Fault slip analysis of Quaternary faults in southeastern Korea

Transcription

1 Gondwana Research 9 (2006) Fault slip analysis of Quaternary faults in southeastern Korea Youngdo Park *, Jin-Han Ree, Seung-Hak Yoo 1 Department of Earth and Environmental Sciences, Korea University, Seoul , South Korea Received 5 December 2004; accepted 20 June 2005 Available online 9 January 2006 Abstract The Quaternary stress field has been reconstructed for southeast Korea using sets of fault data. The subhorizontal direction of the maximum principal stress (r 1 ) trended ENE and the direction of the minimum principal stress (r 3 ) was nearly vertical. The stress ratio (U =(r 2 r 3 )/ (r 1 r 3 )) value was Two possible interpretations for the stress field can be made in the framework of eastern Asian tectonics; (1) The r Hmax trajectory for southeast Korea fits well with the fan-shaped radial pattern of maximum principal stress induced by the India Eurasia collision. Thus, we suggest that the main source for this recent stress field in southeast Korea is related to the remote India Eurasia continental collision. (2) The stress field in Korea shows a pattern similar to that in southwestern Japan. The origin for the E W trending r Hmax in Japan is known to be related to the mantle upwelling in the East China Sea. Thus, it is possible that Quaternary stress field in Korea has evolved synchronously with that in Japan. We suggest further studies (GPS and in situ stress measurement) to test these hypotheses. D 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Quaternary faults; Fault slip analysis; Korea; Eastern Asia; Japan 1. Introduction It is generally thought that East Asian Quaternary to recent tectonics are controlled mainly by two factors: eastward extrusion of continental blocks due to the India Eurasia collision and subduction of the Pacific and Philippine Sea plates (e.g. Molnar and Tapponnier, 1975; Heki et al., 1999). One question concerning the neotectonics of northeast Asia is which was more important to the neotectonics of the Korean peninsula, which is located near to the Nankai Trough and the Japan Trench where the Philippine Sea plate and the Pacific plate, respectively, are being subducted. The Korean peninsula has traditionally been considered to be seismically stable because of weak and rare seismicity relative to Japan where subduction-related seismic activity is intense. However, several active faults have been identified during an extensive geologic survey in southeast Korea (e.g. Kyung, 1997; Okada et al., 1998; Ree et al., 2003). Ages of activities for some of these faults have been precisely * Corresponding author. Present address: Research Division, Heesong Geotek, Yoonhwa building, 16-3 Yangjae-dong, Seocho-gu, Seoul , South Korea. Tel.: ; fax: address: ydpark@korea.ac.kr (Y. Park). determined by direct measurement of fault motion (e.g. electron spin resonance or ESR dating; Lee and Schwarcz, 2001) or by measuring the ages of displaced sediment (e.g. optically stimulated luminescence or OSL dating; Ree et al., 2003). So far, twenty-four Quaternary faults have been found in southeast Korea. We describe here the results of stress inversion based on slip data from these faults. The timing of faults is constrained by field relations, radiometric and OSL dating. One of the prerequisites for paleostress analysis of fault slip data, i.e. synchronicity of fault activity, is met due to their age. We compare the local stress field derived from Quaternary fault slip data with the current regional stress field and plate kinematics of Asia, and suggest that the recent crustal deformation of Korea was induced by the distant collision of India with Eurasia or was related to the mantle upwelling in the East China Sea as suggested by Seno (1999). 2. Quaternary faults in southeastern Korean peninsula Southeastern Korea contains Precambrian basement gneisses, Cretaceous rocks, and Tertiary sedimentary and volcanic rocks. The Cretaceous rocks include granites, volcanics, and a thick succession of sedimentary rocks deposited in lacustrine environment (Chough et al., 2000). The Gyeongsang Basin, where the Cretaceous sedimentary rocks are deposited, X/$ - see front matter D 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi: /j.gr

2 Y. Park et al. / Gondwana Research 9 (2006) might be a pull-apart basin associated with strike-slip faulting due to the northward subduction of the Izanagi plate in the early Cretaceous (Chun and Chough, 1992). From the late Eocene to Miocene, the Japan Sea (or the East Sea) was formed by extensional deformation in this region, presumably due to the upwelling of a deep mantle-driven hot plume (Maruyama et al., 1997). From the Pliocene to the present day, the major tectonic process of eastern Asia has been the formation of the Amurian Plate, bounded by the Baikal rift to the west and by the eastern Japan Sea to the east. The fragmentation of Asia into smaller plates such as the Amurian Plate is thought to be a consequence of the collision between the India and Eurasia plates (Molnar and Tapponnier, 1975; Zonenshain and Savostin, 1981). Recently, Ree et al. (2003) suggested that some Quaternary reverse faults in southeast Korea were formed by compressional deformation related to plate convergence at the eastern Japan Sea where the Amurian Plate starts to subduct underneath the Japanese Islands (Tamaki and Honza, 1985). Fig. 1 is a topographic relief map of southeastern Korea synthesized from digital elevation maps. Five subparallel NNE trending lineaments (A, B, C, D, and E) and one NNW trending lineament (F) are very obvious in Fig. 1. Along these lineaments, subsidiary faults, mostly with a strike-slip sense, are observed. Thus, these lineaments appear to be the Fig. 1. Topographic relief map of southeastern Korea (A: Milyang Fault, B: Moryang Fault, C: Yangsan Fault, D: Dongnae Fault, E: Ilkwang Fault, F: Ulsan Fault). Localities and orientation of 24 Quaternary faults are also shown (lower hemisphere projection, see Table 1 for details). The histogram represents the frequency of deviation angles (see text for details).

3 120 Y. Park et al. / Gondwana Research 9 (2006) topographic expression of large-scale strike-slip faults. Quaternary faults have been discovered along the Yangsan (C) and Ulsan Faults (F). The ages of faulting events for some of the Quaternary faults have been determined by direct dating of fault gouge material (Lee and Schwarcz, 2001). The ESR ages for the Quaternary faults range from 1.3T0.1 Ma to 0.13T0.01 Ma. Some faulting events were determined indirectly using the age of sediments crosscut by the faults. This age represents a maximum age for faulting. OSL dating of quartz grains in sediments as well as 14 C radiometric dating of peat layers has been conducted to determine the ages of sediments crosscut by faults (Kyung and Chang, 2001; Ree et al., 2003). Some of the ages for sediments turned out to be very young (2 to 32 ka; Kyung and Chang, 2001; Ree et al., 2003). The attitudes and localities of the Quaternary faults are also shown in Fig. 1. These faults can be classified into two groups on the basis of the angle of fault and the rake (pitch) of the slickenside lineation on the fault plane (Angelier, 1994). The first group contains NNE-striking high angle strike-slip faults, and the second group contains NNE-striking low angle oblique to dip-slip faults (Fig. 2). The sense of slip was determined using field relations when clear offset markers were present. When there were no clear offset markers, fault rock samples prepared in low-viscosity resin were cut perpendicular to fault plane and parallel to slickenside lineation, and polished Fig. 2. Orientation of Quaternary faults. (a) Strike-slip faults and (b) dip-slip reverse faults on the basis of fault-classification scheme (c) after Angelier (1994). Lower hemisphere projection.

4 Y. Park et al. / Gondwana Research 9 (2006) Table 1 Details for the Quaternary faults Fault location Individual fault name Orientation of fault plane (strike, dip) Orientation of slickenside lineation (trend, plunge) Shear sense Country rock lithology Fault rocks Fault core thickness 1 Yoogye N26E,46SE N74E,32 Reverse Volcanic rocks Gouge cm * 2 Bangok N45E,85NW S42W,25 Dextral Sandstone Gouge cm * 3 Byeokgye N5W,80NE N5W,0 Dextral Andesite Gouge cm * 4 Oeosa N32E,62SE N68E,50 Reverse K-fd porphyry Cataclasite 10 cm * 5 Wangsan N48E,42SE N76E,30 Reverse Andesite Gouge cm 28 m 6-1 Madong 1 N22E,69SE S50E,68 Reverse Volcanic rocks Gouge cm * 6-2 Madong 2 N32W,64NE S66E,48 Reverse Granite Gouge cm * 7 Guereung N20E,45SE N70E,38 Reverse Granite Gouge cm * 8 Janghang N40E,85NW N38E,5 Dextral Andesite Gouge 10 m * 9 Malbang N12W,72SW N84W,71 Reverse Granite Breccia 3 cm * 10-1a Gaegok 1a N15W,85NE S22E,53 Reverse Granite Gouge 20 cm * 10-1b Gaegok 1b N6E,84NW S18W,62 Reverse Granite Gouge 80 cm * 10-2a Gaegok 2a N10W,86SW S3E,60 Reverse Granite Gouge cm * 10-2b Gaegok 2b N33W,72NE S70E,60 Reverse Granite Gouge 9 13 cm * 10-3 Gaegok 3 N26W,40NE N62E,40 Reverse Granite Gouge 20 cm * 11 Ipsil N20W,82NE N13W,42 Dextral Granite Gouge 180 cm * 12 Wonwonsa N20W,52SW S44W,49 Reverse Granite Gouge cm * 13 Eupchon N15E,40SE S49E,37 Reverse Volcanic rocks Gouge cm 6 m 14 Suryeom N40E,44SE S26E,41 Reverse Volcanic rocks Gouge 2 15 cm 1.2 m 15 Sangcheon N15E,85SE N22E,55 Dextral Granite Cataclasite cm * 16-1 Kacheon 1 N4E,82SE S1E,82 Dextral Granite Gouge 10 m * 16-2 Kacheon 2 N30E,84SE N33E,20 Dextral Granite Cataclasite 30 m * 16-3 Kacheon 3 N4E,85SE S3W,85 Dextral Granite Gouge 30 m * 17 Joil N42E,82SE N45E,20 Dextral Volcanic rocks Gouge cm * *: Undetermined net displacement. Total displacement sections were made to determine the sense of slip. Fault rock microstructures indicating sense of slip include composite fabrics such as S/C and shear bands, offsets along secondary fracture sets, and rotated fragments or pebbles (Ree et al., 2003). These slip sense indicators show very clear kinematic trends; (1) high angle strike-slip faults have a dextral sense, and (2) low angle faults have a reverse sense (Fig. 2). Details of each Quaternary fault are given in Table Fault slip analysis 3.1. Method In order to reconstruct the stress field responsible for the development of the Quaternary faults, a fault slip analysis was carried out. Fault slip analysis (Angelier, 1994) is a method for finding the direction of three principal stresses (r 1 >r 2 >r 3, compressive stress being positive) as well as the stress ratio {U =(r 2 r 3 )/(r 1 r 3 )}. The input data for the analysis include the orientation of each fault plane, the rake of the slickenside lineation on each fault plane, and the sense of slip (see Table 1). The fault slip analysis for reconstructing paleostress is based on two main assumptions; (1) the slip on a fault plane occurs in the direction of the maximum resolved shear stress (Bott, 1959), and (2) the stress field is homogeneous throughout the duration of fault activity as well as in the area where the analysis was made. In this study, a grid search method (program SLICK.BAS; Ramsay and Lisle, 2000) was used for fault slip analysis of the Quaternary faults. This method finds the direction of principal stresses and the stress ratio by a trial-and-error approach with incremental changes in the values for each of four variables consisting of trend and plunge of r 1, rake of r 2 on the plane normal to r 1, and the stress ratio (U). The best-fit solution is chosen with a minimum average deviation angle which is defined as an angle between the real slickenside lineation and the direction of maximum shear stress calculated from the given solution on the fault plane Results The directions of the principal stress axes determined from fault slip analysis are shown in Fig. 3. The maximum Fig. 3. Directions of principal stress axes of paleostress determined from fault slip analysis (lower hemisphere projection).

5 122 Y. Park et al. / Gondwana Research 9 (2006) principal stress axis (r 1 ) trends ENE (250-) with a low plunge angle (8-) while the minimum principal stress axis (r 3 ) is nearly vertical (plunge, 79-; trend, 029-) and the intermediate principal stress axis (r 2 ) trends 159- and plunges 7-. The calculated value for the stress ratio (U) is The direction of the maximum principal stress (r 1 ) is subparallel to that determined from the focal mechanism solution of recent earthquakes in the Korean peninsula and adjacent seas (Jun, 1990; Ree et al., 2003). The orientation of the principal stress axes and the value of the stress ratio were applied to each plane to find the direction of maximum shear stress on each fault plane. It was then possible to obtain the deviation angle between slickenside lineation and the calculated direction of the maximum shear stress. These results are shown in stereograms in Fig. 1. The average deviation angle for the twenty-four faults is and roughly 60% of fault data have a deviation angle smaller than the average deviation angle, while several faults have deviation angles larger than Discussion 4.1. Interpretation of the results We have estimated the orientation of principal axes and the relative magnitude of principal stresses for the Quaternary faults in southeastern Korea. The estimated results are evaluated using the Mohr diagram for reduced stress tensors (Etchecopar et al., 1981). The distribution of the twenty-four Quaternary faults on the Mohr diagram (Fig. 4) shows a widely scattered pattern. In general, the scattered pattern is interpreted to be the result of multiple faulting events. This is because varying stress orientations as well as the stress ratio during multiple events will selectively activate fault planes with suitable orientations for slip. When a single value for stress orientation and ratio is mistakenly chosen for an area with multiple faulting events and different stress fields, a pattern similar to that in Fig. 4 will appear. Thus, the simplest interpretation for the scattered pattern in Fig. 4 would be multiple faulting. However, the interpretation of multiple faulting events is problematic because of two major reasons. Firstly, the faulting events occur within a relatively narrow time span that is well constrained to the Quaternary period. Furthermore, there have been no major changes in remote stress during the Quaternary. Processes that can lead to changes in the stress field (e.g. changes of plate motion) have not been reported in eastern Asia during the Quaternary. Thus, a hypothesis of multiple faulting events, based on the assumption of changes in the remote stress field, is unlikely. Secondly, most values for the deviation angle between the slickenside lineation and the calculated direction of maximum shear stress are relatively small (Fig. 1, inset and Fig. 4). We have obtained an average deviation angle of for twenty-four faults and about 60% of the fault population have a smaller deviation angle than the average. We have repeated fault slip analysis with the data set excluding larger deviation angles (>18.4-, n =6) in order to test if the orientations of the principal stresses were different. We have done this with the premise that the fault data sets with large deviation angles might be formed by another faulting event and should be excluded. We found very similar results to the aforementioned results (plunge and trend of r 1 =7-, 247-; plunge and trend of r 2 =4-, 157-; plunge and trend of r 3 =82-, 037-; U = 0.60; average deviation angle = 12.5-). Since the deviation angles are small for both of the analyses and the stress orientations are similar, it is difficult to assume that multiple faulting events with varying remote stress occurred in this area. An alternative explanation for the scattered pattern in Fig. 4 can be proposed by considering the strength of the fault zones. The scattered data in Fig. 4 fall into two groups: one has a higher shear stress to normal stress ratio (Group I in Fig. 4), and the other has a lower shear stress to normal stress ratio (Group II in Fig. 4). If all of the fault activity occurred under the same stress field, Group I faults may result from higher friction while Group II faults result from Fig. 4. Mohr diagram for the reduced stress tensor obtained in this study. Symbols on the Mohr diagram represent the orientation of fault planes (poles to faults). Numbers and parenthesized numbers, respectively, represent fault numbers (Table 1) and deviation angle of each fault.

6 Y. Park et al. / Gondwana Research 9 (2006) lower friction. Group I faults have similar characteristics; dip-slip reverse faults with dip angles ranging from 40- to 55-. On the other hand, dextral strike-slip faults with dip angles higher than 80- belong to Group II faults. If the orientation of the principal stress axes is also considered (Fig. 3), most Group II faults are oriented roughly normal to r 1. This suggests that faults (Group II) with orientations unsuitable for slip may have low frictional strengths. A similar hypothesis has also been proposed for the San Andreas fault zone where concurrent thrusting and strikeslip deformation occurred (Mount and Suppe, 1987; Miller, 1998). The strike-slip faults oriented at high angles to the r 1 direction are interpreted to have a lower frictional strength (Mount and Suppe, 1987). While most of the faults have small deviation angles, some show angles as large as 50-. One possible explanation for this result is fault-to-fault interaction (Pollard et al., 1993) in this area as several large-scale faults are present (A to F in Fig. 1) Current plate kinematics in eastern Asia Fig. 5a illustrates the directions of maximum horizontal stress (r Hmax ) and the directions of compression axes (P axes) from seismological studies in east Asia. The ENE WSW trending r Hmax direction determined from fault slip analysis in this study is subparallel to the r Hmax direction which is interpreted to result from the continental collision between the Indian and Eurasian plates (Fig. 5b; Xu et al., 1992; Zoback, Fig. 5. Plate kinematics and stress field in east Asia. (a) Comparison of the maximum horizontal stress direction among this study, seismic studies (Xu et al., 1992; Ree et al., 2003), and inferred direction in Zoback (1992). SJSFZ: Southern Japan Sea Fault Zone. MTL: Median Tectonic Line. (b) Stress field in Asia due to India Eurasia collision (compiled from Xu et al., 1992; Zoback, 1992; Tamaki and Honza, 1985).

7 124 Y. Park et al. / Gondwana Research 9 (2006) ). On the other hand, our result is quite different from the r Hmax direction in Japan (eastern Honshu), which is related to the subduction of the Pacific plate and the Philippine Sea plate (Fig. 5a). Although Korea is near the margin of the Eurasian Plate, the stress field does not seem to be related to the subduction of oceanic plates. This may be explained through the existence of active deformation zones in Japan. Much of the shortening component of deformation by plate motion is accommodated in the Japanese arc and subduction zone, while the shearing component is accommodated by the Southern Japan Sea Fault Zone and the Median Tectonic Line (Fig. 5a; Itoh et al., 2002). The r Hmax direction in the region between the Southern Japan Sea Fault Zone and the Median Tectonic Line is roughly parallel to that of the velocity vectors of the plates here (Fig. 5a). However, the r Hmax direction becomes very different across the Southern Japan Sea Fault Zone, indicating accommodation of the oblique component of the plate motion by active deformation along the Southern Japan Sea Fault Zone. This leads to a possibility that the Korean peninsula was affected mainly by the collision of India with Eurasia, and that the deformation due to the subduction of oceanic plates was accommodated by deformation in the Japanese Island Arcs. An alternative interpretation for the results of fault slip analysis is that the r Hmax direction in Korea has changed synchronously with that in southwest Japan. The r Hmax direction in Kyushu and central-southwestern Honshu is roughly E W trending based on earthquake focal mechanisms and in situ stress measurements (Seno, 1999). Thus the interpretation by Seno (1999) is different from that by Itoh et al. (2002). Seno (1999) suggested that the E W trending r Hmax direction is produced by the mantle upwelling in the East China Sea and related viscous drag by the spreading flow laterally from the upwelling plume. Since the r Hmax directions in southeastern Korea and southwestern Japan are subparallel, there is a possibility that the stress field in Korea changed simultaneously with that in Japan. Further neotectonic studies, including GPS and in situ stress measurement, are suggested to verify these contrasting hypotheses. 5. Conclusions (1) Fault slip analysis for the twenty-four Quaternary faults in southeastern Korea shows ENE WSW trending r Hmax. (2) Most dip-slip faults show higher shear stress to normal stress ratios (s/r n ) on Mohr diagrams for the reduced stress tensor, while most of strike-slip faults show lower values of s/r n. This difference may reflect deformation partitioning; horizontal shortening deformation by activation of dip-slip faults and strike-slip deformation by activation of high angle strike-slip faults. The unsuitably oriented high angle strike-slip faults may have been activated due to their low frictional strengths, similar to the San Andreas fault system. (3) The direction of r Hmax reconstructed in southeastern Korea is parallel to that extrapolated to South Korea from the India Eurasian collision. This suggests the possibility that the tectonics of the Korean peninsula has been controlled by the remote collision of India with Eurasia. Deformation due to the proximal subduction of oceanic plates is accommodated in the Japanese Islands. (4) The stress field in southwestern Japan is interpreted to be subparallel to that in Korea (e.g. Seno, 1999). The mechanism for this observation is interpreted to be related to the mantle upwelling in the East China Sea. 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