Characteristics and Mechanism of the Dangra Yun Co and Xuru Co NS-Trending Graben in the Gangdese, Tibet
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摘要: 当惹雍错-许如错南北向地堑位于青藏高原冈底斯中部, 由一组南北向高角度正断层与夹其中间的当穹错、当惹雍错、许如错等3个湖泊盆地构成.地堑发育于中新世晚期, 一直活动到全新世.沿着地堑边缘分布中新世碱性火山岩.白榴石响岩的K-Ar年龄为12.6Ma, 其岩石化学特征显示出陆内伸展构造环境.地堑内部发育晚更新世至全新世的湖相沉积物.当惹雍错-许如错地堑是在青藏高原板内构造隆升过程中, 层流加厚的下地壳热垫作用导致上地壳发生伸展作用, 随着青藏高原地壳物质的东流, 南北向伸展作用转向东西向伸展作用, 形成近南北走向的地堑.Abstract: The Dangra Yun Co and Xuru Co NS-trending graben located at central section of the Gangdese, is controlled by NS-trending high-angle normal faults and consists of the Dangqung Co, Dangra Yun Co and Xuru Co. Miocene alkati volcanic rock distributed along the graben indicates an intraplate extensional tectonic environment by its petrochemical and geochemical analyses. A 12.6 Ma age is determined for leucite phonolite by the K-Ar method, and it is found that graben occurred at the late time of Miocene (about 13 Ma), and remained active until Holocene. There are Middle Pliocene to Holocene lacustrine deposits in the graben. The Dangra Yun Co and Xuru Co graben, as well as other NS-trending grabens, is the results of extension of upper crust driven by the thermal upderplating of the lower crust flowing and thickening. S-N extension became E-W extension with the change of flow direction in the lower crust.
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图 1 西藏冈底斯中部当惹雍错-许如错南北向地堑略图
1.地堑; 2.逆冲断裂; 3.走滑断裂; 4.韧性剪切带; 5.正断层; 6.同位素年龄值; 7.Ms=6.7~6.9级地震震中; 8.Ms=5.7~5.9级地震震中; 9.Ms=4.7~4.9级地震震中; 10.温泉或热泉; Q.第四系; N.中新世碱性火山岩; E3.渐新世日贡拉组; E1-2.林子宗群火山岩系; K-J.侏罗-白垩系碎屑岩、碳酸盐岩夹火山岩; C-P.石炭-二叠系; γ5.燕山期二长花岗岩; γ6.燕山期花岗闪长岩; ALT.阿尔金断裂; SKS.昆仑断裂; JSR.金沙江缝合带; JLF.鲜水河断裂; BNS.班公湖-怒江缝合带; YZS.雅鲁藏布江缝合带; MCT.主中央逆冲断裂带; MBT.主边界逆冲断裂带(图 1a引自李亚林等, 2005)
Fig. 1. The geological map of Dangra Yun Co and Xuru Co NS-trending graben
图 3 当惹雍错-许如错地堑剖面(位置见图 1)
1.第四纪中更新世-全新世湖积层; 2.中新世碱性火山岩; 3.渐新世沉积-火山岩系; 4.林子宗群火山岩系; 5.白垩系碎屑岩; 6.石炭-二叠系浅变质岩系; 7.燕山期花岗岩; 8.喜山期花岗岩
Fig. 3. Structural section of Dangra Yun Co and Xuru Co graben
表 1 青藏高原地堑沉积物与断层年代学数据
Table 1. The sedimentary and faults ages of grabens in the Qinghai-Tibet plateau
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