Hyeongseong Cho

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.

Anisotropy of Magnetic Susceptibility (AMS) of the Quaternary Faults, SE Korea: Application to the Determination of Fault Slip Sense and Paleo-stress Field

한반도 남동부 일원에는 주요 지질구조선(양산단층대, 울산단층, 연일구조선, 오천단층계 등)을 따라 제4기 단층들이 도처에 분포하고 있으며, 이들의 기하와 운동학적 특징은 한반도의 제4기 지각변형사와 현생응력상태 등을 이해하는 데 중요한 정보들을 제공한다. 이번 연구에서는 진티단층, 모화단층, 수성지2단층 그리고 왕산단층을 대상으로 대자율이방성 방법을 적용하여 단층가우지의 미세구조를 분석하고 단층의 운동감각을 해석하였으며, 그 결과를 바탕으로 단층암의 미세구조 발달과정과 제4기 응력장에 대해 논의하였다. 대자율이방성 측정을 통해 구해진 자기미세구조로부터 단층의 운동감각을 분석한 결과, 왕산단층을 제외한 나머지 세 단층은 모두 역이동성이 우세한 단층운동에 의한 미세구조가 발달되어 있다. 단층면에 거의 평행한 편평형의 미세구조가 발달하는 점과 이방성정도가 클수록 편평형의 정도가 증가하는 특징에 근거할 때, 단층가우지의 미세구조는 점진적인 변형에 의한 최종응력변형이 기록된 것으로 해석된다. 또한, 최후기에 재활된 역단층운동으로 기존의 단층가우지 내에 존재하는 미세구조들은 모두 지워지고 최후기의 운동에 의한 미세구조만이 기록된 것으로 판단된다. 단층가우지의 자기미세구조 분석을 통해 유추된 운동감각으로부터 구해진 고응력장은 동북동-서남서 방향의 압축력이며, 이 응력장은 지진원 메커니즘, 수압파쇄시험 그리고 단층 지구조분석결과를 통해 구해진 한반도 남동부의 고응력장과 부합된다. 결론적으로 한반도 일원은 제4기 동안 동-서 내지 동북동-서남서 방향의 압축성 응력장이 지배적이었던 것으로 해석된다. 【The Quaternary faults are extensively observed along major inherited fault zones (i.e. Yangsan Fault System, Ulsan Fault, Yeonil Tectonic Line, Ocheon Fault System) in SE Korea. Their geometry and kinematics provide a very useful piece of information about the Quaternary crustal deformation and stress field in and around Korean Peninsula. Using magnetic fabrics (AMS), we attempted to determine the slip senses of Jinti, Mohwa, Suseongji2, and Wangsan faults and then interpreted the fabric development process of fault gouge and the characteristics of stress field during the Quaternary. All the magnetic fabrics of the faults, except the Wangsan Fault, consistently indicate a dominant reverse-slip sense with weak strike-slip component. Most of the oblate fabrics are nearly parallel to the fault surface and the anisotropy degrees generally increase in proportion to the oblatenesses. These results suggest that the fabrics of the fault gouges resulted from a progressive deformation due to continuous simple shear during the last reactivation stage as reverse faulting. It is also interpreted that the pre-existing fabrics were overwhelmed and obliterated by the re-activated faulting. Paleostress field calculated from the fault slip data indicates an ENE-WNW compressive stress, which is in accord with those determined from previous fault tectonic analysis, focal mechanism solution, and hydraulic fracturing test in and around Korean Peninsula.】

Occurrences of Fe-Ti Ore Bodies and Mafic Granulite in the Sancheong Anorthosites, Korea

Fe-Ti ore bodies and mafic granulite occur in the Sancheong anorthosites, south Korea. In order to determine their petrogenetic relationship and to classify the Fe-Ti ore bodies, we have synthetically analyzed characteristics in the field, such as distribution and occurrence, and petrologic features through detailed outcrop sketches. The ore bodies are divided into the regular vein dike- and irregular veinlet swarm types, according to their characteristics of contact with the anorthosites and internal structures. The former shows the tabularly intrusive contact and the pervasively ductile-sheared interior, while the latter, the irregularly tortuous contact and the almost intact interior. Most of the ore bodies are cross-cutting the foliation of the anorthosites and possess abundant anorthositic xenoliths, indicating their intrusion after the formation of foliation in the anorthosites. The mafic granulite, also bearing abundant anorthositic xenoliths, shows interior foliations nearly parallel to intrusion contact, and has abundant ilmenites approximately the same as those of the Fe-Ti ore bodies in chemical composition. And its intrusion into adjacent anorthosites is observed and the intrusion is finally changed into an irregular veinlet swarm type ore body. It is, thus, interpreted that the granulite in the study area was the host material of Fe-Ti ore bodies.

Double injection events of mafic magma into supersolidus Yucheon granites to produce two types of mafic enclaves in the Cretaceous Gyeongsang Basin, SE Korea

Petrographic and geochemical features of the Cretaceous Yucheon granites and their mafic microgranular/magmatic enclaves (MMEs), SE Korea, reveal that the MMEs originated from magma mixing. Mesoscopic and microscopic features indicate that mechanical mixing operated heterogeneously to produce the MMEs with a wide range of sizes and textures. Chemical compositions of amphibole, biotite, and plagioclase rims of both the MMEs and host granites are almost identical, indicating that chemical homogenization took place to some extent after the mechanical mixing. Plagioclase cores, however, have various compositions depending on the host rocks and/or sampling locations, suggesting their sluggish re-equilibration. The MMEs are divided into Type A (low TiO2, very fine-grained, chilled margins) and Type B (high TiO2, fine- to medium-grained, no chilled margins). The lower TiO2 MMEs cooled more rapidly and interacted with granitic magma for a shorter period of time than the higher TiO2 MMEs. Additionally, the former are less enriched in HREEs than the latter. Zoned plagioclase has two zones of increased An content. These features are indicative of double injection events of mafic magma. A previous model explains the magma mixing as resulting from the generation of a slab window due to Kula-Pacific ridge subduction. The model cannot, however, explain the eastward younging of the granites in Korea, necessitating a new, more elaborate model of Cretaceous geodynamics and magmatism in East Asia.

Magnetic fabric (anisotropy of magnetic susceptibility) constraints on emplacement mechanism of clastic dikes

Clastic dikes are generally classified into neptunian and injected dikes. Using the magnetic fabrics (AMS: anisotropy of magnetic susceptibility), we attempt to classify the clastic dikes in the late Cretaceous Dadaepo Basin, SE Korea, and interpret their emplacement mechanisms. The neptunian dikes exhibit a typical oblate sedimentary fabric which makes a sharp contrast with the injected dikes. The fabrics of the injected dikes are greatly influenced by current conditions and transportation types of filling materials. Based on the AMS fabrics, they are classified into four types. (1) VP (vertical flow-parallel) type is formed by imbrication of long-axis of grains in low- to moderate-energy vertical flow of a Newtonian fluid and characterized by a bilateral symmetry of fabrics across the dike. (2) VT (vertical flow-transverse fabric) type results from grain rolling in vertical high-energy flow and is characterized by subvertical k2 and subhorizontal k1 axes on the dike plane. (3) HP (horizontal flow-parallel) type is formed by imbrication of long-axis of grains in horizontal low- to moderate-energy flow, resulting in subvertical k3 and subhorizontal k1 and k2 axes. (4) HT (horizontal flow-transverse) type is formed by grain rolling in horizontal high-energy flow, resulting in streaked k2-k3 on the dike plane and horizontally clustered k1 axes. The AMS fabrics of each type can be a significant indicator for flow direction. Based on abundant AMS fabrics formed by high-energy current, coexistence of paleoseismic structures, and tectonic setting of the basin, earthquake-induced liquefaction is the most plausible trigger for the dike formation.

How to assess the value of geological heritage

지질공원을 구성하는 핵심 요소인 지질유산들을 발굴하고, 그 가치를 올바르게 평가하는 방법이 중차대한 문제로 부각되고 있다. 이번 연구는 기존에 제안된 지질유산 가치평가표의 평가항목들을 보다 명료화 및 구체화하고, 점수부여 기준의 객관화 및 정량화를 위한 방안을 논의하여, 보다 개선된 가치평가표를 제안하였다. 또한, 학술 및 교육적 가치의 대표성, 희소성, 다양성 항목, 경관적 가치의 특이성, 자연성 항목, 본질적 가치의 규모, 온전성 항목 그리고 보전 및 관리목표의 세부항목에 대해서는 가치평가의 주관성을 최소화하는데 기여할 수 있는 정량적 점수부여 근거 및 구체적인 산정기준을 제시하였다. 역사 및 문화적 가치, 기능적 가치, 경제적 가치 등의 세부항목에 대해서는 가치평가의 정확성과 신뢰성을 높이기 위한 방향과 점수부여 기준 및 점수부여시 고려해야 할 요소들에 대해 논의하였다. 마지막으로 지질유산의 등급 산정과 관련해서는 학술 및 교육적 가치와 경관적 가치의 두 세부 항목으로 구성된 새로운 형태의 지질유산 등급 분류표를 제안하였으며, 가중치 부여와 기타 항목들의 병기 등을 고려한 다양한 유형의 등급 분류표를 제시하였다.

Determination of Rock Cleavages Using AMS (Anisotropy of Magnetic Susceptibility): a Case Study on the Geochang Granite Stone, Korea

In granite quarry, stones are generally quarried along easily separating planes called as ‘rock cleavage’. Because orientation and characteristics of the rock cleavage are directly involved with easy quarrying, it is the most important factor on selecting a direction of digging. Using AMS (anisotropy of magnetic susceptibility), we attempt to interpret rock fabrics in Geochang Granite Stone (JS, SD, AR, GD, BW, MD quarry) and discuss about determination of rock cleavages and correlation between the rock fabrics and cleavages. Based on mean susceptibility, thermo-susceptibility curves, and hysteresis parameters, Ti-poor MD and/or PSD magnetites are the main contributor to AMS of the granite stones. The systematic magnetic foliations with sub-vertical dip angle are developed in the whole granite quarries. In most of the granite quarries, the magnetic foliations are significantly consistent with grain plane. In the BW quarry, which has higher P J values than the others, the magnetic foliations coincide exceptionally with *Corresponding author Tel: +82-51-510-2248 E-mail: moonson@pusan.ac.kr 210 조형성·김종선·김건기·강무환·손영관·이윤수·좌용주·손 문 J. Petrol. Soc. Korea rift plane. These results suggest that rock cleavages in granite stone are related to rock fabrics meaning shape and spatial arrangement of crystals. Magnetic fabrics analysis using AMS method, therefore, can be a quantitative and effective tool for determination of rock cleavages in granite quarry.

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.