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.