The Tectonic Evolution Of Basin-orogrnic Belt From Mesozoic To Cenozoic In The Northern Section Of Longmenshan

The nothern section of Longmenshan front belt is located between SE-directed Qingchuan-Pingwu fault and Guangyuan-Dayi fault, it extends along Guangyuan to Anxian and splits the Sichuan Basin in the east, the Qinling orogenic belt in the northeast and the Songpan-Ganzi orogenic belt in the west, it is a critical region to understand the sedimentary basin-orogenic belt system in their formation and evolution in the Mesozoic-Cenozoic.After the systematic study of the north part of Lonmenshan, the structural evolution in the Mesozoic-Cenozoic can be divided into 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 late Triassic, to be influenced by Indosinian movement, Longmen trust belt began to uplift and continuely squeeze from northwest to southeast, the depocenter of western Sichuan foreland basin continue to reduce from northwest to southeast, the Xujianghe Formation and the upper Jurassic strata is unconformity, which indicates that in this stage there are a Fold deformation.From a regional point of view,compare with the south part of the Longmenshan, the north part formed mainly closed fold in this stage. U-Pb ages was 200-400Ma in study area are mainly from Songpan-Ganzi fold belt and the west margin of Yangtze craton, paleocurrent indicates that the source mainly from west to east.(3) In the Early and Middle Jurassic, detrital zircon with the peak age about 2502±4Ma、1806±11Ma and 820±5Ma are mainly from Yangtze craton and Qinling orogenic belt, paleocurrent is northeast, it suggests that the source are mainly from Yangtze craton and Qinling orogenic belt.(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 in this stage.(5) In the Cenozoic, the east part of Tibet Plateau began to rapidly uplift, and occurred thrust from northwest to southeast, as a result, Longmenshan and its foreland basin widely developed piggyback thrust nappe, accompanied by large scale uplift and formation shorten.