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Tectonophysics
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Permo-Triassic changes in bulk crustal shortening direction during deformationand metamorphism of the Taebaeksan Basin, South Korea using foliationintersection/inflection axes: Implications for tectonic movement at theeastern margin of Eurasia during the Songrim (Indosinian) orogeny
Hyeong Soo Kim a,⁎, Jin-Han Ree b
a Department of Earth Science Education, Kyungpook National University, Daegu 702-701, South Koreab Department of Earth and Environmental Sciences, Korea University, Seoul 136-701, South Korea
The Permo-Triassic Songrim (Indosinian) orogeny in South Korea was a major tectonic event involving compli-cated continental collisions at the eastern margin of Eurasia. Previous studies have examined the structural andmetamorphic features of the Songrim orogeny in each of the Paleozoic terranes of the orogenic belt (i.e., theTaebaeksan Basin, the Okcheon Basin, and the Imjingang Belt), but correlations of these features among theterranes remain uncertain. The aim of this paper is to reveal deformation history including bulk crustal shorteningdirections in the Taebaeksan Basin, and to correlate the tectono-metamorphic evolution of the Taebaeksan Basinwith other Phanerozoicmobile belts in eastern Asia based on a combined analysis of foliation intersection/inflectionaxes (FIA) trends andmetamorphic P–T and T–t (time) paths. The orientations and relative timing of FIA preservedas inclusion trails within porphyroblasts of andalusite, chloritoid, garnet, and staurolite reveal two age groups ofinclusion trails in the Pyeongan Supergroup at the northeasternmargin of the Taebaeksan Basin. Thesemicrostruc-tures indicate the development of earlyNNW–NNE-trending structures and fabrics, followedby later E–W-trendingones. These observations suggest a change in the orientation of bulk crustal shortening from E–WtoN–S during theSongrimorogeny. Based on the similarmicrostructures and temperature–timepaths of the three Paleozoic terranes,we interpret that the E–W bulk crustal shortening influenced the eastern part of the Korean Peninsula duringthe early stages of the Songrim orogeny, presumably related to amalgamation between the proto-Japan terraneand the eastern margin of Eurasia, whereas the N–S bulk crustal shortening was stronger in the western part ofthe peninsula during the later stages of the orogeny, related to collision between the South and North China blocks.
The Korean Peninsula records evidence of two Phanerozoic orogenicevents: the late Permian to Triassic Songrim orogeny, related to conti-nental collision between the North and South China blocks, and theJurassic Daebo tectonic event, represented by thin-skinned contraction-al deformation in a continental magmatic arc setting (Chough et al.,2000; Han et al., 2006; Ree et al., 2001; Sagong et al., 2005). Theseevents are recorded mainly in three mobile belts within Korea: theTaebaeksan and Okcheon Basins, and the Imjingang Belt (Fig. 1).
The kinematics of the amalgamation of tectonic provinces in Korearemains poorly understood. Previous studies have proposed that theGyeonggi Massif and the Okcheon Basin were juxtaposed againstthe Yeongnam Massif and the Taebaeksan Basin along a continentaltransform fault (the South Korean Tectonic Line; Fig. 1) during theSongrim orogeny, and that the Imjingang Belt is an eastward extension
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of the Qinling–Dabie–Sulu collisional belt of China (e.g., Chang, 1996;Cho et al., 2007; Chough et al., 2000; Ernst and Liou, 1995; Ree etal., 1996, 2001; Yin and Nie, 1993). According to this model, theGyeonggi Massif and the Okcheon Basin belong to the South Chinablock, while the Yeongnam Massif and Taebaeksan Basin are part ofthe North China block. Others have argued that the Nangrim Massifand Pyeongnam Basin of North Korea (North China block) were amal-gamated with the Gyeonggi Massif, the Okcheon Basin, the YeongnamMassif, and the Taebaeksan Basin (South China block) along with theImjingang Belt as a collision belt (Kwon et al., 2009). Alternatively, aPermo-Triassic suture zone may run through the Gyeonggi Massif(Oh, 2006; Oh et al., 2009). These tectonic models remain debated be-cause of insufficient information on the deformation history and timingof tectonometamorphic phases of the three Phanerozoic mobile belts.Furthermore, no previous study has sought to correlate deformationstructures among the belts.
Inclusion trails within porphyroblasts (which represent a matrixfoliation that was overgrown by the porphyroblast) reveal theinter-relationships between multiple long-lived deformation and
Fig. 1. (a) Schematic tectonic map showing major three Paleozoic mobile belts (Taebaeksan, Okcheon Basins, and Imjingang Belt) with the Precambrian massifs (Nangrim,Gyeonggi, and Yeongnam Massifs), and other geologic features in the Korean Peninsula. (b) Simplified geological map and cross-section of the Taebaeksan Basin and northeasternpart of the Okcheon Basin. Note, SKTL is the South Korean Tectonic Line.(b) is modified from Chough et al. (2000).
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metamorphic events (e.g., Aerden, 1994; Bakker et al., 1989; Bell andJohnson, 1989; Bell and Rubenach, 1983; Ilg and Karlstrom, 2000;Johnson, 1999; Solar and Brown, 1999; Williams, 1994; Zwart,1962), and provide information on the kinematics of paleo-tectonicmovements (e.g., Aerden and Sayab, 2008; Bell et al., 1995; Bell etal., 2012; Shah et al., 2011). Such inclusion trails, with variousgeometries, are found in metasedimentary rocks of the northeasternTaebaeksan Basin, within porphyroblasts of andalusite, chloritoid,garnet, and staurolite.
This paper presents data on the orientations of inclusion trailswithin porphyroblasts in the late Paleozoic Pyeongan Supergroupfrom the northeastern margin of the Taebaeksan Basin, using the foli-ation intersection/inflection axes (FIA) approach (Bell et al., 1995).The aim of the study is to reconstruct the multi-phase deformationhistory of the metapelites in the Taebaeksan Basin during the Permo-Triassic Songrim orogeny based on the FIA data and mesoscopic struc-tures. Also we infer the orientation of bulk crustal shortening duringthe Songrim orogeny. The Permo-Triassic tectono-metamorphic evolu-tion of the Taebaeksan Basin is correlatedwith that of other Phanerozoicmobile belts (i.e., the Okcheon Basin and the Imjingang Belt) based on acombined analysis of FIA trends and metamorphic P–T and T–t (time)paths. The results provide a better understanding of the tectonic evolu-tion of the Permo-Triassic orogeny related to continental collision atthe eastern margin of Eurasia.
2. Tectonic and metamorphic background
The Korean Peninsula consists of eight tectonic provinces(from north to south): the Nangrim Massif, the Pyeongnam Basin, theImjingang Belt, the GyeonggiMassif, the TaebaeksanBasin, theOkcheon
Basin, the Yeongnam Massif, and the Gyeongsang Basin (Fig. 1). Thethree massifs are dominated by Precambrian basement (high-gradegneisses and schists). The PyeongnamBasin contains Paleozoic sedimen-tary sequences similar to those of the Taebaeksan Basin (see below).The Imjingang Belt is an E–W-trending fold-and-thrust belt that consistsmainly of metasedimentary rocks of inferred Devonian age subjected toBarrovian-type regional metamorphism (Ree et al., 1996). The OkcheonBasin is a NE–SW-trending fold-and-thrust belt that consists of lateProterozoic–Paleozoic metasedimentary (Lee, 1999; Lim et al., 2005,2006) and metavolcanic rocks. The Gyeongsang Basin is a Cretaceouscontinental back-arc basin that contains non-marine sedimentary andvolcanic rocks (Chough and Sohn, 2010).
The Taebaeksan Basin consists of the Cambrian–Ordovician JoseonSupergroup (mainly carbonates with minor siliciclastics) and the lateCarboniferous to early Triassic (?) Pyeongan Supergroup (siliciclasticswith subordinate carbonates). The basin contains two sets ofmajor con-tractional structures: (1) N–S to NNE–SSW-trending folds and thrust,related to continental collision during the Permo-Triassic Songrimorogeny, and (2) E–W-trending folds and thrusts related to the Daebotectonic event and/or another tectonic event of the late Cretaceous toearly Tertiary, in a continental magmatic arc setting (Fig. 1b; Cluzel etal., 1990; Chough et al., 2000; Kim, 1996; Kim et al., 1994; Ree et al.,2001).
Kim et al. (2012) reported that the late Paleozoic Pyeongan Super-group in the northeastern Taebaeksan Basin records four deformationevents (D1–D4) that produced two sets of N–S (D1) and E–W (D3)trending generations of upright folds and associated axial plane cleav-ages (S1 and S3) plus two sub-horizontal crenulation cleavages (S2and S4) related with only minor folds. Kim et al. (2012) suggestedthat these deformation events occurred during the Songrim orogeny
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in an active continental margin setting (e.g., arc-related foreland basin)in Permo-Triassic time.
The Pyeongan Supergroup in the northeastern Taebaeksan Basin (thepresent study area) includes the Manhang, Guemcheon, Jangseong, andHambaeksan formations in ascending order (Fig. 2; Lee and Chough,2006). The Manhang and Geumcheon formations consist mainly ofblack slates, dark gray phyllites, mica schist withminormetapsammites,and crystalline limestone. The Jangseong Formation is composed ofmetapsammites and metapelites with Permian plant fossils (Lee, 1999)
Fig. 2. Geological map and cross-section of the study area in the northeastern flank of the Ta(S0, S1, S2, and S3), lineations (L20,1 and L32) , and F1 and F3 fold axis are also shown.
and coal seams. The Hambaeksan Formation consists of meta-arenites.The metapelites in the study area have experienced low-temperature(LT)/medium-pressure (MP) regional metamorphism followed byhigh-temperature (HT)/low-pressure (LP) thermal metamorphism(Kim and Ree, 2010). These metamorphic events resulted in theformation of three metamorphic zones (Fig. 2): (1) And+Ms+Chl±Mrg±Cld±Ky (zone I; mineral abbreviations from Kretz, 1983);(2) Grt+St+Bt±And±Pl (zone II); and (3) And+Sil (Ky)+St+Bt±Grt±Pl (zone III). The peak metamorphic P–T conditions for the LT/MP
ebaeksan Basin. Three metamorphic zones (I, II, and III) and structural fabrics, foliations
Fig. 4. Photomicrographs of andalusite (samples OG51 and 39) and chloritoid (OG26) porphyroblasts showing textural relationships between internal/external foliations and thechanges of asymmetry of inclusion trails with the porphyroblasts. (a)–(c) photos are taken under cross polarized light (XPL) with gypsum plate, and others are plane polarized light(PPL). Single barbed arrow and number represent orientation of strike and way up of each thin section.
Fig. 3. (a) Schematic diagram of a thin slice through the center of a porphyroblast drawn perpendicular to spiral axis of inclusion trails. The intersection of successive sub-vertical andsub-horizontal foliations defined the foliation intersection axes (FIA). (b and c) Sketches show how FIA trend, if generated by the intersection of the near vertical and near horizontalplanes as shown in (a), is independent of the orientation of the direction of thrusting on the horizontal foliation and can form perpendicular to the relative direction of bulk shortening.(a) is from Bell et al. (1995).
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a Mineral abbreviations from Kretz (1983).b d.c.c.=differentiated crenulation cleavage.
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regional metamorphism were ca. 5.0–6.0 kbar at 550–580 °C during theTriassic, with the timing constrained by the deposition of the PyeonganSupergroup containing the Permian plant fossils and the intrusion ofJurassic leucocratic granite (Kim and Ree, 2010).
3. Definition and geological significance of FIAs
The FIA technique is based on the principle that any cylindrical struc-ture will exhibit an opposite asymmetry depending on the viewingdirection. This allows the axes of curved inclusion trails to be determinedfrom sets of radial vertical thin sections (for details, see Bell et al., 1995).During the past two decades, this approach has been successfully ap-plied to reconstruct the history of deformation and metamorphism inmajor orogenic belts of various ages, in Australia (e.g. Ali, 2010; Belland Mares, 1999; Sayab, 2006), the Appalachians (Bell and Welch,2002; Kim and Bell, 2005; Sanislav and Bell, 2011; Timms, 2003; Yeh,2007), the Varsican belt (Aerden, 1994, 1998, 2004), the Himalayas(Bell and Sapkota, 2012; Shah et al., 2011), and collision belts inCentral Asia (Bell and Chen, 2002). FIA data have also been employedto deepen our understanding of the relationships among deformation,metamorphism, and magmatism (e.g., Bell and Hayward, 1991; Belland Johnson, 1992; Bell and Newman, 2006; Fay et al., 2008; Hickeyand Bell, 1999; Kim and Jung, 2010; Lee, 2000; Sanislav and Bell, 2011;Stallard and Hickey, 2001), and even plate motion (e.g., Aerden andSayab, 2008; Sanislav and Bell, 2011; Shah et al., 2011).
According to these studies, sigmoidal and spiral-shaped inclusiontrails within a porphyroblast are produced by successive overgrowth ofmultiple sub-vertical and sub-horizontal crenulation cleavages. There-fore, FIAs are generally subhorizontal and should lie normal to the direc-tion of bulk shortening that was responsible for them (Fig. 3a). ThisFIA is independent of the orientation of stretching lineation developedon shallowly dipping foliations (Fig. 3b,c; Bell et al., 1995). Gyrostatic be-havior of porphyroblasts (e.g., Fay et al., 2008) implies that FIA trendscan be preserved through the effects of subsequent ductile deforma-tions. Consequently, regionally consistent FIA trends in an orogenicbelt are interpreted to record bulk crustal shortening directions andchanges thereinwith time. In some cases, these have been directly relatedto vector of relative plate-motion (e.g., Aerden and Sayab, 2008; Bell andSapkota, 2012; Bell et al., 1995; Shah et al., 2011).
4. FIA trends in the Taebaeksan Basin
4.1. FIA trends of D1 and D2 porphyroblasts
Syn-D1 andalusite and chloritoid porphyroblasts in the presentstudy area occur only in metamorphic zone I, and post-D2 andalusiteoccurs in zone II (Kim et al., 2012). The syn-D1 andalusite and chloritoidmainly contain sigmoidal inclusion trails (S0; Fig. 4a–f), and post-D2
andalusite contains inclusion trails that define a differentiated crenula-tion cleavage (S0/S1 and S2) (Fig. 4g–i). S0 is a sedimentary bedding andbecame parallel to S1 due to extreme deformation, S1 is an axial planefoliation of the first-generation folds, and S2 is a crenulation cleavageof S0/S1. The asymmetry of sigmoidal inclusion trails within andalu-site and chloritoid porphyroblasts shows a switch between verticalthin sections oriented at 020° and 030°, and at 000° and 010°, respec-tively (Fig. 4a–f). Thus, the FIA trends of the syn-D1 andalusite andchloritoid porphyroblasts are 025° and 005°, respectively (Table 1).The differentiated crenulated cleavage preserved within post-D2 an-dalusite porphyroblasts in sample OG39 shows a switch in asymme-try, from clockwise to anticlockwise, between vertical thin sectionswith orientations of 160° and 170° (Fig. 4g–i), indicating a FIA orien-tation of 165° (Table 1).
The matrix foliation (S1) is sub-parallel to S0 in the limbs of F1 folds,due to intense shortening during F1 (Fig. 2). S0 and S1 in the matrix arecrenulated by the sub-horizontal S2, yielding a top-to-the-southeastshear sense (Fig. 5a,b; sample OG68) and top-to-the-northwest shear
sense (Fig. 5c,d; sample OG81) in the left- and right-side limbs of F1, re-spectively, when looking toward the NE. Thus, the FIA trend for thecrenulation cleavage is 010° (Table 1).
4.2. FIA trends of D3 and D4 porphyroblasts
Garnet and staurolite porphyroblasts inmetamorphic zones II and IIIcontain straight and sigmoidal inclusion trails. The garnet porphyroblastpreserves straight S2 inclusion trails that become slightly folded towardthe margin of the crystal (Fig. 5e). The inclusion trails within stauroliteporphyroblast are straight and sub-parallel to the external foliation(S3; Fig. 5f). The external foliation is anastomosing and curvedaround the staurolite. Thus, the garnet porphyroblast grew post-D2 tothe early D3. The staurolite porphyroblasts grew during early D3, andwere deformed by D4 (Kim et al., 2012). FIA trends for the garnet andstaurolite are 095° and 110°, respectively (Table 1). However, staurolitealso occurs within the garnet porphyroblasts as inclusions (Fig. 5e),suggesting that staurolite growth was initiated prior to garnet growth(Kim and Ree, 2010). Andalusite porphyroblasts that contain S3 in therim formed during syn-D4 and/or post-D4 deformation, and the FIAtrend for these inclusion trails is 100° (Table 1).
5. Relative timing of FIA sets: changes of bulk crustal shorteningdirections
We measured 40 FIA trends from chloritoid, andalusite, garnet, andstaurolite porphyroblasts in 23 samples from the Pyeongan Supergroup(Table 1). The FIA trends can be grouped into two sets based onorientation: set 1 (NNW–NNE) and set 2 (ENE–ESE) (Fig. 6a). The
Fig. 5. (a and b) Photomicrograph (PPL) and line diagram from a vertical thin section of sample OG68 show inclusion trails (S0) within syn-D1 andalusite porphyroblasts connectedto the matrix foliation (S1), and the S1 partially offset by shearing with dextral shear sense on the sub-horizontal S2 cleavage. (c and d) Photomicrograph (XPL) and line diagramfrom a vertical thin section of sample OG81 show matrix foliation relationships between S0, S1, and S2. S0, compositional layers between psammitic and pelitic layers, is subparallelto slaty cleavage (S1). S2 is sub-horizontal crenulation cleavage. (e) Photomicrograph (PPL) shows S2 inclusion trails within garnet porphyroblast that partially connected to thematrix foliation (sample OG2). (f) Photomicrograph (XPL) of syn-D4 staurolite preserving sub-vertical S3 foliation in sample OG3. (g and h) Photomicrographs (XPL and PPL) show-ing post-D2 andalusite (core) and syn-D4 and/or post-D4 andalusite (rim) in sample OG39. Small black circles in (d) represent analyzed spots (see Fig. 7). Single barbed arrow andnumber represent orientation of strike and way up of each thin section.
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andalusite porphyroblasts contain both FIA sets, set 1 (350°–030°) andset 2 (085°–120°), based on modal peaks and the relative timing of FIAsidentified in the cores and rims of porphyroblasts. Two FIA sets werealso for andalusite porphyroblasts containing single and multiple FIAs(Fig. 6a). The FIA trends of garnet and staurolite porphyroblasts are alsogrouped into sets 1 and 2 (Fig. 6a). Chloritoid porphyroblasts in sampleOG26 yield a FIA trend of 005°, corresponding to set 1 (Table 1). The 9FIA trends measured from the matrix in zone I belong mainly to set 1(Fig. 6a).
In zone I, the trends of FIAs of set 1, which represent the intersectionlineation between S0 and S1/S2 inclusion trails, appear to be consistentwith the trend of fold axes of F1 folds andwith intersection lineation be-tween S0,1 and S2 (L20,1; Figs. 2 and 6b). The FIAs in set 2 represent the
intersection lineation between S2/S3 and S3/S4, and occur mainly withinporphyroblasts of andalusite, garnet, and staurolite in zones II and III(Table 1). The trends of FIAs in set 2 tend to be oriented sub-parallelto F3 fold axes and to L32 (Figs. 2 and 6b).
5.1. Relative timing of FIA sets
Samples that contain porphyroblasts with two different FIA trendsfrom core to rim are important because they provide information onthe relative timing of porphyroblast growth and of different FIA sets.In andalusite porphyroblasts in samples OG12, OG32, OG33, and OG46,FIAs in porphyroblast cores have trends of between 350° and 030°(set 1), whereas those in rims lie between 095° and 110° (set 2)
Fig. 6. (a) Rose diagrams showing two FIA trend sets from metasedimentary rocks in the study area. (b) Maps showing distribution of FIA sets 1 and 2 of andalusite, chloritoid,garnet, and staurolite porphyroblast and the matrix. Rose diagrams for single and multi FIA trends in andalusite porphyroblasts.
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(Table 1). This relationship suggests that andalusite porphyroblastscontaining FIAs of set 1 grew prior to those containing FIAs of set 2.This relative timing of FIA sets is consistent with the relative timing ofthe growth of andalusite porphyroblasts: syn-D1 and post-D2 andalusiteporphyroblasts contain FIAs of set 1, defined by the intersection betweenS0 and S1/S2 inclusion trails (Fig. 4a–c, g–i); in contrast, andalusitecontaining FIAs of set 2 formed during D4 (Fig. 5g,h).
The occurrence ofmultiple episodic growth stages of andalusite is alsoapparent from compositional variations in Fe content from core to rimwithin individual porphyroblasts. In sample OG39, the Fe content showsan abrupt increase at the boundary between the core (post-D2 andalusite)and the rim (syn-D4 and/or post-D4 andalusite; Fig. 7a). The Fe content ofsyn-D1 and post-D2 andalusite porphyroblasts ranges from 0.014 to 0.021and from0.020 to 0.036 atomsper formula unit (a.p.f.u), respectively, andthat of syn-D4 and/or post-D4 andalusite porphyroblasts ranges from0.027 to 0.040 a.p.f.u (Table 2; Fig. 7b). In general, the Fe content of anda-lusite shows a gradual increase from syn-D1 to syn-D4 and/or post-D4
porphyroblasts (Fig. 7b). Therefore, the microtextural and compositionaldata indicate that FIA set 1 predates set 2.
In summary, the interrelationship between inclusion trails andmatrixfoliations indicates that porphyroblasts in the study area contain earlyFIAs that trend NNW–SSE to NNE–SSW (set 1) and later FIAs that trendENE–WSW to ESE–WNW (set 2). Andalusite porphyroblasts overgrewboth FIA sets, reflecting generally E–W(set 1) andN–S (set 2) bulk crustalshortening events, whereas garnet and staurolite porphyroblasts grewmainly during the later N–S bulk crustal shortening.
6. Discussion
6.1. Change in the direction of bulk crustal shortening during thePermo-Triassic Songrim orogeny in the Taebaeksan Basin
Two FIA sets, NNW–NNE (set 1) and ENE–ESE (set 2), were pre-served within different porphyroblasts in three metamorphic zones
Fig. 7. (a) Profile diagram for Fe content of post-D2 and syn- or post-D4 andalusite insample OG39. Analyzed spots from the core (FIA set 1) to the rim (FIA set 2) areshown in Fig. 5h. (b) Plot of Al vs. Fe per 5 oxygen atoms for andalusite preserving FIAsets 1 and 2.
Table 2Representative compositions of syn-D1, post-D2, and syn/post-D4 andalusite porphyroblasts.
a Total Fe as FeO.⁎ Number in the parentheses represents total number of analyses.
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(Fig. 6). FIA set 1was overprinted by FIA set 2 duringD3 and/or D4 that isrecorded in staurolite and andalusite porphyroblasts. However, varia-tion of FIA set 2 of andalusite porphyroblasts is different from that ofstaurolite porphyroblasts. The spreading of FIA trends can be differentamong different porphyroblasts due to difference of relative growthtiming during sequential deformation events and this has been discussedin the literature (e.g. Bell et al., 1998; Kim and Bell, 2005;William, 1994;Yeh, 2007).
As mentioned above, the direction of bulk crustal shortening at thetime of FIA formation is inferred to be oriented perpendicular tothe FIA trend. The change in the direction of bulk crustal shorteningfrom E–W to N–S occurred during the late Permian to Triassic in theTaebaeksan Basin (Fig. 8a), with the timing of this change constrainedby the deposition of the Pyeongan Supergroup (e.g., Cheong, 1969;
Park, 1989) and intrusion of Jurassic leucocratic granite (ca. 160 Ma;Lee, 1992).
The first bulk shortening event, oriented E–W, probably producedthe roughly N–S-trending F1 folds and the N–S-trending L20,1 (Figs. 2and 8a). Therefore, the E–W bulk shortening deformed the PyeonganSupergroup, presumably during the early stages of LT/MP metamor-phism (ca. 480 °C and 5.0 kbar) with D1 and D2 as part of the Songrimorogeny (Fig. 8), resulting in regional NNE–SSW to N–S striking foldsand thrusts in the Taebaeksan Basin (Fig. 1).
The second period of bulk crustal shortening, oriented N–S, pro-duced E–W-trending F3 folds during D3 deformation (Fig. 8a). The D3
deformation events also produced garnet and staurolite porphyroblastswith E–W FIAs occurring mainly in metamorphic zones II and III (Kimand Ree, 2010; Fig. 6b). Previous studies reported that E–W-trendingfolds in the Taebaeksan Basin resulted from later orogenic events,during the middle Jurassic to Cretaceous (Chough et al., 2000; Kim etal., 1994, 1996); however, the P–T conditions at the time of formationof E–W-trending structures (e.g., F3 folds and the growth of garnetand staurolite porphyroblasts that contain FIAs of set 2; Fig. 8a) in thestudy area suggest that the N–S bulk crustal shortening may also havebeen part of the Songrimorogeny, because F3 folds and FIAs of set 2 pre-date the intrusion of a Jurassic leucocratic granite (e.g., Kim et al., 2012).
6.2. Correlation of Permo-Triassic tectono-metamorphic evolution betweenthe Taebaeksan Basin, the Okcheon Basin, and the Imjingang Belt
The Okcheon Belt is a NE–SW-trending fold-and-thrust belt,bounded by the Gyeonggi Massif to the northwest and the YeongnamMassif to the southeast (Fig. 1). The Okcheon Belt can be divided intotwo basins: the Okcheon Basin to the southwest and the TaebaeksanBasin to the northeast, based on mainly lithology, structure, andmetamorphic grade (Chough et al., 2000). Three Paleozoic–Mesozoicorogenic events are recorded in the Okcheon Basin: (1) the Caledonian(Okcheon) orogeny in the Silurian–Devonian (Cluzel, 1991; Cluzel etal., 1990) or in the early Permian (Kim, 2005; Kim et al., 2007);(2) the Songrim orogeny in the late Permian–Triassic (Cho and Kim,2005; Cluzel, 1991; Cluzel et al., 1990); and (3) the middle Jurassicto early Cretaceous Daebo tectonic event (Cluzel, 1991; Cluzel et al.,1990; Sagong andKwon, 1998), although the existence of the Caledonianorogeny in the Korean Peninsula remains debated. Cho and Kim (2005)
Fig. 8. (a) Comparison of the pressure–temperature–deformation (P–T–d) paths combined with FIA trends of the three late Paleozoic terranes in South Korea: Taebaeksan Basin(Kim and Ree, 2010), the Okcheon Basin (Cho and Kim, 2005; Min and Cho, 1998), and the Imjingang Belt (Cho et al., 2007; Kim, 2002; Kim and Jung, 2010). The black and gray coloredthick arrows in the rose diagrams represent the direction of the crustal bulk shortening in each terrane. (b) Comparison of the late Paleozoic temperature–time (T–t) paths for the threelate Paleozoic mobile belts. Although timing of the peak temperature is a little bit different for each terrane, the trend of cooling history is almost identical.Numbers in the bottom of the boxes refer to the chronologic source data: 1=Cliff et al. (1985), 2=Min et al. (1995), 3=Oh et al. (1995), 4=Cho et al. (1996), 5=Ree et al.(1996), 6=Park andCheong (1998), 7=Cho et al. (1999), 8=Cheong et al. (2003), 9=Kimet al. (2007), 10=Choet al. (2007), 11=Min et al. (1995). See text formoredetailed explanation.
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argued that the Okcheon and Songrim orogenic events in the OkcheonBasin occurred at ~285 Ma and 250–220 Ma, respectively, based onradiometric age data and metamorphic P–T conditions (Kim et al.,2007). They suggested that the early Permianmedium-pressure regionalmetamorphism, which involved the growth of garnet, staurolite, andkyanite porphyroblasts (4.2–9.4 kbar and 490–630 °C), was overprintedby Triassic regional retrograde metamorphism (1.0–5.0 kbar and350–500 °C; Min and Cho, 1998) under conditions of the greenschist toepidote–amphibolite facies. However, it remains uncertain whether thetwo metamorphic events in the Okcheon Belt can be discriminated.The peak metamorphic conditions of the Caledonian orogeny in theOkcheon Basin are similar to those in the Pyeongan Supergroup in theeastern part of the Taebaeksan Basin (Kim and Ree, 2010). Thus, it is pos-sible that the Songrim orogeny in the Okcheon Basin occurred in thePermian.
The regional NE–SW-trending fold-and-thrust in the Okcheon Basinis considered to have resulted from a series of progressive NW–SE bulkshortening events (e.g., Chang, 1988; Cluzel et al., 1990; Koh and Kim,1995); however, Lee (2000) suggested that NE–SW to ENE–WSW bulkcrustal shortening occurred before the NW–SE bulk shortening andmay have been associated with the Permo-Triassic Songrim orogeny,based on FIA trends within garnet and staurolite porphyroblasts(Fig. 8a). This ENE–WSW bulk shortening event may correlate with FIAset 1 of the present study area, although FIAs obtained in the OkcheonBasin show a wide range of trends (Fig. 8a).
As stated above, the Imjingang Belt may represent a zone of Permo-Triassic continental collision at the eastern margin of Eurasia (Cho etal., 2007; Kim, 2008; Kwon et al., 2009; Ree et al., 1996). Devonianmetasedimentary rocks of the Imjingang Belt record Barrovian-typemetamorphism that involved the growth of biotite, garnet, and staurolite
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porphyroblasts (Kim, 2002; Kim and Jung, 2010). The peakmetamorphicconditions for themetapelites in the Imjingang Belt were ca. 610–700 °Cat 9.0–11.0 kbar (Cho et al., 2007; Kim and Jung, 2010), and peakmeta-morphism is estimated to have occurred at 263–230 Ma (Fig. 8), basedon Sm–Nd and Rb–Sr mineral isochron ages and monazite chemicalages (Jeon and Kwon, 1999; Ree et al., 1996), and a SHRIMPU–Pb zirconage (Cho et al., 1996, 2007).
Previous studies identified three deformation events in theImjingang Belt (Jung et al., 1999, 2002; Ree et al., 1996): an initialcontractional deformation (Dn−1), the main contractional deformation(Dn), and extensional ductile shearing (Dn+1). The contractional defor-mation produced south-vergent structures with top-up-to-the-south(reverse) shear sense in the Jingok unit of the Imjingang Belt (Dn−1
and Dn; Jung et al., 1999; Ree et al., 1996). These structures wereoverprinted by top-down-to-the-north (normal) shearing, indicatingthat the contractional deformation was followed by extensional defor-mation (Dn+1). Kim and Jung (2010) reported that garnet and stauro-lite porphyroblasts grew during the NNW–SSE to NNE–SSW bulkcrustal shortening, as deduced from FIA trends (Fig. 8a). These shorten-ing directions are almost identical to those recorded during the D3 and
Fig. 9. Schematic diagrams illustrating the late Paleozoic tectonic evolution with directionPermo-Triassic orogeny. The eastern boundary of the South China block is an active contiThe Pyeongan Supergroup in the Taebaeksan Basin started to deposit in an active continTaebaeksan and Okcheon Basins deformed under E–W and ENE–WSW crustal bulk shortening(b) The early stage (SO2) of the Permo-Triassic orogeny (around 250–220 Ma). Mostmobile beand Metcalfe, 2005). The Imjingang Belt and the Taebaeksan Basin affected by N–S crustal buindicates cessation of subduction. H–O: Hida–Oki terrane, P-J: proto-Japan superterrane, SG: SSchematic diagrams modified from de Jong et al. (2009).
D4 deformations in the Taebaeksan Basin, although the peak metamor-phic conditions of the Imjingang Belt are higher than those of thePyeongan Supergroup in the present study area (Fig. 8).
Asmentioned above, the P–T paths of regionalmetamorphism in theOkcheon Belt (Okcheon and Taebaeksan Basins) and in the ImjingangBelt are Barrovian-type clockwise paths that developed between ca.280 (?) and 220 Ma (Fig. 8b; Cho and Kim, 2005; Cho et al., 2007;Kim et al., 2001). Furthermore, these three regions have experiencedsimilar contractional deformation events (E–Wand/or N–S bulk crustalshortening) and subsequent exhumation during the late Permian toJurassic (Fig. 8b; e.g., Jung et al., 1999; Kim and Jung, 2010; Kim et al.,2011; Lee, 2000).
6.3. Implications for Permo-Triassic tectonics at the eastern margin ofEurasia
The similarity in the characteristics and timing of metamorphism/deformation that affected late Paleozoic metasedimentary rocks inthe Taebaeksan Basin, the Okcheon Basin, and the Imjingang Belt(Fig. 8) suggests that these terranes were deformed under a similar
s of crustal bulk shortening of the Korean Peninsula. (a) The early stage (SO1) of thenental margin in the late Mississippian to the late Permian (around 320 to 260 Ma).ental margin setting (Kim et al., 2012) around the middle Pennsylvanian. Then theduring the early stage of LT/MP metamorphism (Kim and Ree, 2010) in the late Permian.lts and cratons have amalgamated around this time (de Jong et al., 2009; Oh, 2006;Wakitalk shortening (Kim and Jung, 2010; Kim and Ree, 2010). Dashed line with open trianglesongpan Ganzi subduction–accretion complex. Paleo‐latitudes for reference only.
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tectonic setting during the Permian to Triassic. Furthermore, the direc-tions of horizontal bulk crustal shortening during the Songrim orogenyprovide information on paleo-tectonic movements in the region adja-cent to the Korean Peninsula, as well as enabling correlations betweenthe tectono-metamorphic evolutions of the three terranes.
As mentioned above, the Permo-Triassic Songrim orogeny had astrong effect on the Okcheon and Taebaeksan Basins and on theImjingang Belt (Chough et al., 2000; Ree et al., 1996), as well as onthe Gyeonggi Massif (Kwon et al., 2009; Oh, 2006; Oh et al., 2009).Thus traditionally, the Triassic metamorphism and deformation inthe three Phanerozoic mobile belts have been interpreted as a prod-uct of collision between the North and South China blocks, and thecollision belt may be laterally continuous with the Hida–Oki terraneof southwest Japan (e.g., Oh, 2006; Oh and Kusky, 2007; Osanai et al.,2006). However, de Jong et al. (2009) proposed that the late Permianto middle Triassic metamorphism recorded in the Hida–Oki terranewas associated with collision between the proto‐Japan superterraneand the active margin of East Asia, as inferred from age data and geo-chemical data (Fig. 9; Arakawa et al., 2000, 2001; de Jong et al., 2006,2009; Hiroi et al., 1998 and references therein). The collision resultedin E–W contractional deformation in proto-Japan and in the easternpart of the Korean Peninsula.
We therefore infer that the E–W horizontal bulk crustal shorteningin the Taebaeksan and Okcheon Basins during the first stage of thePermo-Triassic Songrim orogeny (SO1) may have resulted from colli-sion between the East Asian continental margin and the proto-Japanterrane (Fig. 9a).
During the second stage of the Songrim orogeny in the early Triassic(250–220 Ma), tectonic movement may have changed abruptly to N–Sbulk crustal shortening (SO2; Fig. 9b), interrupted by gravitationalcollapse and sub-isothermal (at 480–500 °C) decompression duringD2 deformation (Fig. 8; Kim, 2012; Kim and Ree, 2010; Kim et al.,2012). N–S crustal shortening during the second stage of thePermo-Triassic Songrim orogeny (SO2) produced E–W-trending foldsand thrusts in the Taebaeksan Basin and in the Imjingang Belt (Choughet al., 2000; Jung et al., 1999; Kim, 1996; Ree et al., 1996). The exactage of the change in bulk crustal shortening direction (from E–W toN–S) remains unclear, although it can be assumed that the transition oc-curred just before the high-pressure peak of metamorphism in theImjingang Belt (ca. 250 Ma; Cho et al., 1996, 2007; Ree et al., 1996),because the metamorphism was coeval with N–S crustal shortening inthe Imjingang Belt.
7. Conclusions
The interrelationship between foliation intersection/inflection axes(FIA)within andalusite, chloritoid, garnet, and staurolite porphyroblastsand matrix foliations indicates that porphyroblasts in the late PaleozoicPyeongan Supergroup at the northeastern margin of the TaebaeksanBasin reveal two age groups of inclusion trails. These microstructures in-dicate the development of earlyNNW–NNE-trending structures followedby later E–W-trending ones. These observations suggest a change in theorientation of bulk crustal shortening from E–W to N–S during thePermo-Triassic Songrim (Indosinian) orogeny.
Based on the similar deformation and metamorphic features of theTaebaeksan Basin, the Okcheon Basin, and the Imjingang Belt, E–Wbulkcrustal shortening influenced the eastern part of the Korean Peninsulamainly during the first stage of the Permo-Triassic Songrim orogeny(SO1), whereas N–S bulk crustal shortening resulted in strong deforma-tion in the western part of the peninsula during the second stage ofthe Permo-Triassic Songrim orogeny (SO2). The peakmetamorphic con-ditions tend to increase toward the western part of the peninsula(Fig. 8), with ca. 580 °C at 6.0 kbar in the Taebaeksan Basin (Kim andRee, 2010), ca. 600 °C at 8.0 kbar in the Okcheon Basin (Cho and Kim,2005; Min and Cho, 1998), 700 °C at 9.0–11.0 kbar in the ImjingangBelt (Cho et al., 2007; Kim and Jung, 2010), and 850 °C at 21.0 kbar in
the southwestern part of the Gyeonggi Massif (Oh et al., 2004, 2005).Consequently, the Songrim orogeny in the Korean Peninsula is probablynot only related to the amalgamation of terrane fragments at the easternAsiatic margin (e.g., collision of the Sino–Korean continent with theHida–Oki terrane), which resulted in SO1, but also to collision betweenthe North and South China blocks (SO2; Fig. 9).
Acknowledgments
This research was supported by the Basic Science Research Programthrough the National Research Foundation of Korea (NRF), funded bythe Ministry of Education, Science and Technology (2010-0009429)and partly by Kyungpook National University Research Fund 2012 toH.S. Kim and partly by NRF 2010-0024206 to J.-H. Ree. We are pleasedto have the opportunity to contribute this paper to celebrate an interna-tional conference in honor of Prof. Tim Bell. We are grateful for helpfuland constructive reviews by Koen de Jong and anonymous reviewer,and thank Domingo Aerden for valuable comments as well as editorialhandling.
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