지리산 일대의 연성전단변형

Ductile Shear Deformation around Jirisan Area, Korea

Abstract

영남육괴 남서부인 지리산지구의 편마암복합체에 발달하는 연성전단변형의 기하학적 및 운동학적 특성과 연성전단변형을 받은 우백질편마암에 대해 SHRIMP 저어콘 U-Pb 연대측정을 수행하고 연성전단변형의 시기에 대하여 논의하였다. 편마암의 엽리를 연성변형에 의해 생성된 전단엽리들이 절단하고 있으며, 광역적인 분포를 나타내고 있어 이 지역의 편마암은 두 번 이상의 강력한 연성변형을 겪었음을 지시한다. 전단엽리면에 발달한 신장선구조는 북동 방향에 완만한 침강각을 보여 주향이동성 전단운동의 존재를 지시하며, 신장선구조를 포함하고 전단엽리면에 직교하는 단면에서는 우향의 전단감각이 우세하게 발달한다. 연성변형전단의 공간적 분포는 남북 내지 북북동 방향이 우세한 우향의 전단대가 중앙부 및 동부에서 대상 분포로 발달된 특성을 보이며, 서쪽부에서는 호남전단대의 일반적인 방향성인 북동 방향으로 발달하고 있다. 강한 전단변형을 받은 우백질편마암의 2개의 시료에서 구한 U-Pb 일치연령은 각각 $1,868{\pm}3.8Ma$와 $1,867{\pm}4.0Ma$로 연구지역 일원에서 보고된 편마암류들의 U-Pb 연대측정 결과와 일치한다. 연구지역에서 분포하는 전단변형을 받은 편마암류, 편마암류를 관입한 변형된 염기성 암맥과 화강암류 그리고 변형 특성이 관찰되지 않는 화성암류들의 관입 시기를 고려할 때, 연성전단 변형작용은 약 260 Ma-230 Ma 동안 섭입작용과 관련된 화강암질 화성활동 이후인 약 230~220 Ma에 한반도 충돌조산운동과 관련되어 발생하였다.

In the Jirisan area of the Yeongnam Massif, Korea, several ductile shear zones are developed within Precambrian gneiss complex (Jirisan metamorphic rock complex). The ductile shear zones have a general NS- and NNE-striking foliation with westward dipping directions. The foliation developed in the shear zones cut the foliation in gneiss complex. The stretching lineations are well developed in the foliated plane of the shear zone, showing ENE-trend with gentle plunging angle to the ESE direction. Within shear zone, several millimetric to centimetric size of porphyroclasts are deformed strongly as a sigmoid form by ductile shearing. The sigmoid patterns of porphyroclasts in the shear zones indicate the dextral shearing. The spatial distribution of ductile shear zone is characterized by the dominant NS- and NNE-striking dextral sense in the central and eastern regions respectively. In the western part, it develops in NE-striking dextral sense which is the general direction of the Honam shear zone. The U-Pb concordant ages obtained from the two samples, the strongly sheared leucocratic gneiss, are $1,868{\pm}3.8Ma$ and $1,867{\pm}4.0Ma$, respectively, which are consistent with the U-Pb ages reported around the study area. We supposed that the ductile shearing in the study area is occurred about 230~220 Ma during late stage of the continental collision around Korea and is preceded by granitic intrusion related to subduction during 260~230 Ma, which are supported by compiling the age data from sheared gneiss, deformed mafic dyke intruded gneiss complex, and non-deformed igneous rocks.

Fig. 2. Outcrop photographys of major rocks in the study area.

Fig. 3. Outcrop of charnockite developed around Daehagyo (bridge) in the eastern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 4. Outcrop of charnockite developed around Daehagyo (bridge) in the eastern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 5. Outcrop of leucocratic gneiss developed around Honggyeri in the northeastern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 6. Outcrop of granulite developed around Banggok-ri in the northeastern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 7. Outcrops and simplified block diagram of outcrop developed around Geumcheon valley in the southern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 8. Mylonized and folded granitic gneiss developed around Bulil Fall in the southern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 9. Outcrop developed around Dancheon valley in the southern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 10. Outcrop of migmatite within blastoporphyritic granitic gneiss developed around Baekmudong valley in the northern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 11. Outcrop of blastoporphyritic granitic gneiss developed around Dalgung valley in the northwestern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 12. Outcrop of blastoporphyritic granitic gneiss developed around Baemsagol valley in the northwestern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 13. Outcrop of highly sheared quartzite developed around Cheonggye in the northeastern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 14. Outcrop of leucocratic granitic gneiss developed around Bonghwasa Temple of Sangi-ri in the southeastern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 15. Outcrop of leucocratic granitic gneiss developed around Baegamdongcheon of Sangi-ri in the southeastern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 16. Outcrop of leucocratic gneiss developed around Jungsanri in the eastern part of the Jirisan area, Yeongnam massif, Korea.

Fig. 17. Representative cathodoluminescence images of zircon grains and U-Pb concordia diagram from two leucocratic gneiss samples. The ²⁰⁷Pb/²⁰⁶Pb age of individual analytical spots, denoted by solid circles, are given.

Fig. 18. Photography showing the kinematic characteristics related to the ductile shear deformation in each outcrop.

Fig. 1. (a) Tectonic province map of the southern part of the Korean Peninsula (modified from Kang and Lee, 2016) and (b) Geological map of the Jirisan area in the Yeongnam Massif, Korea (modified from Choi et al., 1964; Kim et al., 1964a, 1964b; Kim and Kang, 1965; Nam et al., 1989; Son et al., 1964).

Table 1. SHRIMP zircon U–Pb isotopic data for the analyzes samples from this study