• 자원환경지질
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• 제16권3호
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• pp.223-243
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• 1983

palaeomagnetism is a major implement to define tectonic provinces and to estimate their past relative position quantitatively. In this sort of investigation it is tacitly assumed that the magnetization of the rocks under study was acquired at the time of their formation. However, because of the possibility of secondary alteration and/or replacement of magnetic minerals, this assumption is not always legitimate. To secure reliable palaeomagnetic data it is therefore fundamental to identify the carrier mineral of magnetization. This paper reviews magnetic mineralogy relevant to palaeomagnetism of terrestrial rocks. Under the heading of each mineral its genesis, crystal structure, magnetic properties, criteria for ore microscopic determination and secondary alteration are summerized. This paper should also be helpful in application of magnetic mineralogy to geothermometry and oxygen barometry in igneous petrology, diagenesis and provenance study in sedimentary petrology, metamorphic temperature determination and genesis study of ore deposits.

• 자원환경지질
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• 제17권2호
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• pp.141-147
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• 1984

For the discussion of palaeogeomagnetic field direction and the movement and rotation of geotectonic blocks it is prerequisite to eliminate the secondary remanent magnetization component out of the NRM vector of given rock samples. Among various techniques alternating field and thermal demagnetization methods are most easily applicable and most widely used ones. Physical principle underlying these methods is the concept of relaxation time: Either with an action of external magnetic field or with an elevated temperature relaxation time of magnetic minerals can be drastically shortened. It has been furthermore shown experimentally that the secondary remanent magnetization can be more easily demagnetized than the primary remanent magnetization. Through careful stepwise demagnetization it should also be possible to discriminate the kind of various remanent magnetizations. In addition to the introduction to the underlying physical principles and experimental results of the alternating field and thermal demagnetization this paper gives various practical tips in earring out the demagnetization experiments. Each alternating field and thermal demagnetization instrumentation which is easily adaptable to domestic palaeomagnetic laboratories is also introduced. This paper provides a compact and practical introduction to the methods in palaeomagnetism and would be of interest to present workers and to those who want to embark on research in this field.

• 자원환경지질
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• 제22권2호
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• pp.167-171
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• 1989

Palaeomagnetic studies accompany calculation and plotting of pole position. This paper explains three graphical methods for the determination of pole position and plotting problem. It also derives the numerical formula for pole calculation and explains the method how the pole plotted on the rear hemisphere can be transformed to the frontal hemisphere, which is not clarified elsewhere.

• 자원환경지질
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• 제34권4호
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• pp.355-373
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• 2001

옥천비변성대내 예미지역 23개지점으로부터 총 256개의 정향 코어시료를 채취하여 잔류자기 연구를 실시하였다. 연구지역은 태백과 영월사이의 두위봉형 퇴적지역이며 지리적 중심좌표는 37.18$^{\circ}$N, 128.61$^{\circ}$E이다. 일부 시료들에서는 교류 자기세척이 효과를 발휘하였으나 대부분의 경우 고온 열 세척이 특성잔류자기를 추출하는 주요 수단이었다. 캄브리아기 지층(화절층)의 특성잔류자기는 현재자기장 방향과 다르며 100%의 지층경사보정에서 최대의 군집을 이루어 1차잔류자기의 가능성을 제시하였으나 통계학적 습곡검사를 통과하지는 못하였다. 오도비스기 지층 (막골석회암, 고성석회암)의 경우에는 잔류자기 강도가 매우 낮았으며 또한 심하게 재자화되어 있었다. 석탄기 지층(홍점통)의 특성잔류 자기는 습곡검사와 역자화검사를 통과하는 습곡이전의 1차잔류자기이었는데 교류 세척과 열 세척으로 나타난 각각의 안정 잔류자기 방향이 서로 일치를 보여서 1차잔류자기라는 확신을 굳혀주었다. 이 석탄기 지층(홍점통)의 특성잔류자기는 습곡검사와 역자화검사를 통과하는 습곡이전의 1차잔류자기이었는데 교류 세척과 열 세척으로 나타난 각각의 안정 잔류자기 방향이 서로 일치를 보여서 1차잔류자기라는 확신을 굳혀주었다. 이 석탄기 지층의 특성잔류자기는 한노두에서도 지층의 상하부에 따라 방향이 상당히 다르게 나타나는데 이는 홍점통이 퇴적되는 동안 지구의 자극이 이동되었던 데에 기인한 것으로 해석된다. 석탄기 지층을 상하부로 구분하여 평균 특성잔류자기 방향을 산출하고 이로부터 고자기학적 북극의 위치를 구하여 중국의 것과 비교하면 북중국지괴(NCB)의 것과 매우 유사한 반면 남중국지괴(SCB)의 것과는 판이하게 다르다. 따라서 예미지역은 석탄기동안에 북중국지괴의 일부였거나 최소한 이에 근접하여 있었음을 알 수 있다. 페름기 지층(사동통, 고방산통)으로부터는 현재자장방향으로 재자화된 습곡이후의 2차잔류자기만이 검출되었다.

• 자원환경지질
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• 제29권4호
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• pp.509-522
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• 1996

A total of 460 palaeomagnetic samples was collected from the Tertiary Chongja-Ulsan basins and surrounding area in the southeastern part of Korean peninsula. All samples were stepwise demagnetized by either alternating field or thermal method. It was found that most sample-sites have ChRM declination which has been rotated clockwise from the north-south reference direction of Tertiary East Asia, although other two extrusive sample-sites within the Chongja sedimentary basin show counterclockwise rotation of ChRM declination. Fold tests for the site-mean ChRMs of the latter two sites reveal insignificant result and negative result with 95% confidence level, respectively. The amount of the clockwise deflection of declination varies from about $20^{\circ}$ upto about $80^{\circ}$ according to the block to which each sample-site belongs. The amount of the counterclockwise deflection is about $20^{\circ}$. It is concluded that the clockwise ChRM rotation has been caused by dextral simple shearing accompanied by NNW-SSE spreading of the East Sea which has been active until about 16Ma, and that the counterclockwise rotation is a result of sinistral simple shearing associated with WNW-ESE contraction in the Korean Strait-SW Japan region at about 15 Ma.

• 자원환경지질
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• 제29권3호
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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.

• 자원환경지질
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• 제31권1호
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• pp.31-43
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• 1998

A total of 469 granitic samples were collected from 44 sites in the Ulsan fault area, southeast Korea. According to the previous petrographic studies, the granitic rocks have been divided into four groups (Hornblende biotite granodiorite, Hornblende granite, Biotite granite and Alkali-feldspar granite). NRM intensities, values of low field magnetic susceptibility, and magnetic behaviors during stepwise demagnetization experiments suggest rather a three-fold classification: In this scheme, Hornblende granite and Biotite granite are grouped together, as they did not show any significant differences in magnetic characteristics. Based on the Ishihara (1979)'s criterion, Alkali-feldspar granite is classified as ilmenite-series granite, whereas others are classified as magnetite-series granite. In the eastern part of the study area including the Tertiary basin area, declinations of site-mean characteristic remanent magnetizations (ChRMs) show clockwise deflection of more than 30 from the reference direction of east Asia. Both along and in the adjacent region of the Ulsan fault-line, however, no deflection of remanent direction was observed. A boundary line between the deflected and undeflected site-mean ChRMs is defined in this study, which runs roughly parallel to the Ulsan fault-line at the distance of about 6km eastward from the fault-line. We suggest that this newly found boundary line, which we call Yonil tectonic line, released dextral simple shear stress acted in the southeastern part of the Korean peninsula during the opening stage of the East Sea in the Early Cenozoic.

• 자원환경지질
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• 제26권2호
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• pp.193-206
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• 1993

A total of 111 independently oriented core samples were drilled at 12 sites in fue Kimpo area ($37.70^{\circ}N$, $126.55^{\circ}E$) of the Taedong Supergroup. The Taedong strata are composed of sandstone, conglomeratic sandstone, shale and thin coal seams. The age of the strata is known to be Late Triassic-Early Jurassic according to freshwater Esfuerites and plant fossil (Dictyophyllum-Clathropteris flora) contents. Through AF and thermal demagnetization, an area-mean ChRM direction of $D=48.3^{\circ}\;I=40.3^{\circ}\;{\alpha}_{95}=7.9^{\circ}\;k=59.5$, n=7 was obtained. It passed fold and reversal test in the formation-mean level. Fold test was not significant in the area-mean level. The palaeomagnetic north pole calculated from the area-mean lies at $46.3^{\circ}N$, $222.0^{\circ}E$ with dp=5.7, $dm=9.5^{\circ}$. This pole position is very similar to those of the South China Block (SCB) in Triassic times. Palaeolatitude of the Kimpo area in the Taedong times was $23.0^{\circ}N$, again very similar to the palaeolatitude of the South China Block in the Late Triassic. This low latitude of the study area at the time of deposition explains the tropical-subtropical nature of fossil contents of the Taedong Supergroup.

• 자원환경지질
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• 제29권3호
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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.

• 자원환경지질
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• 제26권4호
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• pp.519-539
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• 1993

A total of 264 independently oriented core samples were collected from 26 sites in the southwestern part (the Naktong Trough) of the Cretaceous $Ky{\check{o}}ngsang$ Basin in south Korea. The sampled formations comprise the sedimentary Shindong and the Hayang Groups of the Lower Cretaceous age. Alternating field and thermal demagnetizations were conducted. Characteristic remanent magnetization (ChRM) was relatively easily isolated in each formation except in the Chinju formation, from which only remagnetization circles were observed. Even though an extensive use of the fold test was not possible due to the nearly homoclinal nature of the strata in the area, we believe that the ChRM of each formation is of primary origin based on the following grounds: The in-situ ChRM direction of each formation is different from the present geomagnetic field direction. Fisherian precision parameter becomes enhanced through the tilt correction in all formations, closely to the values required for a positive fold test. Three out of the five studied formations pass the reversal test. The mean palaeomagnetic pole position from the studied area is found to be statistically different from the contemporary pole from the Chinese block exclusive of the Shandong area. The difference in magnetic declination suggests a $14.5^{\circ}$ (${\pm}10.5^{\circ}$) clockwise rotation of the studied area relative to the Chinese block comprising the west of the Tan-Lu fault. On the other hand, any significant difference in magnetic inclination and concurrent palaeolatitude is not observed between the studied area and China as well as the other area (Taegu-Andong area) in the $Ky{\check{o}}ngsang$ Basin. The dual nature of the magnetic polarity confirmed in all formations suggests an older than 124 Ma (Neocomian or older) age of the studied sedimentary strata.