**2. Flat slab subduction**

nonmarine domains [12, 19]. The evolution of the continental arc ultimately produced a derivation of Korean-derived detrital sediments in the Pacific-side regions, such as in the Inner

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

**Figure 1.** (a) Simplified tectonic map of East Asia, and (b) close-up of South Korea showing the major tectonic provin‐ ces with the study area (boxed) (modified after Egawa and Lee [7]). BG, Bansong Group; CB, Chungnam Basin; GB, Gyeongsang Basin; GG, Gimpo Group; GM, Gyeonggi Massif; NG, Nampo Group; OB, Okcheon Belt; TB, Taebaeksan

The tectonism, magmatism, and sedimentation of South Korea have been systematically well reviewed and summarized by Korean geologists [4, 12, 19, 26, 27]. However, such work has included only limited description and minor discussion on Jurassic basinal evolution because of the very limited distribution and publication of research in comparison with studies related to other Phanerozoic basins (Fig. 1). In contrast, many of Jurassic structural and igneous events

Zone of Southwest Japan [20, 21, 22, 23, 24, 25].

Basin; YM, Yeongnam Massif.

have been reported and detailed [4, 5, 18, 28, 29, 30, 31].

## **2.1. Evidence of flat slab subduction in and around Korea**

Recent igneous studies suggest that the Mesozoic continental arc evolution was triggered by the flat slab subduction of the western paleo-Pacific plates underneath the East Asian continent [6, 45, 46]. According to the observation of modern subduction zones in the Andes, there is a close relationship between flat slab subduction, crustal shortening and thicken‐ ing, and inlandward-migrating magmatism [47, 48, 49, 50]. Subducted slab dip is fundamen‐ tally constrained by slab buoyancy. Therefore, a slab with oceanic plateaus or ridges is flatly subducted over a long distance, while steeper subduction occurs when such features are absent [49, 51].

Evidence for the subduction of such buoyant oceanic materials is found in the Mesozoic accretionary complexes along the eastern margin of Asia, stretching a distance of several thousand kilometers, and is seen particularly in Japan and Russian Far East [2, 52]. These complexes generally consist of oceanic plateau basalts and deep marine deposits, which were accreted and underplated underneath the Asian continental crusts during subduction [53, 54, 55]. Paleomagnetic analysis has revealed that the Japanese Islands were geologically connected 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 migration [47, 48, 59].

and two peninsula-wide orogenies, the Songnim and Daebo orogenies, occurred almost contemporaneously with the Indosinian and Yanshanian orogenies, respectively. It has therefore been conventionally interpreted that these Chinese and Korean orogenies progressed

East Asia-Wide Flat Slab Subduction and Jurassic Synorogenic Basin Evolution in West Korea

http://dx.doi.org/10.5772/56770

65

It is necessary to reiterate here that the Songnim and Daebo orogenies are represented by a regional metamorphism related to the Chinese final assembly and by the evolution of the continental-magmatic arc associated with the paleo-Pacific subduction, respectively. Such facts therefore provide an alternative interpretation: there were distinct orogenic gaps between South China and Korea (Fig. 2) [8, 18]. This implies that when the Triassic flat slab subduction has already initiated the Indosinian orogeny in South China, the Songnim regional metamor‐ phism in Korea was then caused by the ongoing final amalgamation of the Chinese continental blocks. The subsequent Daebo continental-magmatic arc evolution then occurred 60 m.y. later than the compressional Indosinian orogeny, and by this time South China was already in the

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

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

under the same subduction processes [2, 4, 61].

phase of the extensional Yanshanian orogeny.

during the orogeny [63, 64].

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

orogeny (180–170 Ma; U–Pb sphene and Rb–Sr whole-rock ages) [5].

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

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.

and two peninsula-wide orogenies, the Songnim and Daebo orogenies, occurred almost contemporaneously with the Indosinian and Yanshanian orogenies, respectively. It has therefore been conventionally interpreted that these Chinese and Korean orogenies progressed under the same subduction processes [2, 4, 61].

It is necessary to reiterate here that the Songnim and Daebo orogenies are represented by a regional metamorphism related to the Chinese final assembly and by the evolution of the continental-magmatic arc associated with the paleo-Pacific subduction, respectively. Such facts therefore provide an alternative interpretation: there were distinct orogenic gaps between South China and Korea (Fig. 2) [8, 18]. This implies that when the Triassic flat slab subduction has already initiated the Indosinian orogeny in South China, the Songnim regional metamor‐ phism in Korea was then caused by the ongoing final amalgamation of the Chinese continental blocks. The subsequent Daebo continental-magmatic arc evolution then occurred 60 m.y. later than the compressional Indosinian orogeny, and by this time South China was already in the phase of the extensional Yanshanian orogeny.
