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Geosciences Journal Vol. 3, No. 1, p. 49-54, March 1999 Qualitative interpretation of Bouguer anomaly in the southern part of the Korean peninsula K w a n g Sun Choi G.V.R. K u m a r Ki Y o u n g K i m Department of Earth Science Education, Pusan National University, Pusan 609-735, Korea (e-mail: [email protected]) Oil & Natural Gas Corporation Limited, Chennai 600-008, India (e-mail: [email protected]) Department of Geophysics, Kangwon National University, Chunchon 200-701, Korea (e-mail: [email protected]) ABSTRACT: Regional gravity anomaly maps of Korea have been used for geologic information on the nature of crust and isostasy. We made an attempt to interpret the Bouguer map based on the nature, quality, and characteristics of anomalies and integrate the data with the geologic and tectonic features. Seismicity is also considered for the correlation with the Bouguer map. It reveals that the gravity anomaly map helps delineate major tectonic boundaries. It is observed that the gravity high zones are associated with earthquake activity and lineaments in their proximity. Key words: Bouguer anomaly, tectonic boundary, seismicity 1. INTRODUCTION Gravity methods are used for the analysis of geologic structures and evolution of the earth's crust. Though detailed surveys are always advantageous, most of the objectives initially require a broad and regional approach to establish the basic scientific framework. Regional gravity anomaly maps are particularly useful for mapping geographic distribution and configuration of the basement rocks, structural and lithologic provinces, zones of crustal weakness, mass imbalances within the lithosphere, geometrical configuration of sedimentary basins, and the distribution of extrusive and intrusive rocks. In Korea, gravity data were first measured using a pendulum in 1927 (Kumagai, 1953). Regional gravity surveys have been carried out using modem gravimeters since late 1950s (Woollard and Rose, 1963). An updated Bouguer anomaly map has been compiled by Choi and Shin (1996) using gravity measurements at more than 5000 stations in the southern part of the Korean peninsula (Fig. 1). For gridding the data, gravity data of Fukuda (1990) are used in the East Sea and OSU91A in the northern part of the peninsula (Choi et al., 1993). It was concentrated in filling the gap and merging new observations with existing gravity data and prepared a new version. The gravity measurements have been reduced to mean sea level using a Bouguer density of 2.67 gm/cc. Terrain effects have been corrected at each station for 100 x 100 knq 2 in area. The Bouguer anomaly map (Fig. 1) with a contour interval of 5 mGal shows several distinctive features of gravity anomaly in the southern part of the Korean peninsula. The major gravity lows from this map are ascribed to Sobaeksan (S) area and Kangwon-do (K) province, respectively. The large linear gravity trend with a maximum value of 60 mGal in the offshore is due to diminishing depth of crust (Choi, 1988). He also prepared a residual anomaly map and found that negative anomalies coincide well with the granitic and volcanic areas, as the density of these rocks is comparatively less than that of the Precambrian gneiss. Lee (1979) used Bouguer anomalies to calculate the depth to the Moho and his studies indicate that the Korean peninsula is almost in isostatic equilibrium and the mean crustal thickness is about 35 km. On the other hand, Kwon and Yang (1985) indicated from their studies through linear regression that the crust of the Korean peninsula seems not to be in perfect equilibrium but slightly undercompensated. Choi et al. (1993) analysed gravity data and deciphered that the crustal thickness in and around Korea is on the order of 16 to 41 km. Kyung (1989) studied the relation between the topography and the seismicity of the Korean peninsula. He also observed that the epicenters of earthquakes are distributed dominantly along the areas of high gradient of Bouguer and residual anomalies. Many researchers have utilized the Bouguer anomaly map for the nature of the crust and isostasy. Nonetheless, a complete description of Bouguer anomaly interpretation is not available in the previous works, in concordance with the regional geology. 2. QUALITATIVE I N T E R P R E T A T I O N OF BOUGUER A N O M A L Y MAP 2.1. G e n e r a l F e a t u r e s The Bouguer anomaly map presents many closures 50 Kwang Sun Choi, G.V.R. Kumar, and Ki Young Kim Fig. 1. Regioal Bouguer anomaly map in the southern part of Korean peninsula. Contour interval is 5 mGal. This map presents many closures around gravity 'highs' and 'lows'. The isoanomaly lines follow a NNE-SSW trend and are in accordance with the general geologic strike of Korea. around gravity 'highs' and 'lows'. Some of the Bouguer anomalies do not appear to have been caused by surface geology. The isoanomaly lines in the map generally follow a N N E - S S W trend and are in accordance with the general geologic lineament of Korea. A gravity low with a magnitude of - 2 5 reGal and N E SW direction is seen near south of Taejon. This low appears to be caused by a downwarp in the basement and can be attributed to the crustal thickening. As the density of granites is less than the Precambrian rocks of the surrounding area, the huge thickness of these granites gives rise to a low gravity anomaly. This explanation corroborates with the geological explanation of Sobaeksan massif in this area. The southeastern part of this low is faulted as the contours show a steep gradient with its downthrown side southeastwards. Northwestern and western sides of this low are represented by disturbed pattern of anomaly contours. There might be a possibility for extrusive/ intrusive activity in these areas causing localized density variations. A gravity low with a magnitude of - 2 0 reGal with its strike (almost N E - S W ) is noticed near southeast of Interpretation of Bouguer anomaly in Korea 51 Chunchon. This low is most likely due to the granitic intrusion in the Precambrian basement. Although the gravity anomalies in the above two major gravity lows show a little resemblance in their magnitude and strike direction, it is not possible to make any genetic relation between these gravity lows because they could have been formed at different geologic times. In the southeastern part, in the proximity of Taegu, pairs of gravity lows and highs are observed. Although these are of low-relief, the highs can be attributed to volcanic or sedimentary rocks of low porosity and the lows to granites or Tertiary sediments. The average values of density for low-porosity sedimentary rocks, basalts, andesites, granites, and Tertiary sediments are 2.60, 2.90, 2.61, 2.55, and 2.03, respectively (Min and Chung, 1985). Around Seoul, a very varied nature of gravity anomaly pattern having small and large amplitudes suggests that the area comprises metamorphic terrain with rocks of different densities. A broad zone along the east coast with seaward gradient close to 10 mGal/km trends N - S upto latitude 36~ and then N W - S E which reflects the offshore transition of continental crust. The gradual increase in magnitude of Bouguer anomalies indicates a gradual decrease in thickness of the crust. The attitude of this flock of contours trending N - S direction and having increased values towards the coast also suggests that there exists a fault along the eastern side of the east coast. There are many locations in the southeast, northeast, and south-central parts of Korea where dislocations of the narrow belts of high gradients are observed. These dislocations can be interpreted as originating from some major or minor strike-slip faults with horizontal movements. Some of the prominent faults including Yongduri (F1), Andong (F3), and Yangsan (F4) faults have been marked with the help of typical nature of the gravity contours that associate with the faulting (Fig. 2). The fault F2 appears as an assumed one on the geologic map of 1:1,000,000 scale (Korea Institute of Geology, Mining and Materials, 1995), whereas F3 has no surface indication. 2.2.1. Kyonggi massif zone (Z1) This zone is characterized by irregular pattern of contours, relative lows and highs with small relief, trending NNE-SSW. Precambrian rocks associated with granites as intrusives and extrusives at some places are the geological manifestations of the gravity signatures. The zone may contain small and complex minor fold structures at places because of the intrusive activity. 2.2. Zoning Earthquakes in Korea are classified as intraplate seismicity (Lee and Na, 1983). Figure 3 shows epicenters of both historical and instrumental earthquakes having intensity equal to or more than V, which occurred in Korea from A.D. 2 to 1987. It is apparent that higher seismicity is associated with many of the faults and tectonic boundaries. Most seismic activities appear to be associated with the gravity-high zones and are confined to the tectonic boundaries. Evidently, the gravity lows and highs are forming pairs at many places and are separated by After qualitatively analysing the Bouguer anomaly map in terms of gravity lows, highs, and linear trends for possible explanation of subsurface causatives, zoning is introduced on the basis of nature, pattern and characteristics of gravity anomalies to describe various tectonic boundaries (Fig. 2). Seven zones have been clearly identified. The description of each zone is provided hereunder. 2.2.2. Nonmetamorphic zone (Z2) This zone is characterized by long wavelength anomalies, low ampfitude but large relief and trends NNE-SSW. Jurassic granites in this area give rise to this type of anomaly deportment. 2.2.3. Okchon belt zone (Z3) This zone shows narrow anomalies of moderate amplitude. It comprises mainly metamorphic rocks and trends NNE-SSW. 2.2.4. Taebacksan zone (Z4) This zone is characterized by a low gravity anomaly of the order of - 2 0 mGal. This low trends N E - S W with small lows and highs. The Triassic granites with low density are most likely responsible for the low values. 2.2.5. Sobaeksan zone (Z5) This zone is characterized by large wavelength, very low amplitude with high relief and trends NNE-SSW. Massive granitic intrusion attests to the gravity signatures. 2.2.6. Kyongsang Basin (Z6) This zone is classified as many separated lows and highs with small magnitude and no particular trend. These lows are explained due to low-density Bulguksa granites and the highs to sedimentary rocks of the Cretaceous. 2.2.7. Yongdong-Kwangju depression zone (Z7) This zone is of high magnitude with a large range and no particular trend. Cretaceous continental sediments and volcanic rocks are responsible for the gravity anomalies. 3. SEISMICITY 52 Kwang Sun Choi, G.V.R. Kumar, and Ki Young Kim Fig. 2. Zoning map of Bouguer anomaly. Seven tectonic boundaries and five faults are indicated with solid and dashed lines, respectively. Zones 1 through 7 are Kyonggi massif (Z1), Nonmetamorphic zone (Z2), Okchon belt zone (Z3), Taebaeksan zone (Z4), Sobaeksan zone (Z5), Kyongsang Basin (Z6), Yongdong-Kwangju depression zone (Z7), respectively. faults (Fig. 2). The large size and areal extent of these anomalies can reasonably be explained by these contacts extending deep into the crust. Had the loci of the available earthquake data been plotted on the crustal thickness map, it would have been clear whether these contact zones extend into the Moho. Moreover, the focal mechanism solutions for the available earthquake data provide the type of fault mechanism existing in and around the contact zones. Nonetheless, it is apparent that the tectonic boundaries in Korea are associated with fault lineaments and these fault zones mark the gravity-high and -low axes. It is worth mentioning here that Kyung (1993) has examined a close correlation between the seismicity and the Bouguer gradient and concluded that deviatoric stresses may be concentrated in such high gradient areas and earthquakes may occur more easily in those areas than others. Interpretation of Bouguer anomaly in Korea 53 Fig. 3. Epicenter zoning map in the southern part of Korean peninsula. Epicenter zones I through IV trend NNE-SSW and coincide with the general geological lineament, whereas Zone V trends N-S. Only epicenters having an MMI V or greater are marked with empty circles (modified from Lee and Jin, 1989). The size of circle increases with the intensity scale. 4. E P I C E N T E R Z O N I N G M A P (EZM) The seismicity map of Korea helps delineate epicenter zones that are constructed solely on the basis of density of epicenters (Fig. 3). It has been considered that at least one epicenter having a modified Mercalli intensity (MMI) V or greater should fall into the zone which is associated either with gravity high or a tectonic lineament. It is called 'epicenter zoning map' (EZM) and no resemblance to seismic zoning map in which isoseismicity is plotted. Lee and Jin (1989) evaluated seven seismic source zones in Korea based on the correlation between seismicity and geotectonic features. Lee (1998) also delineated seven seismic source zones using digital elevation method. However, the methodology adapted by them is different from that of EZM. The EZM helps delineate areas that are seismically active and deserve fimher studies. The EZM of Korea shows five epicenter zones. Zones I through 1V show a N N E - S S W trend and coincide with the general geological strike of Korea, whereas Zone V shows a 54 Kwang Sun Choi, G.V.R. Kumar, and Ki Young Kim N - S trend. Although the intensities are moderate and are around MMI V, a considerable number of earthquakes are marked in these zones. 5. D I S C U S S I O N Based on seismological studies Lee and Na (1983) opined that the fault systems in the Kyongsang Basin are seismically active, especially the Yangsan Fault where neotectonic activity is observed. They also suggest that seismicity is more obscurely related to Paleozoic and Mesozoic faults because geochemical processes in the crust generally augment the strength of rocks in the fault area. Contrary to this observation, a seismicity map shows large concentration of epicenters in the areas occupied by Paleozoic and Mesozoic rocks. On the other hand, our observations reveal that these high seismicity areas are associated with gravity-high anomalies. These gravity-high anomalies can be explained by crustal thinning or the upper mantle density anomalies. The large concentration of epicenters in Zones I through IV suggests that mechanical strains arise from displacement of separate blocks of lithosphere and the strain is building up because of friction. As the volume of combined forces exceeds the strength of friction, the strain is released in the form of earthquakes and may cause development of new faults within the crust if these earthquakes have deep foci. This is true in Zone V, as the trend of the zone is discordant with the general strike of the area. For these reasons, it is desirable (1) to acquire detailed gravity data for an authentic crustal depth map, (2) to establish seismograph stations in the vicinity of seismically active areas, and (3) to carry out seismic refraction studies across the major lineaments to study the nature of crust. 6. C O N C L U S I O N S The regional Bouguer anomaly map of Korea is useful for delineating various tectonic features. The major tectonic elements deciphered from this map corroborate with the geological observations. Seven zones are identified based on characteristic features of gravity anomaly and their boundaries are associated with faults. The data on earthquake epicenter delineate most of these tectonic boundaries and gravity-high zones. It is important to acquire geophysical data across the faults in the vicinity of major tectonic lineaments. ACKNOWLEDGMENTS: This work was partly supported by the Institute of Research and Development for Energy Resources at Kangwon National Univeristy. The second author wishes to place on record his gratitude to KOSEF, for award of a Postdoctoral Research Fellowship and to Oil & Natural Gas Corporation Limited (ONGC), India for granting him leave. The authors are grateful to the editor, S.K. Chough, for his helpful comments on the manuscript and Prof. Czango Baag, Prof. Yeonghwa Kim, and Dr. Jun Hee Lee at Kangwon National University for proffering useful and instigating discussions. Messrs. H.G. Kim and N.C. Woo deserve special appreciation for rendering help at various stages of this work. REFERENCES Choi, K.S., 1988, Gravity and crustal structure. In: Lee, D.S. (ed.), Geology of Korea. Geological Society of Korea & Kyohak-Sa Publishing Co., Seoul, p. 264-269. 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