Recurrent events on a Quaternary fault recorded in the mineralogy and micromorphology of a weathering profile, Yangsan Fault System, Korea

Quaternary Research 64 (2005) 221 – 233 www.elsevier.com/locate/yqres

Recurrent events on a Quaternary fault recorded in the mineralogy and micromorphology of a weathering profile, Yangsan Fault System, Korea Gi Young Jeonga,*, Chang-Sik Cheongb a

Department of Earth and Environmental Sciences, Andong National University, Andong 760-749, Republic of Korea b Geochronology Team, Korea Basic Science Institute, Taejon 305-333, Republic of Korea Received 18 October 2004 Available online 18 July 2005

Abstract Recurrence characteristics of a Quaternary fault are generally investigated on the basis of field properties that are rapidly degraded by chemical weathering and erosion in warm humid climates. Here we show that in intense weathering environments, mineralogical and micromorphological investigations are valuable in paleoseismological reconstruction. A weathering profile developed in Late Quaternary marine terrace deposits along the southeastern coast of the Korean Peninsula was disturbed by tectonic movement that appears to be a simple one-time reverse faulting event based on field observations. A comparative analysis of the mineralogy, micromorphology, and chemistry of the weathering profile and fault gouge, however, reveals that both the microfissures in the deformed weathering profile and larger void spaces along the fault plane were filled with multi-stage accumulations of illuvial clay and silt minerals of detrital origin, suggesting a repetition of fissuring and subsequent sealing in the weathering profile as it underwent continuous mineralogical transformation and particle translocation. We reconstruct a sequence of multiple faulting events unrecognized in previous field surveys, which requires revision of the view that the Korean Peninsula was tectonically stable, during the Late Quaternary. D 2005 University of Washington. All rights reserved. Keywords: Quaternary; Fault; Recurrence; Weathering; Gouge; Mineralogy; Micromorphology; Clay; Soil; Terrace

Introduction Earthquakes occur frequently on, along active continental margins near coastal regions where populations and modern facilities are generally dense. The threat of future earthquakes can be assessed through study of geologic and geomorphic features created during recent surface ruptures (e.g., McCalpin and Nelson, 1996; Michetti and Hancock, 1997; Yeats et al., 1997; Lettis and Kelson, 2000; Rockwell, 2000). One important factor in assessing the capability of a particular tectonic source is its recurrence, which is investigated by combined study of the structure, stratigraphy, geochronology, and geomorphology of deformation features. In humid temperate to tropical climates, however,

* Corresponding author. Fax: +82 54 823 1627. E-mail address: [email protected] (G.Y. Jeong).

chemical weathering and erosion rapidly degrade macroscopic field evidence as well as datable materials useful for paleoseismological interpretations (e.g., McCalpin and Nelson, 1996; Michetti and Hancock, 1997; Lettis and Kelson, 2000). Weathering includes the formation and transformation of fine-grained minerals and the dissolution of soluble minerals (e.g., Nahon, 1991; Birkeland, 1999). The voids created by mineral dissolution or tectonic disturbance allow the vertical migration and accumulation of fine soil minerals. Instantaneous deformation events inevitably interfere with on-going continuous pedological changes occurring in the weathering profile. Thus, mineralogical and micromorphological analyses of a disturbed weathering profile may provide potentially important clues in defining recurrence intervals of earthquakes. The Korean Peninsula has been long presumed to be relatively safe from earthquakes. No loss of lives has been

0033-5894/$ - see front matter D 2005 University of Washington. All rights reserved. doi:10.1016/j.yqres.2005.05.008

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reported in Korea of dense population since the instrumental recording of earthquakes began in 1905 (Lee, 1987). The presumed seismic safety, however, has been reconsidered by the recent discovery of several tens of Quaternary faults around the Yangsan Fault System, implicating a long-term recurrence of the earthquakes (KIGAM, 1998; Kyung et al., 1999; Lee et al., 1999; Chang, 2001; Chwae et al., 2001; Kyung and Chang, 2001; Lee and Schwarcz, 2001; Choi et al., 2002a,b; Ryoo et al., 2002; Choi, 2003, 2004). Among them, much attention has been paid to surface faults that cut Late Quaternary marine terrace deposits in the southeastern coast where nuclear power plants and heavy industrial complexes have been constructed since the early 1970s (KIGAM, 1998; Lee et al., 1999; KINS, 2003; Ree et al., 2003). Their recurrences and timing constraints are still poorly known because of the lack of materials for radiometric dating, intense chemical weathering, and erosion. We carried out mineralogical and micromorphological approaches for a deformed weathering profile and associated fault gouge to find recurrence characteristics from a microscopic point of view and report here a more complex cataclastic history than was previously interpreted solely from field observations.

important indicators of fault movements such as fault scarp and colluvial wedge deposits have been eroded. The ages of marine terraces are critical in constraining the timing of the latest faulting event. Terrace I (3 – 5 m) was formed obviously during the Holocene from the radiometric dating of peat layers (Kim, 1973, 1990; Lee, 1985). The age of terrace II exceeds the range of radiocarbon dating (52,000 yr) (Kim, 1990). Optically stimulated luminescence (OSL) dating yielded an age of 54,000 – 73,000 yr for Terrace II (18 – 20 m) (Choi et al., 2003a,b), whereas tephrochronologic correlation suggests deposition during marine oxygen isotope stage (MIS) 5e (124,000 – 130,000 yr) (Inoue et al., 2002). The precise age of Terrace III is debatable because of the very rare occurrence of materials for isotopic dating. Ree et al. (2003) reported OSL ages ranging 32,000 –58,000 yr for Terrace III, which are younger or comparable to the OSL ages of the lower Terrace II, requiring abnormally high uplift rates (0.8 –1.7 mm/yr). Cheong et al. (2004) reported older OSL ages for Terrace III ranging from 68,000 to 92,000 yr and suggested them as a younger limit of sedimentation timing. Although these discrepancies have not been resolved, the upper limit of the age of Terrace III is broadly constrained as MIS 7 (190,000 – 244,000 yr) or older if the upper limit of the age of the Terrace II is set to MIS 5e based on tephrochronology.

Quaternary faults and marine terraces in the southeastern coast of Korea

Field description of weathering profile and fault

Emergent marine terraces occur along the southeastern coast of the Korean Peninsula from Busan to Pohang, in the vicinity of distinct tectonic lineaments called the Yangsan Fault System (Fig. 1a). The average altitude of the inner edges of the terraces in the middle part of the southeastern coast is higher than those of corresponding terraces in the northern and southern parts (Choi, 2003, 2004), implying localized tectonic activity. Quaternary reverse faults cutting these terraces were recently found along the middle part (Lee et al., 1999; Chwae et al., 2001). One of them, the Suryum fault, cuts marine Terrace III at an altitude of 45 – 55 m, with a total displacement of ca. 1.2 m (Figs. 1a and b). It strikes 35-N –45-E and dips 40– 45-SE. Slickenside lineations plunge towards 55-S – 78-E at 38 –45-. The regional geologic setting of the Suryum fault consists of Cretaceous granite, mudstone, siltstone, arkosic sandstone, Tertiary volcanics, and unconsolidated Quaternary sediments. At the Suryum site, the bedrock underlying paleo-beach gravels is Tertiary bentonite transformed from basaltic tuff via diagenesis. At this locality, debate focuses on the number of fault movements and the timing of the most recent deformation which define the capability of a particular tectonic source (Sower et al., 1998). Previous investigations did not find any field evidence of multiple movements (KIGAM, 1998; Lee et al., 1999; KINS, 2003; Ree et al., 2003), but the multiple movements should not be excluded because

After uplift, the gravel deposits were subjected to weathering in a humid temperate climate. Present annual average temperature and precipitation are 12.2-C and 1091 mm, respectively, but the climate probably varied through the glacial – interglacial cycles. Two units of weathered gravels that were not deformed by faulting were recognized from the weathering profile of paleobeach gravels (Fig. 1b): (1) a Unit I (ca. 170 cm thick) in the lower part of the profile with preserved original sedimentary features, and voids between chemically decomposed pebbles cemented with yellowish brown pedogenic clay (10YR 6/3) (Jeong et al., 2002); (2) a Unit II (ca. 30 – 70 cm thick) in the upper part of the profile, a brown soil horizon (5YR 3/4) which lost its original sedimentary features due to severe chemical weathering, mineral translocation, and bioturbation. The two units are also distinctive in their clay mineralogy. Although the identification of soil horizons and soil classification were not conducted in detail, Unit I appears to be comparable to a Cox horizon, whereas Unit II consists mostly of a Bt horizon beneath a thin A horizon (Birkeland, 1999). Fresh gravels are almost absent through the weathering profile but very locally preserved in the allophane-cemented sandy gravel layer (Fig. 1b). Jeong et al. (2002) showed that the allophane, formed by the weathering of bytownite sands derived from underlying bentonite, protected pebbles from weathering.

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Figure 1. Setting and field appearance of the studied Quaternary reverse fault at the Suryum site, SE Korea. (a) Study site near Yangsan Fault System, the most prominent set of lineaments in South Korea, shown with thick black lines at left. Distribution of marine terrace deposits is shown at right (Lee et al., 1999). Interval of contours is 10 m. (b) Quaternary reverse fault cutting a weathering profile of paleo-beach gravels overlying bentonite. Fresh pebbles are preserved in allophane-cemented sandy gravel layers. Lettered circles along the fault indicate locations of samples in following panels c to g. Numerics indicate the locations of samples for chemical analysis in Table 1. Arrows along the fault indicate sense of movement. Details of the boxes in Units I and II are given in Figures 2c and h, respectively. (c) Brown gouge developed within Unit I with slickenside cutting about at the center. (d) Deformed Unit I near fault. Slab made from epoxy-impregnated sample. Layers dip steeply near fault by dragging. (e) Shiny slickenside composed of brown gouge developed on the footwall of the deformed Unit I. Arrow indicates slip direction of bentonite hanging wall. (f) Fragments of laminated clay (LC) associated with bentonite fragments within the brown gouge. Details of the box are given in Figure 6a. (g) Green gouge sheared as indicated by arrows; wall rock is Tertiary bentonite.

The reverse fault cuts both the weathered gravels and the underlying bentonite bedrock (Fig. 1b). Because both Unit II and the colluvial wedge below the scarp have been eroded, exposures in the trench lacked definitive stratigraphic evidence of multiple deformations, and the fault appeared to be a simple reverse fault. The gouge occurring

along the fault plane consists of brown clay-rich sticky material that has been sharply cut by slickensides with subparallel foliations (Fig. 1c). Pebbles scattered within the gouge were oriented roughly parallel to the direction of fault displacement. Although the pebbles in both deformed Unit I and the brown gouge were strongly weathered to the same

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degree as those from undeformed Unit I, their original shapes are well preserved. Near the fault plane, sediment layers of Unit I, originally horizontal, dip at a high angle (Fig. 1d). Locally, slickensides cutting the thin brown gouge are very shiny (Fig. 1e). In the trace of the fault, the width of the gouge varies from only a few millimeters in the middle part (Fig. 1e) to several centimeters in the upper and lower parts (Figs. 1c and f). Fragments of dark brown laminated clay (5YR 3/2) and bentonite are present within the brown gouge, particularly between bentonite walls in the lower part of the outcrop (Fig. 1f). The brown gouge is discontinuously replaced by gouge of dusky yellow-green color (5GY 6/2) in the lowermost part of the fault outcrop (Fig. 1g).

Samples and analytical methods Weathered gravels, fault gouges, and bentonite were sampled from trench. Samples for a micromorphological analysis were trimmed in situ to a cube with a chisel, taken off after insertion into a plastic box, and sealed airtightly with a lid. All the information on the orientation of the sample was marked directly on the external wall of the box. Samples for mineralogical and chemical analysis were stored in a zipper bag. Fresh gravels were separated from an allophane-cemented sandy gravel with scalpel blade and tweezers in laboratory (Jeong et al., 2002). For a mineralogical identification, bulk samples were ground in agate mortar with pestle in natural wet state for X-ray diffraction (XRD) analysis. For the identification of clay minerals, clays under 2 Am were separated by sedimentation. Random mounts of bulk samples and oriented mounts of clays were analyzed by a Rigaku (Japan) D/MAX2200 XRD instrument equipped with a diffracted-beam monochromator, a Cu target operating at 40 kV/30 mA, and standard processing softwares. Sequential XRD scans were performed for the oriented mounts first after solvation with ethylene glycol and then progressively higher heat treatments. For a micromorphological analysis, polished thin sections were prepared from original bulk samples that had been impregnated with epoxy resin diluted with acetone under vacuum after air drying (Jeong and Kim, 1993), and observed using a polarizing microscope and a JEOL (Japan) JSM 6300 scanning electron microscope in a back-scattered electron (BSE) image mode. Secondary electron images were obtained from the air-dried original samples after gold coating using a JEOL JSM 6700F field emission gun scanning electron microscope. The chemical compositions of weathered gravels, fresh gravel, gouges, and bentonite were analyzed for samples of 0.3 – 1 kg weight by Activation Laboratories, Ontario, Canada with a Thermo Jarrell Ash ENVIRO II inductively coupled plasma emission spectrometer. Clay aggregates in the polished thin section were analyzed with a Cameca (France) SX51 electron microprobe analyzer after carbon coating. Chemical analyses of the clay particles of the laminated clay were

conducted by a JEOL 2010 transmission electron microscope equipped with an Oxford energy dispersive X-ray spectrometer. Particle size was measured for bulk samples using a Malvern (England) Mastersizer 2000 laser particlesize analyzer.

Mineralogy and micromorphology of weathering profile and gouge Undeformed weathered gravels (Units I and II) Black, dark green, and dark brown pebbles, derived from thermally metamorphosed Cretaceous sedimentary rocks, are the most abundant in the fresh gravel preserved in the allophane-cemented layer. They contain much finegrained biotite of <100 Am size and subordinate chlorite (Fig. 2a). In Unit I, plagioclase within pebbles has been preferentially leached leaving abundant secondary voids, whereas biotite and chlorite were respectively transformed to regularly interstratified biotite – vermiculite and chlorite – vermiculite (Figs. 2b, e – g). Primary voids between pebbles are filled almost completely with hydrated halloysite clay (Figs. 2c and e). Microscopic observations show globular clusters of short halloysite tubes indicating their in situ growth via chemical precipitation (Fig. 2d) (Jeong, 1998a). The brown soil horizon in Unit II is mostly composed of clay and weathering-resistant quartz-rich silt with scattered sand grains (Fig. 2h). In bulk samples, quartz is most abundant, but in clay fractions, vermiculite weathered from interstratified biotite –vermiculite and chlorite – vermiculite is an important clay mineral co-occurring with dehydrated halloysite, kaolinite, goethite, quartz, K-feldspar, and illite (Fig. 2i). Deformed weathered gravels (Unit I) In deformed Unit I, weathered porous pebbles preserve their original shapes, but halloysite clay was fragmented to form secondary micro-voids, which were filled with two kinds of accumulations of fine mineral particles: early-stage accumulation of clay (EA) and late-stage accumulation of quartz-rich fine silt/clay (LA) (Figs. 3a and b). It is obvious that EA fills microfissures of halloysite clay and encloses its fragments (Fig. 3b). Microstratification is a common feature of EA (inset in Fig. 3b). High-magnification secondary electron images of EA show the oriented deposition of submicron plates of clay minerals, suggesting a detrital origin (Fig. 3c). ‘‘Detrital’’ here refers to a particle transported in suspension excluding in situ crystal growth. LA encloses fragments of both the halloysite clay and EA (Fig. 3d). The fine silt particles of LA are mostly quartz grains bridged with clays (inset in Fig. 3a). The fabrics of LA and Unit II are indistinguishable (compare two insets in Figs. 2h and 3a).

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Figure 2. Micromorphology and mineralogy of undeformed Units I and II of the weathering profile. (a) An unweathered biotite-rich pebble composed of quartz, plagioclase, biotite, K-feldspar, and chlorite. Back-scattered electron (BSE) image of thin section. Sample from the allophane-cemented sandy gravel layer (Jeong et al., 2002). (b) The interior of a weathered biotite-rich pebble in Unit I showing many dissolution voids (black) left after the dissolution of plagioclase (arrows). BSE image of thin section. (c) Overall BSE image of a thin section of Unit I from the box in Figure 1b. Halloysite clay fills the primary voids between weathered porous pebbles. Inset image in white box shows close-up of halloysite clay. (d) In-situ growth clusters of short halloysite tubes in halloysite clay of Unit I. Secondary electron image of an original sample. (e) X-ray diffraction (XRD) patterns of bulk sample and clay-size fraction (<2 Am) of Unit I. d values ˚ units. (f) XRD patterns of interstratified biotite – vermiculite treated with ethylene glycol and heats. (g) XRD patterns of interstratified chlorite – are given in A vermiculite. (h) BSE image of a thin section of Unit II from box in Figure 1b showing quartz-rich fine silt/clay aggregates. Inset image in white box shows magnified view. (i) XRD patterns of bulk sample of Unit II and oriented samples of clay fractions treated with ethylene glycol and heats. Formamide ˚ phase is a mixture of kaolinite and dehydrated halloysite. B = biotite, BV = interstratified intercalation test (Churchman et al., 1984) showed that the 7 A biotite – vermiculite, C = chlorite, CV = interstratified chlorite – vermiculite, DH = dehydrated halloysite, EG = ethylene glycol, G = goethite, H = hydrated halloysite, IL = illite, K = kaolinite, Kf = K-feldspar, P = plagioclase, Pb = weathered pebble, Q = quartz, V = vermiculite.

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Figure 3. Micromorphology of deformed Unit I adjacent to the fault, for the sample in Figure 1d. (a) Fragmentation of halloysite clay followed by early-stage accumulation of infiltrated clay (EA) and late-stage accumulation of coarser quartz-rich fine silt and clay (LA). Inset is a magnified image of LA composed of quartz silt and bridging clay. BSE image of a thin section. (b) EA enclosing halloysite clay fragments. BSE image magnified from box 1 in panel a. Inset shows curved sedimentation laminae of EA. (c) Oriented deposition of platy clay particles to form EA in deformed Unit I. Secondary electron image of original sample. (d) LA enclosing the fragments of both halloysite clay and EA. BSE Image magnified from box 2 in panel a.

Gouges and laminated clay Although the brown gouge appears to be homogeneous in hand specimen, samples are heterogeneous at the submillimeter scale. In the peripheral region of the brown gouge, some massive EA was fragmented, leading to the development of microfissures that later filled with LA (Fig. 4a). In the interior of the brown gouge, silt particles of LA as well as fragments of EA are oriented parallel to the fault displacement (Fig. 4b). Pebbles within the gouge were significantly weathered, as shown by the abundant voids remaining after plagioclase dissolution, but still preserve their rounded shapes (Fig. 4c). The surface layer of slickensides, one millimeter thick, is composed of highly sheared EA and LA (Fig. 4d). Secondary electron image of sample with slickenside shows the compact and sheared aggregates of submicron plates of clay minerals (Fig. 4e). Bulk and clay mineralogies of the brown gouge are equivalent to those of Unit II (Figs. 2i and 4f). The green gouge (Fig. 5a) that formed along the fault within diagenetic bentonite lacks original tuffaceous textures preserved in the bentonite (Fig. 5b) and are very roughly foliated along the displacement. Both the bentonite and the green gouge are almost completely composed of smectite and include minor halloysite formed by the weathering of calcic plagioclase phenocrysts in diagenetic bentonite (Fig. 5c).

Laminated clay occurs locally within the brown gouge. The fissures in the fragmented laminated clay are filled with LA-rich material (Fig. 6a). Some samples display a regular alternation of darker and brighter laminae (Fig. 6b). The bulk mineralogy differs from that of Unit II and brown gouge only by the absence of coarser quartz grains, and the clay mineralogies are indistinguishable (Figs. 2i, 4f, and 6c). Particles of the laminated clay have a wide range of chemical compositions (Fig. 6d). The particle-size distribution of the laminated clay shows a mode at 0.8 Am, but that of Unit II and brown gouges has modes from 4 to 8 Am and a secondary maximum around 0.6– 0.8 Am (Fig. 7).

Chemistry of bulk samples and clay aggregates CaO and Na2O contents greatly decrease from fresh gravel to Units I and II, whereas K2O content decreases only moderately (Table 1). Loss on ignition, Al2O3, Fe2O3, and TiO2 all increase in Unit II. In bulk chemistry, the brown gouge is not distinguishable from Unit II but is clearly different from Unit I, bentonite, and green gouge. Bentonite and green gouge have the same bulk chemistry. Electron microprobe analyses of clay aggregates in the deformed Unit I and brown gouge are plotted in elemental abundance diagrams (Si/Al atom ratio, Fe, K vs. Mg) (Fig.

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Figure 4. Micromorphology and mineralogy of brown gouge. (a) LA filling microfissures of EA in transitional area between Unit I and gouge. BSE image of a thin section from sample in Figure 1c. (b) Preferred orientation of particles and EA fragments in LA. BSE image of a thin section from the sample in Figure 1c. (c) Weathered pebbles with loose fabric due to abundant voids (black) formed by dissolution of plagioclase. BSE image of thin section from sample in Figure 1c. (d) Cross section of slickenside showing shearing of LA and EA. BSE image of a thin section from sample in Figure 1e. (e) Cross section of slickenside showing sheared fabric of platy clay particles. Secondary electron image of sample in Figure 1e. (f) XRD patterns of bulk sample and oriented clay fractions in Figure 1c treated with ethylene glycol and heating.

8). Elemental abundance is the number of cation calculated on the basis of total 44 negative charge of the anions {O20(OH)4} of 2:1 clay minerals although most of the aggregates are mixtures of several clay minerals. The chemical compositions of halloysite clay are clustered near the ideal composition of kaolin minerals (Si/Al = 1, Mg = 0). The laminated clays are higher in Si/Al atom ratio and Mg content. Their average values of Si/Al (1.42) and Mg per 44 anion charge (0.48) approach those of TEM-EDS analysis data of the individual clay particles in the laminated clay samples (Si/Al 1.45, Mg 0.43) (Fig. 6d). The chemical compositions of EA are distinguished from those of

halloysite clay by higher Si/Al atom ratio and Mg, Fe, and K contents, and mostly plot between halloysite clay and laminated clay in the diagrams.

Discussion Weathering of gravel deposits The paleo-beach gravels of Terrace III in the fault outcrop were highly weathered as shown by the welldeveloped brown soil, severely decomposed pebbles, and

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Figure 5. Micromorphology and mineralogy of green gouge and bentonite in Figure 1g. (a) Green gouge lacking original fabric. Thin section, plane-polarized light. (b) Diagenetic bentonite retaining original fabric of basaltic tuff. Thin section, plane-polarized light. (c) XRD patterns of the bulk samples and the oriented samples of the clay fractions treated with ethylene glycol and heating. S = smectite.

abundant clays throughout the profile. The large decrease of CaO and Na2O and increase of Al2O3, Fe2O3, and TiO2 contents in Units I and II in comparison to fresh gravel are attributed to the dissolution of plagioclase, and concomitant formation of halloysite, kaolinite, Fe-oxides/oxyhydroxides, and Ti-oxides. Particularly, the doubling of Fe2O3 content is responsible for the brown color of Unit II. Partial decomposition of biotite and slight weathering of K-feldspar account for the moderate decrease of K2O. In the undeformed Unit I, weathered pebbles with original shapes are engulfed by halloysite (Fig. 2c). This suggests that halloysite formed from a solute supplied by leaching of the pebbles in the upper part of the originally thick profile when the pebbles in the lower part were still relatively unweathered. With continued weathering, the erosive lowering of the ground surface caused the pebbles in Unit I to be leached, forming a porous fabric by dissolution of plagioclase, followed by the transformation of biotite to interstratified biotite – vermiculite and chlorite to interstratified chlorite – vermiculite, while quartz and Kfeldspar remained unaltered. In Unit II, plagioclase has completely disappeared, and interstratified biotite– vermiculite and chlorite –vermiculite derived from weathering of biotite and chlorite have been decomposed into kaolin – vermiculite. We think that silt-sized

interstratified biotite –vermiculite and chlorite – vermiculite particles in Unit I were comminuted to clay size in the course of weathering to kaolin – vermiculite in Unit II. Fe released by the decomposition of interstratified biotite – vermiculite and chlorite – vermiculite was precipitated as goethite. Hydrated halloysite was significantly dehydrated by prolonged exposure to wetting– drying cycles near surface. Illuvial origin of the brown gouge Gouge is a fine-grained breccia formed by the crushing of rocks and minerals brought about by fracturing and frictional sliding during fault movements (Davis and Reynolds, 1996). The constituents of gouge are assumed to be derived from materials in the hanging wall and footwall blocks. All the brown gouges cutting Unit I and the bentonite look like normal gouges of cataclastic origin in outcrop. The mineralogy and chemistry, however, suggests quite a different origin. The mineral composition of the brown gouge (Fig. 4f) is equivalent to that of the brown soil of Unit II (Fig. 2i), but markedly different from that of Unit I (Fig. 2e), bentonite (Fig. 5c), and green gouge (Fig. 5c). The bulk chemical composition of the brown gouge also has a strong similarity to that of Unit II but dissimilar to those of Unit I and bentonite (Table 1). These comparisons imply that the constituents of

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Figure 6. Micromorphology and mineralogy of laminated clays. (a) Fissures in laminated clay (LC) filled with LA (white open spaces were formed by shrinkage of laminated clay during air-drying for epoxy impregnation). Thin section made from sample shown in box, Figure 1f. Plane-polarized light. (b) Thin laminae in laminated clay. (c) XRD patterns of bulk sample and oriented clay samples treated with ethylene glycol and heating. (d) Si/Al vs. Mg contents of clay particles from laminated clays measured by energy-dispersive spectroscopy under transmission electron microscope. Numbers of cations were calculated on the basis of total 44 negative charge of the anions {O20(OH)4} of 2:1 clay minerals. Average values (filled square): Si/Al 1.45, Mg 0.43.

the brown gouge do not simply derive from the mechanical destruction of Unit I sediments and bentonite, but are mostly illuvial accumulations of detrital clay/silt particles transported in suspension downward from the brown soils of Unit II by percolating soil water. Laminated clay provides further evidence for the sedimentation of illuvial particles in the void space along the fault plane. Thin laminae in the laminated clay most likely formed through the episodic sedimentation from clay suspensions migrating deeply down the void space and being collecting into small pockets. Their similar clay mineralogies suggest that the laminated clay is equivalent to the clay components of Unit II and brown gouge, as shown by the similar modes around 0.8 Am common in their size distributions. The wide range of chemical compositions of individual particles of the laminated clay (Fig. 6d) indicates a fine-scale mixture of vermiculite and kaolinite/halloysite within the particles. Such mixtures are known to be common in the late stages of biotite weathering (Coffman and Fanning, 1975; Jeong, 1998b).

The green gouge in the deepest part of the outcrop appears to be a normal gouge, in which smectic particles were mostly derived from the mechanical destruction of wall rock bentonite. They could have formed during the faulting events recorded in the beach gravels. The fragments of bentonite in the brown gouge slightly above the green gouge (Fig. 1f), however, suggest that faulting at a shallow depth was insufficient to form gouge but gave rise to the production of coarse fragments. The green gouges appear to have been formed before exhumation and the deposition of beach gravels as suggested by KIGAM (1998) and Ree et al. (2003). Multiple faulting events Previous works on the Suryum fault believed the brown gouge to be cataclastic origin created by a simple one-time reverse faulting (Lee et al., 1999; Ree et al., 2003). The pedogenic origin of the brown gouge shown in our study, however, requires a new interpretation on the recurrent events of the fault.

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Figure 7. Size distribution of the brown gouges, Unit II, and laminated clay. Note the bimodal distribution for brown fault gouges and Unit II in contrast to the unimodal distribution for the laminated clay. Sample numbers of the brown gouges are given in parentheses.

The pedogenic origin of the clay/silt constituents of the brown gouge suggests the presence of void spaces along the fault plane that channeled soil suspension infiltrating from the surface. The void spaces appear to have been formed as a consequence of the reactivation of a pre-existing fault in the bentonite bedrock, and subsequently sealed with illuvial accumulations of detrital clay and silt of brown color. Shiny slickensides and illuvial origin of the brown gouge (Figs. 1c and e) indicate at least two fault movements, one before and one after the accumulations of detrital clay and silt. However, another event must have occurred because laminated clays were fragmented and enclosed by brown clay/silt materials within brown gouges (Figs. 1f and 6a). The microscopic anatomy of the deformed Unit I and brown gouges reveals more details on the multiple deformation history. The first faulting event is recorded by the fragmentation of halloysite clay in Unit I (Figs. 3a and b). Pebbles and fragments of halloysite clay were entrained into the voids along the fault plane. The microfissures of the deformed Unit I and the void spaces along the fault plane were later sealed with EA that is a microscopic accumulation of detrital clay particles. These particles were infiltrated from the upper part of the weathering profile (Unit II). The high Si/Al ratio and high Mg, Fe, and K contents of EA indicate the presence of vermiculite, illite, and Fe-oxides/oxyhydroxides abundant in the highly weathered brown soil horizons of Unit II. In the larger voids

between more rigid bentonite walls in the deepest part of the fault, illuvial clay particles accumulated locally as macroscopic masses of laminated clay. EA is a microscopic equivalent of the laminated clay, and both clays can be called argillan in terms of soil micromorphology. The chemical compositions of EA, intermediate between halloysite clay and laminated clay (Fig. 8), imply that clay particles infiltrating through the fault zone were partly admixed with halloysite particles derived from the fragmentation of halloysite clay. The second faulting event is indicated by the fragmentation of halloysite clay, EA, and laminated clay. New microfissures and void spaces were developed and sealed with LA (Figs. 3d and 4a) that is rich in fine silt particles imported from upper brown soil horizons. We speculate that a continued lowering of the ground surface after the first faulting by erosion might have allowed infiltration of the coarser, silt-rich LA rather than EA. The third and latest faulting event resulted in the development of sheared fabrics and slickensides in the brown gouge that became the compact and sticky mixtures of halloysite clay, EA, and LA with the characteristics typical of gouge. The pebbles within the brown gouge were weathered after the last faulting event. Our study provides the first direct evidence for multiple movements of Quaternary faults in the southeastern coast of the Korean Peninsula. Movement history of the gouge zone after deposition of beach gravels is summarized by the sequential accumulations of soil particles and their disturbances as follows: (1) first faulting to create void spaces; (2) sealing by the illuvial clays; (3) second faulting that fragmented the illuvial clays, forming new voids; (4) sealing by infiltrated silt-rich soil particles; and (5) third faulting that developed slickensides from the mixture of all the illuvial clay and silt accumulations. Although many Quaternary faults were recently found in the Yangsan Fault Table 1 Chemical compositions (wt.%) of bulk samples from the weathering profile of gravel deposits, gouges, and bentonite Fresh gravela Unit I Unit II Brown gouge Bentonite Green gouge 1b SiO2 68.42 Al2O3 14.70 Fe2O3 3.60 MnO 0.074 MgO 1.18 CaO 1.40 Na2O 3.01 K2 O 3.10 TiO2 0.554 0.10 P2O5 LOIc 3.92 Total 100.05 a

2

3

4

5

6

64.07 18.42 3.92 0.081 0.92 0.21 0.51 2.17 0.584 0.05 7.79 98.74

59.70 18.90 7.60 0.034 1.04 0.04 0.04 1.35 1.047 0.04 10.19 99.98

57.50 19.99 7.26 0.050 1.12 0.08 0.07 1.46 0.953 0.04 11.51 100.02

45.12 48.99 22.23 17.69 10.49 9.08 0.075 0.045 3.02 3.82 0.42 0.61 <0.01 <0.01 0.55 1.01 1.038 0.614 0.07 0.04 17.19 17.05 100.16 98.89

Fresh gravel sample was separated from the allophane-cemented sandy gravel layer of Unit I where pebbles were locally protected from weathering (Jeong et al., 2002). b Location of sample marked in Figure 1b. c Loss on ignition.

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Figure 8. Electron microprobe analyses of halloysite clay (circle) in Unit I, EA in deformed Unit I (triangle), and EA in brown clay gouge (cross), together with that of laminated clays (square). Analysis performed on aggregates of fine clay free of silt grains in polished thin section. Numbers of cations were calculated on the basis of total 44 negative charge of the anions {O20(OH)4} of 2:1 clay minerals.

System, movement history has not been reported in detail. Multiple movements of a few of the known Quaternary faults were suggested by electron spin resonance (ESR) dating of gouge (Lee and Schwarcz, 2001), cross-cut relationship of the slickensides on the fault plane (Ryoo et al., 2002), and stratigraphic correlation of bore hole data (Choi et al., 2002a). However, they are usually based on the evidence imprinted in the latest stage of faulting history. Our study gives an insight into whole history of faulting on a micro-scale in the Suryum fault of relatively small displacement. Recurrence intervals are rather shorter than general assumption, requiring the re-evaluation of the seismic safety around the Yangsan Fault System. Applications to paleoenvironmental reconstruction Pedological studies of weathering profiles and buried soils are commonly used in the correlation of soil horizons across faults, the identification of fault recurrence times and

intervals, and age estimation (Amit et al., 1996; McCalpin and Berry, 1996; Birkeland, 1999; Rockwell, 2000). However, mineralogical and micromorphological methods have been rarely applied in these investigations. Soil micromorphology has been developed to aid in soil genesis and classification, and has also been applied to deciphering sequences of soil-forming events and changes in environmental conditions (Kubie¨na, 1938; Bullock, 1983; Fitzpatrick, 1984; Reheis, 1987; Kemp, 1999). However, most studies focused on the solum, including A, E, and B horizons, and rarely extended to the underlying C horizons (saprolite or weathered rock/sediments preserving original structures/fabrics). Our study shows that in the weathering profile disturbed by tectonic movements, clay and silt particles originating in the intensively weathered soil horizons could be easily transported through and accumulated in the C horizon even down into the bedrock via microfissures and void spaces of fault, thus recording a sequence of disturbances. Therefore, micromorphological

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investigations combined with mineralogical analysis of the entire weathering profile, including bedrock and fault gouges, can provide a useful approach to the detailed reconstruction of Quaternary tectonic movements. Our new approach combining mineralogy, micromorphology, and chemistry appears to become more powerful by using microscopic analytical tools such as electron microscopy and microchemical analysis in addition to a normal optical observation of thin sections. The approach can be applied for the reconstruction of paleoenvironmental changes to any geologic medium undergoing a translocation of fine particles from the regions of faster weathering of humid tropical climates to the regions of slower weathering such as loess plateau of China (Reheis, 1987; Kemp, 1999; Kemp et al., 2001).

Conclusions Sequential accumulations of fine illuvial particles through fault zones and their disturbances record multiple faulting events on a micro-scale which were not recognized during the field investigations. Our study highlights a critical role for mineralogical and micromorphological investigations of the whole weathering profile and fault gouges in reconstructing relative movement history of a Quaternary surface fault. Particularly such an approach may be valuable in the intense chemical weathering environments lacking suitable materials for radiometric dating and stratigraphic indicators. This approach has further potential in the reconstruction of environmental history when combined with numerical dating, stratigraphic correlation, and mapping of soil catenas. The Korean Peninsula has long been considered to be a stable tectonic setting compared to other regions lying near plate boundaries. Together with recent investigations on the activity of the Yangsan Fault System in southeastern Korea, the result of this study raises questions regarding the inferred tectonic stability of the Korean Peninsula, during the Late Quaternary.

Acknowledgments This research was supported by Korea Institute of Nuclear Safety. Bong Ho Lee assisted in field and laboratory. Comments from M. Reheis, an anonymous reviewer, and A. Gillespie improved the manuscript.

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