• Economic and Environmental Geology
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• v.40 no.2 s.183
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• pp.171-189
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• 2007

A study of anisotropy of magnetic susceptibility (AMS) was undertaken on Cretaceous granitic, volcanic and sedimentary rocks in the eastern region of the Yangsan fault, southeast Korea. A total of 542 independently oriented core samples collected form 77 sites were studied. The main magnetic mineral in granitic rocks is magnetite according to the magnitude of bulk susceptibility, high-temperature susceptibility variation and isothermal remanent magnetization. Both of magnetic lineation and foliation with NE-SW trends are revealed in the granitic rocks, while volcanic rocks show scattered directions and sedimentary rocks show only load foliation parallel to the bedding planes. The following evidences read to the conclusion that both magnetic fabrics in the granitic rocks have been obtained by a tectonic stress before full solidification of the magma: (i) A fully hardened granitic rocks would get hardly any fabric, (ii) Difference of the magnetic fabric trends with those of the geological structures in the granitic rocks themselves formed by brittle deformation after solidification (e.g. patterns of small-faults and joints), (iii) Kinking of biotite and undulose extinction in quartz observed under the polarizing microscope, (iv) Discordance of magnetic fabrics in the granitic rocks with those in the surrounding rocks. The NE-SW trend of the magnetic foliations suggests a NW-SE compressive stress of nearly contemporaneous with the emplacement of the granitic rocks. The compression should have caused a sinistral strike-slip movement of the Yangsan Fault considering the trend of the latter. As the age of the granitic rocks in the study area is reported to be around $60\sim70$ Ma, it is concluded that the Yangsan fault did the sinistral strike-slip movement during this time (L. Cretaceous Maastrichtian - Cenozoic Paleocene).

• Journal of the Korean Geophysical Society
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• v.2 no.3
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• pp.169-178
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• 1999

Low-field anisotropy of magnetic susceptibility (AMS) was measured with 247 samples from 17 sites of Pre-Cambrian anorthositic rocks in the Hadong-Sanchong area, southwestern part of the Ryongnam Block. Tectonic stress-direction is defined by the minimum susceptibility (k3) direction, and flow-direction by the maximum susceptibility (k1) direction. Five sites rendered self-consistent NW-SE site-mean tectonic stress-direction. Even though a general fold test for every site was not possible due to the homoclinal nature of the bedding attitudes, a site with various bedding attitudes shows far better clustering of the k3-direction before the bedding-tilt correction. The in-situ NW-SE tectonic stress-direction is consistent over the study area and compatible with petrographic foliation observed in metamorphic rocks in and arround the study area, suggesting a regional compressive force acted after the emplacement of the anorthositic rocks. On the other hand, flow-directions obtained from six sites varies from site to site. Strong-field IRM experiments show predominance of titanomagnetites over a small amount of hematite in some samples.

• The Journal of the Petrological Society of Korea
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• v.24 no.3
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• pp.209-231
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• 2015

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 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.

• Economic and Environmental Geology
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• v.34 no.1
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• pp.133-146
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• 2001

We report the results of structural field observation and measurement of anisotropy of magnetic susceptibility (AMS) of the diamictitic Hwanggangri Formation distributed in Chungju-Suanbo area of the Okchon Belt, Korea. The outcrops of the Hwanggangri Formation show two types of cleavage in general: slaty cleavage (SI) and crenulation cleavage (5z). 5] cleavage is, however, well observable only in the notheastem (NE) part of study area, while overwhelmed by 52 cleavage in the southwestern (5W) part, indicating stronger later deformation in 5W part of the study area. This partitioning of the study area is corroborated by both IRM and AMS parameters: NE part of the study area is characterized by higher IRM intensity, higher bulk magnetic susceptibility, higher AM5 degree, and by oblate shape of magnetic susceptibility ellipsoid. Their values become drastically lowered toward southwest, and reach to a stable minimum in the whole 5W part of the study area. In addition, degree of both metamorphism and deformation tends to increase gradually from northeast toward southwest and also from northwest toward southeast in the study area. Based on the distribution pattern of the principal axes ( $k_1, k_2, k_3$ axes) of magnetic anisotropy ellipsoids revealed in the NE part of the study area, three episodes of deformation ( $D_1, D_2, D_3$ ) are recognized: D_1$ deformation produced $S_2$ cleavage with NE-5W trend, which is caused by a strong NW-SE tlattening of a coaxial pure shear. $D_2$ deformation produced 5z cleavage characterized by a non-coaxial deformation. It was caused by a ductile or semi-ductile thrusting toward NW and concurrent sinistral shearing along $S_2$ cleavage plane. Lastly, $D_3$ deformation produced tlexural folding of all previous structures with a nearly horizontal NE fold axis. Distribution pattern of the principal axes of magnetic anisotropy ellipsoid from the SW part of the study area, on the other hand, does not show any coherency among sites or samples. We interpret that this dispersed pattern of $k_1, k_2, k_3$ axes together with lower anisotropy strength indicates that magnetic fabrics in the SW part have been disturbed either by a superposition of strong deformation/metamorphism or by a kind of reciprocal strain due to an overlapping of $D_1$ and $D_2$ or by both processes.

• 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.