretaceous andesite after the closing of the Jinan Basin (Lee, 1992). Isotopic ages of the intrusive andesite are Ma (whole rock K-Ar: Ma, zircon FT: 70 Ma, and apatite FT: 69 Ma; Shin and Jin, 1995). The depth to the basement in the southeastern part of the basin is deeper than that in the northwestern part and thickness of the sedimentary rock reaches up to 1.5 km (Baag and Kwon, 1994). The Jinan Group consists of four stratigraphic units: the Mandeoksan, Dalgil, Sansudong, and Maisan formations (Fig. 2-2). The Mandeoksan Formation (ca. 800 m thick) comprises mainly sandstones, whereas the Dalgil and Sansudong formations (ca. 600 m thick each) are mostly composed of shale. The Maisan Formation (ca m thick) consists of clast-supported conglomerates (Lee, 1992). Lee (1992) interpreted that four lithostratigraphic units of the Jinan Group were deposited coevally. The depositional age of the Jinan Group was reported to be Barremian (ca. 120 Ma; Yi et al., 1998). Lee (1992) interpreted that the Jinan Group was deposited in alluvial fan, fan delta, alluvial plain and lacustrine settings, and was uplifted in the Late Cretaceous by transpression due to the northwestward subduction of the Izanagi Plate. The present altitude of the Jinan Basin sediment is higher (ca. 400 m) 26

38 Fig Geological map of the Jinan Basin, Korea (modified from Lee 1992). Apatite FT sample locations are marked by filled circles with sample numbers and IC sample locations by open circles with KI values. 27

39 than that of its surrounding basement rocks, and flower structures are well observed in the Jinan Group (Lee, 1992). Based on a pollen alteration study on the Sansudong Formation, Yi et al. (1998) suggested that the maximum temperature of the Jinan Basin sediment was higher than 200 C EXPERIMENTAL METHODS IC analysis was carried out to constrain the maximum paleo-temperature of the Jinan Group. Eight shale samples from the Sansudong and Dalgil formations were crushed, ultrasonically disaggregated and centrifuged. The <2 µm fraction treated with ethylene glycol at 60 C for 8 hours was analyzed using a Rigaku Model D/Max-3c diffractometer with Ni-filtered CuKα radiation through θ with a scan speed of 0.5 2θ/min at operating conditions of 40 kv/30 ma. The measured Kübler Index (KI) values were calibrated by a Crystallinity Index Standard (CIS) scale using standards of Warr and Rice (1994). For apatite FT analysis one sandstone sample from the Mandeoksan Formation, and one conglomerate and four granite cobble samples from the Maisan Formation, each weighing about 5-10 kg, were collected (Fig. 2-2). The conventional heavy mineral separation was carried out using heavy liquid and magnetic techniques. Apatite grains were handpicked and mounted in epoxy resin. They were polished with diamond paste and etched chemically in 0.6% HNO 3 at 32.0±0.5. Apatite grains were dated by the external detector method, counting induced tracks in high-quality muscovite external detectors. Thermal neutron irradiation was carried out at NAA-1 facility in the 28

40 HANARO reactor of the Korea Atomic Energy Research Institute, and fluences were monitored by counting tracks in muscovite external detectors over NIST SRM612 glasses. This facility has a high Cd ratio of 250 for Au (Lim and Lee, 2000), which satisfies the criteria recommended by Hurford (1990). After irradiation, the external muscovite detectors were detached and etched in 48% HF at 32±0.5 for 4 min. Fission tracks were counted using a Nikon Optiphot-II microscope with a dry 100 objective and a total magnification of A computer-automated microscope stage system (Dumitru, 1993) was used to translate between sample and detector. The zeta calibration approach (Hurford, 1990; Hurford and Green, 1983) was used with a ζ value of 351.4±20.3 (2σ) for apatite (Choi and Lee, 2006). The conventional central age, Poissonian error, and P (χ 2 ) (Galbraith, 1981; Green, 1981) were calculated using a ZETAAGE program (Brandon, 1996). To constrain cooling patterns of Jinan Basin sedimentary rocks, confined FT lengths were measured on the apatite mineral surfaces which were polished parallel to the crystallographic c-axis (i.e., Laslett et al., 1982). The AFTSolve program (Ketcham et al., 2000) was used for modeling the cooling history. As apatite FT annealing characteristics are known to be controlled by chemical composition (Green et al., 1986), F and Cl contents of dated apatite grains were determined using a JEOL JXA-8900R electron microprobe with wavelength dispersive system RESULTS 29

41 The analyzed samples (<2µm) are mainly composed of illite, chlorite and quartz (Fig. 2-3). Measured KI values range between 0.15 and 0.43 (Fig. 2-2), corresponding to diagenetic to epizone (c.f. Kübler and Jaboyedoff, 2000). KI values do not show any specific basin-wide trend, but three samples in the Sansudong Formation have relatively low KI values. 94 apatite grains from six samples were analyzed and the apatite FT data are presented in Table 2-1. The radial plots of apatite single grain ages are displayed in Figure 2-4. All apatite FT ages are concordant (within 1σ error limit) at ca. 68 Ma with narrow and unimodal population. All samples pass the χ 2 test at a probability P (χ 2 ) > 5% (Galbraith, 1981; Green, 1981), which means that all age components belong to a single age group. In all samples, track lengths of apatite grains show a unimodal distribution and mean track length ranges between 11.8 and 12.4 µm with a standard deviation between 1.9 and 2.3 µm (Fig. 2-4). Apatite grains have low Cl concentrations ranging from 0.0 to 0.1 wt.% (average: 0.05 wt.%) while F concentrations ranging from 3.5 to 4.0 wt.% (average: 3.7 wt.%), which indicates that the analyzed Jinan Basin apatites have little variations of chemical composition and are classified as fluorapatite, usually included in granitic rocks INTERPRETATION KI values and maximum burial temperatures The KI values of the Jinan Basin sediments do not seem to be related with the present volcanic rock distribution. There is little evidence of significant hydrothermal 30

42 Table 2-1. Apatite fission track analytical results of the Jinan Basin. Sample No. of Spontaneous track Induced track Dosimeter glass Rock* γ P(χ 2 ) Age ± 1σ Track length data code grains Ns ρ s Ni ρ i Nd ρ d (%) (Ma) N length(µm) 1σ (µm) Gp ± Gp ± Gp ± Ss ± Gp ± Cg ± All analyses by external detector method using 0.5 for the 4π geometry factor. * Ss: sandstone, Gp: granite cobble in conglomerate, Cg: conglomerate. Ns=Number of spontaneous tracks counted to determine ρs; ρs=density of spontaneous tracks ( 10 6 /cm 2 ); Ni=Number of induced tracks counted in a muscovite external detector to determine ρi; ρi=density of induced tracks in a sample ( 10 6 /cm 2 ); Nd=Number of induced tracks counted in a muscovite detector to determine ρd; ρd=density of induced tracks in NIST-SRM612 dosimeter glass ( 10 6 /cm 2 ); γ=correlation coefficient between ρs and ρi; P(χ 2 )=the probability of obtaining χ 2 value for n degrees of freedom where n= No. of grains -1; N=number of measured track lengths 31

43 Fig Representative X-ray diffraction patterns for <2µm, air-dried samples from the Jinan Basin. Peak intensity is measured in counts per second (CPS). Ch: chlorite; I: illite; Q: quartz; I+Q: illite+quartz; KI: Kübler Index. 32

44 33 Fig Radial plots (Galbraith, 1990) and track length distributions of apatite FT analytical results of the Jinan Basin. All FT ages are central ages with an error of ±1σ. For radial plots, the position on the x-scale records the uncertainty of individual age estimates, whilst each point has the same standard error on the y-scale (illustrated as ±2σ). The age of each grain can be determined by extrapolating a line from the origin on the left through the grain s x, y co-ordinates to intercept the radial age scale. The further the data point plots from the origin, the more precise the measurement. Shaded zone represents the depositional age of the Jinan Basin.

45 alteration, suggesting that the main thermal influence on the Jinan Basin sediments seem to have been related to burial in association with subsidence. Three samples have quite lower KI values ( ) than the others ( ) which might have been affected by higher than normal thermal alteration. The former seems to be related to the depth of burial, whereas the latter to enhanced thermal alteration due to an extra heat source or to mixing populations between diagenetic and detrital illite. If KI values had been affected by Upper Cretaceous volcanism, the lower KI values should indicate paleotemperature influenced by a heating event, whereas the higher KI values may represent the influence of burial depth. However, two samples collected from locations short distance apart (~1 km) show quite large difference in KI values (0.43 and 0.15 in Fig. 2-2), which can be interpreted that these two samples were not affected by the same thermal event. Rather, the sample with the low KI value might have been caused by the mixing of detrital illite. Thus, the samples with the lower KI values ( ) are not dealt with in the evaluation of maximum paleotemperatures. The higher KI values ( ) used in this study correspond to the upper diagenetic zone to lower anchizone (Kübler and Jaboyedoff, 2000). IC is not only a function of temperature but probably also of other factors such as fluid pressure, stress, lithology, etc. (Frey, 1987). Thus, relationship between IC and temperature derived from the other basins cannot be applied for calibration of maximum temperature in the Jinan Basin estimated from their IC values. The relationship between IC and maximum paleotemperature derived by Underwood et al. (1993) and Kosakowski et al. (1999) was proposed by considering correlations of IC not with 34

46 measured temperature but indirectly with other temperature-sensitive indicators such as vitrinite reflectance, although correlations between vitrinite reflectance and temperature also have uncertainties in addition to that of correlations between vitrinite reflectance and IC (Kübler and Jaboyedoff, 2000). Accordingly, there is no proof that application of the calibration between IC and temperature to the Jinan Basin provides accurate paleotemperature. However, even such rough approximations still help to assume maximum temperature input necessary for thermal modeling in the following section. Underwood et al. (1993) evaluated correlation between KI values and maximum temperature for condition that have moderately high geothermal gradient and relatively rapid heating event (ex: Cenozoic Shimanto Belt), whereas Kosakowski et al. (1999) introduced the equation for hydrothermal systems by combining previous IC values. Note that the Kosakowski et al. (1999) s formula was calibrated to standard sample of Frey (1987) and represents more reasonable paleo-temperatures in a lower temperature zone than that of Underwood et al. (1993), but the former did not use CIS scale whereas the latter did. From the available IC values ( ), Kosakowski et al. (1999) s and Underwood et al. (1993) s calibrations provide temperature ranges of C (mean: 207 C) and C (mean: 287 C), respectively. Because the Cretaceous Korean Peninsula is known to have relatively high geothermal gradient and the Jinan Basin seems not to have been experienced significant hydrothermal alterations (c.f. Lee and Lee, 2001), paleotemperatures estimated from the Underwood et al. (1993) s equation may be more applicable for the purpose of this study. A mean paleotemperature of ca. 287 C is in good agreement with that of the fossil-pollen alteration study of Yi et al. 35

47 36 Fig Representative thermal history of the Jinan Basin calculated by apatite FT length modeling results of the Jinan Basin (using a AFTSolve program of Ketcham et al. 2000). The histogram of FT length represents the measured data of each sample. n: number of grains; GOF: goodness of fit. The cooling curves were constructed by apatite FT length modeling and the heating curves by depositional age (120 Ma) and maximum paleo-temperature ( C) at 105 Ma.

48 (1998) and is well above the apatite partial annealing zone (APAZ) temperature (ca C; Gallagher et al., 1998). Thermal history of the Jinan Basin All samples show high P (χ 2 ) values (>88%) and ages much younger than the depositional age of the Jinan Basin. This suggests total annealing of inherited tracks during basin evolution, considering maximum paleotemperature higher than the apatite closure temperature (ca. 100 C; Donelick, 1991). Because temperature is the most important factor of track annealing (Gallagher et al., 1998), the Jinan Basin apatite FT ages indicate that the sediments were cooled below the apatite FT closure temperature at ca. 68 Ma. This age is older than the average apatite FT age of widely distributed Jurassic granites in South Korea (ca. 59 Ma; Jin et al., 1987), which means cooling of the Jinan Basin sediment earlier than most areas of the southern Korean Peninsula. Mean confined track length of the Jinan Basin apatites is shorter than 12.5 µm and their track lengths are broadly distributed (Fig. 2-4). Such length distribution and mean track length are not like those of a fast cooling case which is characterized by narrow length distribution and a long mean track length (>14 μm; Green et al., 1989). Inverse thermal modeling from combining confined FT lengths with FT ages was carried out to infer a detailed cooling pattern. For thermal modeling of the Jinan Basin, the depositional age of ca. 120 Ma and the maximum paleo-temperature of C (325 C is limit of maximum temperature in the modeling program) were used as constraints. The timing of the maximum paleotemperature was constrained according to 37

49 Xie and Heller (2009), who showed that a typical tectonic subsidence pattern of strikeslip basins is characterized by short life span (<10 m.y.) and subsiding depth of 2-4 km. Based on this information, it was assumed that the Jinan Basin underwent subsidence up to 10 m.y. after its formation and that it resided at the maximum burial depth with burial heating for ca. 5 m.y. The thermal history of the Jinan Basin was modeled using maximum burial between 110 Ma and 100 Ma. 105 Ma was used as the maximum burial timing, whose result is similar to those using 110 and 100 Ma. The result suggests two cooling episodes with a prolonged residence in APAZ in between (Fig. 2-5). Thermal modeling result reveals rapid and short-lived deep subsidence after deposition, rapid cooling down to APAZ during Ma, and slow cooling after 30 Ma. This result suggests that the Jinan Basin sediment experienced burial heating by tectonic subsidence and sediment loading. The cooling rate of the Jinan Basin sediments during Ma is estimated to be ca. 11 C/Ma, corresponding to the exhumation rate of ca. 0.3 mm/yr, assuming a geothermal gradient of the Korean Peninsula during the Late Cretaceous being ca. 35 C/km (e.g. Lim and Lee, 2005) due to active continental arc volcanism. This result agrees with the narrow age range of intrusive andesite (76-69 Ma) in the Jinan Basin, suggestive of their rapid cooling in shallow crustal level. 2.3 THERMAL HISTORY OF EAST ASIAN CONTINENTAL MARGIN THERMAL HISTORY OF THE OTHER CRETACEOUS BASINS IN KOREA To understand the regional tectonism, thermal history of the Jinan Basin is compared with those of the other Cretaceous basins in Korea. Brief geological settings and thermal 38

50 histories of the other studied Cretaceous basins are summarized here. More detailed information on thermal history of each basin can be referred to corresponding references (see below). Gyeongsang Basin The Gyeongsang Basin is known to be formed by the oblique subduction of the Paleo-Pacific plate beneath the Eurasian Plate, but its formation mechanism is not yet clearly recognized (Lee, 1999a). Its basin-fill, the Gyeongsang Supergroup, comprises three groups, the Sindong, Hayang, and Yucheon groups in stratigraphically ascending order. During the early stage of the basin evolution, the Sindong Group was deposited (Aptian-Albian; Lee et al., 2010) along the western boundary of the Gyeongsang Basin with a thickness of ca m (Chang, 1975). The deposition of the Hayang Group was followed by sporadic volcanism and eastward basin extension (Chang, 1975). Then, the Yucheon Group, composed dominantly of volcanic rocks with subordinate volcaniclastic rocks, was deposited in the Late Cretaceous by the climactic continental arc magmatism (Lee et al., 1987). Thermal history of the Gyeongsang Basin was studied by Shin and Nishimura (1993), Lee and Lee (2001), Lim et al. (2003), and Lim and Lee (2005). Shin and Nishimura (1993) analyzed apatite, zircon, and sphene FT ages of Jurassic granites in the basement of the Gyeongsang Basin and suggested slow uplift (mean rate: ~0.04 mm/yr) of the southeastern Korean Peninsula from the Jurassic to the Cretaceous. Lee and Lee (2001) interpreted that Gyeongsang Basin sediment experienced the metamorphic grade of late 39

51 40 Fig Thermal histories and apatite FT ages of the Cretaceous basins and Palgongsan Granite in the Korean Peninsula (Pungam and Yeongdong basins: Choi and Lee, 2006; Palgongsan Granite: Lim and Lee, 2005; Gyeongsang Basin: Lim et al., 2003). Solid dots are FT ages and dashed lines represent thermal history inferred for Cretaceous basins. Solid line indicates thermal history of the Jinan Basin based on apatite FT time-temperature modeling. Note that the age-based thermal history of the Jinan Basin is quite different from that of modeling result.

52 diagenetic zone to high anchizone by both burial and emplacement of Upper Cretaceous plutonic rocks. According to a FT study by Lim et al. (2003) on Sindong Group sandstones, the Sindong Group was heated into the zircon partial annealing zone until ca. 80 Ma and cooled below the apatite closure temperature at ca. 60 Ma. Lim and Lee (2005) studied the Upper Cretaceous Palgongsan Granite, the largest pluton located in the Middle Western part of the Gyeongsang Basin. It shows a simple cooling pattern with ca. 53 and 65 Ma of apatite and zircon FT ages, respectively, suggestive of cooling by regional uplift and exhumation processes (Lim and Lee, 2005). Thermal history of the Cretaceous granites in the Korean Peninsula provides information about the depth of the Sindong Group in the Gyeongsang Basin. They intruded into the Gyeongsang Basin during Ma with emplacement depths of less than 3 km (Cho and Kwon, 1994). For example, the thermal history of the Cretaceous Palgongsan Granite, which intruded into the Sindong and Hayang groups at 84 Ma (Jwa and Choi, 2002) at a depth of ca. 2.8 km (Cho and Kwon, 1994), provides information about the detailed thermal history of the Sindong Group. After emplacement the Palgongsan Granite was rapidly cooled down to the apatite FT closure temperature until ca. 53 Ma due to the large thermal contrast with the country rocks (Lim and Lee, 2005). This means that the current outcrop level of the Sindong Group of the Gyeongsang Basin was situated at shallow crustal depth level of ca. 3 km around 84 Ma and stayed there until the Miocene (fig. 4 of Lim and Lee, 2005). Assuming the geothermal gradient of the Sindong Group in Late Cretaceous time being ca. 35 C/km and temperature equilibrium between the Palgongsan Granite and the Gyeongsang Basin at 41

53 ca. 53 Ma, the corrected thermal history of the Sindong Group is suggestive of rapid cooling prior to 84 Ma (Fig. 2-6). Yeongdong Basin The Yeongdong Basin is a pull-apart basin located in the central part of South Korea (Fig. 2-1) and occupies an area ca. 40 km long and 12 km wide (ca. 540 km 2 ; Lee et al., 1991). Kim and Hwang (1986) subdivided the sediment fill (ca m thick) into the Mangyeri, Saniri, Dongjeongri, Baekmasan, Wonchonri, and Myeongyundong formations. The Mangyeri, Saniri, and lower Dongjeongri formations belong to Neocomian age (ca Ma), while the upper Dongjeongri, Baekmasan, Wonchonri, and Myeongyundong formations are considered to be Aptian to Albian in age (ca Ma; Chun et al., 1993). Thermal history of the Yeongdong Basin was studied by Choi and Lee (2006) using apatite and zircon FT analyses on coarse sandstones from the Dongjeongri, Baekmasan and Myeongyundong formations and basement granite. Zircon FT ages of the Yeongdong Basin range from ca. 83 to 64 Ma, but apatite FT ages show consistency at ca. 63 Ma. Based on little evidence of hydrothermal alteration of the basinfill, the Yeongdong Basin is interpreted to have been heated to the zircon partial annealing zone during burial and cooled down to the apatite FT closure temperature at 63 Ma (Fig. 2-6). Pungam Basin 42

54 The Pungam Basin is located in the eastern central Korean Peninsula (Fig. 2-1). Pungam Basin sediment is m thick and occupies an area ca. 7 km wide and 20 km long in the NE-SW direction. Its evolutionary history is quite different from the other Cretaceous basins in the Korean Peninsula. The Pungam Basin was formed as a transpressional basin at a gentle restraining bend developed along the Gongju- Eumseong Fault System (Lee, 1998). Alluvial fan and lacustrine environments dominated during deposition of the basin-fill, but its upper part was mostly eroded out during exhumation (Cheong and Kim, 1999). The stratigraphy and depositional age of the Pungam Basin are not established well. Radiogenic isotope age data (84-70 Ma by whole-rock K-Ar dating; Kim, 1998) from the intruded andesite and volcanic pebbles in sedimentary rocks constrain the depositional age of the sediment indirectly and imply continuous volcanic activity during the basin evolution (Cheong and Kim, 1999). According to the thermal history reconstruction of the Pungam Basin by Choi and Lee (2006), zircon FT ages of the Pungam Basin range from ca. 89 to 70 Ma, while FTs of apatite were totally annealed and cooled below ca. 100 C at ca. 50 Ma (Fig. 2-6). Such apatite FT age is younger than any of those of the Korean Cretaceous basins, which suggests that they were probably affected by Cenozoic regional exhumation of the Korean Peninsula (i.e. Han, 2002) UPLIFT OF THE CRETACEOUS BASINS IN KOREA AND SUBDUCTION DIRECTION CHANGE OF THE OCEANIC PLATE 43

55 Thermal histories of the studied Korean Cretaceous basins show subtle discrepancy in their apatite FT ages (Fig. 2-6). Apatite FT age of widely distributed Jurassic granites in Korea provides a proper reference of regional exhumation to compare such ages. Jurassic granites have consistent mean apatite FT age of ca. 59 Ma regardless of the distribution, except for those in the easternmost Korean Peninsula uplifted in the mid- Cenozoic (Jin et al., 1987). Their similarity of apatite FT ages with the Sindong Group in the Gyeongsang Basin and their exposure during the Cretaceous time (Shin and Nishimura, 1993) indicate that these Jurassic granites might have experienced regional exhumation in the Late Cretaceous and their thermal history might be analogous with that of the Sindong Group, which is interpreted to have been cooled by exhumation (Lim and Lee, 2005). Thermal histories of the Yeongdong and Pungam basins should not be directly compared with those of the Jinan and Gyeongsang basins because they are based on FT ages only and these ages might not indicate timing of a certain thermal event. Hence, their thermal histories were inferred by circumstantial evidence. The cooling history of the Yeongdong Basin sediment, whose apatite FT age and geographic location are in the middle between those of the Jinan Basin and the Sindong Group of the Gyeongsang Basin, is supposed to be similar to that of the Jinan and Gyeongsang basins. The Pungam Basin has an apatite FT age that is not related to those of the other Cretaceous basins in the Korean Peninsula because this transpressional basin was formed when the others were already uplifted to shallow crustal level. The apatite FT age of the Pungam Basin was rather affected by the later Cenozoic uplift episode associated with the 44

56 adjacent Taebaeksan Mountains to the east (Han 2002), the backbone of the Korean Peninsula developing from north to south along the eastern part of the peninsula. Difference in thermal histories between the Jinan Basin and the Sindong Group of the Gyeongsang Basin suggests different cooling processes of the two basins. The Jinan Basin sediment was interpreted to have been uplifted more than ca. 400 m than the surrounding basement rocks by transpression in Late Cretaceous time (Lee, 1992). However, it is not a sufficient displacement to explain the ca. 165 C cooling of the basinfill during ca. 15 million years between 95 and 80 Ma. The Jinan Basin is interpreted to have been uplifted by transpression prior to and exhumed further by the regional exhumation that caused the cooling of the Sindong Group. The Cenozoic cooling of the Cretaceous basins since the late Oligocene seems to be related to the regional exhumation due to the opening of the East Sea (Japan Sea) (Han, 2002), which occurred during the late Oligocene to early Miocene (Kimura and Tamaki, 1986). Further researches are needed to clarify the Cenozoic tectonics of the sedimentary basins. As discussed above, the Upper Cretaceous uplift of the Cretaceous strike-slip basins are interpreted to have been related to coeval change of the oceanic plate subduction direction, because the latter generated compressional force for regional uplifting. Subduction direction of the Izanagi Plate was changed from oblique to normal since ca Ma (Lithgow-Bertelloni and Richards, 1998). The coincidence in timing between the rapid cooling of the Jinan Basin and rapid change of convergence direction of the Izanagi Plate at around 90 Ma (Tokiwa, 2009) suggests that the Ma cooling 45

57 of the Jinan Basin seems to have been related to compression caused by the orthogonal subduction of the Izanagi Plate. Thus, the cause of uplift of the Jinan Basin can be ascribed to such transpressional forces. However, the orthogonal subduction of the Izanagi Plate sustained until the end of Cretaceous (74-64 Ma; Lithgow-Bertelloni and Richards, 1998), whereas rapid uplift of the Korean Cretaceous basins was completed prior to ca. 80 Ma. If the changing subduction direction was an exclusive cause of the regional uplift, the continental margin would have been kept uplifting until 64 Ma. Moreover, this transpressional force cannot explain the cause of the uplift of the Sindong Group of the Gyeongsang Basin whose formation mechanism is different from these pull-apart basins. This indicates that there was a certain mechanism causing this active continental margin to be regionally uplifted during the beginning stage of the orthogonal subduction of the Izanagi Plate UPPER CRETACEOUS RIDGE SUBDUCTION AND REGIONAL UPLIFT OF THE EAST ASIAN CONTINENTAL MARGIN The cause of regional uplift of the Korean Cretaceous basins can be inferred from comparison of the thermal history of these basins with those of adjacent areas such as northeastern China and southwestern Japan. Southwestern Japan consists of several accretionary complexes, which formed since the Paleozoic and was located close to the east of the Korean Peninsula during the Jurassic-Cretaceous (Fig. 2-7; Lee, 2008). The Ryoke, Sambagawa, and Shimanto belts correspond to the Cretaceous accretionary complexes (Nakajima, 1997). The Ryoke and Sambagawa belts are interpreted to have 46

58 been exhumed in the Late Cretaceous (Kamp and Takemura, 1993; Shinjoe and Tagami, 1994). Since 90 Ma, an oceanic ridge existed between the Izanagi-Kula Plate and the Pacific Plate was being subducted beneath the Asian continent near southwestern Japan until the end of the Cretaceous (Maruyama et al., 1997). Subduction of the young and buoyant oceanic plate with high speed and normal angle to the continental margin generated remarkable igneous activity (Takahashi, 1983) and resulted in exhumation of the Sambagawa and Ryoke belts (Maruyama et al., 1997). In this period, Median Tectonic Line in Japan was reactivated and trench deposition of the Upper Cretaceous Shimanto Belt occurred (ca Ma; Taira et al., 1980; Yamakita and Otoh, 2000). A recent apatite and zircon FT thermochronologic study on Phanerozoic granitoids in the Yanji area, northeastern China, which is located in more inland area than the Korean Peninsula also suggests that Upper Cretaceous and late Cenozoic (95-80 Ma and 15-0 Ma, respectively) exhumation of the Phanerozoic granitoids was likely related to the subduction of the Pacific Plate (Li et al., 2009). Figure 2-7 shows the thermal histories of the Jinan and Gyeongsang basins, granitoids of the Yanji area, and the Ryoke, Sambagawa, and Shimanto belts (Hara and Kimura, 2008; Hasebe et al., 1993; Kamp and Takemura, 1993; Li et al., 2009; Shinjoe and Tagami, 1994; Suzuki and Adachi, 1998; Wallis et al., 2009). Except for the Shimanto Belt which started exhumation in the middle Eocene, rapid cooling timings of the Korean Cretaceous basins, the Yanji granitoids, and the Ryoke and Sambagawa belts are concentrated near ca. 90 Ma, and match well with the initial depositional timing of the Shimanto belt and the timing of the Izanagi-Pacific ridge subduction. The uplift period of Ma in the East Asian 47

59 48 Fig a) Paleogeographic map of the East Asian margin at ca. 90 Ma (modified from Lee, 2008; Maruyama et al., 1997; Wallis et al., 2009). Locations of thermal history studies are marked by acronyms (JB-Jinan Basin, GB- Gyeongsang Basin, YJ-Yanji Granitoids, RB-Ryoke Belt, SB-Sambagawa Belt, and SHB-Shimanto Belt). b) Thermal histories of the Cretaceous Gyeongsang and Jinan basins, Yanji granitoids, and cooling histories of the Ryoke, Sambagawa, and Shimanto accretionary belts in Japan. References and analytical methods of thermal history curves are as follows: JB this study; GB Lim and Lee (2005), apatite & zircon FT; YG Li et al. (2009), apatite FT; RB1 Kamp and Takemura (1993), apatite & zircon FT; RB2 Suzuki and Adachi (1998), CHIME monazite; SB1 Aoki et al. (2010), Shinjoe and Tagami (1994), zircon FT; SB2 Wallis et al. (2009), eclozite Lu-Hf dating; SHB1 Hara and Kimura (2008), IC, illite K-Ar dating, & zircon FT; SHB2 Hasebe et al. (1993), apatite & zircon FT.

60 continental margin is also well agreed with subduction timing of the Izanagi-Pacific ridge beneath the Sambagawa belt (85-80 Ma; Wallis et al., 2009), although response of the continental margin might have begun earlier than that of the oceanic side. Especially, the Korean Cretaceous basins and Yanji granitoids have the similar timing of cooling but the Yanji granitoids show slower cooling, suggestive of lesser regional influence of the ridge subduction than in the proximal Korean Peninsula. The coincidence of timing of these tectonic events suggests that during the early Late Cretaceous, the subduction of the Izanagi-Pacific ridge provided force for the regional exhumation of the Korean Peninsula as well as the entire East Asian continental margin. The subduction of the Izanagi-Pacific ridge migrated northeastwards with time (Hara and Kimura, 2008) and thus the Korean Peninsula could have escaped from the influence of such ridge subduction, resulting in the end of regional exhumation at ca. 80 Ma. 2.4 CONCLUSIONS Thermal histories of the Cretaceous basins in the Korean Peninsula were studied to understand response of the East Asian continental margin to the subduction of the Paleo- Pacific plate. Sedimentary rocks in the Cretaceous transtensional Jinan Basin, Korea were analyzed first by IC and FT dating to reconstruct their burial and cooling history, and the results were compared with those of the other Cretaceous non-marine basins in the Korean Peninsula. The Jinan Basin sediments have a consistent apatite FT age of ca. 68 Ma with maximum paleotemperature of ca. 287 C, and the detailed cooling pattern inferred from FT-length modeling suggests two cooling events during ca Ma and 49

61 after ca. 30 Ma. The thermal history of the Jinan Basin reveals that the Jinan Basin was cooled down mainly by regional exhumation during ca Ma, but it was uplifted slightly earlier than the regional exhumation represented by the cooling history of the Sindong Group of the Gyeongsang Basin, probably by transpressional force due to the subduction direction change of the paleo-pacific (Izanagi) plate. Another pull-apart basin, the Yeongdong Basin, is assumed to have a similar thermal history to that of the Jinan Basin. Comparison of these results with the thermal histories of northeastern China and the coeval accretionary complexes in southwestern Japan reveals that the Upper Cretaceous regional exhumation of the East Asian continental margin was facilitated by the subduction of the Izanagi-Pacific Plate ridge which occurred at ca. 90 Ma. The regional uplift of the East Asian continental margin including the Korean Peninsula was completed at ca. 80 Ma when the Izanagi-Pacific Plate ridge subduction moved away from this region. 50

62 3. DETRITAL APATITE THERMOCHRONOLOGY OF SOUTH KOREAN RIVER SEDIMENTS 3.1 INTRODUCTION Thermochronology is a study of quantitative reconstruction of the thermal history of a rock using radiometric dating methods which have various closure temperatures, such as 40 Ar/ 39 Ar, fission track (FT), (U-Th)/He, etc. (e.g., Berger and York, 1981). Based on thermal sensitivity of these methods, dated minerals/rocks provide their cooling ages, which mean the elapsed times since they had been cooled enough to retain daughter products of radiogenic isotopes. Among these methods, FT dating method is a powerful tool for low temperature thermochronology, due to its unique daughter product (fission damage trails) and low closure temperature (ca. 100 C for apatite; Donelick, 1991). Because FT dating belongs to single grain age dating method, it can be applied to apatite and/or zircon grains in the sedimentary rocks as detrital thermochronology. Detrital thermochronology analyzes ages of sediment grains and compares their age components with age patterns of the hinterland and source areas for provenance analysis, landscape evolution study, exhumation study, etc. (Lisker et al., 2009 and references therein). Since the Mesozoic, the Korean Peninsula has experienced tectonic events mainly associated with subduction of the paleo-pacific plates: compression during the Early Mesozoic and the Early Cenozoic and extension during the Late Mesozoic (Chang, 1995). These major tectonic events of the Korean Peninsula might have left thermochronologic signatures in rocks and minerals as corresponding age components. 51

63 Fig a) The distribution of the mountain ranges in the southern Korean Peninsula (modified after KRIHS, 2009). B) Drainage basins of the Han, the Geum, and the Nakdong rivers in South Korea (modified after Choi et al., 2012). Sample locations are marked with circles. 52

64 Detrital minerals having such signatures are transported and gathered at the mouth of a river with thermal history of its drainage area. Thus, in this study, FT analyses were carried out on detrital apatite grains in three major river sediments of South Korea to estimate overall apatite FT ages of the rocks in their drainage areas. Then, the analytical results are compared and contrasted with previous studies to characterize the thermal history of Korea since the Mesozoic. 3.2 GEOLOGICAL SETTING The Korean Peninsula is located in the East Asian continental margin and comprised Precambrian granitic gneiss and metasedimentary rocks, Paleozoic basins, Mesozoic igneous rocks, and Cretaceous basins. As a part of the East Asian continental margin, the Korean Peninsula has experienced major tectonic events since the Mesozoic. There were arc magmatism, orogeny, and regional exhumation during the Mesozoic by the subduction of the paleo-pacific plates beneath the Asian continent (Choi and Lee, 2011; Choi et al., 2012; Maruyama et al., 1997). Arc magmatism was active throughout the Mesozoic in Korea (Chapter 1), but notable orogenic events occurred two times in the southern Korean Peninsula: during the Triassic-Late Jurassic with NE-trending folds and thrusts (Daebo orogeny) and during the Late Cretaceous-Early Paleogene with EWtrending folds and thrusts (Bulguksa orogeny) (Kim, 1996). In the Cenozoic, NStrending folds, thrusts, and mountain ranges (c.f. Taebaeksan Range) were formed along the eastern part of the Peninsula (Kim, 1996; Lee, 1999b), although their formation mechanism is poorly understood. Uplift or denudation of South Korea during the 53

65 Mesozoic is not well recognized. However, differences in emplacement depth between the Jurassic (~12-28 km) and Cretaceous (< 10 km) granitoids in South Korea (Cho and Kwon, 1994) suggest the Late Jurassic exhumation/uplift elevate Jurassic plutons to the shallow crustal level. The Late Cretaceous regional exhumation might be caused due to subduction of the Izanagi-Pacific plate ridge beneath the Asian continent at 90 Ma as is discussed at Chapter 2. The Han, the Geum, and the Nakdong rivers are major rivers in the southern Korean Peninsula and their drainage areas are separated by the mountain ranges (Fig. 3-1). The drainage area of the Han River occupies the middle part of the Korean Peninsula, which comprises Precambrian basements, the Paleozoic Taebaeksan Basin, and Jurassic granitoids (Shin and Jin, 1995). The Geum River drainage area comprises Precambrian basements, the Paleozoic Okcheon Belt, the Paleozoic granitoids, the Jurassic granitoids and sedimentary rocks, and Cretaceous plutonic, volcanic, and sedimentary rocks. The Nakdong River flows through the Taebaeksan Basin, Precambrian basements, Jurassic granitoids, and the nonmarine Cretaceous Gyeongsang Basin. 3.3 EXPERIMENTAL METHODS For apatite FT analysis medium-coarse sized sandy sediments, each weighing about 5-10 kg, were collected from rivermouths of the Han, the Geum, and the Nakdong rivers were collected. The conventional heavy mineral separation was carried out using heavy liquid and magnetic techniques. About 1000 apatite grains were handpicked and mounted in epoxy resin. Then they were polished with diamond paste and etched 54

66 chemically in 0.6% HNO 3 at 32.0±0.5 C. Apatite grains were dated by the external detector method, counting induced tracks in high-quality muscovite external detectors. Thermal neutron irradiation was carried out at NAA-1 facility in the HANARO reactor of the Korea Atomic Energy Research Institute, and fluences were monitored by counting tracks in muscovite external detectors over NIST SRM612 glasses. This facility has a high Cd ratio of 250 for Au (Lim and Lee, 2000), which satisfies the criteria recommended by Hurford (1990). After irradiation, the external muscovite detectors were detached and etched in 48% HF at 32±0.5 C for 4 min. Fission tracks were counted using a Nikon Optiphot-II microscope with a dry 100 objective and a total magnification of A computer-automated microscope stage system (Dumitru, 1993) was used to translate between sample and detector. The zeta calibration approach (Hurford, 1990; Hurford and Green, 1983) was used with a ζ value of 351.4±20.3 (2σ) for apatite (Choi and Lee, 2011). The conventional central age, Poissonian error, and P (χ 2 ) (Galbraith, 1981; Green, 1981) of detrital apatite grains were calculated. 3.4 RESULTS & INTERPRETATION 121 apatite grains among 1000 grains analyzed yielded FT ages (Table 3-1) and their probability-density plots are displayed in Figure 3-2. Induced track density (ρi) of the Geum River samples ( ) is lower than those of the Han ( ) and the Nakdong ( ) river samples, which means lower U content of the Geum River apatites than the others probably due to differences in source rocks in their catchment 55

67 Table 3-1. Apatite fission track analytical results of Korean river sediments Sample code No. of grains Spontaneous track Induced track Dosimeter glass Ns ρ s Ni ρ i Nd ρ d P(χ 2 ) (%) Age ± 1σ (Ma) age dispersion Weighted mean age ± 2σ (Ma) Han River GP-s ± GP-s ± MSR-b ± MSR-b ± Geum River b ± b ± Nakdong River ESD-s ± ESD-s ± ESD-b ± ESD-b ± π 57.7 ± ± 6.9 Total: 62.3 ± 3.6 Ns=Number of spontaneous tracks counted to determine ρs; ρs=density of spontaneous tracks ( 10 6 /cm 2 ); Ni=Number of induced tracks counted in a muscovite external detector to determine ρi; ρi=density of induced tracks in a sample ( 10 6 /cm 2 ); Nd=Number of induced tracks counted in a 74.4 ± 5.8 muscovite detector to determine ρd; ρd=density of induced tracks in NIST-SRM612 dosimeter glass ( 10 6 /cm 2 ); P(χ 2 )=the probability of obtaining χ 2 value for n degrees of freedom where n= No. of grains -1 56

68 Fig Probability-density plots presenting detrital apatite FT ages of a) the Han, b) the Geum, c) the Nakdong, and d) the total river sediments. W.M.: weighted mean age. 57

69 areas. Central ages of the river samples range between Ma and weighted mean age of all apatite grains is 62 Ma. Weighted averages of the Han, the Geum, and the Nakdong river sediments are 58, 62, and 74 Ma, respectively. Apatite FT ages of the Han and the Nakdong river sediments show relatively high age dispersion ( and , respectively) and do not pass the χ 2 test at a probability P (χ 2 ) >5% (Galbraith, 1981; Green, 1981), which means that age components belong to more than one age group. This results in younger weighted mean ages of the Han and the Nakdong river samples than their central ages. On the other hand, central and weighted mean ages of the Geum River apatite grains overlap each other, because their ages belong to one age group. Probability-density plots of apatite single grain ages show age populations skewed to Late Cretaceous-Early Cenozoic ages. The highest age peaks of the Han, the Geum, and the Nakdong river samples are at ca. 49, 54, and 68 Ma, respectively. Deconvolution of the apatite FT age populations were carried out using the BinomFit program (Brandon, 1996) for further interpretations of apatite age components in each river s drainage area (Table 3-2 and Fig. 3-3). Apatite single grain ages of the Han River sediments can be grouped as three age populations with peaks at 52 Ma (48.6%), 96 Ma (17.5%), and 154 Ma (33.8%). The Geum River apatite grains have only one age population with a peak at 68 Ma. Age populations of the Nakdong River apatite grains can be unmixed as three groups with peaks at 58 Ma (28.1%), Ma (62.9%), and 218 Ma (9.0%). 3.5 DISCUSSION 58

70 3.5.1 PROVENANCE OF APATITE FT AGE COMPONENTS Except for minor Triassic age peak age from the Nakdong River samples, apatite FT age components of the three rivers can be grouped as three age populations: the Late Cretaceous-Early Paleogene, the Middle Cretaceous, and the Late Jurassic (Fig. 3-3). Note that most of these apatite FT ages may represent exhumation events, but apatite FT ages of the Nakdong River sediments, can involve FT ages of apatite grains of the Late Cretaceous ages from the igneous rocks within the Gyeongsang Basin. The Late Cretaceous-Early Paleogene age component This age component, included in all river sediments, seems to represent the overall apatite FT age reset by regional exhumation, considering apatite FT ages of rocks in the drainage areas. Previously reported apatite FT ages range from 78 to 29 Ma in the middle to southern parts of the Han River drainage area (Choi and Lee, 2006; Han, 2002; Jin et al., 1987; Shin and Jin, 1995), whereas igneous and sedimentary rocks in the Geum River drainage area yield apatite FT ages ranging Ma (Choi and Lee, 2006; Jin et al., 1987; Shin and Jin, 1995), and those in the Nakdong River drainage basin range from 74 to 31 Ma (Jin et al., 1987; Lim and Lee, 2005; Lim et al., 2003; Shin and Jin, 1995). Accordingly, the youngest age population representing the regional exhumation of the Han, the Geum, and the Nakdong river drainage areas corresponds to the long-term cooling rate of ca. 1.7, 1.3, and 1.5 C/Ma, respectively, assuming apatite FT closure temperature as 100 C (Donelick, 1991) and annual mean temperature of South Korea as 13 C. 59

71 Table 3-2. Age peaks and their percentages in the deconvoluted apatite FT age components of Korean river sediments River No. of grains P1 (Ma) P2 (Ma) P3 (Ma) P4 (Ma) fraction (%) fraction (%) fraction (%) fraction (%) Han River Geum River Nakdong River

72 Fig Deconvoluted probability-density plots of detrital apatite FT ages of a) the Han, b) the Geum, c) the Nakdong river sediments. The shaded are ±1 SE envelopes for observed probability-density plots and the red lines are deconvoluted apatite FT age components. 61

73 The Middle Cretaceous age component This age component is included in the Han and the Nakdong river samples, with peaks at 96 and 104 Ma, respectively. This age is quite older than the other apatite FT ages affected by the regional exhumation mentioned above. This means that the host rocks had been positioned at shallower depth than the apatite partial annealing zone (APAZ) during the Late Cretaceous exhumation. Thus, considering the spatial distribution of the two drainage basins this age component may have been derived from the divide between the Han and the Nakdong river drainage areas. The northern part of the Sobaeksan Range, NE-trending mountain range is known to be formed by orogeny during the Jurassic (Daebo orogeny; Kim, 1996; Lee, 1999b). The Late Jurassic age component This age component occurs only in the Han River sample. In the southern Han River drainage area, rocks including apatite ages with a peak at 154 Ma have not been reported previously. The previously reported apatite FT ages are exclusively from its middle and southern parts (Choi and Lee, 2006; Han, 2002; Jin et al., 1987; Shin and Jin, 1995). Thus, apatite grains with this age component may have been derived from the northern part of the drainage area, without resetting of their FT ages by later tectonic events. This suggests that the influence of the Late Cretaceous regional exhumation during the Izanagi-Pacific ridge subduction was up to the middle part of the Korean Peninsula. This interpretation is supported by the distribution of the Upper Cretaceous 62

74 volcanic rocks related to the ridge subduction at that time in the Korean Peninsula (c.f., Shin and Jin, 1995). Thus, this age component indicates that the present surface of the Korean Peninsula reached to the shallow crustal level by the Late Jurassic exhumation during the Jurassic Daebo orogeny, considering differences in emplacement depths between the Jurassic and Cretaceous granitoids described above THE PALEOGENE EXHUMATION OF THE KOREAN PENINSULA The long-term cooling rates inferred from the Late Cretaceous-Early Paleogene age components differ among the river sediments. The Geum River sample shows slower cooling rate (ca. 1.3 C/Ma) than those of the Han (ca. 1.7 C/Ma) and the Nakdong (ca. 1.5 C/Ma) river samples. The Geum River drainage area, which is located the western part of Korea and does not include high mountain ranges, seems to have been mainly affected by the Late Cretaceous exhumation due to the Izanagi-Pacific ridge subduction. This suggests that the eastern part of the Korean Peninsula might have experienced higher exhumation event than the western part of the Korean Peninsula. To confirm the effect of the Cenozoic exhumation, cooling histories of the Jurassic and Cretaceous granites widely distributed in the southern Korean Peninsula was reconstructed by t-t modeling using FT ages and FT length distributions of Jin et al. (1987) with the AFTSolve program (Ketcham et al., 2000). Five granitoid samples (sample codes 102, 112, 125, 127, and 128 in Jin et al., 1987) yielded meaningful modeling results indicative of the Cenozoic regional exhumation in Korea (Fig. 3-4). 63

75 The Cretaceous (102) and the Jurassic (112 and 125) samples located in the east of the Taebaeksan Range, seem to have been cooled since ca Ma, corresponding to the cooling rate of ca. 1.0, 4.0, and 2.3 C/my, respectively. The cooling rate of the Cretaceous sample (102) is relatively lower than those of the Jurassic samples (112 and 125). This might be caused by difference in emplacement depth and timing among them. The Cretaceous granitoids in the southern Korean Peninsula were emplaced at the shallow crustal level (Cho and Kwon, 1994), which may cause rapid cooling of the Cretaceous granite (102) due to high thermal contrast with the country rocks before 60 Ma, resulting in a relatively slower cooling rate during Ma. On the contrary, the cooling rates of samples in the west of the Taebaeksan Range are smaller during the same time period. The Jurassic granitoid (127), the western part of the Taebaeksan Range has a cooling rate of ca C/my during Ma. The cooling rate of another Jurassic granitoids (128) in the central Korean Peninsula during Ma is estimated to be ca C/my. However, since 31 Ma the cooling rate of the sample 127 increased to ca. 2.1 C/my, while the sample 128 has maintained the previous rate until ca. 10 Ma. This suggests that the Paleogene exhumation in the Korean Peninsula occurred earlier preferentially in its easternmost part (samples 102, 112, and 125) during Ma and then in its central part (sample 127) from 30 Ma to the present, resulting in younger of apatite FT ages than its western part. 3.6 CONCLUSIONS FT analyses of detrital apatite grains from the Han, the Geum, and the Nakdong river 64

76 Table 3-3. Rb/Sr, K/Ar, and apatite FT ages of Mesozoic granites and their cooling rates during Ma (modified from Jin et al., 1987). sample code elevation (m) Rb/Sr age (Ma) ± 2σ K/Ar age (Ma) ± 2σ apatite FT age (Ma) ± 1σ cooling rate during Ma ( C/Ma) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

77 66 Fig Representative thermal histories of the Jurassic and Cretaceous granitoids in the southern Korean Peninsula, calculated by apatite FT length modeling using the data of Jin et al. (1987). The histogram of FT length represents the measured data of each sample. n: number of grains; GOF: goodness of fit. The shaded are envelopes with a merit function value of 0.05 and the thick solid lines are the single t-t paths that best reproduce the measured data. The cooling curves were constructed by apatite FT length modeling and the cooling curves by K/Ar and/or Rb/Sr biotite ages of the granitoids from Jin et al. (1987) and references therein.

78 sediments yielded central ages of Ma. The deconvolution of the apatite FT age populations revealed that the apatite ages of these three river sediments are composed of three age components: the Late Cretaceous-Early Paleogene, the Middle Cretaceous, and the Late Jurassic. The Late Cretaceous-Early Paleogene age component suggests average apatite FT age related to the regional exhumation. The long-term cooling rate of each river drainage area is estimated to be ca. 1.7, 1.3, and 1.5 C/Ma in the Han, the Geum, and the Nakdong rivers drainage areas, respectively. The Middle Cretaceous age component comes from the mountain range dividing the drainage areas of the Han and the Nakdong rivers and represents the Jurassic Daebo orogenic event. The Late Jurassic age component only occurs in the Han River samples, probably derived from the northern part of the Han River drainage area and seems to be related with the Jurassic orogenic event. This geographic distribution indicates that the area north of the middle Korean Peninsula was not affected by the Late Cretaceous exhumation event that was most likely caused by the Izanagi-Pacific plate ridge subduction. The Paleogene exhumation event occurred in the eastern part of the Korean Peninsula preferentially, on the basis of differences in the long-term cooling rates of the river drainage areas and T-t modeling results of widespread granitoids using data of Jin et al. (1987). 67

79 4. PROVENANCE OF THE SOUTHEASTERN YELLOW SEA SEDIMENTS 4.1 INTRODUCTION Zircon is a common accessory mineral in most rocks and is a frequently studied component of detrital assemblages because of its physical and chemical resistance. Zircon is also a mainstay for geochronological determinations due to its high U/Pb ratio at the time of formation and the ability of the zircon crystal to retain the daughter product of U and Th radioactive decay. Thus, the age distribution of detrital zircons in sediment or sedimentary rock has been applied for evaluation of their provenance (Cawood et al., 2003; Dodson et al., 1988; Link et al., 2005; Nelson, 2001 amongst others). Age-spectra of detrital zircons in sediments are determined to investigate their source terrane and distinguish sediments of different provenances. Recently, there are several tries to compare detrital zircon age populations between different sediments quantitatively (Fletcher et al., 2007; Guynn, 2006). The Yellow Sea is an epicontinental sea submerged due to post-glacial sea-level rise in the Holocene (Yang et al., 2003; Fig. 4-1a). Sandy sediments along its western and eastern margins are thought to have been supplied from rivers in the surrounding land regions, whose pathways had been extended to the southwestern part of Jeju Island during the glacial period (Park, 1999). The paleo-fluvial sediments were reworked during the transgressions, forming huge sand ridges and veneers (Li et al., 2001; Liu et al., 1989; Shinn et al., 2007). 68

80 Fig (a) Map of rivers feeding the Yellow Sea. North Korean rivers include the Aprok, the Daedong, and the Cheongcheon rivers, while South Korean rivers include the Hantan, the Han, the Geum, the Yeongsan, the Seomjin, and the Nakdong rivers. (b) Bathymetric chart and grain size distribution of the Yellow Sea (modified from Milliman et al., 1989; Shinn et al., 2007; Uehara and Saito, 2003; Wellner and Bartek, 2003). 69

81 Various methodologies have been applied to investigate the provenance of the Yellow Sea sediments such as clay mineralogy (Lee and Chough, 1989; Park and Khim, 1992; Qin et al., 1989), heavy mineralogy (Lee and Chough, 1989; Lee et al., 1988; Qin et al., 1989), carbonate mineralogy (Alexander et al., 1991; Milliman et al., 1985), geochemistry using various elements and isotopes (Liu et al., 2009; Xu et al., 2009; Yang and Youn, 2007; Yang et al., 2003; Yang et al., 2004; Youn and Kim, 2011), magnetic property (Liu et al., 2010; Liu et al., 2003; Wang et al., 2010), seismic profile (Liu et al., 2002; Liu et al., 2004), and so on. Most of the studies, however, are related to sedimentation of fine-grained deposits during the post-glacial transgression stage, and provenance of sandy sediments in the Yellow Sea has been rarely investigated in spite of their vast distribution. Also, there exist different opinions about the provenance of fine sediments in the southeastern Yellow Sea. Thus, this study aims to determine provenance of the sandy sediments in the southeastern Yellow Sea using detrital zircon U-Pb dating by LA-ICP-MS and compare these geochronological data with fine-grained sediments in the region GEOLOGICAL SETTING OF THE YELLOW SEA The Yellow Sea is an epicontinental sea located between China and the Korean Peninsula with an average depth of 45 m (Fig. 4-1b). The drainage systems feeding the Yellow Sea are mainly the Yellow and the Yangtze rivers in eastern China and rivers in the Korean Peninsula including the Aprok, the Daedong, the Cheongcheon, the Hantan, the Han, the Geum, the Yeongsan, the Seomjin, and the Nakdong rivers. The annual 70

82 discharge of sediments to the Yellow Sea is estimated to be more than 10 9 tons (Lee and Chough, 1989). Annual sediment input of the Yellow River is ca tons (Milliman and Meade, 1983) and ca. 90% of them are derived from the loess deposits, which occupy about half of the whole Yellow River drainage area (Ren and Shi, 1986). The Yangtze River drainage basin contains Paleozoic carbonate rocks, felsic metamorphic rocks, and Quaternary clastic rocks (Zhang et al., 1990) and annually provides ca tons of sediment to the Yellow Sea. Bulk of the Yangtze Riverderived sediment is transported southwards along the Chinese coast (Li et al., 2005; Milliman et al., 1985). Korean rivers contribute sediment to the Yellow Sea only ca tons annually (Schubel et al., 1986). Korean river basins consist mainly Cretaceous and Jurassic granites, and Precambrian gneiss with minor proportions of limestone, schist, volcanic rocks, and phillites (Chough et al., 2000; Lee et al., 1988). These differences in geology of drainage basins of the Yellow Sea produce a smooth, low-gradient coast in the western part, while its eastern part is steep, rocky and highly indented (Choi and Dalrymple, 2004; Liu et al., 1992). Sediments in the Yellow Sea consist of mud, silt, and sand (Fig. 4-1b). Muddy deposits are dominant in the central part, while sand and muddy sand blanket the eastern and western parts (Yang et al., 2003). Large-scale sandy deposits are distributed in the southwestern and eastern-southeastern Yellow Sea, forming unique radial tidal sand ridges, as a remnant deposit of winnowing by tidal currents and/or shoreface erosion during glacio-eustatic sea-level fluctuations in the Holocene (Li et al., 2001; Liu et al., 1989; Shinn et al., 2007; Yang, 1989). The seafloor deepens southeastwards with a NW- 71

83 SE trough in the central part and its water depth becomes ca. 130 m at the southeasternmost part (Fig. 4-1b; Shinn et al., 2007). The currently submerged area of the Yellow Sea Basin was subaerially exposed during the Last Glacial Maximum (LGM), when sea level was 130 m below the present level (Qin et al., 1989). The timing of transgression was different between the western side (ca. 5-7 ky BP; Qin et al., 1989) and the eastern side (before 9 ky BP; Lee and Yoon, 1997; Uehara and Saito, 2003) due to the different seafloor topography. Many researches have been carried out on the provenance of the Yellow Sea sediments with various methodologies, and among them some different interpretations were raised, especially for sediments in the southern part (Yang et al., 2003). But, there is a general consensus that fine sediments in the near-shore area of the Yellow Sea have been supplied from adjacent rivers; from the Yellow River in the northern part (Liu et al., 2004; Youn and Kim, 2011; Zhao et al., 1990), from the Yellow and Yangtze rivers in the western part (Alexander et al., 1991; Kim et al., 1999; Ren and Shi, 1986), and from the Korean rivers in the eastern part (Kim et al., 1999; Lee et al., 1988). Also, it is generally known that Korean rivers have provided sandy sediments to the northern Yellow Sea between the Shandong Peninsula and the Korean Peninsula (Kim et al., 1999; Liu et al., 2009). Fine-grained deposits in the remote areas from the surrounding landmass, the central and the southern Yellow Sea are interpreted to have been derived from the Yellow River (Cho et al., 1999) and Yangtze and paleo-yellow rivers (Liu et al., 2003; Youn and Kim, 2011), respectively. Fine sediments in the southeastern Yellow Sea are interpreted to have been derived from the Yellow River (Cho et al., 72

84 1999), or Korean rivers (Lee et al., 1988), or shelf bedrock during the transgression (Jiang et al., 2000). On the other hand, several studies (Nohara et al., 1999; Qin et al., 1989; Wang et al., 2010) suggest that fine sediments in the Yellow Sea seem to have been mostly derived from the Yellow and Yangtze rivers. 4.3 EXPERIMENTAL METHODS Seven surface sediments, each weighing 5 kg, were collected from the southeastern Yellow Sea for detrital zircon age dating (Fig. 4-1b). Zircons were extracted using conventional heavy mineral separation processes and then were handpicked under a binocular microscope. Since the sample size for detrital studies should be at least grains to obtain statistical confidence (Dodson et al., 1988; Link et al., 2005; Vermeesch, 2004), 100 zircon grains per each sample were embedded in PFA Teflon sheet and were polished to expose mineral surfaces. The laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) used in this study is Thermo Elemental PlasmaQuad3 housed at the Earthquake Research Institute, the University of Tokyo. This instrument is equipped with S-option interface and CHICANE ion lens (Orihashi et al., 2008). The laser ablation system is a New Wave UP213 system, which utilizes a frequency quintupled Nd-YAG 213 nm wavelength. The details of analytical procedure are after Lee et al. (2010). He gas was employed as a carrier gas and small amount of N 2 gases (0.8 ml/min) was mixed into the carrier gas. Operational settings of ICP conditions, such as torch position, gas flow rates or lens settings were optimized to maximize 208 Pb signal intensity by laser ablation 73

85 of SRM610 glass standard. The instrument sensitivity obtained in this study was cps/µg g -1 for Pb with the ablation pit size of 30 µm, energy density of J/cm 3, pulse width of 4 ns nominal and pulse repetition rate of 10 Hz. Single spot ablation was achieved by a 3-second pre-ablation and 20 seconds for data acquisition. The interelement fractionation during the analysis was monitored by analyzing fragment of the zircon standard which has a concordant isotope-dilution thermal ionization mass spectrometry age of ± 0.4 Ma (Wiedenbeck et al., 1995). The standard was analyzed once for every five unknowns, so the quality of the analyses was closely controlled. The lead isotopic ratios were corrected for common Pb using the measured 204 Pb, assuming an initial Pb composition according to Stacey and Kramers (1975) and uncertainties of 1.0%, 0.3%, and 2.0% for 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb, respectively. Data processing was carried out using the ISOPLOT program (Ludwig, 2003). 207 Pb/ 206 Pb ages were used for zircon grains older than 1000 Ma to construct probability-density plots. 4.4 RESULTS Among 700 zircon grains analyzed 453 concordant or slightly discordant (<10%) U-Pb and Pb-Pb zircon ages were obtained and are presented in Table 4-1 and plotted on probability-density plots (Fig. 4-2). All zircon grains but four have Th/U ratios larger than 0.1, indicating that they are of igneous origin (Hartmann et al., 2000; Vavra et al., 1999). All samples yield > 60 concordant to slightly discordant zircon U-Pb ages, 74

86 covering age population present as little as 5% at the 95% confidence level (Dodson et al., 1988; Link et al., 2005), and thus number of zircon grains analyzed in this study is sufficient to obtain the representative age population of each sample. Zircon grains in most samples except G15 have a wide age range from Archean to Cenozoic, suggestive of a wide range of igneous rock ages in the source area. Characteristically, sample G15 does not contain zircon grains younger than the Late Cretaceous. Results of detrital zircon U-Pb ages of each sample, from northeast to southwest, are described below and summarized in Figure G3 A total of 61 concordant detrital zircon ages was obtained from the easternmost sample of the southeastern Yellow Sea. Most of zircons have Paleoproterozoic ages between 2092 and 1758 Ma (39%), however the majority of these grains have a ca to ca Ma age range with a peak at 1881 Ma. Another major age population ranges from 204 to 157 Ma (23%) with peaks at 186 and 170 Ma. Other minor age groups are Triassic ( Ma, 8%), Early Paleoproterozoic ( Ma, 5%), and Late Archean-Early Paleoproterozoic ( Ma, 5%). 2. G7 G7 yielded 61 concordant to slightly discordant grains sufficient for provenance analysis. The strongest peak age is represented by ca Ma with a Paleoproterozoic age range between 2013 and 1696 Ma (41%). Another 75

87 prominent age group has a ca. 224 Ma to ca. 172 Ma age range (20%) with age peaks at 206 and 185 Ma, with subordinate age populations are Late Cretaceous (73-66 Ma, 5%), Late Jurassic ( Ma, 5%), and Late Archean-Early Paleoproterozoic ( Ma, 5%). 3. G15 68 analyses were obtained from G15 zircon grains. The presence of Neoproterozoic component is indicated by a cluster of zircon ages between 788 and 696 Ma (6%). The strongest age population ranges between 2085 and 1686 Ma (49%) with a peak at 1867 Ma. The second major age group has a ca. 239 Ma to ca. 176 Ma age range (12%) and other minor age groups are Early Cretaceous ( Ma, 1%), Early Paleoproterozoic ( Ma, 9%), and Late Archean ( Ma, 3%). 4. G30 The diminishment of Paleoproterozoic component is indicated in 67 grain ages of G30. The data show the most prominent age population between 223 and 151 Ma (36%) with peaks at 203, 183, and 161 Ma, followed by Neoproterozoic ages between 840 and 654 Ma (24%) with peaks at 771 and 741 Ma. Other age groups belong to Late Paleoproterozoic ( Ma, 13%) and Late Archean-Early Paleoproterozoic ( Ma, 10%). 76

88 5. G20 A total of 60 analytical results was obtained from G20 zircons. The most prominent age group comprises Neoproterozoic zircons with age ranging between 826 and 701 Ma (23%). The highest peak age is 747 Ma. A minor Neoproterozoic age also occurs between 574 and 538 Ma with a peak at 558 Ma. Other major age groups are composed of zircons with Paleoproterozoic ( Ma, 15%), middle Triassic-Early Jurassic ( Ma, 13%), and middle-late Jurassic ( Ma, 10%) in age. Zircons with Early Paleoproterozoic-Late Archean age ( Ma) comprise 8% of the total population. 6. G40 74 analyses were obtained from G40 sediment. Neoproterozoic age group forms the largest proportion of the population (31%), spanning the time interval between ca. 823 and ca. 607 Ma with a peak at 746 Ma. The second age group is consisted of zircon ages from ca. 225 Ma to ca. 184 Ma (26%). Other minor ages belong to Late Jurassic ( Ma, 1%), Late Paleozoic ( Ma, 7%), Early Paleozoic ( Ma, 3%), Late Paleoproterozoic ( Ma; 9%), and Late Archean-Early Paleoproterozoic ( Ma; 7%). 7. G62 77

89 Table 4-1. U-Pb isotopic data for detrital zircon grains of the southeastern Yellow Sea seidments determined by LA-ICP-MS Sample Th/U 206 Pbc* 207 Pb/ 206 Pb Error 206 Pb/ 238 U Error 207 Pb/ 235 U Error Disc** 238 U- 206 Pb age Error 235 U- 207 Pb age Error 207/206 Pb age Error DiscPb** Name (%) 2σ 2σ 2σ (%) (Ma) 2σ (Ma) 2σ (Ma) 2σ (%) G3 (n=61) G ± ± ± ,749.5 ± ,808.9 ± ,879 ± G ± ± ± ,655.4 ± ,746.2 ± ,857 ± G n.d ± ± ± ± ± 8.4 G n.d ± ± ± ,774.7 ± ,833.5 ± ,902 ± G ± ± ± ± ± 8.6 G n.d ± ± ± ± ± 11.0 G n.d ± ± ± ± ± 11.5 G n.d ± ± ± ,010.2 ± ,036.4 ± ,064 ± 60.8 G ± ± ± ,680.5 ± ,715.3 ± ,759 ± 53.3 G ± ± ± ,528.0 ± ,517.9 ± ,504 ± 51.1 G ± ± ± ,110.5 ± ,176.4 ± ,240 ± G n.d ± ± ± ,786.7 ± ,826.0 ± ,872 ± 56.7 G ± ± ± ,875.0 ± ,891.4 ± ,910 ± 56.2 G ± ± ± ,847.4 ± ,860.4 ± ,876 ± 52.7 G n.d ± ± ± ± ± 9.2 G n.d ± ± ± ± ± 6.9 G n.d ± ± ± ,885.3 ± ,931.8 ± ,983 ± 54.3 G n.d ± ± ± ± ± 33.2 G n.d ± ± ± ± ± 12.7 G n.d ± ± ± ,183.5 ± ,354.9 ± ,508 ± G ± ± ± ,684.8 ± ,769.6 ± ,872 ± G ± ± ± ,159.8 ± ,257.8 ± ,348 ± G n.d ± ± ± ,216.6 ± ,361.1 ± ,489 ± G ± ± ± ,784.2 ± ,822.3 ± ,867 ± 29.6 G n.d ± ± ± ± ± 12.2 G n.d ± ± ± ± ± 11.1 G n.d ± ± ± ,876.3 ± ,964.9 ± ,060 ± G n.d ± ± ± ± ± 9.0 G n.d ± ± ± ± ± 19.9 G n.d ± ± ± ,413.4 ± ,465.0 ± ,509 ± 42.2 G n.d ± ± ± ,858.3 ± ,875.4 ± ,895 ± 27.4 G n.d ± ± ± ,882.6 ± ,875.1 ± ,867 ± 30.1 G n.d ± ± ± ± ± 9.2 G ± ± ± ,203.9 ± ,246.1 ± ,285 ± 32.6 G n.d ± ± ± ,851.5 ± ,884.8 ± ,922 ± 31.7 G n.d ± ± ± ,789.2 ± ,833.3 ± ,885 ± 33.8 G n.d ± ± ± ± ± 8.5 G n.d ± ± ± ± ± 16.6 G n.d ± ± ± ,820.5 ± ,921.9 ± ,034 ± G ± ± ± ,716.2 ± ,826.0 ± ,954 ± G ± ± ± ,592.6 ± ,636.1 ± ,670 ± 29.0 G n.d ± ± ± ± ± 18.5 G ± ± ± ,818.6 ± ,837.2 ± ,859 ± 21.8 G n.d ± ± ± ± ± 17.0 G n.d ± ± ± ± ± 11.5 G n.d ± ± ± ± ± 8.1 G n.d ± ± ± ± ± 14.6 G ± ± ± ± ± 10.7 G n.d ± ± ± ± ± 24.7 G n.d ± ± ± ,764.3 ± ,826.5 ± ,899 ± G n.d ± ± ± ± ± 7.3 G n.d ± ± ± ,829.3 ± ,855.3 ± ,885 ± 41.5 G n.d ± ± ± ± ± 34.4 G ± ± ± ,809.8 ± ,847.0 ± ,890 ± G n.d ± ± ± ± ± 16.2 G n.d ± ± ± ± ± 15.2 G n.d ± ± ± ± ± 4.4 G ± ± ± ± ± 26.5 G n.d ± ± ± ,820.9 ± ,851.7 ± ,887 ± G n.d ± ± ± ± ± 25.3 G n.d ± ± ± ,765.5 ± ,827.7 ± ,900 ± G7 (n=61) g n.d ± ± ± ± ± 10.5 g n.d ± ± ± ± ± 19.4 g ± ± ± ,763.8 ± ,798.3 ± ,839 ± 37.3 g n.d ± ± ± ± ± 8.9 g n.d ± ± ± ± ± 14.7 g n.d ± ± ± ± ± 21.7 g ± ± ± ,762.1 ± ,806.2 ± ,858 ± g ± ± ± ,725.3 ± ,788.9 ± ,864 ± g n.d ± ± ± ± ± 10.9 g ± ± ± ,853.8 ± ,866.5 ± ,881 ± 28.4 g n.d ± ± ± ± ± 9.1 g n.d ± ± ± ± ± 37.6 g n.d ± ± ± ± ± 9.4 g n.d ± ± ± ,362.9 ± ,442.6 ± ,510 ± g n.d ± ± ± ,761.0 ± ,806.0 ± ,859 ± 36.8 g ± ± ± ± ± 29.4 g ± ± ± ,690.2 ± ,755.0 ± ,834 ± g n.d ± ± ± ± ± 7.8 g n.d ± ± ± ± ± 16.3 g n.d ± ± ± ± ± 37.4 g n.d ± ± ± ,820.6 ± ,867.6 ± ,921 ± g n.d ± ± ± ,820.8 ± ,862.1 ± ,909 ± g n.d ± ± ± ,239.7 ± ,345.3 ± ,439 ± g n.d ± ± ± ,790.0 ± ,849.5 ± ,918 ± g n.d ± ± ± ,663.8 ± ,678.8 ± ,698 ± 43.6 g ± ± ± ,785.6 ± ,821.0 ± ,862 ± g ± ± ± ,804.3 ± ,848.3 ± ,899 ± g n.d ± ± ± ,635.4 ± ,735.4 ± ,859 ± g n.d ± ± ± ± ± 11.9 g n.d ± ± ± ,659.1 ± ,754.5 ± ,871 ± g n.d ± ± ± ± ± 10.5 g n.d ± ± ± ,782.1 ± ,816.3 ± ,856 ± 36.4 g n.d ± ± ± ± ±

90 g n.d ± ± ± ,042.0 ± ,096.4 ± ,151 ± g n.d ± ± ± ,782.6 ± ,825.8 ± ,876 ± g n.d ± ± ± ± ± 9.6 g n.d ± ± ± ± ± 20.6 g n.d ± ± ± ± ± 13.6 g n.d ± ± ± ± ± 7.4 g ± ± ± ± ± 5.5 g ± ± ± ,079.8 ± ,170.2 ± ,257 ± g n.d ± ± ± ± ± 15.7 g ± ± ± ,831.8 ± ,844.5 ± ,860 ± 70.4 g n.d ± ± ± ± ± 13.2 g n.d ± ± ± ± ± 15.7 g ± ± ± ,715.0 ± ,763.6 ± ,822 ± 54.1 g ± ± ± ± ± 12.0 g n.d ± ± ± ± ± 13.0 g ± ± ± ,612.3 ± ,724.7 ± ,865 ± g ± ± ± ,783.7 ± ,841.0 ± ,907 ± 54.2 g ± ± ± ,780.5 ± ,821.1 ± ,869 ± 53.1 g n.d ± ± ± ,846.7 ± ,869.7 ± ,896 ± 53.5 g ± ± ± ,755.3 ± ,797.0 ± ,846 ± 52.7 g n.d ± ± ± ± ± 16.3 g n.d ± ± ± ± ± 12.2 g n.d ± ± ± ± ± 14.0 g ± ± ± ,863.3 ± ,862.1 ± ,862 ± 83.4 g n.d ± ± ± ,803.7 ± ,809.8 ± ,817 ± 97.9 g ± ± ± ,245.9 ± ,350.7 ± ,444 ± g n.d ± ± ± ± ± 14.0 g n.d ± ± ± ± ± 12.1 G15 (n=68) LA n.d ± ± ± ± ± 22.0 LA ± ± ± ± ± 31.1 LA ± ± ± ,895.3 ± ,966.9 ± ,044 ± 47.4 LA ± ± ± ± ± 19.9 LA n.d ± ± ± ,698.0 ± ,784.7 ± ,888 ± LA ± ± ± ± ± 65.2 LA n.d ± ± ± ,157.0 ± ,182.9 ± ,208 ± 53.1 LA n.d ± ± ± ,964.3 ± ,939.8 ± ,914 ± 46.1 LA n.d ± ± ± ± ± 22.6 LA ± ± ± ,760.4 ± ,793.4 ± ,833 ± 39.2 LA ± ± ± ,899.2 ± ,836.0 ± ,766 ± 39.8 LA ± ± ± ,752.9 ± ,808.6 ± ,874 ± LA n.d ± ± ± ± ± 11.8 LA ± ± ± ,803.3 ± ,827.8 ± ,856 ± 34.5 LA ± ± ± ,186.4 ± ,209.6 ± ,232 ± 51.7 LA n.d ± ± ± ± ± 10.4 LA ± ± ± ,660.7 ± ,785.8 ± ,936 ± LA ± ± ± ,895.2 ± ,917.4 ± ,942 ± 53.7 LA ± ± ± ,796.9 ± ,807.2 ± ,820 ± 54.1 LA n.d ± ± ± ,869.4 ± ,924.4 ± ,985 ± 49.6 LA ± ± ± ,603.1 ± ,671.7 ± ,760 ± LA ± ± ± ,990.7 ± ,005.1 ± ,020 ± 47.0 LA ± ± ± ,310.3 ± ,339.5 ± ,366 ± 61.7 LA n.d ± ± ± ,803.3 ± ,856.3 ± ,917 ± 48.9 LA ± ± ± ± ± ,060 ± LA ± ± ± ,719.4 ± ,775.0 ± ,842 ± 48.0 LA n.d ± ± ± ± ± 54.2 LA n.d ± ± ± ,657.4 ± ,760.7 ± ,886 ± LA ± ± ± ,723.5 ± ,750.8 ± ,784 ± 40.5 LA ± ± ± ,326.0 ± ,482.0 ± ,613 ± LA n.d ± ± ± ± ± 29.1 LA n.d ± ± ± ,795.5 ± ,808.2 ± ,823 ± 62.6 LA n.d ± ± ± ,296.6 ± ,384.2 ± ,461 ± LA ± ± ± ,736.0 ± ,793.7 ± ,862 ± LA ± ± ± ,998.2 ± ,145.1 ± ,290 ± LA n.d ± ± ± ± ± 17.4 LA ± ± ± ± ± 16.1 LA n.d ± ± ± ± ± 27.5 LA ± ± ± ,800.5 ± ,836.3 ± ,878 ± LA ± ± ± ,842.6 ± ,847.5 ± ,854 ± 38.0 c? LA n.d ± ± ± ± ± 33.0 LA n.d ± ± ± ,461.0 ± ,564.2 ± ,647 ± LA ± ± ± ± ± 48.2 LA ± ± ± ,861.2 ± ,864.5 ± ,869 ± 36.8 c? LA ± ± ± ± ± 18.4 LA ± ± ± ,659.9 ± ,765.7 ± ,894 ± LA ± ± ± ,916.9 ± ,871.6 ± ,822 ± 50.3 c? LA ± ± ± ,747.9 ± ,807.5 ± ,878 ± LA n.d ± ± ± ,761.3 ± ,815.9 ± ,880 ± LA ± ± ± ,737.5 ± ,799.2 ± ,872 ± LA ± ± ± ± ± 8.2 LA n.d ± ± ± ± ± 23.7 LA ± ± ± ± ± 15.1 LA n.d ± ± ± ± ± 22.8 LA n.d ± ± ± ± ± 33.3 LA ± ± ± ,703.7 ± ,751.0 ± ,808 ± LA n.d ± ± ± ± ± 20.0 LA n.d ± ± ± ,124.4 ± ,246.9 ± ,361 ± LA n.d ± ± ± ± ± 34.2 LA ± ± ± ,822.4 ± ,846.8 ± ,875 ± 33.3 LA n.d ± ± ± ,491.0 ± ,502.5 ± ,512 ± 46.9 LA n.d ± ± ± ,900.3 ± ,912.7 ± ,927 ± 34.9 LA n.d ± ± ± ,838.6 ± ,849.5 ± ,862 ± 46.7 LA n.d ± ± ± ,594.1 ± ,616.8 ± ,647 ± 46.0 LA n.d ± ± ± ± ± 8.0 LA ± ± ± ,657.2 ± ,730.5 ± ,821 ± LA n.d ± ± ± ± ± 9.6 LA n.d ± ± ± ,960.2 ± ,915.9 ± ,869 ± 32.5 G20 (n=60) g n.d ± ± ± ± ±

91 g n.d ± ± ± ± ± 12.1 g n.d ± ± ± ± ± 5.8 g n.d ± ± ± ± ± 13.6 g n.d ± ± ± ± ± 11.9 g n.d ± ± ± ± ± 9.0 g n.d ± ± ± ± ± 9.3 g ± ± ± ,817.6 ± ,848.2 ± ,883 ± 32.1 g n.d ± ± ± ,719.0 ± ,770.3 ± ,832 ± g n.d ± ± ± ± ± 13.7 g n.d ± ± ± ± ± 18.6 g ± ± ± ,120.6 ± ,232.0 ± ,336 ± g n.d ± ± ± ± ± 24.8 g n.d ± ± ± c? ± ± 7.3 g ± ± ± ± ± 6.7 g ± ± ± ± ± 20.3 g ± ± ± ,760.0 ± ,819.1 ± ,888 ± g ± ± ± ,862.1 ± ,927.9 ± ,000 ± g n.d ± ± ± ± ± 63.6 g n.d ± ± ± ± ± 40.1 g n.d ± ± ± ± ± 6.2 g n.d ± ± ± ± ± 27.2 g n.d ± ± ± ± ± 7.2 g n.d ± ± ± ± ± 10.3 g n.d ± ± ± ,779.2 ± ,846.6 ± ,924 ± g n.d ± ± ± ± ± 35.4 g n.d ± ± ± ± ± 35.1 g ± ± ± ,783.5 ± ,830.3 ± ,885 ± g n.d ± ± ± ± ± 46.8 g n.d ± ± ± ± ± 8.4 g n.d ± ± ± ± ± 5.3 g n.d ± ± ± ± ± 24.1 g n.d ± ± ± ± ± 14.4 g n.d ± ± ± ± ± 12.2 g n.d ± ± ± ± ± 5.6 g n.d ± ± ± ± ± 12.4 g n.d ± ± ± ± ± 23.0 g ± ± ± ,522.6 ± ,559.7 ± ,590 ± 62.1 g n.d ± ± ± ± ± 36.6 g ± ± ± ,768.4 ± ,784.7 ± ,804 ± 42.8 g n.d ± ± ± ± ± 29.8 g n.d ± ± ± ± ± 61.5 g n.d ± ± ± ± ± 48.1 g n.d ± ± ± ,752.8 ± ,805.1 ± ,867 ± g n.d ± ± ± ± ± 39.3 g n.d ± ± ± ± ± 7.7 g n.d ± ± ± ± ± 32.4 g n.d ± ± ± ,419.2 ± ,431.5 ± ,443 ± 50.5 g n.d ± ± ± ± ± 60.2 g ± ± ± ± ± 20.4 g n.d ± ± ± ,815.8 ± ,871.7 ± ,935 ± g ± ± ± ,199.7 ± ,326.2 ± ,440 ± g n.d ± ± ± ± ± 11.5 g n.d ± ± ± ± ± 21.7 g n.d ± ± ± ± ± 12.7 g n.d ± ± ± ± ± 27.2 g n.d ± ± ± ± ± 34.8 g n.d ± ± ± ,162.2 ± ,259.3 ± ,349 ± g n.d ± ± ± ,345.2 ± ,389.7 ± ,428 ± 53.1 c? g ± ± ± ,714.1 ± ,761.4 ± ,819 ± G30 (n=67) g n.d ± ± ± ± ± 9.9 g n.d ± ± ± ± ± 8.1 g n.d ± ± ± ,943.5 ± ,990.7 ± ,041 ± 66.1 g n.d ± ± ± ± ± 11.2 g n.d ± ± ± ± ± 19.3 g n.d ± ± ± ± ± 23.6 g n.d ± ± ± ,637.4 ± ,737.2 ± ,860 ± g ± ± ± ± ± 10.1 g n.d ± ± ± ± ± 10.3 g n.d ± ± ± ± ± 8.9 g n.d ± ± ± ± ± 13.2 g n.d ± ± ± ± ± 33.1 g n.d ± ± ± ,587.0 ± ,665.2 ± ,766 ± g n.d ± ± ± ± ± 3.8 g ± ± ± ,233.5 ± ,336.0 ± ,428 ± g n.d ± ± ± ± ± 7.8 g n.d ± ± ± ± ± 39.8 g n.d ± ± ± ± ± 4.9 g n.d ± ± ± ± ± 11.0 g n.d ± ± ± ± ± 12.2 g n.d ± ± ± ± ± 8.7 g n.d ± ± ± ± ± 35.2 g ± ± ± ± ± 14.6 g n.d ± ± ± ± ± 9.2 g n.d ± ± ± ± ± 8.2 g n.d ± ± ± ± ± 28.2 g n.d ± ± ± ± ± 26.9 g ± ± ± ,238.2 ± ,330.8 ± ,413 ± G n.d ± ± ± ,531.3 ± ,549.8 ± ,576 ± 39.7 G n.d ± ± ± ,252.0 ± ,362.2 ± ,459 ± G n.d ± ± ± ± ± 56.1 G n.d ± ± ± ± ± 13.1 G n.d ± ± ± ± ± 74.7 G n.d ± ± ± ± ± 10.6 G n.d ± ± ± ± ± 62.9 G n.d ± ± ± ± ± 34.1 G n.d ± ± ± ± ± 11.9 G n.d ± ± ± ± ± 15.0 G n.d ± ± ± ,402.1 ± ,487.3 ± ,558 ± G n.d ± ± ± ± ±

92 G n.d ± ± ± ± ± 45.0 G n.d ± ± ± ± ± 9.0 G n.d ± ± ± ± ± 11.9 G ± ± ± ,846.2 ± ,848.3 ± ,851 ± 96.1 G n.d ± ± ± ,729.4 ± ,804.9 ± ,894 ± G n.d ± ± ± ± ± 58.2 G n.d ± ± ± ,535.0 ± ,561.1 ± ,583 ± G ± ± ± ± ± 38.3 G ± ± ± ± ± 48.8 G n.d ± ± ± ± ± 46.8 G ± ± ± ,858.5 ± ,864.2 ± ,871 ± 56.2 G n.d ± ± ± ± ± 14.5 G ± ± ± ,326.7 ± ,379.2 ± ,425 ± 72.0 G n.d ± ± ± ± ± 17.0 G n.d ± ± ± ± ± 15.8 G n.d ± ± ± ± ± 16.6 G n.d ± ± ± ± ± 14.7 G n.d ± ± ± ± ± 37.3 G n.d ± ± ± ± ± 29.8 G ± ± ± ,694.8 ± ,771.6 ± ,864 ± G ± ± ± ,661.3 ± ,750.2 ± ,859 ± G n.d ± ± ± ,802.0 ± ,845.7 ± ,896 ± 50.8 G n.d ± ± ± ± ± 49.7 G ± ± ± ± ± 32.9 G n.d ± ± ± ± ± 16.7 G n.d ± ± ± ± ± 2.7 G n.d ± ± ± ,191.5 ± ,351.0 ± ,493 ± G40 (n=74) G n.d ± ± ± ,571.3 ± ,649.8 ± ,752 ± G n.d ± ± ± ± ± 11.0 G ± ± ± ,721.3 ± ,715.0 ± ,708 ± 59.6 G n.d ± ± ± ± ± 45.3 G n.d ± ± ± ± ± 31.5 G n.d ± ± ± ± ± 31.3 G n.d ± ± ± ± ± 41.4 G n.d ± ± ± ± ± 27.8 G n.d ± ± ± ± ± ,114 ± G n.d ± ± ± ± ± 27.9 G n.d ± ± ± ± ± 52.7 G n.d ± ± ± ± ± 9.1 G n.d ± ± ± ± ± 10.5 G n.d ± ± ± ± ± 25.4 G n.d ± ± ± ± ± 20.8 G n.d ± ± ± ± ± 45.6 G ± ± ± ± ± 16.9 G ± ± ± ± ± 20.3 G n.d ± ± ± ± ± 59.9 G n.d ± ± ± ± ± 10.6 G n.d ± ± ± ± ± 35.1 G n.d ± ± ± ± ± 34.6 G n.d ± ± ± ,251.4 ± ,369.9 ± ,474 ± G n.d ± ± ± ± ± 16.7 G n.d ± ± ± ,437.5 ± ,505.7 ± ,562 ± 67.3 G n.d ± ± ± ± ± 9.7 G n.d ± ± ± ± ± 10.2 G ± ± ± ± ± 15.4 G n.d ± ± ± ± ± 10.5 G n.d ± ± ± ± ± 14.4 G n.d ± ± ± ± ± 29.6 G n.d ± ± ± ± ± 11.9 G n.d ± ± ± ± ± 28.3 G n.d ± ± ± ± ± 30.9 G n.d ± ± ± ± ± 16.9 G n.d ± ± ± ± ± 21.4 G n.d ± ± ± ± ± ,088 ± G n.d ± ± ± ,759.6 ± ,833.8 ± ,920 ± G ± ± ± ± ± 10.2 G n.d ± ± ± ± ± 48.8 G n.d ± ± ± ,847.4 ± ,834.0 ± ,819 ± 51.6 G n.d ± ± ± ± ± 37.6 G n.d ± ± ± ± ± 19.4 G ± ± ± ± ± 32.5 G n.d ± ± ± ± ± 14.1 G n.d ± ± ± ± ± 17.0 G n.d ± ± ± ± ± 11.6 G n.d ± ± ± ,366.4 ± ,448.1 ± ,517 ± G n.d ± ± ± ± ± 43.1 G n.d ± ± ± ± ± 11.1 G ± ± ± ,640.2 ± ,739.4 ± ,862 ± G ± ± ± ,912.2 ± ,895.6 ± ,878 ± 41.6 G n.d ± ± ± ± ± 24.6 G n.d ± ± ± ± ± 34.9 G n.d ± ± ± ± ± 74.9 G n.d ± ± ± ± ± 48.1 G n.d ± ± ± ± ± 15.0 G ± ± ± ± ± 62.6 G ± ± ± ± ± 23.0 G n.d ± ± ± ± ± 9.9 G n.d ± ± ± ± ± 9.1 G n.d ± ± ± ± ± 10.6 G n.d ± ± ± ± ± 53.3 G n.d ± ± ± ± ± 31.5 G ± ± ± ,388.6 ± ,412.2 ± ,433 ± 63.8 G ± ± ± ,580.0 ± ,540.9 ± ,510 ± 54.4 G n.d ± ± ± ± ± 29.6 G n.d ± ± ± ,894.9 ± ,910.4 ± ,928 ± 42.4 G n.d ± ± ± ± ± 16.2 G n.d ± ± ± ,755.7 ± ,853.8 ± ,966 ± G ± ± ± ± ± 6.0 G n.d ± ± ± ± ±

93 G n.d ± ± ± ± ± 48.4 G n.d ± ± ± ± ± 2.8 G62 (n=62) G n.d ± ± ± ± ± 17.7 G n.d ± ± ± ± ± 80.3 G n.d ± ± ± ± ± 25.9 G n.d ± ± ± ± ± 47.7 G n.d ± ± ± ± ± 25.8 G n.d ± ± ± ± ± 62.0 G n.d ± ± ± ± ± 54.6 G ± ± ± ± ± 10.4 G n.d ± ± ± ± ± 9.8 G n.d ± ± ± ± ± 35.1 G n.d ± ± ± ± ± 12.6 G ± ± ± ± ± 48.4 G n.d ± ± ± ± ± 39.6 G n.d ± ± ± ± ± 30.1 G n.d ± ± ± ± ± 13.8 G n.d ± ± ± ± ± 43.6 G n.d ± ± ± ± ± 13.5 G n.d ± ± ± ± ± 44.9 G n.d ± ± ± ± ± 5.7 G ± ± ± ,526.3 ± ,529.1 ± ,532 ± 27.6 G n.d ± ± ± ± ± 13.2 G n.d ± ± ± ,552.0 ± ,550.6 ± ,550 ± 90.5 G ± ± ± ± ± 12.9 G ± ± ± ,809.5 ± ,825.2 ± ,844 ± 74.5 G n.d ± ± ± ± ± 16.6 G n.d ± ± ± ± ± 12.8 G ± ± ± ,864.6 ± ,994.1 ± ,132 ± G n.d ± ± ± ± ± 50.2 G n.d ± ± ± ± ± 12.5 G n.d ± ± ± ± ± 10.6 G ± ± ± ± ± 19.9 G n.d ± ± ± ,368.3 ± ,437.1 ± ,496 ± 71.3 G n.d ± ± ± ± ± 42.5 G n.d ± ± ± ± ± 10.7 G n.d ± ± ± ± ± 10.6 G n.d ± ± ± ± ± 16.0 G n.d ± ± ± ± ± 17.9 G n.d ± ± ± ,576.4 ± ,541.1 ± ,514 ± 73.1 G n.d ± ± ± ,981.5 ± ,994.7 ± ,009 ± 71.4 G n.d ± ± ± ± ± 9.2 G ± ± ± ± ± 9.0 G n.d ± ± ± ± ± 24.9 G n.d ± ± ± ,957.9 ± ,971.2 ± ,986 ± 65.3 G n.d ± ± ± ± ± 35.4 G n.d ± ± ± ± ± 6.0 G n.d ± ± ± ± ± 37.6 G n.d ± ± ± ± ± 29.7 G n.d ± ± ± ± ± 9.9 G n.d ± ± ± ± ± 25.3 G ± ± ± ± ± 10.0 G ± ± ± ,339.0 ± ,572.1 ± ,706 ± G n.d ± ± ± ± ± 21.9 G ± ± ± ,790.6 ± ,792.5 ± ,795 ± 60.2 G n.d ± ± ± ± ± 47.3 G n.d ± ± ± ± ± 22.8 G n.d ± ± ± ± ± 39.0 G n.d ± ± ± ± ± 33.1 G n.d ± ± ± ,158.7 ± ,193.4 ± ,227 ± 62.0 G n.d ± ± ± ± ± 10.1 G ± ± ± ± ± 23.2 G n.d ± ± ± ,394.3 ± ,611.9 ± ,736 ± G n.d ± ± ± ± ± 22.5 * Percentage of 206 Pb contributed by common Pb on the basis of 204 Pb. Value of common Pb was assumed by Stacey and Kramers (1975) model; n.d. : no detection of 204 Pb. ** Degree of discordance (%); negative numbers and blanks show normal discordant and concordanct within 2σ of the analytical error, respectively. 82

94 This westernmost sample of the southeastern Yellow Sea yielded 62 zircon U-Pb ages. The large proportions of zircons have ages between 858 and 627 Ma (21%) and between 228 and 189 Ma (21%) with peaks at 734 Ma and 200 Ma, respectively. Another major population has a ca. 443 to ca. 379 Ma age range (13%) with a peak at 409 Ma. Other minor zircon groups have Early Cretaceous ( Ma, 8%), Late Neoproterozoic ( Ma, 3%), Late Paleoproterozoic ( Ma, 5%), and Late Archean-Early Paleoproterozoic ( Ma; 6%) ages. 4.5 INTERPRETATION & DISCUSSION ZIRCON AGE POPULATIONS OF SOUTHEASTERN YELLOW SEA SEDIMENTS Considering that sandy sediments were presumably transported by paleo-fluvial processes and subsequently reworked during the transgressions, zircon age distribution of the southeastern Yellow Sea sediment needs to be focused on major age groups (>10%), as population size of the minor age groups (<10%) of detrital grains in a fluvial sediment is not repeatable even though the presence itself is significant (Link et al., 2005). This reduces possibility for misinterpretation of their provenances by modification of the original zircon age distribution due to reworking during the postglacial transgression. The selected zircon age distributions are subdivided into age groups of the Mesozoic, Paleozoic, Neoproterozoic, Paleoproterozoic, and Neoarchean (Figs. 4-2 and 4-3). These 83

95 84 Fig Probability-density plots of the detrital zircon U-Pb ages in the southeastern Yellow Sea sediments. Note two age scales.

96 85 Fig Proportions of detrital zircon age groups in the southeastern Yellow Sea sediments. KP represents the Korean Peninsula and SC represents South China.

97 age groups show gradual changes in their relative proportions according to sample locations in the E-W direction; westward increasing proportions of Late Triassic-Early Jurassic, Middle Paleozoic, and Early Paleozoic-Neoproterozoic ages, and diminishing proportions of Paleoproterozoic ages. The broad and unique range of Paleozoic- Neoproterozoic zircon ages observed in the western samples, shown as numerous peaks in Figure 4-2, suggests that the source terrain for these samples was spatially large in extent or diverse in its crystallization ages, or recycled from a sedimentary source that originally tapped a large source terrain PROVENANCE OF THE SOUTHEASTERN YELLOW SEA SEDIMENTS Mesozoic igneous rocks on the East Asian continental margin were mainly generated by arc magmatism due to the subduction of the paleo-pacific Plate (Choi et al., 2012; Kim, 1996; Maruyama et al., 1997), which had been active throughout the Mesozoic era. The most likely source igneous rocks for the Mesozoic zircons in the study area seem to be located in the coastal region of South China and the Korean Peninsula (Okada, 2000; Shin and Jin, 1995). Igneous rocks of Paleozoic age are well defined in China due to several collision events (Zhang et al., 2004), but are little known in the Korean Peninsula except for detrital zircons of Paleozoic in age (Cho, 2007; Cho et al., 2010). Neoproterozoic igneous rocks having zircon peak ages in the studied samples are not common in the East Asian continent. Several studies reported Neoproterozoic igneous rocks of ca Ma from China and the Korean Peninsula (Cho, 2001; Kwon et al., 1995; Peng et al., 2012; Zhao et al., 2011 and references therein). Paleoproterozoic, 86

98 especially Ga, and Archean ages are common in Precambrian rocks of the North China and the Korean Peninsula (Darby and Gehrels, 2006; Zhai et al., 2007). Considering the above-mentioned age distributions of basement rocks in the East Asian continent, it seems that three eastern samples of the southeastern Yellow Sea sediments (G3, G7, G15) were mainly derived from the Korean Peninsula, whereas two western samples (G40, G64) were supplied mostly from South China. The samples in the middle part, G20 and G30, seem to represent the mixture of these two sources. Quantitative comparison of zircon age distributions with those from sediments of rivers draining to the Yellow Sea provide better information of provenance, because zircon age distributions of river sediments can be quite different from the basement geology of their drainage area (c.f., figs. 7(c) and (g) of Rino et al., 2008). Recent studies on major river sediments around the Yellow Sea (Iizuka et al., 2010; Rino et al., 2008; Wu et al., 2007b; Yang et al., 2009; Yang et al., 2007) provided detrital zircon U- Pb age distributions of the river sediments. Detrital zircon U-Pb ages of South Korean river sediments are from Choi et al. (2012) and the authors unpublished data. Zircon age distributions of the major river sediments supplied from the China Craton and the Korean Peninsula were statistically compared among the major age groups (>10%) of the southeastern Yellow Sea sediments using Kolmogorov-Smirnoff (K-S) test (Press et al., 1986) and the results are shown in Table 4-2 and Figure 4-4. A P value is determined by the K-S test and where P > 0.05, one is unable to reject the null hypothesis, with 95% confidence, that two age populations were selected randomly from the same parent population. Based on the P values of these sediments, the detrital zircon 87

99 Table 4-2. P values for K-S test of age distributions of detrital zircon U-Pb ages of southeastern Yellow Sea sediments. P values > 0.05 are highlighted. Luan River Yongding River Yellow River Hanjiang River Yangtze River G62 G40 G20 G30 G15 G7 G3 South Korean rivers North Korean rivers Luan River Yongding River Yellow River Hanjiang River Yangtze River G G G G G G G South Korean rivers North Korean rivers : Yang et al. (2009); 2: Yang et al. (2007); 3: Iizuka et al. (2010); 4: Choi et al. (2012) and Choi et al. (unpublished); 5: Wu et al. (2007). 88

100 Fig Possible provenance discrimination of southeastern Yellow Sea sandy sediments based on their P values in Table

101 age distributions can be subdivided into two different groups; 1) G3, G7, G15, and Korean river sediments and 2) G20, G30, G40, G62, and Hanjiang and Yangtze river sediments whose catchment basins are in South China (Fig. 4-4). Sediments of Group 1 might have been derived exclusively from the southern Korean Peninsula except G15, of which provenance also includes the northern Korean Peninsula. The Yangtze and Hanjiang rivers might supply sandy sediments to Group 2 samples except for G40, of which provenance might be also different from G30 due to sediment mixing with Group 1. Accordingly, an additional local source, other than the Yellow River (Table 4-2), might have provided sand grains to G40 and probably to G20 and G62. Shelf bedrock eroded during the transgression may have been a possible source (c.f., Jiang et al., 2000). This result is consistent with that inferred from the basement rock geology in this study. Sediment supply from North China through the Yellow River seems insignificant to the southeastern Yellow Sea sandy sediments, which is not in agreement with the result of the previous study on the fine sediments in the same area (Cho et al., 1999), probably due to a long distance between these source regions and the southeastern Yellow Sea as well as mostly silty grain size of the Yellow River sediment load THE CAUSE OF E-W VARIATION OF DETRITAL ZIRCON AGE SPECTRA E-W variation of detrital zircon age populations in the southeastern Yellow Sea sediments might have been caused by differences in amounts of sand grains supplied from two main provenances. Rising sea-level first occurred in the eastern part (Lee and Yoon, 1997; Uehara and Saito, 2003) might have affected a sediment supply from the 90

102 Korean Peninsula selectively. Assuming that the topography and hydraulic conditions of sediment transport of the Yellow Sea have not been significantly changed during the post-glacial transgression and accepting the present grain size distribution in the Yellow Sea, the successive coastline positions of the Yellow Sea can be depicted as shown in Figure 4-5. According to the reconstructions, sandy sediment might have been supplied to the locations of water depth of ca. 80 m from the Korean Peninsula and of water depth of ca. 40 m from South China. Rising sea level caused trapping of sandy sediments in the coastal zones and prevented supplying them to the southeastern Yellow Sea. As the sea level rising started from the eastern part of the Yellow Sea, sandy sediment transport from the Korean Peninsula had been reduced earlier than that from South China. At the beginning of the sea level rising after deglaciation, the Korean Peninsula might have supplied sandy sediments to the eastern part (G3, G7, and G15) and the mixing zone (G20 and G30), while rivers from South China might have transported sediments to the western part (G40 and G62) and the mixing zone (G20 and G30). Then, when the sea level reached the level m lower than the present, the sandy sediment supply from the Korean Peninsula to the deepened paleo-valley at the mixing zone was cutoff, while rivers of South China might have continuously provided sandy sediments to the valley. Thus, sea level rising and associated reworking during the transgression might have controlled the relative proportions of sediment supply between South China and the southern Korean Peninsula, resulting in a gradual E-W variation of detrital zircon age populations in the surface sediments of the southeastern Yellow Sea. 91

103 92 Fig Paleo-coastlines of the Yellow Sea during the Holocene successive transgressive stages, assuming that its topography has not been changed sea level since the last glacial stage: (a) -100m, (b) -80m, (c) -60m, and (d) -40m lower than the present.