**4. Conclusions**

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

**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.

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

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

Daebo orogeny.

*3.2.2. Hydrothermal alternation*

anthracitization after tectonic burial.

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 Chinese final amalgamation.

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 illitization and anthracitization, regardless of the stratigraphy.
