Geometry And Kinematics Of The Tangwangzhai Multistage Thrust-nappe Structure In Northern Longmen Shan Thrust Belt

The Tangwangzhai thrust-nappe structure is located in northern of NE-trending Longmen Shan thrust belt, eastern margin of the Tibetan Plateau. It has long been thought to be an important mark of uplifting of the eastern margin of Tibetan Plateau. However, the main thrust fault, the fold style and multi-period structure kinematics characteristics indicate that it is a result of multi-period thrusting and overprinting.This paper takes the Tangwangzhai thrust-nappe structure as research object. We collect data of multi-period foliation, lineation, and related faults and folds for refined geometry and kinematics structural analysis. We analyze the superposition relation among all periods of deformation. Through dividing deformation stages and preliminarily confirming deformation time, a new evolution process has been determined. 1. The Tangwangzhai thrust-nappe structure underwent 3 stages of deformation since Mesozoic. The D1 includes three imbricated nappes from north to south. Along NW trending kink part or axial plane of overturned folds develops spaced cleavages dipping NE and local phyllitic foliation(S1). Curved thrust faults(F1) with NW-SE trending and SW-ward bulging and related striation(L1) indicate a top-to-the SW thrusting. The extension mode is piggy-back. Initiating at about 235 Ma and ending at 210 Ma, the D1 may associated with the Late Triassic collision between the North and South China Blocks. The D2 deformation only exists in upper and middle nappes. It marked by recumbent folds(f2) based on D1 deformation and related SE-dipping axial-plane cleavage(S2), crenulation cleavage, S-C fabrics and stretching lineation(L2). It is a SE-ward normal ductile shear deformation due to the postorogenic collapse, which occurred at Early Jurassic(197-183Ma). The D3 includes three imbricated nappes from west to east and also has an imbricated style. NE-SW trending spaced cleavage(S3) along moderately tight folds(f3), NE-SW trending thrust faults(F3) and corresponding striation(L3), and local brittle-ductile shear zone indicate top-to-the-SE thrust and the extension mode is piggy-back. It may concerned with the SE-ward lateral extrusion process of Songpan-Ganzi fold belt and the clockwise-rotated subduction of Yangtze plate in Indosinian and Yanshanian, and the collision between India and Tibet in Himalayan. 2. The Tangwangzhai thrust-nappe structure underwent strong compression in the direction of southeast. It has a shortening rate of 60.3%, and the moving distance of Siqingchi nappe could be at least 21.6km.