KR9700364
Overview of the Tectonic Environment in Korea with Reference to HLW Disposal
Chun-Soo Kim KAERI
1. Introduction It is worldwidely accepted by most of the countries with nuclear energy programs that deep geological disposal in a stable geological environment is a feasible means of isolating HLW for very long time. The geological condition of the Korean peninsula, a major link between the Pacific active margin and the Asian mainland, should be reviewed in terms of the stability over geological time scales for a deep geological repository. Comprehensive understanding of the tectonic evolution of the peninsula will be great help to develop the technical feasibility and performance assessment methodology for HLW disposal in geologic formation. A significant research is recently carried out as an integrated geoscience approach on the tectonics and geodynarnics of the Eastern Asian continent. However, many hypotheses on tectonic evolution should be proved by further studies. This short paper is summarized on the long stability of the Korean peninsula from data available at present. Most of the information are based on Geology of Korea(1987), Geology of Korea(1996), and Tectonic Evolution of Eastern Asian continent(1997).
2. Tectonic Setting
2.1 Tectonic Evolution The Korea peninsula is located in the area where the Eurasian continent is contacted with the west Pacific mobile belt. Whereas the Japan archipelago is characterized by active mobile belt, the Korean peninsula has a close affinity with the Asian continent in geology and tectonic setting(Fig. 1). In a broad category, the peninsula occupying the eastern margin of the Korea-China platform belongs to a part of the shield area regarded as stable land of cratonic nature, but has some differences from the stable platform. In platforms, deep rifts are usually absent and major uplifts/troughs do not have linear structures, and igneous activity is usually slight without granitic intrusions. On the other hand, deep rifts in the peninsula cut the basement and troughs formed in the Paleozoic period are usually linear, and Mesozoic and Cenozoic troughs are filled with thick continental clastic sediments. Moreover, the orogenies occurred were accompanied by volcanic and granitic intrusion, and neotectonic movements are relatively intensive. These tectonic characteristics are considered as the marginal geosyncline phenomena of platform with superimposed tectonic elements during the Meso-Cenozoic era. The Korea-China platform was converted into a stable massif after the crustal movement in the late Lower Proterozoic era(1.8 Ga ago)which is known as a great transitional age in crust development. Unlike other platforms in the world, The Korea-China platform was under a strongly mobilized state after having been converted into a platform. There were the rift type mobile belts such as Imjingang and Okchon Fold belts during the middle Paleozoic age, foreland basin in the margin of the platform during the late Meso-Cenozoic age, and the tectonic sturctures of the Meso-Cenozoic age were superimposed on the peninsula. The tectonic evolution of the Korean peninsula can be divided into three stages as follows : • Ancient geosyncline stage : Archean(2.6 Ga) - Proterozoic(1.8 Ga) • Stable platform stage •' Middle Proterozoic(1.7 Ga) - Late Paleozoic(260 Ma) • Stage influenced by the movement of west Pacific plate : Mesozoic(230 Ma) - Holocene(present) For the geotectonic units of the peninsula, the Mesozoic orogeny is most important. In the wide area of East Asia, magmatism, volcanism, and the deformation/metamorphism of the sedimentary covers of platform occurred intensively during the Mesozoic era. The tectonic movement in this era is explained as a transitional development of platform into geosyncline stage. The Mesozoic tectonic activity started in the Middle Triassic period is the most vigorous crustal movement in the entire Korean peninsula, accompanied by folding, fault bock movement and igneous intrusion. These phenomena are special tectonic movements which are not found in the platform or geosyncline zone in general. It is considered that more intensive orogeny and fault blocks followed by magmatism within the continent occurred due mainly to the influence of the subduction of the west Pacific plate. Such tectonic movements intensified gradually from the north to the south and from the west to the east, resulted from the eastward subduction zone of the Pacific plate. The tectonic movements in the peninsula are considered to be divided into three correlated stages, one after the other from the beginning of the Mesozoic era as follows : • Middle-Upper Triassic(210-180 Ma) : - Songnim disturbance in Korea, - Indosinian movement in China - Akiyoshi movement in Japan • Middle-Upper Juras sic (180-136 Ma) - Daebo orogeny in Korea - Early phase of the Yenshan movement in China - Mid-phase of the Sakawa movement in Japan - Nevadan orogeny in North America • Upper Cretaceous-Paleocene( 120-40 Ma) - Bulguksa disturbance in Korea - Last stage of the Yenshan movement in China - Alpine orogeny in Europe - Laramide orogeny in North America After the Mesozoic tectonic movements, the whole of Korea had been uplifted. The Cenozoic tectonic activity was not intensive as the previous tectonic movements and is generally represented by mafic to intermediate volcanic activities in limited areas. Pleistocene alkali volcanic eruption took places in NNE trending rift zones as well as the Mt. Paektu, UUung and Cheju islands. 2.2 Tectonic Provinces The tectonic framework of the Korean peninsula is considered to consist of the eastern extension of the Korea-China platform in the northern part of the peninsula and the northeastern extension of the Yangtze platform in the southern part. However, some differences exist in geologic settings and structural events between the peninsula and the continent, and thus the further study is necessary to reach the conclusive correlation. Since the tectonic province was first described in 1933, many revision made on the basis of the geological time unit rather than structural distinction(Fig. 2). More recently the division of tectonic units was proposed by the stabilization time of the original crust. Among the tectonic units proposed, massif and fold belts which are more relating to radioactive waste disposal are as follows ; • Archean - Early Proterozoic massif - Rangnim massif (North Korea) - Kwanmo massif (North Korea) - Kyonggi massif (South Korea) - Ryongnam massif(South Korea) • Upper Proterozoic-Upper Paleozoic fold belt - Okchon fold belt(Upper Proterozoic; South Korea) - Imjingang fold belt(Middle Paleozoic; North Korea) - Tumangang fold belt(Upper Paleozoic! North Korea) The Kyonggi massif is located in the central part of the Korean peninsula and bounded by Imjingang fold belt and Okchon fold belt in the north and south, respectively. In the oldest folded basement of Kyonggi massif, metasomatic granites are widely developed and highly metamorphosed schists and migmatites in the metasomatic outliers exhibit mostly amphibolite to granulite facies. Also, the metasedimentary rocks consisting of quartzite, quartz schist are extensively exposed in the southwestern part of the Kyonggi massif. The fault structure has largely two directions. One is parallel to the tectonic direction of NE distributed in the south of the massif, the other has a NNE direction in the north of the massif. The Ryongnam massif, located in the southern part of the peninsula, is bounded by the Okchon fold belt on the north. On the southeast, it contacts to the basement of Cretaceous sedimentary basin(Kyongsang basin). The massif consists of gneiss and schist in early Proterozoic metamorphic group and is characterized by a ploymetarnorphic region due to orogenies in many times. The crystalline basement is exposed mainly on the northeastern part and various kinds of gneiss, migmatite and crystalline schist are developed in the southwestern part. Fault systems developed are in NE-NNE, NWW and EW-NEE directions. Faults with a NE direction proedominate on the southwestern part of the peninsula and faults with a NNE direction increase gradually towards the southeast. Faults with a EW-NNE direction are characterized as thrusts inclined to the south. The Okchon fold belt is extended over 450km in length and 50 ~ 80km in width, between the Kyonggi and Ryongnam massifs. The fold belt is divided into two regions on the basis of metamorphic fades, metamorphic zone in the southwestern region and nonmetamorphic zone in the northeastern region.
3. Igneous Activity The intrusive and effusive rocks in the Korean peninsula were formed from the Archean to the Quaternary and the plutonic rocks are occupied by nearly one-half of the peninsula(Fig. 3). The majority of igneous rocks in Korea are granites and their varieties. Intermediate plutonic rocks of mainly diorite are exposed as small stocks in the southern part of the inland and along the east coast of the northern part. The distribution of mafic and ultramafic plutonics is almost limited to the tectonic regions. Among these plutonic rocks, the rock mass suitable for a host rock of repository based on the areal extent and tectonic environment is summarized in Table 1 .
4. Volcanism Volcanism in the peninsula has occurred thoughout most of the geological ages from Archean to Holocene with igneous activity. But the most prominent volcanism was taken place in the period of the Upper Jurassic- Lower Cretacepus(160~100Ma ago). Volcanic activities in these periods took place along large fault zones and were vigorous during sedimentation in the Mesozoic basin. Volcanic activities in the basin play an important role in establishing the stratigraphy sequences of the Mesozoic basins. The Cretaceous volcanism continued up to the early Tertiary(50Ma ago). But it was relatively inactive throughout the Paleogene and intermittently in the Neogene. The volcanism was continued to the Quaternary in several areas, i.e. Mt. Paektu, Chugaryong fault zone and Cheju island. According to historical records, several volcanic activities in Korea were dated between 1000 and 1600 AD.
5. Seismicity
In terms of plate tectonics, the Korean peninsula is located within the Eurasian plate. The Pacific plate subducts under the Eurasian plate in Japan and under the Indo-Australian plate in New Guinea, while the Indo-Australian plate collides with the Eurasian plate in the Himalayas. Also the Philippine plate subducts under the Eurasian plate along the Philippine trench. Therefore, earthquakes in Korea are ascribed to the intraplate seismicity (Fig. 4). In Korea, historical earthquake records date back to 2 AD and the instrumental data were collected from 1905. Based on historical seismicity, the southeastern part and the southwestern part of the peninsula turned out to be seismic zones, whereas the seismicity of the Rangnim Massif and Paektu volcanic zone in the northern part is remarkable weaker than the other tectonic provinces(Fig. 5). The distribution of earthquakes associated with the instrumental data shows that the seismicity is stronger in the southern and western parts of the peninsula than in the other regions(Fig. 6). However, deep sources of earthquakes are distributed in the northeastern part and in the East sea. In general, the close correlation between seismicity and Quaternary tectonics in the Korean peninsula can not be found. Quaternary tectonics are rather limited to volcanic eruptions in certain regions. Most of the earthquakes might occur in zone of weakness, mainly along faults. Particularly, earthquakes having MM intensity more than 4 are related with fault structures. It appears that many of the faults created during the Mesozoic orogenies have generated earthquakes afterwards. Therefore a number of major faults and tectonic boundaries of the peninsula may be active even though they were formed before the Cenozoic. The intraplate seismicity is characterized by a high degree of irregularity both in space and time. The depths of earthquake sources are estimated to 10— 12km below ground in general. The most destructive earthquakes since 1905 were Ssanggyesa earthquake on July 4, 1936 and Hongsung earthquake on October 7, 1978. The MM intensity of these earthquakes is VII and the magnitude of the Hongsung earthquake turned out to be about 5.2. According to the historical earthquake records, the strengthening period in Korea is for about 500 years and the weakening period for about 200 years. This tendency have alternated each other from 2 AD to the present.
6. Conclusion Korea is located in a stable platform which was subjected to major periods of significant tectonic movement between about 180 and 100 million years ago. And then subsequent tectonic activities have been diminished and limited to in local areas. In the present stage, the distruptive natural phenomena are characterized as sudden activities, i.e. volcanisim and seismicity, in the localized particular areas, which can be avoided by excluding certain areas in the early stage of siting. The plutonic intrusion in Korea occurred from the Proterozoic-Cretaceous era, resulting in a large areal extent. The wide distribution of plutonic rock is important consideration as the potential host rock, allowing flexibility of siting. The fracture zones developed in the peninsula are more or less systematic and most of the fracture zones in large scale has been reactivated subsequently along pre-existing fractures. Thus the competent host rock mass for a repository will be available in the distributed area of plutonic rocks. REFERENCES
[1] Geology of Korea(1987) (ed. Lee, D.S.), Geological Society of Korea, Kyohaksa Co., Seoul, 514p. [2] Geology of Korea(1996) (chief ed. Paek, R.J.), Institute of Geology, State Academy of Sciences DPR of Korea, Foreign Languages Books Publishing House, Pyongyang, 631p. [3] Tectonic Evolution of Eastern Asian Continent(1997) (ed. Lee, Y.I. and Kim, J.H.), International Symposium on the Occasion of the 50th Anniversary of the Geological Society of Korea, Sept. 24-25, 1997, Seoul, Korea, 273p. [4] Cluzel, D,. Lee, B.Y. and Cadet, J.P., 1991 Indosinian dextral ductile fault system and synkinematic plutonism in the southwest of the Ogcheon belUSouth Korea). Tectonophysics, 194:131-151. [5] Chwae U. and Jun, M.S.Q996) The seismotectonic interpretation of Korean Peninsula, in 1996 Symposium on seismology in East Asia, KIGAM, Oct. 15-18, 1996 Daejon, Korea. [6] Lee, K.(1996) Earthquakes of Korea, in 1996 Symposium on seismology in East Asia, KIGAM, Oct. 15-18, 1996 Daejon, Korea. Table 1. Characteristics of plutonic rocks as potential host rock of HLW repository
«impkx t4 («£ru&*ve active?
Granite bodies by - Garnet-biotite gneissic In graduation with granitization, metasomatism in granite Archean Early-Proterozoic, Archean to Jirisan complex, folding stage, partly dykes of - Pophyroblastic granite penetrated by dykes Kyonggi complex Early Proterozoic magmatic origin - Leuco granite - Pegmatitic granite
Bunchon granite Batholith, stock of contemporaneous - Biotite granite Injector intrude Archean (2100+50 Ma) or post stages of geosyncUne fold - Leuco granite & Lower Proterozoic, Hongjesa - Granodiorite overlain by Lower-Middle granite gneiss - Quartzdiorite metasediinentary group Proterozoic (1800-1750 Ma) Sosan granite gneiss (1420±70 Ma)
Batholith, stock along fracture in - Gabbro diorite Penetrate Upper Paleozoic, Deabo granites platform - Biotite granite overlain by Upper Jurassic Jurassic (180-140 Ma) - Fine granied two mica granite
Intrusion in forms of stock or plate - Quartz diorite Penetrate Upper Jurassic Buiguksa granite along fracture in platfrom - Graphic granite and Certaceous, overlain Cretaceous (-135 Ma) - Granoporphyry by Upper Cretaceous- - Alkali granite Paleocenc 118°E MONGOLIA
- 40° N
X4AEAN PlATfttRJA
I18«E
Fig. 1. General tectonics setting of the Korean Peninsula[4] 1. Tuman Basin 2- Kwanmobong Massif 3. Tanchon Folded Ben 3-1. Amnok Folded Belt 4. Nangnim Massif 5. Pyongnam Basin 6. Kyonggi Massif 6-L Ongjin Basin 6-2 Ctiungnam Depression Zone 6-3. Ktngju Depression Zone 7. Okch'on Foided Belt 7-L Okchon Neogeosyndinal Zone 7-2 Okch'on Paleosyndinal Z«.;e 8. Ryongnam Massif 8-1. Taebaeksan Zo« 8-2. Chinsan Zone 9. Kyongsang Basin 9-1. Yongdong-Kwangju Depression Zone 10. Yonrl Basin 11 Cheju Volcanic Island A. KilchuMyongction Graoen B. Ch'ugaryong Fault Zone
Fig. 2. Detailed tectonic provinces in Korea[l] TT
UGENO
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Fig. 3. Distribution of granitic rock mass in KoreaQ] 1904 1/1 0: 0 -- 1992 12/ II 24: 0 M: 4.5- ».8
7 •• e ' :
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60
Fig. 4. Seismicity map of shallow earthquakes (focal depth<60 km and magnitute>4.0) in E. Asia(1964~1992)[5] 126 128 130
42 -42
40 -40
38- -38
36- -36
34. -34
0 60 100 200KM
124 126 128 130
Fig 5. Epicenters of Korean historical earthquakes having MMI equal to or greater than V during A.D. 2-1905[6] 124 126 128 130 r Magnitude
42 42
40 40
38 38
36 36
• o
34 34
0 50 100 200KM
124 126 128 130
Fig. 6. Epicenters of Korean instrumental earthquakes having magnutude equal to or greater than 4.0 during A.D. 1905-1993[6] Overview of the Tectonic Environment in Korea with Reference to HLW Disposal
October, 1997
Chun Soo Kim
KAERI, Korea XISAICH VOI LANK H(X>:' PUITONK H.tXT- GEOLOGIC TIME (Ma Ago) Dec. 31
A| AH LH
n-NQZOIC
Dec. 25
MESOZOIC
Dec.
:n
PALLOZOIC
TVou. 13
PROTEROZOIC
July 17_
ARCHEAN Jan. 1 CALENDAR : YEAR -egend of geological map of Korea(l 1,000,000). 5 Archean Craton
Rift H tr Accreted Crust 3c rt> tor i-i s VD (u a> 50 rt vo O Island Arc; Accreting o o Subduction Zone n M p) Ocean Island Chain o re
O s Oceanic Lithosphere rt O o ••d a M Mid-ocean Ridge o ^> o p) n> D. W ft) M •O (B rt W rt) Rift; Passive Margin O.g l-h Q- i-l O Archean Craton S i—i Paleoproterozoic igneous — rocks fr\ Paleoproterozoic igneous rocks Neoproterozoic groups Mesoproterozoic groups 3 Paleoproterozoic groups • Archean groups
Precambrian groups in Korea, northeastern China and Japan. Sokcho EAST SEA OF KOREA
WEST SEA OF KOREA
25 50km I I
Tectonic schema of Kyonggi Massif. (T) Kimpho-Kansong Fault Zone, (2) Kongju Fault, (3) Wonsan-Seoul Fault Zone. The legend is the same as Fig.7.3. Only 1 means the Kyonggi and Sosan Groups and 2 points to the Chunchon Group. hhk
SOUTH SEA OF KOREA Tectonic Schema of Ryongnam Massif, Okchon Fold Belt, Raktonggang Basin, Yong-il Basin and Samchok Basin. The legend is basically the same as Fig.7.3. Besides, the following ones are added. (A) Okchon Fold Belt, (B) Ryongnam Massif, (C) Raktonggang Basin, (D) Yong-il Basin, (|) Samchok Basin. Tertiary Tertiary strata Cretaceous — Tertiary III[1 H Granitic rocks & Porphyries intrusion Yuch'on Group Cretaceous I I Hayang Group Shindong Group Pre- Cretaceous [I Basement roc (>__ Zfi jo k SimpUfied geologic map of the Kyongsang Basin. Distribution of the Cretaceous sedimentary and volcanic rocks. Several selected sedimentary basins described in text are: Kyongsang Basin(l), Yongdong Basin(2), Chinan Basin (3), Umsong Basin(4), P'ungam Basin (5), Basin along Chaeryong River (A), Basin near Pydnyang (B), and Basin near Onsong (C). RUSSIA F.AST SEA OF KOHEA YO CD SOUTH SEA Of KOREA Jtju 1(1 Deep fracture diagram deciphered on space photo (1:4,000,000). A. Clear fracture, B. Unclear Fracture, C. Circular structures. EAST/CHINA SEA s Schematic development model of the Yellow Sea Basin(Cretaceous-Tertiary) (after Jun et al., 1994) 129 130 Correlationship between geological map and distribution of local events during 1988-1996. RUSSIA C H f EAST SEA OF KOREA WKnsan Ullung Is. o I WEST SEA OF KOREA Tok Is. 00 SOUTH SEA OF KOREA 66km Jeju Is. Fracture diagram of Korea. 1. First class fractures, 2. Second class fractures, 3. Third class fractures, 4. Fractures determined by space photograph and geo- physical data, 5. Normal fault, 6. Reverse fault, 7. Thrust, 8. Wrench fault, 9. Active fault. Names of fault zones: 1. Susongchon, 2. Kyongsong-Kilju, 3. Musan-Kimchaek, 4. Sodusu, 5. Puktaechon, 6. Jasong-Myong- chon, 7. Jangpari, 8. Kosochon, 9. Phungso, 10. Pujongang, 11. Amnokgang, 12. Wiswn-Huchang, 13. Kanggye, 14. Chong- chongang, 15. Kumjingang, 16. Changsong-Unsan, 17. Taeryonggang, 18. Tongnim-Unjon, 19. Jangnim, 20. Namgang, 21. Hwangju-Koksao, 22. Jaeryonggang, 23. Unpba-Haeju, 24. Kujang-Kumya, 25. Taedonggang-Ryesonggang, 26. Rimjingang, 27. Sinwon-Osan, 28. Kanghwa-Kosong, 29. Wonsan-Scoul, 30. Hongchon, 31. Kongju, 32. Jangsong-Mungyong, 33. Okchon, 34. Samchok, 35. Tanyang, 36. Yongyang, 37. Hwasangdong, 38. Ryangsan. 39. Milyanggang, 40. Samchong-Sangju, 41. Posong-Sachon. • Curriculum Vita© of Chun-Soo Kim 0 Education • Bachelor of Science in Geology, Seoul National University, Korea, 1972 • Master of Science in Geological Engineering, University of Missouri-Rolla, USA, 1977 • Doctor of Philosophy in Engineering Geology, McGil! University, Canada, 1984 t> Experience Or, C.S.Kim has joined in KAERI since 1988. Major research areas and experiences include fracture hydrogeology, synthesis of geologic/hydrogeologie/liydrochemical data, evaluation of hydrogeological properties of earth materials. He also has experiences in slurry pond reclamation, and characterization of groundwater system for urban development and land use. * |> Other • Board member of the Korean Society of Groundwater Environment : 1993 - Present • Editorial member of the Geological Society of Korea : 1992 - Present •Editorial member ai Korean Society of Engineering Geology : 1991- 1997 •Member of Interniuionai Association of Engineering Geology • 1988 - Present