• The Journal of the Petrological Society of Korea
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• v.27 no.1
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• pp.67-72
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• 2018

The K-Ar ages of a sericite quartz schist in the lower Jangsan Formation along the Okdong fault zone reported by Yun (1983) have attracted attention again because of their potential to constrain the depositional timing of the Jangsan Formation. The oldest age of $562{\pm}2Ma$ among three reported K-Ar ages in the schist led to the claim that the depositional period of the lowermost Jangsan Formation in the Joseon Supergroup is late Neoproterozoic. Its depositional age is important for understanding the tectonic evolution of the Korean Peninsula including the formation and evolution histories of its sedimentary basins. Thus, the reliability and geological meaning of three K-Ar ages in the original paper (Yun, 1983) were revisited in the review. Quartz grains in the analyzed sample contain a considerable amount of excess Ar, and therefore it is inappropriate to use the ages as a basis for a depositional age constraint of the Jangsan Formation. The timing of mylonitization in the schist is recalculated as ~170 Ma.

• Economic and Environmental Geology
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• v.25 no.1
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• pp.61-72
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• 1992

The Danyang area consists of the thrust and folded sedimentary rocks of Paleozoic and Mesozoic Era. The area is bounded by major tectonic units which are the Gagdong Thrust to the west and the Okdong Fault to the east. According to the structural analyses, the area is affected by polyphase deformation. This study establishes deformational sequence in the area. Mylonite zone along the Okdong Fault corresponds to the first generation of structures ($D_1$). $D_1$-structures are discrete shear zone in the Jangsan Formation and bedding parallel extensional deformation in the Cambro-Ordovician sequences. $D_2$-structures were formed prior to the sedimentation of the Jurassic Bansong Group, which are the NW-trending fold and linear structures. After sedimentation of the Bansong Group, the area is strongly affected by the Daebo Orogeny which produces NE-trending thrusts, folds and linear structures. Earlier structures were tightened and rotated toward NE. Some thrust faults did not propagate into the Bansong Group. It is suggested either the Bansong Group acted as a decoupling horizon or rest on unconformably on the thrust faults. The area is weakly affected by $D_4$-event of which structures are E-W trending folds and faults. The Jugryeong Fault clearly cut the earlier folds and thrust faults. The rocks within the fault zone were sliced and rotated during the strike-slip movements. Block rotation and transpressional features can be commonly observed.

• The Journal of the Petrological Society of Korea
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• v.4 no.2
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• pp.144-152
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• 1995

The Jangsan Quartzite of the Joseon Supergroup and the foliated granite (so-called granitlc gneiss of presumed Precambrian age) of the Yeongnam massif are in direct contact at Cheondong-ri area, 6 km @SE of Danyang. sllthough it has been thought traditionally that the Jangsan Quartzite overlies unconformably the f&ted granite, it is difficult to interpret the contact as an unconformity smce the basal conglomerate in- the lower part of the Jangsan Quartzite does not have any clast of the foliated granite, Rather, recent structural studies of this area indlcate that the contact is a ductile shear zone. However, the sense and age of the shear movement are still problematic. Our mesoscopic and microscopic studies of &tre Cheondong-11 semi-brittle shear zone involving foliated cataclasite and phyllonite, which is a pa& of the Ogdong fault, indlcate a top-to-the northeast shearing, i.e., dextral strike slip. We also performed Pb-Pb dating for the age-unknown foliated granite, since the age of deformed granite ccarr emtrain the maximum age of deformation. The whole rock and feldspar Pb isotape data for the foliated granite and a micaceous xenolith define an isoc chron age of $2.16{\pm}0.15$ Ga ($2{\sigma}$;MSWD=4.4) which is interpreted as the emplacement age of the granite. This early Proterozoic age agrees with those of Precambrian igneous activity In the Yeongnam massif reported previously. The obtaiPrfid gge confirms the traditional idea about the age of the foliated granite and indicates that other methd(s) should be employed to constrain the age of the shear movement.

• Economic and Environmental Geology
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• v.22 no.3
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• pp.285-291
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• 1989

The Okdong Fault is situated in Okdong-Hamchang area, the central part of Korea. The area consists of Precambrian gneisses and granitoids, Paleozoic clastic and carbonate rocks, and Mesozoic clastic rocks and igneous intrusives. The Okdong Fault is situated along contact boundary between the lowermost Cambrian Basal Quartzite and Precambrian basements. Mylonites occur as narrow zone which is extended over 100km and is restricted to within 10m-30m along the Okdong Fault. The main features of mylonites are quartz mylonite derived from Cambrian Basal Quartzite and mylonitic granitoids from Precambrian granitoids. Movement sense is deduced as a sinistral strike-slip movement with evidence of rotation of sheared porphyroclasts, rotation of fragments and S/C-bands. The mylonite zone has been reactivated as fault which reveals oblique-slip movement. The fault resurges as faults which reveals normal(to the NW) and reverse(to the SE) dip-slip movement. Normal faults are dominant in the northern and southern part and reverse or thrust faults are dominant in the central part of the Okdong Fault. The thrust movement can be correlated with the Daebo Orogeny of Jurassic Period. Granites and dyke rocks intruded into Paleozoic and Precambrian rocks during Cretaceous Period.

• Economic and Environmental Geology
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• v.25 no.2
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• pp.179-190
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• 1992

The Yeongchun area is located at the central part of the Danyang Coalfield, where Precambrian granitoids, Cambro-Ordovician Choseon Supergroup, Carboniferous-early Triassic Pyeongan Supergroup, middle Triassic-Jurassic Bansong Group and extrusive tuffs are exposed. The rocks in the area underwent four phases of deformation, which are (a) $D_1$ : Movement of the Okdong Fault, (b) $D_2$ : Formation of NW-SE trending folds and stretching lineations, (c) $D_3$: Movement of the Gagdong Thrust Fault and associated structures of NNE-SSW trending folds, and (d) $D_4$ : E-W trending strike-slip faults and folds. During the $D_3$-event, flexural slip deformation intensively affected rocks in the area. Strain measurements show relatively low strain intensity in the area. The types of strain ellipsoid are prolate in the hangingwall area and those near to the footwall area range from plane strain to weak oblate. The oblate type is developed in the region far from the footwall area.