• The Journal of the Petrological Society of Korea
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• v.23 no.2
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• pp.75-103
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• 2014

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.

• Economic and Environmental Geology
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• v.29 no.3
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• pp.357-367
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• 1996

A total of 113 samples (basalts, tuffs, and siltstones from coal-bearing sediments) was collected from 14 sites of the Tertiary Changgi basin in southeastern Korea, and studied palaeomagnetically. Site-mean declination of the ChRM from 5 sites was found to be deflected clockwise about $30^{\circ}$. Other 5 sites showed no vertical-axis deflection of ChRM direction. In consideration of previous palaeomagnetic data from other Tertiary basins in the vicinity, it is interpreted that the deflection of ChRM directions has been caused by NNW-SSE simple shear associated with the opening of the East Sea, and the time of rotation should be about 16 Ma. Other 2 sites showed counterclockwise deflection of site-mean ChRM. These sites might be located among lager tectonic blocks which were rotating clockwise. AMS (anisotropy of magnetic susceptibility) study revealed $NE{\rightarrow}SW$ directed magnetic lineation at two tuffaceous sites. This might indicate flow direction of tuffs during the time of deposition. Most of the other sites showed load-foliation lying subparallel to the bedding plane. This must have been caused by gravitational loading acted vertically to the strata.

• Korean Journal of Heritage: History & Science
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• v.51 no.2
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• pp.78-91
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• 2018

Sangjogam, located in Goseong, Gyeongsangnam-do, was designated as Natural Monument #411, because of its diverse geological heritage, such as fossils, ripple marks, dykes, and columnar joints. In the area, Byeongpungbawi, with its beautiful columnar joints vertical to the bedding plane of the underlying sedimentary rocks and spectacular coastal view, was named after its overall shape reminiscent of a huge folding screen. The purpose of this study was to investigate the formation process of the columnar joints using the anisotropy of magnetic susceptibility (AMS) method. AMS measurements showed that the k1 and k3 values representative of directions of the long and short axes of a magnetic particle at each point strongly clustered, and the oblate magnetic foliation structure in Byeongpungbawi developed during sill-type intrusion rather than lava flow. In summary, Byeongpungbawi was produced by sill-type intrusion along the bedding plane of the underlying sedimentary layer, and the subsequent formation of columnar joints was accompanied by the cooling and contraction of intruding rhyolite magma. This study potentially provides a basic research tool in understanding the formation mechanism of columnar joints which are widely distributed in southern Korea.

• Economic and Environmental Geology
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• v.29 no.3
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• pp.369-380
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• 1996

In order to determine the emplacement time and stratigraphic position of basaltic rocks in the central part of Pohang basin, palaeomagnetic investigations were conducted on 111 samples collected from 6 sites. Formation mean built from site-mean ChRM directions is $d=340.7^{\circ}$, $i=52.2^{\circ}$ (${\alpha}_{95}=6.0^{\circ}$, k=91) in the geographic coordinates. If "tilt" correction is applied regarding the plane perpendicular to the side walls of columnar joint as palaeohorizon, the formation mean becomes $d=328.7^{\circ}$, $i=43.8^{\circ}$ (${\alpha}_{95}=17.1^{\circ}$, k=13). It is to be noticed that the formation mean is deflected significantly (about $20{\sim}30^{\circ}$) counterclockwise from the Tertiary reference geomagnetic field of Eurasia, independent of "tilt" correction. This situation is very different from that of clockwise rotation of ChRM directions which has been ubiquitously observed in other Tertiary basins of south Korea, and indicates sinistral regional simple-shearing during the emplacement times of the basaltic rocks. Considering previous palaeomagnetic and AMS (anisotropy of magnetic susceptibility) data from other Tertiary regions of south Korea, the time of this sinistral shearing should be 15 Ma or directly thereafter, and this time point represents inversion from extensional to compressional tectonic regime. Magnetic foliation data obtained in this study indicate WNW-ESN compression during or directly after emplacement of the basaltic rocks, while it has known from regional tectonic study that the time of the WNW-ESN compression is about 15 Ma, when SW Honshu was rotated and the Korean strait was narrowed.

• Korean Journal of Mineralogy and Petrology
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• v.36 no.1
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• pp.55-72
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• 2023

This study aims to identify the fault zone architecture and geometric and kinematic characteristics of the Yeongdeok Fault, based on the geometry and kinematic data of various structural elements obtained by detailed field survey and anisotropy of magnetic susceptibility (AMS) of the fault rocks. The Yeongdeok Fault extends from Opo-ri, Ganggu-myeon, Yeongdeok-gun to Gilgok-ri, Maehwa-myeon and Bangyul-ri, Giseong-myeon, Uljin-gun, and cuts various rock types from the Paleo-proterozoic to the Mesozoic with a range of 4.6-5.0 km (4.77 km in average) of right-lateral offset or forms the rock boundaries. The fault is divided into four segments based on its geometric features and shows N-S to NNW strikes and dips of an angle of ≥ 54° to the east at most outcrops, even though the outcrops showing the westward dipping (a range of 54°-82°) of fault surface increase as it goes north. The Yeongdeok Fault shows the difference in the fault zone architecture and in the fault core width ranging from 0.3 to 15 m depending on the bedrock type, which is interpreted as due to differences in the physical properties of bedrock such as ductility, mineral composition, particle size, and anisotropy. Combining the results of paleostress reconstruction and AMS in this and previous studies, the Yeongdeok Fault experienced (1) sinistral strike-slip under NW-SE maximum horizontal principle stress (σ_Hmax) and NE-SW minimum horizontal principle stress (σ_Hmin) in the late Cretaceous to early Cenozoic, and then (2) dextral strike-slip under NE-SW maximum horizontal principle stress (σ_Hmax) and NW-SE minimum horizontal principle stress (σ_Hmin) in the Paleogene. It is interpreted that the deformation caused by the Paleogene dextral strike-slip movement was the most dominant, and the crustal deformation was insignificant thereafter.

• The Journal of the Petrological Society of Korea
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• v.28 no.3
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• pp.171-193
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• 2019

This study aims to identify the geometry and internal structures of the Yeongdeok Fault, a branch fault of the Yangsan Fault, by detailed mapping and to characterize its kinematics by analyzing the attitudes of sedimentary rocks adjacent to the fault, slip data on the fault surfaces, and anisotropy of magnetic susceptibility (AMS) of the fault gouges. The Yeongdeok Fault, which shows a total extension of 40 km on the digital elevation map, cuts the Triassic Yeongdeok Granite and the Cretaceous sedimentary and volcanic rocks with about 8.1 km of dextral strike-slip offset. The NNW- or N-S-striking Yeongdeok Fault runs as a single fault north of Hwacheon-ri, Yeongdeok-eup, but south of Hwacheon-ri it branches into two faults. The western one of these two faults shows a zigzag-shaped extension consisting of a series of NNE- to NE- and NNW-striking segments, while the eastern one is extended south-southeastward and then merged with the Yangsan Fault in Gangu-myeon, Yeongdeok-gun. The Yeongdeok Fault dips eastward with an angle of > $65^{\circ}$ at most outcrops and shows its fault cores and damage zones of 2~15 m and of up to 180 m wide, respectively. The fault cores derived from several different wall rocks, such as granites and sedimentary and volcanic rocks, show different deformation patterns. The fault cores derived from granites consist mainly of fault breccias with gouge zones less than 10 cm thick, in which shear deformation is concentrated. While the fault cores derived from sedimentary rocks consist of gouges and breccia zones, which anastomose and link up each other with greater widths than those derived from granites. The attitudes of sedimentary rocks adjacent to the fault become tilted at a high angle similar to that of the fault. The fault slip data and AMS of the fault gouges indicate two main events of the Yeongdeok Fault, (1) sinistral strike-slip under NW-SE compression and then (2) dextral strike-slip under NE-SW compression, and shows the overwhelming deformation feature recorded by the later dextral strike-slip. Comparing the deformation history and features of the Yeongdeok Fault in the study area with those of the Yangsan Fault of previous studies, it is interpreted that the two faults experienced the same sinistral and dextral strike-slip movements under the late Cretaceous NW-SE compression and the Paleogene NE-SW compression, respectively, despite the slight difference in strike of the two faults.