Journal of the Geological Society of Korea (지질학회지)

The Geological Society of Korea (대한지질학회)
권호 표지

ESR dating of fault gouge from the Gacheon 1 site on the Yangsan fault zone

양산단층대 가천 1지점의 단층비지 ESR 연대측정

  • Published : 2012.12.31
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Abstract

We carried out ESR dating of 10 samples of fault gouge collected from the Gacheon 1 site of the Yangsan fault zone, Korea. We tested the consistency of ESR ages along the fault gouge zone developed on the same fault strand about 25 m extent. ESR ages estimated from two bands of fault gouge (Ga 1-1 and Ga 1-2 from an outcrop; Ga T-2 and Ga T-3 from trench 1) were too closely-spaced in time to be distinguished by the ESR dating method. We also found that ESR ages of three samples (Ga 1-1, Ga T-3, Ga T-5) collected along the same band of fault gouge about 25 m in length show consistent ESR ages, equivalent doses and dose rates. The fault in Gacheon 1 site (outcrop, trench 1 and 2) of the Yangsan fault zone were reactivated, adding well-defined new bands to the older band of fault gouge. During this adding of new bands, the older gouge band was inactive, which represents the Type I faulting mode. The fault gouge zone about 30 cm thick in trench 3 was homogeneous and all the structures were overprinted across the whole zone leading to difficulty in recognizing reactivation (Type III faulting mode). Faulting mode in the fault gouge zone shows variations along the same fault strand. ESR dates of seven samples from the fault strand in Gacheon 1 site range from 1,400 ka to 1,000 ka and three samples were in saturation. Results of this ESR age estimates suggest that the fault strand in this site was formed before the Quaternary period and was reactivated at least two times within the Quaternary period. Some normal stress and displacement are needed in order to reset the ESR signals during fault movement. Later fault movement along the discrete fault surface between fault gouge band and higher terrace deposits may not have zeroed ESR signals significantly due to low normal stress to the fault plane at surface environment.

양산단층대의 가천 1지점에 발달된 단층에서 채취한 10개의 단층비지 시료를 이용하여 ESR 연대 측정을 하였다. 약 25 m의 연장을 가지는 단층에서 ESR 연대가 서로 일치하는지를 검증하였다. 하천 노두에서의 두 개의 단층비지띠(Ga 1-1, Ga 1-2)와 트렌치 1에서의 두 개의 단층비지띠(Ga T-2, Ga T-3)는 오차 범위 내에서 4개 시료 모두의 ESR 연대가 일치하여 각각의 단층비지띠의 활동시기를 구분할 수 없었다. 약 25 m의 단층 연장선 내에 있는 동일한 단층비지띠(Ga 1-1, Ga T-3 및 Ga T-5)의 ESR 연대, 상응조사량 및 조사율은 오차 범위 내에서 서로 일치한다. 가천 1지점의 하천노두, 트렌치 1 및 2에 있는 단층비지대는 단층비지 진화양식 I에 의해 진화하였으며, 새로운 단층비지띠가 오래된 단층비지띠에 첨가되어 단층비지대의 폭이 단층이 재활동 할 때마다 증가하였다. 트렌치 3에 있는 30 cm 폭의 균질한 단층비지대는 단층비지 진화양식 III에 의해 진화했다. 위의 자료는 단층비지 진화양식이 단층의 주향방향을 따라 바뀌고 있음을 보여준다. 분석한 10개의 시료 중 3개는 이미 ESR 신호가 포화되어 있어 ESR 연대를 구하지 못하였으며, 나머지 7개의 ESR 연대는 100만년~140만년 전의 범위에 있다. ESR 연대 자료는 가천 1지점의 단층은 제4기 이전에 형성되었으며, 제4기에 적어도 두 번 이상 재활동 한 것으로 해석된다(130~140만 년 전, 100~120만 년 전). 단층비지대와 제4기 고위 단구 역 층과의 경계부에 발달된 명확한 단층면을 따라서 단층이 재활동 했을 가능성이 있으나, ESR 신호가 충분히 줄어들지 않아 ESR 연대 측정법으로는 그 시기를 알 수가 없다.

Keywords

References

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