암석학회지 (The Journal of the Petrological Society of Korea)

한국암석학회 (The Petrological Society of Korea)
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경상북도 북후면-평은면 지역에 발달된 예천전단대의 구조적 특성

Structural characteristics of the Yecheon Shear Zone in the Pukhumyeon-Pyeongeunmyeon area, Gyeongsangbukdo, Korea

  • 강지훈 (안동대학교 지구환경과학과)
  • Kang, Ji-Hoon (Department of Earth and Environmental Sciences, Andong National University)
  • 투고 : 2014.04.21
  • 심사 : 2014.05.07
  • 발행 : 2014.06.30
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초록

소백산육괴의 중앙부에 위치하는 경상북도 북후면-평은면 지역에서 주 구성암류인 선캠브리아기 변성퇴적암류와 중생대 화강암에 대한 변형된 암석 및 미구조 연구를 통하여 (동)북동-(서)남서 방향으로 발달하는 우수 주향 이동성 예천전단대의 상대적인 형성시기, 변형온도, 초압쇄암대의 분포 등을 파악하였다. 연성 변형된 주요 암석구조는 전단변형에 의해 형성된 압쇄구조면과 신장선구조 그리고 편마구조면 내지 압쇄구조면이 습곡되어 형성된 (동)남동-버전스의 완침강-중경사 밀착습곡 등이 있다. 연성전단변형 동안에 형성된 장석들의 특징적인 미구조는 플레임 퍼어사이트, 밀메카이트, 미세단열, 미세킹크, 킹크대, 파동소광 등이 인지된다. 재결정된 석영집합체의 c-축 배열은 rhomb와 basal가 탁월한 미끄럼계로 작용하여 형성된 single girdle과 Type I crossed girdle 형태를 보여준다. 전단운동감각 결정면상에서 인지되는 전단운동감각은 우세한 방향성인 북서-경사와 부수적인 방향성인 남동-경사 압쇄구조면상에서 상부-북동-이동과 상부-남서-이동을 각각 보여준다. 이러한 특징적인 암석 및 미구조로부터 예천전단운동은 밀착습곡이 형성된 이후 발생하였으며, 예천전단운동이 발생할 당시의 변형온도는 $350{\sim}450^{\circ}C$이었음을 알 수 있다. 선캠브리아기 변성퇴적암류의 대상 분포와 변형된 선캠브리아기 변성퇴적암류의 장석과 중생대 화강암의 석영의 입도 분포로부터 불노봉과 박달산 북부를 통과하는 (동)북동 방향의 예천전단대의 초압쇄암대가 설정된다.

The dextral strike-slip Yecheon Shear Zone of (E)NE-(W)SW trend is developed in the Pukhumyeon-Pyeongeunmyeon area, Gyeongsangbukdo, in the central part of Sobaeksan Massif, Korea which mainly consists of Precambrian metasedimentary rocks and Mesozoic granite. This paper researched the relative formation time, the deformation temperature, and the distribution of ultramylonite zone of Yecheon Shear Zone from the analysis of their deformed rock structures and microstructures. There are the mylonite foliation and stretching lineation by shear deformation, and the (E)SE-vergence gentle plunging, moderately inclined tight fold defined by the gneissosity and mylonite foliation as ductilely deformed main rock structures. Flame perthites, myrmekites, microfractures, microkinks, kinkband, and undulatory extinction are recognized as characteristic microstructures in feldspars during ductile shear deformation. Patterns of c-axis fabrics of recrystallized quartz aggregate display single girdle and type I crossed girdle where the rhomb and basal slip systems are predominant. The shearing senses on sense-of-shear plane show top-to-the-NE and top-to-the-SW on the predominant NW-dipping and subordinate SE-dipping mylonite foliations, respectively. These characteristic rock structures and microstructures suggest that the Yecheon ductile shearing occurred under $350{\sim}450^{\circ}C$ deformation temperature after the tight folding. The (E)NE trending ultramylonite zone of Yecheon Shear Zone, which passes through the Pulnobong and the north part of Pakdalsan, is established from the zonal distribution of Precambrian metasedimentary rocks and the grain-size distribution of feldspars in the Precambrian metasedimentary rocks and quartzs in the Mesozoic granite.

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