Typical Cenozoic Tectonic Association And Its Formation Mechanism In The Eastern Margin Of Tibetan Plateau

The eastern margin of the Tibetan plateau has always been the focus of debate in the international geologists and the ideal region for studying deformation mechanism during the plateau uplift.On the basis of previous studies,we focused on structural styles,assemblages and their formational mechanism,and carried out regional geological mapping and analyzed fault styles,assemblages,basin-mountain couples and their formation.For this purpose,five transects were selected from north to south along the eastern margin of the plateau for structural analysis and fission track analysis.Furthermore,we designed and conducted sandbox modeling experiments to simulate fault re-activiting,assemblage and deformational sequence,based on understanded tectonic models with investigated boundary conditions.After comprehensive analysis of the above research results,we conclude that the eastern margin of Tibetan plateau has experienced three main tectonic stages?Dc₁¹?Dc₁²?Dc₂?during Cenozoic.They are:1)stage Dc₁¹?55-15 Ma?,continued formation of the Tibetan plateau after the India-Asia collision,caused nearly horizontal northeast-southwest compression in the eastern margin of the plateau.This consistent pressure reactivated the preexisted Trassic Longmen Shan fault?ancient Wenchuan-Maowen fault?,and formed the new Cenozoic Longmen Shan faults?including Wenchuan-Maowen fault,Beichuan-Yingxiu fault and Guanxian-Anxian fault?.Which are characterized by dextral strike-slip and thrust movement.However,thick layers of Mingshan conglomerate formation(E_1-2m)only deposited in south of the Longmen Shan foreland basin.2)stage Dc₁²?15-5 Ma?,the Xianhuihe fault was activated on the basis of the pre-existed fault?weak zone?within the Songpan-Ganzi terrane,acting as a large-scale sinistral strike-slip fault.A conjugate fault system was produced by combining the new generated Xianshuihe fault and the Longmen Shan faults together.Meanwhile,the southern segment Longmen Shan thrust belt was modified by eastward movement of the Xianshuihe fault,and changed from dextral strike-slip fault to thrust fault-dominated,causing rapid uplift and formation of high mountains?e.g.Gongga Mountain?.3)stage Dc₂?5-0 Ma?,the Longmen Shan faults showed both dextral strike-slip and eastward thrusting movement,accompanied with large thickness Dayi conglomerate formation?N₂d?deposited in the northern part of the Longmen Shan foreland basin,representing three large alluvial fans.Meanwhile,lots of blind reverse faults generated beneath the Longmen Shan foreland basin.Based on the above analysis,we propose that formation and evolution of the Cenozoic fault system in the eastern margin of Tibetan plateau,is mainly due to northeastward compression caused by India-Asia collision,and staged reactivation of pre-existing faults.