대한지리학회지 (Journal of the Korean Geographical Society)

대한지리학회 (The Korean Geographical Society)
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방사성 우주기원 동위원소를 이용한 티벳고원에서 가장 오래된 제4기(?) 빙성퇴적물인 Bunthang sequence의 절대 연대측정과 이의 고환경 및 지반운동에 대한 의미

Absolute Age Determination of One of the Oldest Quaternary(?) Glacial Deposit (Bunthang Sequence) in the Tibetan Plateau Using Radioactive Decay of Cosmogonic $^{10}Be$ and $^{26}Al$, the Central Kavakoram, Pakistan: Implication for Paleoenvironment and Tectonics

  • 성영배 (미국 신시내티 대학교 지질학과)
  • 발행 : 2007.06.30
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초록

티벳고원의 서쪽, 중부 카라코람의 인더스 강이 지나는 Skardu 근처에서 발견된 약 1.3km의 두께를 보이는 Bunthang 시퀀스의 절대 연대측정을 우주 기원 동위원소인 $^{10}Be$과 $^{26}Al$의 비를 이용해서 측정했으며 약 2백 65만년전에 급격하게 퇴적된 것으로 확인되었다. 이러한 퇴적시기는 지금까지 발견된 가장 오래된 직접적인 빙하활동의 증거로서 이전의 고지자기 연구와도 일치한다. Bunthang 시퀀스는 아래에서부터 빙퇴석, 호성 퇴적물, 하천 퇴적물 그리고 다시 호성 퇴적물로 이루어지며 어떠한 단층운동의 증거도 발견되지 않는 점으로 미루어 볼 때 분지의 생성은 빙하의 하방침식에 의해서 만들어졌으며 빙하의 후퇴와 더불어 proglacial과 paraglacial 프로세스에 의해서 생성된 것으로 판단된다. 이 지역에 있어서 신생대의 활발한 지반 활동은 인더스 강의 구배를 변형시킴으로써 연구지역과 같은 국지적인 호소퇴적층의 활발한 퇴적을 용이하게 하였으며 또한 지반운동과 빙하의 침식에 의한 사면의 불안정성은 이러한 국지적 퇴적 작용을 더욱 촉발시켰을 것으로 판단된다. 이전의 연구와 본 연구의 결과로써 지난 제4기 동안 빙하의 활동이 약해진 것으로 보건데 지난 마지막 빙기 최성기를 정점으로 티벳고원에 커다란 빙상이 존재했다는 가설은 틀린 것으로 보인다. 이 지역에서 제4기 동안의 빙하 활동의 축소는 희말라야 산맥과 카라코람 산맥 중심의 급격한 융기로 인해 Indian monsoon의 유입이 줄어든 것에서 기인한 것으로 추측된다.

Absolute age of the deposition of 1.3 km-thick Bunthang sequence within the Skardu intermontane basin of the Central Karakoram was determined using radioactive decay of cosmogonic $^{10}Be$ and $^{26}Al$ burial dating. The Bunthang sequence deposited around 2.65 Ma, which is the oldest glaciation in the region. The timing of deposition of the Bunthang sequence is consistent with the previous suggestion that the basin filling took place between Brunhess and Matuyama chrons. Four major sedimentary facies interfinger within the Bunthang sequence: glacial diamict, lacustrine, fluvial and lacustrine facies upward. This sedimentary distinctiveness and the lack of evidence on the faults for alternative pull-apart basin model around the Bunthang sequence, suggest that the depressional basin was formed by deep subglacial erosion during the exrtensive Bunthang Glacial Stage and subsequently the sediments underlain by basal diamict, was quickly deposited by preglacial and paraglacial processes. Temporary ponding of the Indus River due to tectonic uplift in the downstream or blockage by mass movements might make the basin filing more possible. The hypothesis that the single ice sheet developed on the Tibetan Plateau during the global last glacial cycle should be refuted by the existence of the older extensive Bunthang glacier Furthermore, the extensive glaciation during the early Quaternary (and thus progressive decrease in extent with time) suggests that there may have been significant uplift of the Pamir to the west and Himalaya to the south, which would have reduced the penetration of westerlies and Indian summer monsoon and hence moisture supply to the region.

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