Youngbeom Cheon

Geometry and kinematics of the Ocheon Fault System along the boundary between the Miocene Pohang and Janggi basins, SE Korea, and its tectonic implications

Detailed geological mapping and observations of various structural elements were made in order to determine the geometry and kinematics of the Ocheon Fault System (OFS) along the boundary between the Early Miocene Janggi and the Middle Miocene Pohang basins, SE Korea, and to reveal its roles on the basin evolutions. The OFS is a NE-trending relayed fault system composed of a number of NE or NNE-trending normal-slip and sinistral-normal oblique-slip faults, and has a scissor fault geometry decreasing in vertical offset southwestward. The constituent faults created independent grabens or half-grabens on the hanging-walls for the deposition of the Early or Middle Miocene strata. The OFS was initially the northwestern border fault of the Janggi Basin which acted as normal faults by the WNW-ESE tensional stress associated with the NNW-directed dextral simple shear caused by the East Sea opening. Afterwards, it experienced clockwise rotation with change of slip sense from normal-slip to sinistral-normal oblique-slip in response to the progressive dextral simple shear. At about 17 Ma, the shear stress propagating westward was released rapidly by the dextral strike-slip faulting of the NNW-trending Yeonil Tectonic Line (YTL) and the normal faulting of the NNE-trending western border faults of the Pohang Basin. At that time, the depocenter suddenly migrated northward and the depositional environment also changed rapidly from terrestrial to marine due to dramatic subsidence of the Pohang Basin. The Pohang Basin is interpreted to be a pull-apart basin extended at releasing bend/overstep between two PDZs (Principal Displacement Zones), i.e., the YTL and probably the East Korea Fault. The OFS was also reactivated as the eastern border faults of the Pohang Basin. In contrast to the western border faults, the OFS was rotated clockwise and could not be linked with the YTL because of its scissor fault geometry. Our results suggest that the NNW-trending regional dextral shear stress persisted for a considerable period of time in SE Korea during the East Sea opening, supporting the pull-apart opening of the East Sea rather than the fan-shaped opening. Most of the previous studies advocating the pull-apart opening emphasize the role of the NNE-trending strikeslip faults, like the Yangsan fault and OFS, as PDZs. In contrast, this study suggests that the NNE-trending faults in SE Korea acted as major normal faults at releasing bends or stepovers in the NNW-trending dextral fault system during the East Sea opening.

Miocene tectonic evolution of the basins and fault systems, SE Korea: Dextral, simple shear during the East Sea (Sea of Japan) opening

Crustal deformation of SE Korea caused by back-arc opening of the East Sea (Sea of Japan) commenced locally in the Late Oligocene. Intense deformation occurred during the Early Miocene, accompanying extension of parallelogram-shaped pull-apart basins between NNW-striking principal displacement zones, clockwise rotation and northwestward tilting of blocks, and southwestward propagating rifting. At about 17 Ma, the crustal deformation suddenly expanded westward and northward owing to activation of the Yeonil Tectonic Line, resulting in a dramatic change of depositional environment and the subsidence of wedge-shaped pull-apart basins. The Yeonil Tectonic Line and western border faults of the Pohang Basin are the westernmost limit of the Miocene crustal deformation. All the features of crustal deformation in SE Korea support the pull-apart model for the East Sea opening. However, they indicate that the NNW-striking faults such as the Yeonil Tectonic Line acted as the principal displacement zones rather than the NNE-striking Yangsan Fault under a consistent dextral simple shear. At about 16 Ma, the collision of the Philippine Sea Plate with the Japanese Islands caused a tectonic inversion. This inversion resulted in a compositional change of basaltic magma at about 15 Ma and crustal uplift in SE Korea, which caused the cessation of sedimentation at about 10 Ma.

Formation and Evolution of the Miocene Ipcheon Subbasin in Yangbuk-myeon, Gyeongju, SE Korea

입천소분지는 기반암과 단층으로 경계되며 북동 방향으로 길쭉한 형태의 비대칭 지구의 기하를 가지는 분지로 인근 와읍과 어일분지와는 기반암에 의해 격리되어 있는 독립된 소규모 마이오세 분지이다. 분지충전물의 층리는 대부분 북서 내지 서북서 방향으로 경사지며, 분지 북동부에는 데사이트질 화산물질을 다량 포함하는 전기 마이오세 퇴적물이 그리고 남서부에는 사암을 협재하는 비화산성 중기 마이오세 육성 역암이 분포한다. 또한 분지충전물 내 퇴적동시기 공액상 정단층들은 분지가 서북서-동남동 방향으로 확장하였음을 지시한다. 이러한 특징들은 인근 와읍 및 어일분지와 것들과 매우 유사한 것으로, 분지의 확장이 북서부 경계단층에 의해 주도되었으며 분지의 열개가 북서에서 남동으로 전파되었음을 지시한다. 한편, 입천소분지 내에는 어일분지에서 흔히 나타나는 현무암질 화산물질이 관찰되지 않는다. 슬랩과 경하관찰 결과, 입천소분지의 데사이트질 응회암과 응회질 이암은 와읍분지의 용동리응회암과 매우 유사한 특징을 보인다. 중기 마이오세의 비화산성 퇴적층은 와읍과 어일분지 그리고 입천소분지에 공통적으로 분지의 남서부에 분포한다. 따라서 입천소분지의 확장은 22Ma 경 와읍분지의 확장과 함께 시작되어 다량의 데사이트질 화산물질이 유입 되었으며, 이후 어일분지의 주 확장시기인 20~18Ma 사이에는 확장을 멈추었다가, 약 17Ma에 이르러 연일 구조선의 운동과 함께 분지의 열개가 남서쪽으로 전파되면서 중기 마이오세 초의 연일층군에 해당하는 비화산성 역암이 분지의 남서부에 퇴적된 것으로 결론지어진다.The Ipcheon Subbasin is an isolated Miocene basin in SE Korea, which has the geometry of an asymmetric graben elongated in the NE-SW direction. It is in contact with basement rocks by faults and separated from adjacent Waup and Eoil basins by the basement. The strata of the basin fills have an overall homoclinal structure, dipping toward NW or WNW. The basin fills consist of Early Miocene sediments rich in dacitic volcanic and volcaniclastic deposits and Middle Miocene non-volcanic and nonmarine conglomerates intercalated with sand layers, which are distributed in the northeastern and southwestern parts of the basin, respectively. Kinematic analysis of syndepositional conjugate faults in the basin fills indicates WNW-ESE extension of the basin. These features are very similar to those of the adjacent Waup and Eoil basins, indicating that the basin extension was governed by the NE-trending northwestern border faults and that the basin experienced a propagating rifting from NE to SW. Basaltic materials, which occur abundantly in the Eoil Basin, are totally absent in the Ipcheon Subbasin. The observations of the dacitic tuff and tuffaceous mudstone in the subbasin, on slabs and under microscope, suggest that they have lithologies very similar to those of the Yondongri Tuff in the Waup Basin. The Middle Miocene non-volcanic sediments of the Waup and Eoil basins and the Ipcheon Subbasin are distributed consistently in the southwestern part of each basin. It is thus concluded that the extension of the Ipcheon Subbasin began at about 22 Ma together with the Waup Basin and was lulled during the main extension period of the Eoil Basin between 20-18 Ma. At about 17 Ma, the subbasin was re-extended due to the activation of the Yeonil Tectonic Line associated with the propagating rifting toward SW. This event is interpreted to have provided new sedimentation space for the Middle Miocene sediments in the southwestern parts of the Waup and Eoil basins and the Ipcheon Subbasin as well.

Development of the intracontinental, continuous, narrow transpressional zone along the Sinnyeong Fault in the Cretaceous Gyeongsang Basin, SE Korea

The WNW-ESE-striking Sinnyeong Fault, the most conspicuous fault of the Gaeum Fault System in the Cretaceous Gyeongsang Basin of Korea, provides an opportunity to understand the architectures and evolution of an intracontinental transpressional fault zone. We focus on the structural characteristics of the Sinnyeong Fault based on detailed field observations and magnetic fabric analysis. Its main movement is interpreted as sinistral slip with a small reverse component, although it could also have been active before the main movement. The deformation zone is asymmetric about the fault core. Sedimentary strata of the southern deformation zone are more extensively folded and deformed than those of the northern part, and the southern zone is much wider than the northern zone. NW-SE-trending en-echelon folds are limited to the southern periphery of the fault zone, where it is narrow, and underwent rotation toward the fault surface. These contractional deformations along the entire length of the fault are interpreted to have formed approximately coevally with the sinistral faulting. Newly observed WNW-ESE-striking mappable faults show a similar structural feature. Our results suggest that the damage asymmetry resulted from the eastward transport direction and relative uplift of south blocks of the faults under NE-SW compressive stress, which oriented at high angle (60–70°) to pre-existing strike of the faults. During progressive deformation, continuing slip accumulation along vertically tilted and fault-parallel sedimentary strata located directly south of the fault cores was responsible for the larger motions of the southern blocks of the faults rather than the relatively fixed northern blocks. We highlight that the sinistral reactivation of the Sinnyeong Fault formed a continuous, narrow transpressional zone during its long-term evolution.