**3. Synorogenic basin evolution in West Korea**

to the Asian continent before the opening of the Japan Sea in Miocene epoch [6, 56], and that the Jurassic accretionary complex in Southwest Japan was situated next to South Korea during its formation [25, 29], which was initiated in, at the latest, the early Late Triassic period [57] and continued through the Jurassic period [52]. Adakitic granites, which are indicators of slab melting, intruded widely into the Korean continental crusts with an inlandward younging trend during the Jurassic period [5, 8, 18, 58], supporting the interpretation of inlandward slab

64 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

The geology of South China records two major Mesozoic orogenies: the Indosinian orogeny (250–205 Ma) indicated by inlandward-migrating magmatic front with crustal thickening and shortening, and the Yanshanian orogeny (180–66 Ma) characterized by an oceanwardretrograding magmatic front with crustal thinning and stretching [6, 60]. These two orogenic events resulted from a flat slab subduction with a length of 1400 kilometers, and a subsequent slab rollback [6, 45]. The Korean Peninsula is situated just 500 km northeast of South China,

**Figure 2.** Landward and subsequent oceanward migration of subducted slab and magmatic fronts (direction indicat‐ ed with a heavy line) in Korea and South China: (a) Triassic, (b) Jurassic, and (c) Cretaceous periods (modified after Li and Li [6], Choi et al. [8], Egawa [44], Kiminami et al. [46], and Zhou et al. [60]). Migration of magmatic front is linked to the morphology of the subducted slab of oceanic plate. BO, Bulguksa orogeny; DO, Daebo orogeny; EYO, Early Yan‐

shanian orogeny; LYO, Late Yanshanian orogeny; IO, Indosinian orogeny; SO, Songnim orogeny.

migration [47, 48, 59].

**2.2. Orogenic gaps between Korea and South China**

The foregoing flat slab subduction then triggered and drove the Daebo orogeny in Korea, with a significant crustal shortening and thickening [4, 30]. This crustal deformation created an orogenic wedge in middle South Korea, which consists of the southeast- and northwestvergent fold-and-thrust belts (Fig. 3) [62]. The former belt corresponds to a pro-wedge region, which includes the Okcheon Belt and the Taebaeksan Basin, and the latter-mentioned belt developed as a retro-wedge region, which includes the Chungnam region [4, 7, 30, 33]. Such wedge structures were probably formed under a NW–SE-directed compressional setting during the orogeny [63, 64].

The Chungnam Basin (consisting of several separated subbasins―the Ocheon, Oseosan, and Seongju subbasins, and other unnamed) was filled with a Jurassic nonmarine deposit, known as the Nampo Group (Fig. 4). This group unconformably covers the pre-Jurassic metamorphic basement rocks, and was structurally underlain by these rocks due to the postdepositional thrust faulting [40, 41]. The stratigraphy of the Nampo Group is subdivided into the Hajo, Amisan, Jogyeri, Baegunsa, and Seongjuri formations with decreasing age [65, 66]. Among them, the Hajo, Jogyeri, and Seongjuri formations are mainly composed of conglomerate and sandstone, whereas the Amisan and Baegunsa formations are dominated by an alternation of coal-bearing shale and sandstone. In this study, the stratigraphy of the Oseosan Subbasin (as defined by Egawa and Lee [7, 41]) is revised on the basis of the recognition of the Oseosan Thrust, which allows the structurally repetitive distribution of the Hajo and Amisan forma‐ tions (Figs. 4, 5). The depositional age of the Nampo Group is inferred as being between Sinemurian and Aalenian, based on U–Pb zircon dating of regionally metamorphosed basement rocks (230–220 Ma) [35, 37, 38] and felsic lapilli tuff of the Baegunsa Formation (170 Ma) [30], which is synchronous with the magmatic event in the early stage of the Daebo orogeny (180–170 Ma; U–Pb sphene and Rb–Sr whole-rock ages) [5].

**Figure 3.** (a) The possible tectonic arrangement of South Korea and the Inner Zone of Southwest Japan during the Jurassic period (modified after Egawa and Lee [7]). CB, Chungnam Basin; GM, Gyeonggi Massif; MTL-TTL, Median Tec‐ tonic Line–Tanakura Tectonic Line; OB, Okcheon Belt; TB, Taebaeksan Basin; TMAB, Tanba–Mino–Ashio Belt (Jurassic accretionary complex); YM, Yeongnam Massif. (b) Schematic cross section along the b'–b'' section in (a) showing the possible evolution of a continental arc (not to scale).

## **3.1. Basin filling controlled by a tectonic cycle**

Egawa and Lee [7] detailed and classified the nonmarine sedimentary characteristics of the Nampo Group into seven sedimentary facies associations: colluvial fan, alluvial fan, braid‐ plain, delta plain, delta front, offshore lacustrine, and volcaniclastic plain (Fig. 5). A combina‐ tion of these facies associations reveals a vertical cyclic pattern presented by the fining- to coarsening-upward lower and upper sequences of the alluvio-lacustrine system in the Ocheon, Oseosan, and Seongju subbasins. These depositional cycles are subdivided by the thick, progressive colluvial/alluvial fan deposits of the Jogyeri Formation, along with strong interformational unconformities occurring between the Amisan and Lower Jogyeri formations (U1 unconformity) and between the Lower and Upper Jogyeri formations (U2 unconformity).

Such stratigraphic features correspond to typical alluvial basin-filling patterns, and are attributable to tectonically-driven sediment flux or climate-driven diffusivity occurring over a relatively short time-scale [67, 68]). The lack of stratigraphic or temporal variations in the degree of chemical weathering [69], along with the presence of coal deposits [70, 71], indicates little or no climate fluctuation at the time of basin filling. This illustrates that a process of tectonically-driven sediment flux is most likely to have occurred. As variation of sediment flux is an index of tectonic activity, the remarkable gravel progradation of the Jogyeri Formation probably records a time of low sediment flux and quiescent tectonism (Fig. 6) [67, 72, 73]. Under this assumption, therefore, the fine-grained sediments in the

other four formations are interpreted to have been deposited under active tectonism. It is assumed that the phase of Jogyeri gravel progradation reflected the progressive encroach‐ ment of deformation into the foreland [74, 75] due to the subduction-induced crustal shortening. These relationships permit a possible interpretation of the Chungnam Basin as

**Figure 4.** Geological map of the Chungnam Basin (which consists of the Ocheon, Oseosan, and Seongju subbasins) filled with the Jurassic Nampo Group (modified after Egawa and Lee [7]). BT, Baegunsa Thrust; CT, Cheongla Thrust;

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being a piggyback or wedge-top basin [76, 77].

OcT, Ocheon Thrust; OsT, Oseosan Thrust.

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**3.1. Basin filling controlled by a tectonic cycle**

possible evolution of a continental arc (not to scale).

Egawa and Lee [7] detailed and classified the nonmarine sedimentary characteristics of the Nampo Group into seven sedimentary facies associations: colluvial fan, alluvial fan, braid‐ plain, delta plain, delta front, offshore lacustrine, and volcaniclastic plain (Fig. 5). A combina‐ tion of these facies associations reveals a vertical cyclic pattern presented by the fining- to coarsening-upward lower and upper sequences of the alluvio-lacustrine system in the Ocheon, Oseosan, and Seongju subbasins. These depositional cycles are subdivided by the thick, progressive colluvial/alluvial fan deposits of the Jogyeri Formation, along with strong interformational unconformities occurring between the Amisan and Lower Jogyeri formations (U1 unconformity) and between the Lower and Upper Jogyeri formations (U2 unconformity).

**Figure 3.** (a) The possible tectonic arrangement of South Korea and the Inner Zone of Southwest Japan during the Jurassic period (modified after Egawa and Lee [7]). CB, Chungnam Basin; GM, Gyeonggi Massif; MTL-TTL, Median Tec‐ tonic Line–Tanakura Tectonic Line; OB, Okcheon Belt; TB, Taebaeksan Basin; TMAB, Tanba–Mino–Ashio Belt (Jurassic accretionary complex); YM, Yeongnam Massif. (b) Schematic cross section along the b'–b'' section in (a) showing the

66 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

Such stratigraphic features correspond to typical alluvial basin-filling patterns, and are attributable to tectonically-driven sediment flux or climate-driven diffusivity occurring over a relatively short time-scale [67, 68]). The lack of stratigraphic or temporal variations in the degree of chemical weathering [69], along with the presence of coal deposits [70, 71], indicates little or no climate fluctuation at the time of basin filling. This illustrates that a process of tectonically-driven sediment flux is most likely to have occurred. As variation of sediment flux is an index of tectonic activity, the remarkable gravel progradation of the Jogyeri Formation probably records a time of low sediment flux and quiescent tectonism (Fig. 6) [67, 72, 73]. Under this assumption, therefore, the fine-grained sediments in the

**Figure 4.** Geological map of the Chungnam Basin (which consists of the Ocheon, Oseosan, and Seongju subbasins) filled with the Jurassic Nampo Group (modified after Egawa and Lee [7]). BT, Baegunsa Thrust; CT, Cheongla Thrust; OcT, Ocheon Thrust; OsT, Oseosan Thrust.

other four formations are interpreted to have been deposited under active tectonism. It is assumed that the phase of Jogyeri gravel progradation reflected the progressive encroach‐ ment of deformation into the foreland [74, 75] due to the subduction-induced crustal shortening. These relationships permit a possible interpretation of the Chungnam Basin as being a piggyback or wedge-top basin [76, 77].

**Figure 5.** Strato-sedimentological interpretation of the Jurassic Nampo Group in the Ocheon, Oseosan and Seongju subbasins (modified after Egawa and Lee [7]). HJ, Hajo Formation; AM, Amisan Formation; BG, Baegunsa Formation; LJG, Lower Jogyeri Formation; SJ, Seongjuri Formation; UJG, Upper Jogyeri Formation.

#### **3.2. Postdepositional thermal events**

In the late stage of the Daebo orogeny (late Jurassic to earliest Cretaceous time), the orogenic activity was further accelerated by the oblique subduction of the paleo-Pacific plates with strike-slip motion [29, 78, 79, 80], leading to significant crustal shortening and thickening represented by thrust-imbricate stacking [4, 30, 31]. Most of the Daebo granites were synde‐ positionally intruded in the early stage of the orogeny, followed by a quiescent phase of magmatic activity of ca. 60 m.y. before the initiation of the Bulguksa orogeny (Fig. 7) [2, 5]. Such a magmatic hiatus is likely to have resulted from the existence of oceanic plateaus or ridges subducting underneath the East Asian continental crusts [81, 82]. South China, however,

**Figure 6.** Schematic syntectonic evolution of the Chungnam Basin in the depositional stages of (a) the Hajo, (b) Ami‐ san, (c) Lower Jogyeri, (d) Upper Jogyeri, (e) Baegunsa, and (f) Seongjuri formations (modified after Egawa and Lee [7])

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(not to scale).

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**Figure 6.** Schematic syntectonic evolution of the Chungnam Basin in the depositional stages of (a) the Hajo, (b) Ami‐ san, (c) Lower Jogyeri, (d) Upper Jogyeri, (e) Baegunsa, and (f) Seongjuri formations (modified after Egawa and Lee [7]) (not to scale).

**3.2. Postdepositional thermal events**

In the late stage of the Daebo orogeny (late Jurassic to earliest Cretaceous time), the orogenic activity was further accelerated by the oblique subduction of the paleo-Pacific plates with strike-slip motion [29, 78, 79, 80], leading to significant crustal shortening and thickening represented by thrust-imbricate stacking [4, 30, 31]. Most of the Daebo granites were synde‐

**Figure 5.** Strato-sedimentological interpretation of the Jurassic Nampo Group in the Ocheon, Oseosan and Seongju subbasins (modified after Egawa and Lee [7]). HJ, Hajo Formation; AM, Amisan Formation; BG, Baegunsa Formation;

LJG, Lower Jogyeri Formation; SJ, Seongjuri Formation; UJG, Upper Jogyeri Formation.

68 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

positionally intruded in the early stage of the orogeny, followed by a quiescent phase of magmatic activity of ca. 60 m.y. before the initiation of the Bulguksa orogeny (Fig. 7) [2, 5]. Such a magmatic hiatus is likely to have resulted from the existence of oceanic plateaus or ridges subducting underneath the East Asian continental crusts [81, 82]. South China, however, shows no interval of quiescent magmatism between the Indosinian and Yanshanian orogenies, and this is probably related to the slab delamination and rollback that occurred immediately after the flat subduction [6, 45].

**Figure 7.** Inlandward and oceanward migration of the Daebo and Bulguksa granites in South Korea, respectively (modified after Kim [4], Sagong et al. [5], and Park [17]). BO, Bulguksa orogeny; DO, Daebo orogeny.

The Nampo Group has experienced high-grade diagenesis or low-grade metamorphism. This is evidenced by the presence of very high-rank coals (anthracite to meta-anthracite) and by the very high vitrinite reflectance values (5 to 6%) which occur entirely in the Seongju Subbasin [70, 71], as well as the high illitization occurring within the three subbasins which ranges in the thermal grade of anchizone to epizone [40]. Both coal and illite in sediments are commonly used as an indicator of paleotemperature, and Egawa and Lee [40] classified this postdeposi‐ tional thermal event into early and late histories: tectonic burial metamorphism and hydro‐ thermal alteration, respectively.

#### *3.2.1. Tectonic burial metamorphism*

The early tectonic burial resulted from crustal loading induced by the postdepositional basement overthrusting on the Nampo Group (Fig. 8). The grade of mechanical compaction textures in sandstones tends to increase down the sequence (Fig. 9), and the lowermost strata (Hajo Formation) appear to have been deformed in a ductile manner [40, 41, 83]. Similarly, the illite in sandstones shows a down sequence increase in its crystallinity, from anchizone to epizone (Fig. 9). Based on the equations proposed by Underwood et al. [84] and Kosakowski et al. [85], the measured illite crystallinity approximates the possible maximum paleotemperature and total burial depth of the Nampo Group in the Ocheon Subbasin as being 340 °C and 9700 m, respectively, although the total depositional thickness is 3300 m. This estima‐ tion is in good agreement with the observations of ductile deformation, epizonal metamor‐ phism, and basement overthrusting.

**Figure 8.** Conceptual structural models showing the postdepositional crustal shortening and thickening in the Chung‐ nam region (modified after Egawa and Lee [40, 42]) (not to scale). BT, Baegunsa Thrust; CT, Cheongla Thrust; OcT,

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Radiometric dating of illite in sediments is helpful in constraining the latest diagenetic and low-grade metamorphic ages [86, 87], and is used to interpret the timing of regional over‐

Ocheon Thrust; OsT, Oseosan Thrust.

shows no interval of quiescent magmatism between the Indosinian and Yanshanian orogenies, and this is probably related to the slab delamination and rollback that occurred immediately

70 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

**Figure 7.** Inlandward and oceanward migration of the Daebo and Bulguksa granites in South Korea, respectively

The Nampo Group has experienced high-grade diagenesis or low-grade metamorphism. This is evidenced by the presence of very high-rank coals (anthracite to meta-anthracite) and by the very high vitrinite reflectance values (5 to 6%) which occur entirely in the Seongju Subbasin [70, 71], as well as the high illitization occurring within the three subbasins which ranges in the thermal grade of anchizone to epizone [40]. Both coal and illite in sediments are commonly used as an indicator of paleotemperature, and Egawa and Lee [40] classified this postdeposi‐ tional thermal event into early and late histories: tectonic burial metamorphism and hydro‐

The early tectonic burial resulted from crustal loading induced by the postdepositional basement overthrusting on the Nampo Group (Fig. 8). The grade of mechanical compaction textures in sandstones tends to increase down the sequence (Fig. 9), and the lowermost strata (Hajo Formation) appear to have been deformed in a ductile manner [40, 41, 83]. Similarly, the illite in sandstones shows a down sequence increase in its crystallinity, from anchizone to epizone (Fig. 9). Based on the equations proposed by Underwood et al. [84] and Kosakowski et al. [85], the measured illite crystallinity approximates the possible maximum paleotemperature and total burial depth of the Nampo Group in the Ocheon Subbasin as being 340 °C and 9700 m, respectively, although the total depositional thickness is 3300 m. This estima‐ tion is in good agreement with the observations of ductile deformation, epizonal metamor‐

(modified after Kim [4], Sagong et al. [5], and Park [17]). BO, Bulguksa orogeny; DO, Daebo orogeny.

after the flat subduction [6, 45].

thermal alteration, respectively.

*3.2.1. Tectonic burial metamorphism*

phism, and basement overthrusting.

**Figure 8.** Conceptual structural models showing the postdepositional crustal shortening and thickening in the Chung‐ nam region (modified after Egawa and Lee [40, 42]) (not to scale). BT, Baegunsa Thrust; CT, Cheongla Thrust; OcT, Ocheon Thrust; OsT, Oseosan Thrust.

Radiometric dating of illite in sediments is helpful in constraining the latest diagenetic and low-grade metamorphic ages [86, 87], and is used to interpret the timing of regional over‐

**4. Conclusions**

Chinese final amalgamation.

**Acknowledgements**

Mesozoic tectonism, magmatism, and sedimentation in East Asia were fundamentally controlled by a series of flat slab subduction and subsequent slab rollback of the northwestern paleo-Pacific plates, which allowed the evolution of an Andean-type continental arc several thousand kilometers-long. Paleo-Pacific oceanic crusts with buoyant materials (such as oceanic plateaus and ridges) had subducted and migrated inlandward underneath the Asian continent, leading to a significant magmatic progradation and crustal shortening and thickening. The subsequent delamination and rollback of the inland subducted slab resulted in the retrogra‐ dation of the magmatic front, together with crustal stretching and thinning. These dynamic events are closely associated with the evolution of major orogenies in Korea and South China: the flat slab subduction caused the Daebo and Indosinian orogenies, and the slab rollback produced the Bulguksa and Yanshanian orogenies. There is a clear time lag between the flat subduction- and rollback-induced orogenies in Korea and those in South China, which were initiated 60 m.y. and 80 m.y. later in Korea, respectively, probably due to the effect of the

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The Chungnam Basin in central western Korea was filled with a Lower to Middle Jurassic nonmarine succession, known as the Nampo Group, the deposition and structural develop‐ ment of which occurred simultaneously with the evolution of the flat subduction-induced continental-magmatic arc during the Daebo orogeny. An integrated stratigraphic, sedimento‐ logic, diagenetic, and geochronologic analysis has demonstrated that the basin-filling proc‐ esses and subsequent structural and thermal evolution of the Nampo Group were fundamentally controlled by subduction tectonics. The Nampo Group is composed of the two repeated, fining- to coarsening-upward alluvio-lacustrine sequences, separated by an interval of thick breccia–gravel progradation deposits and relative strong proximal unconformities. The observed relationships of the succession provide a record of sedimentation that was most likely controlled by the temporal variations of tectonism during the early stage of the Daebo orogeny. The postdepositional basement thrusting over the Nampo Group then led to a tectonic burial, resulting in low-grade metamorphism. Burial heating is strongly suggested by the down-sequence increase in illitization from anchizone to epizone, and in the degrees of mechanical grain compaction and ductile deformation. The maximum paleotemperature and burial depth of the Nampo Group are estimated to be 340°C and 10 km, respectively, and the extrapolated K–Ar illite dating of 157–140 Ma indicates that the tectonic burial metamorphism was completed at the end of the Daebo orogeny. A subsequent granite intrusion and hydro‐ thermal alteration, probably occurring during the Bulguksa orogeny, have enhanced the

I am grateful to editor Yasuto Itoh and Ana Pantar for their contributions in improving the clarity of this publication. I would also like to thank Yong Il Lee, Daekyo Cheong, and Shigeru

illitization and anthracitization, regardless of the stratigraphy.

Otoh for their academic supports during my postgraduate study in Korea.

**Figure 9.** Simplified diagram showing the structural and diagenetic characteristics in the Ocheon, Oseosan and Seongju subbasins (modified after Egawa [43]). Az, anchizone; Ez, epizone; KI, Kübler Index; TG, thermal grade.

thrusting [88]. A mixture of authigenic (1Md) and detrital (2M1) components of illite is common in argillaceous sediments. Based on this knowledge, Egawa and Lee [42] measured the K–Ar ages of different-size clay fractions from the Amisan shale in the Ocheon Subbasin, and estimated the latest age of authigenic illite to be 157–140 Ma (Fig. 8), by using a linear regression model defined by the detrital amount and the K–Ar age of different size fractions [89, 90, 91]. The estimated age, therefore, is younger than the depositional age of the Nampo Group (~170 Ma) [30] and ranges within the duration of the Daebo orogeny (190–135 Ma) [4], which suggests that the tectonic burial metamorphism of the Nampo Group occurred in the late stage of the Daebo orogeny.

#### *3.2.2. Hydrothermal alternation*

The subsequent hydrothermal alternation was much affected by a magmatic intrusion and hotfluid migration, probably during the Bulguksa orogeny [40]. The coal rank and illite crystal‐ linity of the Seongju sediments plot into a very high thermal grade, with little stratigraphic variation (Fig. 9) [40, 70, 71]. When fluids warmed by pluton migrate along faults and fractures in the basin, they can transfer heat to the basin fills and lead to thermal alteration even at a relatively shallow depth of burial [85, 92, 93]. The Nampo Group in the Seongju Subbasin is highly faulted and folded in places, and there are granite intrusions into the southeastern subbasin (Fig. 4). These structures and intrusions probably enhanced the illitization and anthracitization after tectonic burial.
