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Journal of Structural Geology 42 (2012) 91e103

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Journal of Structural Geology

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The Geounri shear zone in the Paleozoic Taebaeksan Basin of Korea: Tectonicimplications

Seong Yong Lee a, Kyoungwon Min b, Jin-Han Ree a,*, Raehee Han c, Haemyeong Jung d

aDepartment of Earth and Environmental Sciences, Korea University, Seoul 136-701, South KoreabDepartment of Geological Sciences, University of Florida, Gainesville, FL 32611, USAcKorea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Koread School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, South Korea

a r t i c l e i n f o

Article history:Received 22 March 2012Received in revised form18 June 2012Accepted 20 June 2012Available online 28 June 2012

Keywords:Taebaeksan BasinGeounri shear zoneSongrim orogenyPhyllonite40Ar/39Ar age

* Corresponding author. Tel.: þ82 2 3290 3178; faxE-mail address: reejh@korea.ac.kr (J.-H. Ree).

0191-8141/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.jsg.2012.06.009

a b s t r a c t

The Songrim and Daebo orogenies represent two major Phanerozoic tectonic events that are well-preserved across much of the present-day Korean Peninsula. The Songrim orogeny corresponds to theLate PermianeTriassic collision of the North and South China cratons whereas the Jurassic Daebo tectonicevent represents a thin-skinned contractional deformation in a continental arc setting. It is well-established that the Songrim orogeny left a strong imprint on the geological record preserved in themiddle and northernparts of the Korean Peninsula (e.g., the PyeongnamBasin and the Imjingang belt)withonly aminor impact on the geology of southern Korea (e.g., the Okcheon and Taebaeksan basins). It was theDaebo tectonic event, however, which generatedmost of the deformational structures observedwithin theOkcheon and Taebaeksan basins. TheDeokpori thrust in the Taebaeksan Basin is a significant fault structurethat formed during the Daebo tectonic event; no other regional structures related to the Songrim orogenyhave been found in the Taebaeksan Basin. In the vicinity of the Deokpori thrust, we have identified in thisstudy a previously undocumented reverse-slip shear zone, which is named the Geounri shear zone.Microfabrics observed in phyllonite and marble mylonite samples suggest that this shear zone wasdeveloped in a plastic deformation regime atw400 �C, in contrast with the nearby Deokpori thrust, whichformed in a brittle deformation regime. A geochronological analysis of muscovite isolated from phyllonitesamples of the Geounri shear zone, yielded two 40Ar/39Ar age spectrawith a combinedweightedmean ageof 209 � 5 (2s) Ma. Thermal modeling of the Ar data, combined with structural interpretations, suggeststhat thismuscovite 40Ar/39Ar age represents a robust lower age limit for the timing of the Geounri shearingevent, therefore linking formation of the shear zone with deformational events caused by the Songrimorogeny. These results imply that structural features caused by ‘Songrim deformation’ were developed athigher temperatures than structures caused by ‘Daebo deformation’ as foundwithin the Taebaeksan Basin.

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1. Introduction

Understanding the deformational history of the Korean Penin-sula, which comprises several distinct tectonostratigraphic terranesof differing age that were variably deformed from Paleozoic toMesozoic times, is critical in formulating accurate Phanerozoictectonic models of eastern Asia. This is especially true becausesome of these terranes share geological similarities with the NorthChina craton, while others appear related to the South China craton,thereby tying the Korean Peninsula to the crustal-scale geologicalhistory of the surrounding region (e.g., China). In particular, the

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Taebaeksan Basin contains a wealth of geological informationregarding Phanerozoic tectonic evolution, because it contains EarlyPaleozoic, Late Paleozoic, and Early to Middle Mesozoic sedimen-tary sequences intruded by Mesozoic plutons (Chough et al., 2000).

Two major tectonic events are recorded in the geology of theTaebaeksan Basin: (1) the Late Permian to Triassic collisional Son-grim orogeny; and (2) the Middle Jurassic Daebo tectonic event(Chough et al., 2000; Ree et al., 2001; Han et al., 2006). The Songrimorogeny is considered to have resulted from the collision betweenthe North and South China cratons, with the QinlingeDabieeSulucollisional belt of China extending to (correlated with) the Imjin-gang belt (Yin and Nie, 1993; Ree et al., 1996) and/or the Gyeonggimassif (Oh, 2006) of Korea. Following the Songrim orogeny, theKorean Peninsula was affected by the Middle Jurassic to EarlyCretaceous Daebo tectonic event, represented by thin-skinned

S.Y. Lee et al. / Journal of Structural Geology 42 (2012) 91e10392

contractional deformation within a continental magmatic arcsetting (Sagong et al., 2005; Han et al., 2006; Kim and Ree, 2010).

Distinguishing between “Songrim” and “Daebo” structureswithin the Taebaeksan Basin is difficult because both sets ofstructures have a similar orientation and vergence (Kim et al.,1994). The NNEeSSW-striking, brittle Deokpori (Gakdong) thrust(with an ESE vergence) is the most prominent Daebo structure inthe basin. The fault separates the Yeongwol and Taebaek groups ofthe early Paleozoic Joseon Supergroup (Han et al., 2006;Zwingmann et al., 2011). In contrast, Songrim structures in theTaebaeksan Basin have not been identified, although Kim et al.(1994) suggested that any putative Songrim structures shouldoccur as brittle thrusts similar to the Daebo thrusts.

In this study we report a major reverse-slip ductile shear zone(the “Geounri shear zone”) in the Yeongwol area of the TaebaeksanBasin. The shear zone strikes NNEeSSW, has an ESE vergence, and isexposed a few hundred meters west of the sub-parallel Deokporithrust. The aims of the study are to: (1) describe the characteristic,outcrop-scale structures and microfabrics present within theGeounri shear zone; (2) determine the timing of shearing/displacement using 40Ar/39Ar thermochronology; and (3) considerthe implications of the results for regional tectonics in the KoreanPeninsula during the early Mesozoic.

2. Geological setting

The Okcheon belt, comprising the Taebaeksan and Okcheonbasins, is a NEeSW-trending Phanerozoic fold-and-thrust belt thatoccurs in the southern half of the Korean Peninsula, occurring

Fig. 1. Simplified geological map showing tectonic prov

between two Precambrian massifs (Fig. 1). Although the relation-ship between the Taebaeksan and Okcheon basins is still contro-versial, it has been suggested that they belong to the North andSouth China cratons, respectively, and that they were juxtaposedwith one another along a continental transform fault at the end ofthe PermianeTriassic collisional orogeny between the two cratons(Chough et al., 2000; Ree et al., 2001).

TheTaebaeksanBasinconsistsmainlyof theCambrianeOrdovicianJoseon Supergroup and the CarboniferousePermian (and possiblysome Early Triassic) Pyeongan Supergroup, along with the subordi-nate late Early Jurassic Bansong Group (part of the Daedong Super-group) (Fig. 2). The JoseonSupergroupprimarilyconsists of carbonateswith lesser siliciclastics, while the Pyeongan Supergroup is composedmainly of siliciclastics with some carbonates in the lower part of thesuccession. The Bansong Group consists of conglomerate, sandstone,and shale, along with minor pyroclastic rocks, and occurs on thefootwall side of major thrusts in the area, including the Deokpori andGongsuwon thrusts (Fig. 2).

Yoshimura (1940) classified the Joseon Supergroup into theDuwibong (Taebaek Group; Choi, 1998) and Yeongwol (YeongwolGroup; Choi, 1998) types based on lithologic successions and fauna,and noted that the Deokpori thrust occurs along the boundarybetween the two types (Figs. 1 and 2). In particular, shallow-waterendemic trilobites of the Cambrian occur in the Taebaek Group,whereas the Yeongwol Group is characterized deep-water cosmo-politan trilobites of the Cambrian (Kobayashi, 1967; Choi and Kim,2006). The Deokpori thrust, which is at least 250 km long, extendsfrom the Jeongseon area in the northeastern Okcheon belt to theCheongsan area in the south-central Okcheon belt, although it is

inces of East Asia. Modified from Ree et al. (2001).

Fig. 2. Geological map of the northeastern Okcheon belt. CS: Cheongsan; DY: Danyang; YW: Yeongwol. Modified from Ree et al. (2001). An enlarged geological map of the boxedarea is shown in Fig. 3.

S.Y. Lee et al. / Journal of Structural Geology 42 (2012) 91e103 93

crosscut by Cretaceous plutons (Fig. 2). Han et al. (2006) suggestedthat the Deokpori and Gongsuwon thrusts developed duringJurassic tectonic events related to the Daebo orogeny. Theirsuggestion is based on the occurrence of the late Early JurassicBansong Group within the footwalls of thrust faults, radiometricages of pyroclastic rocks occurring locally within the BansongGroup, and the nature of structures along and adjacent to thethrusts (see also Ree et al. (2009), for details on syn-faulting

deformation of the Bansong Group prior to lithification). Recently,Zwingmann et al. (2011) reported that segments of the Deokporithrust were reactivated during the Late Cretaceous.

In the northeastern Yeongwol area, the Geounri shear zoneoccurs within the Ordovician Yeongheung Formation of theYeongwol Group at about 300 m west of the subparallel Deokporithrust (Fig. 3). This ductile shear zone has a thickness of 300e500mand consists of marble mylonite and phyllonite rocks, with

Fig. 3. Geological map and cross-section highlighting bedrock geology in the Geounri shear zone study area. This map area is located in the eastern Yeongwol region. The outcroplocation of Fig. 4 is marked as a red star. DT: Deokpori Thrust. Fm: Formation. SZB: Shear zone boundary. Modified from GICTR (1962).

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phyllonite occurring as a 100e250 m thick layer along the center ofthe shear zone. The Yeongheung Formation is the least understoodformation in the Yeongwol Group mainly because its lithologicsuccession varies from section to section (Choi, 1998). Thus, it isunclear that the carbonate rocks in the footwall of the Geounrishear zone (i.e. between the Geounri shear zone and the Deokporithrust) is a member of the Yeongheung Formation (D.K. Choi, 2011,personal communication).

3. Geounri shear zone

3.1. Outcrop structures

Mylonitic foliation within the marble mylonite is defined bya compositional layering with alternating silicate-rich (<1e15 mmthick) and calcite-rich layers (5e50 mm thick). The foliation strikesNeS to NNWeSSE and dips to the west at a high angle (typically

S.Y. Lee et al. / Journal of Structural Geology 42 (2012) 91e103 95

60e70�). A ridge-in-groove type lineation occurs locally on thefoliation within the marble mylonite and is subparallel to the dipdirection of the foliation (Fig. 4).

The phyllonite shows a strong mylonitic foliation defined bya compositional layering of quartz-rich (1e3 mm thick) andphyllosilicate-rich layers (0.5e2 mm thick). This foliation isconcordant with that of the marble mylonite. A down-dip ridge-in-groove lineation is also present on the foliation plane of phyllonite,and is subparallel to the long axis of ellipsoidal quartz ribbons.Lensoidal blocks of the marble mylonite (tens of centimeters toseveral meters in length) occur within the phyllonite layer. Theasymmetry of shear-band foliation with respect to the myloniticfoliation in the phyllonite, and within the marble mylonite blocks,indicates a top-up-to-the-southeast (reverse) sense of shear (Fig. 5aand b). A relatively thin gouge layer (5e30 cm thick) also existswithin the phyllonite layer, and is subparallel to the mylonitic foli-ation (Fig. 4). This gouge layer presumably represents a local reac-tivation of the Geounri shear zone in the brittle regime after uplift.

3.2. Microfabrics of marble mylonite

All petrographic thin sections used in making microstructuralobservations were cut along a plane perpendicular to the myloniticfoliation and parallel to the observed down-dip lineations found onfoliation surfaces. The marble mylonite consists mainly of calcitewith subordinate quartz, muscovite, and opaque minerals. Each ofthe calcite and silicate minerals displays additional microscopiccompositional layering observable at thin-section scale. Silicate-rich layers consist mainly of muscovite, with some quartz andrare calcite. The muscovite grains, mostly 10e50 mm in length,show sweeping undulose extinction and their basal planes aresubparallel to the mylonitic foliation. The quartz grains, mostly10e20 mm in length, are equiaxed to slightly elongate and alsoshow sweeping undulose extinction.

Fig. 4. (a) Geological strip map across the Geounri shear zone, and an equal area projectionGeounri shear zone in the field. The great circle in the projection plot represents the avera

The calcite-rich layers of the marble mylonite consist ofdynamically recrystallized calcite grains (27e37 mm in size; Fig. 6a,b) with quartz, muscovite, and opaque minerals as minor accessoryphases (<10% combined total modal abundance). Relict calcitegrains, or porphyroclasts (120e500 mm in size), occur locally andcontain thick (10e20 mm), lensoidal mechanical twins, exhibitingfeatures consistent with twin boundary migration (Fig. 6c). Locally,recrystallized calcite grains grew along twin boundaries within therelict porphyroclastic grains. Some of these relict calcite grainsdisplay core-and-mantle structure, with subgrains and new grainsbeing of a similar size (28e34 mm; Fig. 6d).

The recrystallized calcite grains are elongate parallel to themylonitic foliation, with an axial ration of 1.5e1.8. These calcitecrystals show sweeping undulose extinction and most containmechanical twins (Fig. 6b). Some of these mechanical twins showfeatures indicative of twin boundarymigration. Grain boundaries ofthe recrystallized calcites are wavy or lobate, indicating dynamicgrain boundary migration. Locally, the grain boundaries of therecrystallized calcite grains appear straight, and exhibit triplejunctions between neighboring grains, especially when the totalcombined modal abundance of minor accessory phases is less than3%. These microstructures observed within the Geounri shear zoneare in marked contrast to those observed within wall-rockcarbonates outside the shear zone, where abundant fossil frag-ments occur and calcite grains do not contain any deformationsubstructures.

The individual lattice orientations of a large population ofrecrystallized calcite grains was measured using an electron back-scattered diffraction (EBSD) facility housed at the School of Earthand Environmental Sciences, Seoul National University, Korea. AllEBSD patterns were manually indexed using HKL’s Channel 5software (for details on the analytical methods, see Jung et al.(2009)). The pole figures and inverse pole figures determined forthe recrystallized calcite grains show a strong lattice preferred

plot summarizing outcrop structural measurements obtained during mapping of thege foliation. (b) Outcrop photograph of phyllonite and gouge layer.

Fig. 5. (a) Polished slab of phyllonite rock collected from the Geounri shear zone. The slab was cut perpendicular to the mylonitic foliation and parallel to the elongation lineation.(b) Line drawing of (a). (c) and (d) Photomicrograph of polished petrographic thin sections of phyllonite (cross-polarized light). Sigmoidal quartz porphyroclast in the center of (c)indicates a top-up-to-the-east (reverse) sense of shear. Asymmetric pressure shadow wings around opaque minerals and grain-shape foliation (S) of quartz grains oblique to C-foliation in (d) also indicate the top-up-to-the-east (reverse) sense of shear. Ms: muscovite. Qtz: quartz. Op: opaque.

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orientation (LPO), which suggests dominant basal <a> slip withsubordinate rhomb <a> slip (Fig. 7). These slip systems operate ata relatively high temperature (De Bresser and Spiers, 1997).

In summary, calcite grains in the marble mylonite appear tohave been deformed mainly by dislocation creep during ductileshearing, in which the dynamic recovery mechanism wasa combination of grain boundary migration and subgrain rotationrecrystallization processes. The deformation temperature duringthe formation of similar microstructures and LPO of naturallydeformed calcites has been estimated at 350e400 �C (Molli et al.,2000; Oesterling et al., 2007).

3.3. Microfabrics of phyllonite

The central phyllonite within the Geounri shear zone iscomposed mainly of muscovite, quartz, and chloritoid, along withminor opaque minerals (Fig. 8a). The quartz grains found withinquartz-rich layers include porphyroclasts (125e350 mm in size) andmatrix grains (20e50 mm in size). Thin muscovite seams (3e15 mmthick) occur as anastomosing or discontinuous trails within the

quartz-rich layers. The quartz porphyroclasts containing rare sub-grains range from elongate to globular in form, and exhibitsweeping undulose extinction and deformation bands. Some quartzporphyroclasts have transgranular extension fractures that areinfilled by quartz and muscovite. The matrix quartz grains, whichtend to be elongate parallel to the plane of mylonitic foliation, showeither uniform extinction or sweeping undulose extinction, andexhibit serrated or slightly wavy grain boundaries. Sigmoidalquartz porphyroclasts, asymmetric pressure shadow wings aroundopaque minerals and grain-shape foliation (S) of quartz grainsoblique to C-foliation consistently indicate a top-up-to-the-southeast (reverse) sense of shear (Fig. 5c and d).

Muscovite grains within the muscovite-rich layers of the phyl-lonite tend to be oriented with their basal plane parallel to themylonitic foliation, have a grain size of 10e300 mm (rarely up to500 mm), and commonly exhibit sweeping undulose extinction andminor kink bands that collectively suggest pre- or syntectonicgrowth. Some of the muscovite grains are partially retrograded tochlorite. Quartz ribbons (5e20mm long and 0.1e1.5 mm thick) alsooccur within the muscovite-rich layers. Quartz grains (mostly

Fig. 6. Photomicrographs of polished petrographic thin sections of marble mylonite (cross-polarized light). (a) Lobate boundaries and mechanical twins of calcite grains. (b) Close-up of boxed area from (a). (c) Relict calcite grains with thick twins showing twin boundary migration (upper right and left). New calcite grains appear to have grown along sometwin boundaries. (d) Relict calcite grains exhibiting core-and-mantle structure.

S.Y. Lee et al. / Journal of Structural Geology 42 (2012) 91e103 97

30e100 mm in length) within the quartz ribbons are elongateparallel to (i.e., within the plane of) the mylonitic foliation, andexhibit sweeping undulose extinction, deformation bands, andsubgrain boundaries. The grain boundaries of quartz grains withinthe ribbons are lobate or wavy, indicating dynamic grain boundarymigration (Fig. 8b). The lattice pole figures for quartz grains in theribbons show a lattice preferred orientation (Fig. 9), suggesting theoperation of basal <a> and prism <a> slip systems. These micro-fabrics correspond to a deformation regime that is transitionalbetween subgrain rotation and grain boundary migration recrys-tallization; i.e., that described by Stipp et al. (2002), who estimatedthe deformation temperature in this case to be about 500 �C.Furthermore (and in comparison), Song and Ree (2007) estimatedthe deformation temperature of a quartzemuscovite mylonite withsimilar microfabrics to be 400e450 �C.

Chloritoid porphyroblasts (mostly 100e300 mm in length) occurin both quartz- and muscovite-rich layers. Most of these porphyr-oblasts exhibit pressure shadows infilled by quartz, indicatingeither pre- or syntectonic growth of chloritoid (Fig. 10a, b). Inaddition, some chloritoid porphyroblasts overgrow the myloniticfoliation and deformed quartz grains, and do not exhibit pressureshadows. These observations suggest post-tectonic growth forthese particular chloritoid grains (Fig. 10c, d).

4. 40Are39Ar geochronology of phyllonite

4.1. Analytical procedures

To constrain the timing of deformation in the Geounri shearzone, we performed 40Ar/39Ar analysis of muscovite separatesisolated from samples of the phyllonite unit that is central to the

shear zone. A large block of phyllonite sample was collected nearGeounri Bridge, and then prepared following standard mineralseparation procedures including crushing, sieving, and panning.More than 100 muscovite grains of relatively large thickness wereidentified and hand-picked under a stereomicroscope. A 0.635 mgsample (YW-57) of these hand-picked muscovite grains (eachmeasuring w150 mm in size) was then wrapped in Al foil, andplaced in a quartz tube for irradiation. To estimate neutron flux atthe sample locations within the quartz tube, five doses (w1.2 mgeach) of GA1550 biotite standard (Renne et al., 1998, 2010) weresimilarly wrapped and placed in the quartz tube with a spacing ofw10 mm. The quartz tube (OS11) was flame-sealed and irradiatedin the Oregon State University (USA) TRIGA Reactor using the CLICITfacility for 15 h. The remaining analytical procedures for 40Ar/39Arthermochronology were performed at the University of Florida,USA. The irradiatedmuscovite sample was then split into two doses(YW-57a and YW-57b) and each placed within a well in a copperplanchette. Samples were then degassed by incremental heatingusing a CO2 laser. The extracted gas was purified using two GP-50SAES getters and was analyzed with a MAP215-50 mass spec-trometer operating in electron multiplier mode. Procedural blankswere measured before and after each sample analysis. Based onthe total decay constant listed in Steiger and Jäger (1977), theJ-value of the sample location in the tube was calculated at0.0038150 � 0.0000382 (2s). For details of the analytical proce-dures used during 40Ar/39Ar dating, see Foster et al. (2009, 2010).The ages were calculated based on the decay constants and tFCs (ageof Fish Canyon sanidine) of Renne et al. (2010, 2011). The resultingages are slightly older (0.9%) than the ages calculated using thedecay constants of Steiger and Jäger (1977), but this age differencecauses only a very minimal deviation from our age interpretations.

Fig. 7. (a) Pole figures and (b) inverse pole figures of lattice orientations of recrystallized calcite grains in marble mylonite. Fo. N: foliation normal. Lin: lineation. Equal-area, upper-hemisphere projections. We used a half scatter width of 30� to draw pole figures and inverse pole figures. The color coding refers to the density of data points (the numbers in thelegend correspond to multiples of uniform distribution).

S.Y. Lee et al. / Journal of Structural Geology 42 (2012) 91e10398

All uncertainties on the ages are reported at 2s, which incorporatesboth analytical and systematic errors.

4.2. Results

The 40Ar/39Ar age data for YW-57a and -57b muscovite sepa-rates are shown in Table 1 and Fig. 11. The YW-57a muscovite

Fig. 8. (a) Photomicrographs of phyllonite. (b) Elongated quartz grains with lobate boundarieCrossed-polarized light.

fraction yielded a well-defined plateau with a 100% concordantplateau age of 206.1 � 5.5 Ma (Fig. 11). This plateau age overlapswithin error the total fusion age (204.4�12.2 Ma), normal isochronage (202.5 � 50.4 Ma) and inverse isochron age (197.6 � 147.8 Ma)calculated for YW-57a.

In contrast, the YW-57b muscovite fraction yielded a rathercomplicated age spectra with the apparent ages becoming older as

s, deformation bands and subgrains in a quartz ribbon within a phyllonite rock sample.

Fig. 9. Pole figure of lattice orientations of recrystallized quartz grains in phyllonite. Equal-area, upper-hemisphere projections. We used a half scatter width of 30� to draw the polefigure. The color coding refers to the density of data points (the numbers in the legend correspond to multiples of uniform distribution).

S.Y. Lee et al. / Journal of Structural Geology 42 (2012) 91e103 99

the cumulative 39Ar increases (Fig.11). The reason for this pattern ofage spectra is unclear, but we suggest that it is likely related torecoil redistribution of Ar isotopes, particularly 39Ar, during irra-diation. Because the recoil distance of 39Ar is known to bew0.1 mmin most silicates (Turner and Cadogan, 1974), any K-free minerals(e.g., chlorite) interlayered within muscovite grains may receive39Ar atoms produced in the neighboring K-rich muscovite. Duringstepped heating experiments in a laboratory, argon gas in suchinterlayered K-free minerals (e.g., chlorite) may diffuse faster thanin muscovite, meaning that the low temperature steps may bedominated by the interlayered phases yielding systematicallyyoung apparent ages (39Ar-enriched signals). As a consequence, the

Fig. 10. Photomicrographs of chloritoid porphyroblasts in phyllonite. (a) and (c): Plane

high temperature steps are then dominated by Ar emanating fromthe host phases (i.e., muscovite), generating systematically oldapparent ages (39Ar-deficient signals) (Lo and Onstott, 1989; Loet al., 2000; Min et al., 2001). From detailed examination ofmultiple petrographic thin sections of phyllonite samples under anoptical microscope, we detected localized chloritization ofnumerous muscovite grains. The resulting age spectra from suchcomposite samples can also be significantly affected by themorphological relationships between the two interlayered phases.

The age discrepancy between the YW-57a and YW-57b suggestsan unintended, but slightly different, nature of the muscovitefractions in these two separates. One possibility is that the YW-57b

-polarized light. (b) and (d): Crossed-polarized light. Cld: chloritoid. Qtz: quartz.

Table 140Ar/39Ar data for YW-57 muscovite samples.

Run ID Laser [W] 40Ar 2s error 39Ar 2s error 38Ar 2s error 37Ar 2s error 36Ar 2s error % 40Ar* Age[Ma]

2s errorwithout J

YW-57 J ¼ 0.0038150 � 0.00003826457-001 4 1.306711 0.015988 0.041566 0.001227 0.000458 0.000026 0.000040 0.000001 0.000150 0.000035 99.9 204.0 6.86458-002 8 0.676705 0.013555 0.020628 0.000513 0.000269 0.000017 0.000060 0.000011 0.000036 0.000038 99.9 209.0 8.06459-003 12 0.118256 0.012917 0.003105 0.000091 0.000057 0.000009 0.000029 0.000014 0.000186 0.000062 97.5 197.1 162.46460-004 30 0.118645 0.026241 0.000649 0.000034 0.000074 0.000035 0.000028 0.000021 0.000473 0.000089 9.9 105.5 927.76461-001 3 0.687248 0.023259 0.022683 0.001269 0.000338 0.000043 0.000024 0.000013 0.000041 0.000079 99.91 193.9 12.96462-002 5 2.337942 0.089250 0.068269 0.004479 0.000909 0.000087 0.000049 0.000020 0.000082 0.000064 99.92 222.9 16.06463-003 8 0.830203 0.006358 0.023710 0.000471 0.000303 0.000044 0.000079 0.000026 0.000119 0.000047 99.92 223.6 6.36464-004 30 0.172167 0.022165 0.003016 0.000042 0.000063 0.000047 0.000049 0.000029 0.000150 0.000044 99.95 317.5 52.8

S.Y. Lee et al. / Journal of Structural Geology 42 (2012) 91e103100

muscovites contain more abundant interlayered phases than theYW-57a, causing a more complicated recoil redistribution of argonisotopes. An alternative, but less likely, interpretation is that theneutron flux within the original irradiation packet was notcompletely homogeneous, and the two separates representdifferent portions in the irradiation packet.

Although the redistribution of Ar isotopes in the presentsamples is the likely cause of slightly disturbed age spectra for theYW-57b muscovite fraction, it is unlikely that these samplesexperienced considerable recoil loss of 39Ar out of themuscoviteechlorite system during irradiation because the physicaldimensions of the muscovite grains (w150 mm) are significantlylarger than the recoil distance of 39Ar (w0.1 mm). For the YW-57b

Fig. 11. 40Ar/39Ar incremental heating data obtained frommuscovite separates isolatedfrom a sample of the Geounri phyllonite. The YW-57a data yielded a 100% concordantplateau age. In contrast, the YW-57b data shows a systematic increase in apparent ageswith cumulative 39Ar released, which is interpreted as an analytical artifact resultingfrom the slight redistribution of Ar isotopes during irradiation. We interpret theplateau age from YW-57a and the total fusion age from YW-57b to represent the mostreliable muscovite 40Ar/39Ar ages.

muscovite fraction, the total fusion age of 219.9 � 10.0 Ma isindistinguishable from the mean age of 223.5 � 6.2 Ma calculatedfrom the two intermediate and consistent age steps (Fig. 11), whichsuggests that the net recoil loss of 39Ar out of the entire aliquot ofmuscovite grains during sample irradiation was insignificant. Evenwith such minor recoil-driven disturbances, the total fusion age isconsidered to be more reliable than plateau ages (Min et al., 2001),and therefore we interpret the total fusion age of 219.9� 10.0 Ma toindicate the timing of Ar closure in the YW-57b muscovite system.In summary, the plateau age of YW-57a (206.1 � 5.5 Ma) and totalfusion age of YW-57b (219.9 � 10.0 Ma) are considered to be themost reliable age estimates among other types of 40Ar/39Ar agesdetermined on muscovite in this study. Furthermore, the weightedmean of these two ages (209.3 � 4.8 Ma) is regarded as the mostrepresentative muscovite 40Ar/39Ar age in this study, which we alsoconsider to be a robust age constraint that is useful in estimatingthe timing of formation of the Geounri shear zone (as outlinedbelow).

5. Discussion

As stated above, the Deokpori thrust is a major regionalgeological structure in the Korean Peninsula that formed during theJurassic Daebo tectonic event (Han et al., 2006). In the field, theDeokpori thrust outcrops as an approximately 100-m-wide faultzone, consisting of foliated cataclasite, ultracataclasite, and thinlayers of fault gouge. The cataclastic foliation present withinDeokpori fault rocks is defined by a preferred orientation of micasand clay minerals (Han et al., 2006). Grains and grain aggregates ofcalcite and quartz within these fault rocks are angular to sub-angular in form, with intragranular fractures developed withinmost of the grains. In contrast, quartz grains in phyllonite andcalcite grains in marble mylonite of the Geounri shear zone weredeformed mainly by dislocation creep at an estimated temperatureof 400e450 �C. Therefore, we conclude that the phyllonite andmarble mylonite of the Geounri shear zone formed in a plasticdeformation regime, whereas fault rocks of the nearby Deokporithrust fault developed in a brittle deformation regime.

The 40Ar/39Ar age data obtained from syntectonic muscovitegrains in this study provide important clues regarding the timingand temperature conditions of the deformation that occurredwithin the Geounri shear zone during its formation. To investigatethe geological implications of the muscovite 40Ar/39Ar dataobtained from the Geounri phyllonite, we focus on the followingtwo aspects: (1) Is there any inherited Ar present in these musco-vite grains that could affect the 40Ar/39Ar ages? (2) Do the 40Ar/39Arages represent the timing of deformation, or alternatively, thetiming of cooling below the closure temperature of Ar in muscovitelong after formation of the Geounri shear zone?

The alignment of muscovite grains parallel to the plane ofmylonitic foliation and the precipitation of muscovite as well as

Tf

S.Y. Lee et al. / Journal of Structural Geology 42 (2012) 91e103 101

quartz along syntectonic extension fractures within the quartzporphyroclasts in the phyllonite, indicate syntectonic growth ofmuscovite grains. Although these microstructural features suggestthat most of themuscovite grains grew during the shearing event, itis possible that some existed prior to shearing. Accordingly,recrystallization of these previously existing muscovite grainscould have taken placewith syntectonic growth/overgrowth of newmuscovite during the main shearing event. In this scenario, someinherited radiogenic 40Ar may have been incorporated into therecrystallized muscovite grains, yielding systematically oldapparent 40Ar/39Ar ages with respect to the timing of the mainshearing event. Such an inherited 40Ar component has classicallybeen a problem in geochronological studies aimed at constrainingthe timing of a deformational event, particularly if the temperatureconditions of deformation were not high enough to completelyreset the KeAr system (Dunlap et al., 1991; Dunlap, 1997). For theGeounri shear zone, the inferred deformation temperature of400 �C is slightly higher than commonly accepted Ar closuretemperatures in muscovite (300e400 �C) deduced from the cali-bration of muscovite 40Ar/39Ar ages with other thermochronologicconstraints (Purdy and Jäger, 1976; Jäger, 1979; Hodges, 1991; Spear,1993).

One of the primary tasks of 40Ar/39Ar thermochronology is tointerpret natural distributions of argon isotopes in geologicsamples in order to uncover their thermal histories, in aid oflaboratory experiments (McDougall and Harrison, 1999). Argondiffusion in solid phases is commonly described by Fickian type,volume diffusion, whosemathematical solutions for different initialand boundary conditions are available in Crank (1975). Althoughsimple volume diffusion is frequently assumed in thermochronol-ogy, other types of diffusion, as well as complications in applyingthe diffusion properties to resolving a range of geologic issues, aresummarized in Watson and Baxter (2007) and Baxter (2010).

To investigate the possible contribution of an inheritedcomponent to the 40Ar/39Ar system of the Geounri muscovitesamples, we performed volume diffusion modeling and estimatedthe fractional loss of 40Ar (fAr) for several different isothermalheating scenarios. For this numerical simulation, the threefollowing sets of Ar diffusion parameters were used: (1) E¼ 40 kcal/mol, Do ¼ 1.38 � 10�4 cm2/s assuming a plane sheet diffusiongeometry (Robbins, 1972); (2) E ¼ 52 kcal/mol, Do ¼ 0.04 cm2/s foran infinite cylinder model (Hames and Bowring, 1994); and (3)E¼ 63 kcal/mol, Do ¼ 2.30 cm2/s with an infinite cylinder geometry(Harrison et al., 2009). The models of Robbins (1972) and Hamesand Bowring (1994), which are based on the same set of experi-mental data, indicate relatively low retentivity of Ar with theresulting closure temperatures being comparable to previous esti-mates constrained from thermochronologic calibrations. However,the recent work of Harrison et al. (2009) suggests much higher Ar

able 2Ar (Ar fractional loss) calculated for different diffusion parameters and isothermal heatin

Diffusion domainradius [um]

Holding duration [Myr] fAr

Plane sheet geomeRobbins (1972)

10 5 1.0010 1.00

20 5 1.0010 1.00

30 5 1.0010 1.00

40 5 1.0010 1.00

50 5 1.0010 1.00

retentivity, indicating that more efficient preservation of inherited40Ar atoms takes placewithinmuscovite grains thanwas previouslythought.

In addition, the diffusion domain size needs to be established inorder to carry out meaningful thermal modeling studies of Ardiffusion in muscovite. Although the diffusion domains are oftenconsidered to be identical to the shape and size of muscovite grains(Hames and Hodges,1993; Hodges et al., 1994; Hames and Bowring,1994), it is likely that many deformed muscovite grains containmicrostructural discontinuities that can play an important role aspathways for rapid Ar diffusion. Harrison et al. (1985) suggested aneffective diffusion domain dimension of w150 mm for the biotitegrains used in their experiments, and Wright et al. (1991) arguedthat the diffusion domain radius is less than 225 mmeven for biotitegrains with radii up tow600 mm. These proposals are supported bythe variation observed in in-situ laser ablation 40Ar/39Ar agesobtained for different spots on the same basal planes of muscovitecrystals (Kramar et al., 2001; Mulch et al., 2002, 2004). Further-more, from SEM (Scanning Electron Microscopy) examinations ofsieved muscovite grains, Harrison et al. (2009) observed thatmuscovite grains of the same size populations commonly containw5% significantly smaller grains adhering to the large ones due toelectrostatic attraction. For the Geounri samples, we assumed thatthe maximum diffusion length scale is w100 mm, which is two-thirds of the typical muscovite grain size of w150 mm. Finally, wealso assumed that natural isothermal heating took place in theGeounri shear zone, with heating durations lasting from 5 to10 Myr, which is consistent with the likely timescales for suchshear-zone deformation events.

The resulting fAr values for different input parameters are listedin Table 2. The Robbins (1972) and Hames and Bowring (1994)parameters yield a fAr value of 1 suggesting complete Ar degass-ing during the assumed conditions of deformation. There are someexceptions to this result, however, when Harrison et al. (2009)parameters are combined with large diffusion domain dimen-sions. Nevertheless, the simple lattice diffusion model employed inour calculations is likely to yield only a ‘lower limit’ of fAr becausethe recrystallization and deformation processes involved wouldsignificantly enhance Ar diffusion along transient grain boundariesor dislocations (Hames and Cheney, 1997). Furthermore, thecontribution of a relict inherited component to the resulting40Ar/39Ar ages would be even less than the calculated values in ournumerical simulations because the volume portion of pretectonicmuscovite, if preserved, would be very small compared to thesyntectonic, recrystallized muscovite, meaning that a much smallerdiffusion domain radius would need to be used for accuratecalculation of fAr values. It is noteworthy that even the mostretentive parameters of Harrison et al. (2009) would yield morethan 90% Ar loss for a diffusion radius of <30 mm (Table 2).

g scenarios.

try, Cylinder geometry,Hames and Bowring (1994)

Cylinder geometry,Harrison et al. (2009)

1.00 1.001.00 1.001.00 0.891.00 0.981.00 0.691.00 0.861.00 0.561.00 0.721.00 0.461.00 0.62

S.Y. Lee et al. / Journal of Structural Geology 42 (2012) 91e103102

Therefore, we conclude that if any inherited Ar atoms existed inrelic muscovite grains, they would have been almost completelyreleased during deformation.

Another important issue is whether the muscovite 40Ar/39Arages determined for phyllonite samples from the Geounri shearzone represent the timing of plastic deformation during shearing,or alternatively, the timing of significantly younger post-deformational cooling. To test these hypotheses, we calculatedclosure temperatures (Tc) of the Geounri muscovite. For a coolingrate of 10 �C/Myr and a diffusion domain radius of 30e50 mm, Tc iscalculated as 270e280 �C using the least retentive diffusionparameters of Robbins (1972), and 390e405 �C when using themost retentive parameters of Harrison et al. (2009)(Fig. 12). TheHames and Bowring’s (1994) parameters yielded intermediate Tcvalues of 325e340 �C. Although these estimated closure tempera-tures encompass a wide range of values (mainly due to the largescatter of the reported diffusion parameters), most of the calculatedTc values are lower than the estimated peak deformation temper-atures, suggesting that the muscovite 40Ar/39Ar ages represent thetiming of when these grains cooled below their closure tempera-tures at some time after peak deformation, rather than represent-ing the timing of deformation itself. The time gap betweencessation of the hypothesized isothermal heating at w400 �C andclosure of the Ar system in muscovite (median Tc ¼ 325e340 �Cusing Hames and Bowring’s (1994) parameters) is estimated to bew5e10 Myr when monotonous cooling of 10 �C/Myr is assumed.Based on these calculations, we suggest that the muscovite40Ar/39Ar age data provide a robust minimum age of 209� 5Ma forthe timing of peak deformation, but note that this deformationalevent may have ceased sometime (perhaps 5e10 Myr) prior to theclosure of the muscovite Ar system at 209 Ma.

Therefore, we conclude that the Geounri shear zone was likelydeveloped in the late stages of development of the LatePermianeTriassic Songrim orogeny that represents collisionbetween the North and South China cratons. There is muchprevious consensus that deformation caused by the Songrimorogeny was more intense than the Jurassic Daebo tectonic event inthe Imjingang belt and the Pyeongnam Basin of North Korea (see

Fig. 12. A plot of radius of Ar diffusion domain versus closure temperature formuscovite at various cooling rates.

Fig. 1), and vice versa in the Okcheon and Taebaeksan basins ofSouth Korea (e.g., Reedman and Um,1975). At any rate, up until now‘Songrim’ structures have not been clearly identified in the Tae-baeksan Basin, in agreement with traditional viewpoints. In thepresent study, however, our results suggest for the first time, thatthe Songrim orogeny may actually have left as strong an imprint onthe Taebaeksan Basin as the Daebo tectonic event. In addition, ourresults imply that the “Songrim” structures formed at highertemperature conditions than the “Daebo” structures within theTaebaeksan Basin. More regional correlations that indicate thefarther extension of the Geounri shear zone are presently unclearsince the shear zone is cross-cut by the lateral ramp of the Deokporithrust fault to the northeast (see Fig. 2). To understand the largerrole that the Geounri shear zone may have played in the amal-gamation of the tectonic provinces of the Korean Peninsula duringthe Songrim orogeny, it will be necessary to identify new traces ofthe shear zone in each thrust sheet of the Jurassic thrust system,followed by palinspastic restoration of the original regionalgeometry of the shear zone prior to the formation of the thrustsystem.

6. Summary

(1) The Geounri shear zone in the Paleozoic Taebaeksan Basin ofthe Korean Peninsula can be characterized as a steeply dipping(60e70�), reverse-slip, ductile shear zone with a thickness of300e500 m that lithologically consists of marble mylonite, anda 100e250-m-thick phyllonite unit occurring at the shear zonecenter.

(2) Although the Geounri shear zone has a similar orientation andvergence as the nearby Deokpori thrust (the most prominentJurassic structure developed in a regional contractional conti-nental arc setting deformational event), the conditions offormation for these two structures are remarkably different,with the Geounri shear zone forming in a plastic deformationregime and the Deokpori thrust forming in a brittle deforma-tion regime.

(3) Themuscovite 40Ar/39Ar ages (weightedmean¼ 209.3� 4.8Ma)determined in this study, combinedwith thermal modeling andstructural constraints, suggest that themuscovite Ar systemwascompletely reset by deformation during shearing, and couldthen have been subjected to diffusive loss for a long time afterpeak deformation in the Geounri shear zone. Therefore, thetiming of the main deformational event in the Geounri shearzone is inferred to be slightly older than the weightedmean ageof 209.3 � 4.8 Ma determined on muscovite, and the timing ofthis deformation probably corresponds to the later stages ofdevelopment of the Late PermianeTriassic collision between theNorth and South China cratons (the Songrim orogeny in theKorean Peninsula).

Acknowledgments

We thank David Foster for lab assistance and constructivediscussions. We appreciate constructive comments by an anony-mous reviewer and Chris Morley. This work was supported by theNational Research Foundation of Korea fund 2010-0024206 to Reeand partially by National Research Foundation of Korea fund 3345-20100013 to Jung.

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