西藏高原東側鮮水河斷裂帶與丹巴背斜之岩石磁學性質與古應力分析;Rock Magnetism and Paleostress Analysis of Danba Anticline and Xianshuihe Sinistral Fault Zone in the Eastern Tibetan Plateau, China

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Title:	西藏高原東側鮮水河斷裂帶與丹巴背斜之岩石磁學性質與古應力分析;Rock Magnetism and Paleostress Analysis of Danba Anticline and Xianshuihe Sinistral Fault Zone in the Eastern Tibetan Plateau, China
Authors:	賀厚平;Ho, Hou-Ping
Contributors:	地球科學學系
Keywords:	丹巴;鮮水河斷裂帶;應力分析;磁滯曲線;磁黃鐵礦;Danba;Xianshuihe sinistral fault zone;stress analysis;hysteresis loop;pyrrhotite
Date:	2018-01-29
Issue Date:	2018-04-13 11:01:31 (UTC+8)
Publisher:	國立中央大學
Abstract:	丹巴背斜，位在西藏高原東緣的龍門山造山帶與鮮水河左移斷裂帶之間，是一個快速抬升的區域。西藏高原是早新生代以來印澳板塊和歐亞板塊不斷收斂聚合而形成的產物，然而西藏高原下部地殼的塑性材料向東部流動，並受到揚子地塊的阻擋，因此在西藏高原東緣產生相當複雜的地質構造現象。為了要了解這個地區構造複雜的機制，我們利用應力分析與磁學性質量測這兩個方法。首先，我們到野外測量丹巴背斜到鮮水河斷裂帶之間的斷層擦痕，用來分析該地區的應力狀況，得到丹巴背斜的最大水平主應力走向以NE－SW向為主，鮮水河斷裂帶最大水平主應力走向以WNW－ESE向為主，然而折多山山頂並無明顯統一趨勢，可能是其所屬的貢嘎山花崗岩體為一正花狀構造，部分所受應力並非水平或垂直，軌跡雜亂，統一趨勢，值得後續研究正花狀構造底部至頂部的應力軌跡。此外，我們採集了該地區的低度變質岩樣本，分析樣本中的磁滯曲線以了解磁學性質，再利用X光繞射分析與掃描式電子顯微鏡確認其中的磁性礦物種類。根據前人研究，在低度變質岩地層中初現的磁黃鐵礦可以做為近變質帶與淺變質帶的等變質線，有助於地層對比與大地構造的研究，本研究比較在鮮水河斷裂帶兩側皆有出露的三疊系地層，發現在鮮水河斷裂帶與丹巴背斜之間有磁黃鐵礦分布，其餘地區皆無發現，代表鮮水河斷裂帶的東北側三疊紀地層的變質度已達淺變質帶，配合主應力軸相對位置的結果，表示鮮水河斷裂帶東北側長期以來可能受到褶皺逆衝帶與擠壓走向滑移帶的作用而有抬升現象。;Danba antiform, an area with extreme exhumation, locates between Longmenshan orogen and Xianshuihe sinistral fault zone in the eastern Tibetan Plateau. The Tibetan plateau was built by the convergence between Indo-Australian plate and Eurasian plate since early Cenozoic. However, the eastward lower crustal flow under the plateau obstructed by the Yangtze craton soon after this convergence and generated a very complex structural phenomenon. We used two methods, stress analysis and magnetic measurement, to understand the processes and mechanisms of this structural complexity. First, to be aware of the principle compressive stress in this area, we measured slickensides in the field from Danba area to Xianshuihe fault zone to carry out a series of analysis. We then obtained the strike of the maximum principle stress axis is NE-SW in Danba antiform, WNW－ESE in Xianshuihe sinistral fault zone, and no united trend on the peak of Zheduoshan. The possibility is the Gongashan granite which Zheduoshan located is a positive flower structure. Some of the principle stress are not horizontal or vertical here that made the stress trajectories diverse. In addition, in order to understand the magnetic characteristics of low-grade metamorphic rocks in this area, we took samples and processed the rock magnetic measurement of hysteresis loop. Then ensured the component of magnetic minerals by X-ray diffraction analysis (XRD) and Scanning Electron Microscope (SEM). The occurrence of pyrrhotite can be taken as an important isograd between anchizone and epizone in low-grade metamorphic rocks, which is helpful for stratigraphic and structural studies. Our study compared with the Triassic formation which appear both side of Xianshuihe sinistral fault zone, then observed pyrrhotite distributed between Xianshuihe sinistral fault zone and Danba antiform only. No any pyrrotite be found in the other Triassic formation. It means the Triassic formation in the northeast side of Xianshuihe sinistral fault zone is in epizone. The combination of our result shows there are fold-thrust zone and compressive strike-slip zone causing the uplift in the northeast side of Xianshuihe sinistral fault zone in a long period.
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