Structural Style And Kinematics Of The Bikou Thrust Terrane In The Northeastern Margin Of The Tibetan Plateau

The Bikou terrane locates in the northeastern margin of the Tibetan Plateau and in the northern part of the Longmen Shan thrust belt, where is the junction of the Western Qinling orogenic belt, the Songpan-Ganze terrane and the Yangtze block. It is wedge-shaped with the NEE trending direction. The Bikou terrane is separated from the Qinling-Dabie orogenic belt by the nearly E-W trending Mianlue suture to the north, the Songpan-Ganze terrane by the nearly S-N trending Huya Fault to the west, the Longmen Shan thrust belt by the NEE trending Qingchuan-Pingwu Fault to the south and structurally pinched out to the east where the Qingchuan-Pingwu Fault and the Mianlue sture join together. Former researches have considered the Bikou terrane as the northern part of the Longmen Shan thrust belt for a long time, the formation of which was related to the Longmen Shan top-to-the-SE thrusting in the Cenozoic. But the recent research on petrology and regional tectonics suggested that the formation of the Bikou terrane might be related to the foreland thrusting of the Southern Qinling orogenic belt and extrusion structure in the late Triassic. The reason of different opinions is the lack of systematic analysis about the structural style and kinematics about the Bikou terrane and its boundary faults. On the basis of precious researches, the author investigates the structure and kinematics of the Bikou terrane. Three stages of deformation have been indentified. The D1 deformation, which is marked by spaced cleavage(S1), moderately tight fold(f1), mineral lineation(L1) and active faults(F1) in the circumstance of top-to-the-S thrusting,was overprinted by younger deformation. This deformation occurred in the middle and late Triassic,associated with the collision between the North and South China Blocks during the Indosinian orogeny. The D2 deformation produced penetrative cleavage(S2), mineral lineation(L2), recumbent fold(f2), detachment faults(F2) and ductile deformation of different scales in the circumstance of top-to-the-SSE normal shearing. Penetrative cleavage S2 is parallel to S0, which indicates bedding shearing. This deformation occurred in the early and middle Jurassic, associated with the regional extension after the Indosinian orogeny. The D3 deformation is marked by spaced cleavage(S3), lineation(L3), fold deformation(f3) and reactivated thrust faults(F3) in the circumstance of top-to-the-SE thrust and strike-slipping. The D3 deformation included three phases. The first stage was top-to-the-SE thrusting along the reactivated Qingchuan-Pingwu Fault from the late Jurassic to the early Cretaceous. In the second stage, sinistral strike-slipping along the Qingchuan-Pingwu Fault replaced the diminished thrusting and caused the top-to-the-W thrust-nappe structure of the Bikou terrane along the Huya Fault. This stage occurred in the late Cretaceous to Oligocene. In the third stage, the direction of strike-slipping was inversed, which reflected in the dislocation of rivers and landscape caused by the Qingchuan-Pingwu Fault. This stage initiated in Miocene and lasted to now. The D3 deformation was associated with the uplift of the Tibetan Plateau. The Qingchuan-Pingwu Fault and the Huya Fault had different nature in different deformation phases. In D1 deformation phase, the Qingchuan-Pingwu Fault acted as the main thrust fault, at the same time, the Huya Fault might not exist or just formed a series of tensional fracture surface. In D3 deformation phase,the nature of the Qingchuan-Pingwu Fault transformed from thrust fault to strike-slipping fault, on the contrary, the nature of the Huya Fault transformed from strike-slipping fault to thrust fault.