Tectonic Activities in Middle and North Sections of Longmenshan Thrust Belt during Late Indosinian: Evidence from Structural Analysis and Detrital Zircon Geochronology
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摘要: 晚三叠世是四川盆地演化过程中的重要阶段，该时期盆地西部经历了从被动大陆边缘向前陆盆地的演变.为进一步厘定龙门山中北段印支晚期构造的具体活动时间及活动强度，本研究选择龙门山前中北段某地震剖面进行构造几何分析，结果显示龙门山前中北段大规模的逆冲推覆很可能在须家河组四段沉积初期就已经出现，其恢复的推覆位移量至少达到11 km；另外针对研究区晚三叠世地层开展碎屑锆石U-Pb年代学以及沉积学分析，结果显示~800 Ma主峰及沉积学响应表明须家河组三段沉积时期研究区域地层中具有大量来自龙门山中段彭灌杂岩的碎屑物质，反映龙门山中北段基底在须家河组三段沉积时期已经局部隆升并为研究区提供物源.Abstract: It is wildly shared that the western Sichuan basin changed from passive continental margin to a foreland basin during the Late Triassic. In order to depict this transition process as well as the time and intensity of the tectonic activity of Longmenshan thrust belt during Late Indosinian, a seismic section in the middle part of Longmenshan thrust belt was used to perform structural geometric analysis. The results show that the large scale thrusting of Longmenshan probably occurred during the early stage of the deposition time of T3x4 and the total displacement was at least 11 km. This is also supported by the sedimentary record of the detrital zircon U-Pb age from the synchrous strata. The major age peak of ~800 Ma shows great affinity to the Pengguan complex and Baoxing complex from the middle section of Longmenshan belt. It is concluded that the basement in the middle part of Longmenshan belt was uplifted partly during T3x3 and provided materials for the western Sichuan basin.
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图 1 地震测线（A-A’）所处的地质及大地构造位置
Fig. 1. Geological sketch and tectonic position of the seismic line A-A'
图 2 龙门山前中段地震剖面的构造解释方案
图中红色虚线表示印支晚期形成的隐伏断层，红色线条及向上箭头表示钻井位置
Fig. 2. Structural interpretation of seismic section in the fore-middle section of Longmenshan thrust belt
图 3 晚三叠世须家河组样品LMS-01镜下照片
Qm.单晶石英；Kfs.钾长石；Pl.斜长石；Lt.岩屑
Fig. 3. Microscopic photo of sample LMS-01 from Xujiahe Formation in Upper Triassic
图 4 龙门山中段须家河组以及样品古水流向图
Fig. 4. Stratigraphy of Xujiahe Formation in the middle section of Longmenshan thrust belt and the paleocurrent of the sample
图 5 样品LMS-01部分碎屑锆石CL图像
黄色数字为锆石年龄值；橙色数字为该锆石的Th/U值
Fig. 5. Cathode luminescence images of detrital zircons in the sample LMS-01
图 6 样品LMS-01碎屑锆石U-Pb年龄直方图以及谐和图
Fig. 6. Histogram diagram of all detrital zircons in sample LMS-01 and U-Pb concordia with Th/U diagram
图 7 恢复深部隐伏叠加断层转折褶皱后的地震剖面构造解释方案
Fig. 7. Interpretation of the structural section after restoration of the buried fault bend fold
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