| Longmenshan, a very typical thrust nappe in china, is trapped by Songpan-Ganzi fold belt in the west, Sichuan basin in the east and the Qinling Orogenic belt in the northeast.After the systemic study of the middle piece of Longmenshan, the structural style can divided into four main patterns: dome, fault-related folds, thrust structure and slipping structure.The structural evolution of the middle part Longmenshan in the Mesozoic and Cenozoic has four stages: (1) In the middle Triassic, initial Longmenshan and its front areas were formed when an intense tectonic movement happened at Yangtze craton. (2) In the early Rhaetian, there is no Xujiahe Formation in Jiangyou, while it can be found in other areas, which indicates that in this stage the middle part is higher than the north and south Longmenshan. In the Indo-Sinian movement, the strata in the middle and north Longmenshan were deformed, the relationship of Xujiahe Formation and the upper Jurassic strata is unconformity but in the southern part it is disconformity. By the comparison of zircon U-Pb age and Lu-Hf isotope of the adjacent areas, the provenance of early Xujiahe Formation were Songpan-Ganzi fold belt, Yangtze craton, Qinling Orogenic belt and Bikou area, and the late Xujiahe Formation were Songpan-Ganzi fold belt, recycle sedimentary of Longmenshan and Yangtze craton. (3) In the Early and middle Jurassic stage, the sedimentary thickness is steady which represents the inactive structural movement. (4) In the Late Jurassic, the front range of Longmenshan uplifted druing the Yanshan movement, as a result, the north part is so high that it is lack of cretaceous sedimentary, however, in the south part,the relationship between upper Jurassic Lianhuakou Formation and Cretaceous strata is unconformity.The thrust activity began in Himalayan movement, when the cretaceous strata in the middle and southern part were deformed. At the same time, klippes were appeared because of the high degree of uplift. |
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