| The mosozoic thrust belt in the northern segment of the Longmen Shan belt, lieing at the junction of Qinling orogenic belt, Songpan-Garze terrance and Yangtze Plate, having superposed deformed by the cenozoic thrust structure strongly, has long been taken for a part of the NE structural belt in Longmen Shan thrust belt. Althought the mosozoic thrust nappe structure has been recognized, It lacks of research about structural analysis of structural style and kinematics subtly, and can’t distinguish it from the cenozoic thrust structure belt in the actual structural analysis. This article analyzes by structural analysis method detaily and divided the north segment of Longmen Shan thrust belt into five parts from north to south: Xiahe-Lixian thrust belt, Nanping-Wenxian multi-layered thrust belt, Qingchuan-Pingwu fold and thrust belt, Tangwangzhai thrust belt and Longwangmiao thrust belt. With geological survey and synthetical analysis of deformation fabric, structural kinematics and deformation sequences in all of thrust belts, we understand their successively superposed relationship clearly by staging and matching and regconize three periods of structural deformation: Xiahe-Lixian thrust belt grew a mass of north dipping nappe thrust, unsymmetrical fold east-west trending and stretching lineation which direct to south in the D1 period; Nanping-Wenxian multi-layered thrust belt developed low angel thrust fault which thrust to south, ductile shear deformation, closed oblique reversed fold which has north trending axial surface, vast north dipping gently penertration structural foliation S1, boudinage and rootless hook fold in D1 period; Qingchuan-Pingwu fold and thrust belt which dip angel is 50°-70° and trend to NEE-SWW, has trending EW medium tight fold in the hanging wall and grow spaced cleavage, axial plane cleavage and east-west trending broad and gentle fold in D1 period; Tangwangzhai thrust belt grew north dipping, associated NWW-SWW trending broad and gentle fold and lineation in D1 period; Longwangmiao thrust belt developed top-to-the south thrusting and NWW-trending broad and gentle fold. Both Tangwangzhai and Longwangmiao thrust belt was unconformity covered by the upper Jurassic baitianba formation, it can be infered that they had a process of top-to-the south thrusting in the late triassic. In conclusion, the D1 period is top-to-the south piggy-back imbricated thrust structure in the middle-late triassic, the process might associated with the indosinian collision between the North and South China Blocks. The D2 preiod is a positive ductile shear structure, which caused by extension after orogenic belt’s collapse at the 206Ma~154Ma. There has different forms in the leading edge and trailling edge of the thrust belt.It always developed ductile shear zone, detachment fault, recumbent fold etc in the leaing edge, and rift basin in the trailling edge. The D3 period is the top-to-the southeast thrust nappe structure. The fault has low dip, and the belt developed cross fold, axial plane cleavage etc. The figures show that it was caused by compression and thrust from Songpan-Garze block to Yangtze plate at the time of uplift of the Tibetan plateau at the 20~16Ma. |
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