**1. Introduction**

The interplay between oceanic plate subduction and the development of continental margins is of considerable geological interest, and of a particular interest for Asian structural geologists and petrologists is the subduction of the present and ancient Pacific plates, which triggered orogenic development and contributed to crustal evolution in the circum-Pacific regions through the Phanerozoic [1, 2]. Since the Triassic, the northwestern circum-Pacific region (also known as the East Asian continental margin) initiated the evolution of a continental arc stretching several thousand kilometers, which resulted in an East Asia-wide crustal shortening and thickening, orogenic basin formation, and landward magmatic progradation [2, 3, 4, 5, 6, 7, 8, 9]. It is noted that although the paleo-Pacific subduction along this region was also present in Paleozoic time, it did not exert a major tectonic impact on the Asian continents [10, 11, 12], and that this lack of impact was probably related to the fact that the Paleotethys Ocean lay between Laurasia and Gondwana until the Triassic period when the East Asian continental blocks had not been yet assembled [13, 14].

The Korean Peninsula, situated in the middle of the East Asian continental margin (Fig. 1), was plunged into a tectonically active phase in Mesozoic time, and three major orogenies are recorded; the Songnim, Daebo, and Bulguksa [4]. Among these, the Songnim orogeny (260– 220 Ma) is represented by regional metamorphism in a close association with the final amalgamation of Chinese continental blocks in Permian–Triassic period [15, 16]. A drastic tectonic transition followed this orogeny, and the evolution of a continental-magmatic arc occurred during the Daebo (190–135 Ma) and Bulguksa (100–45 Ma) orogenies, which resulted from the flat slab subduction and subsequent slab rollback of the western paleo-Pacific plates, respectively [5, 8, 17, 18]. It is evident that the Songnim–Daebo tectonic transition led to a radical shift of the Korean sedimentary environments, from Paleozoic marine to Mesozoic

© 2013 Egawa; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Egawa; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

It has been conventionally interpreted that Jurassic non-marine basins are interorogenic basins, formed during the period between the Songnim and Daebo orogenies [4, 19, 32]. However, recent radiometric dating of detrital zircon [30, 33, 34] has provided an alternative view of this conventional interpretation and has shown that the depositional age of these Jurassic basins corresponds to the early phase of the Daebo orogeny; indicating a close association with the

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

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

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From recent petrologic analyses and radiometric dating, the Chungnam region in western South Korea (known as the Hongseong Belt) has been interpreted as being an eastern extension of the Qinling–Dabie–Sulu Belt (the collisional belt between the North and South China blocks) (Fig. 1) [15, 35]. In the Chungnam region, it appears that Proterozoic to Paleozoic basement rocks were regionally metamorphosed with a high to ultra-high pressure facies during the Songnim orogeny [35, 36, 37, 38, 39]. The subsequent rapid uplift and denudation of these basement rocks then delivered their detritus into the Jurassic Chungnam Basin [34], which was followed by a structural disturbance during the late stage of the Daebo orogeny [31, 40].

The author of this paper has been studying the Chungnam Basin for several years [7, 40, 41, 42, 43, 44], and has demonstrated that the basin filling and thermal history are closely related to the Daebo continental-arc evolution. This paper presents an overview of the characteristics and mechanisms of Mesozoic flat slab subduction in East Asia, and then summarizes the sedimentary and structural evolution of the Chungnam Basin during the Daebo orogeny, with the intention of promoting a better understanding of the basin-filling processes in West Korea and also of the interplay between basinal and crustal evolution at the active continental margin

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

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

subduction-induced continental arc evolution.

of East Asia.

absent [49, 51].

**2. Flat slab subduction**

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

**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 Basin; YM, Yeongnam Massif.

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

It has been conventionally interpreted that Jurassic non-marine basins are interorogenic basins, formed during the period between the Songnim and Daebo orogenies [4, 19, 32]. However, recent radiometric dating of detrital zircon [30, 33, 34] has provided an alternative view of this conventional interpretation and has shown that the depositional age of these Jurassic basins corresponds to the early phase of the Daebo orogeny; indicating a close association with the subduction-induced continental arc evolution.

From recent petrologic analyses and radiometric dating, the Chungnam region in western South Korea (known as the Hongseong Belt) has been interpreted as being an eastern extension of the Qinling–Dabie–Sulu Belt (the collisional belt between the North and South China blocks) (Fig. 1) [15, 35]. In the Chungnam region, it appears that Proterozoic to Paleozoic basement rocks were regionally metamorphosed with a high to ultra-high pressure facies during the Songnim orogeny [35, 36, 37, 38, 39]. The subsequent rapid uplift and denudation of these basement rocks then delivered their detritus into the Jurassic Chungnam Basin [34], which was followed by a structural disturbance during the late stage of the Daebo orogeny [31, 40].

The author of this paper has been studying the Chungnam Basin for several years [7, 40, 41, 42, 43, 44], and has demonstrated that the basin filling and thermal history are closely related to the Daebo continental-arc evolution. This paper presents an overview of the characteristics and mechanisms of Mesozoic flat slab subduction in East Asia, and then summarizes the sedimentary and structural evolution of the Chungnam Basin during the Daebo orogeny, with the intention of promoting a better understanding of the basin-filling processes in West Korea and also of the interplay between basinal and crustal evolution at the active continental margin of East Asia.
