Geological Architecture And Formation Mechanism Of The Transitional Zone Between Longmenshan Mountains And Sichuan Basin

With its location between the Tibetan Plateau and the Sichuan Basin,the Longmenshan orogenic belt is one of the ideal places for studying multi-stage tectonics of intracontinental tectonic deformation,and it is also one of the key areas to study resource minerals and earthquake disaster prevention.Thus the study of the geological architecture of the Longmenshan has important scientific significance and application value.In this study,high-resolution seismic reflection profiles were combined with near-surface geological data,well-drilling data,hypercenter relocation,geodetic measurements,and fault-bend folding and ductile shear zone theory to reveal the geological structure of the Longmenshan.Special attention was given to quantitative analyses of the structural geometry and kinematics characteristics of the basin-mountain transitional zone.32 new apatite fission-track data were used to constrain the exhumation and deformation time.On the basis of temporal-spatial framework,this study also discussed the genetic mechanism of the complex structural configuration.The apatite fission track dating and thermal history simulation results revealed that the uplift of the Longmenshan frontal fold-and-thrust belts mainly occured in the Cenozoic.Two major stages of rapid uplift are identified: the first stage has occurred in the Paleocene to Eocene(~65-35 Ma)with progressive evolution tendency from NW to SE and the second stage occurred since the Miocene(~15-12 Ma)with sequential evolution tendency from SE to NW.The transitional zone between Longmenshan Mountains and Sichuan Basin is characterized by dip-structural zonation,strike-structural segmentation,and vertically structural stratification.In the dip direction,from northwest to southeast,the deformation level gradually becomes shallow,the involved layers gradually become newer,the deformation degree gradually weakened,and the brittle deformation gradually increased.Along the strike of Longmenshan,from southwest to northeast,folding and thrusting extended range becomes smaller,initiation and finalization tend to be young,and the Cenozoic activity gradually weakened.Vertically,three detachment layers(middle-upper crust ductile deformation layer,Pre-Sinian detachment layer,and Middle-Lower Triassic detachment layer)are developed and determined the stratified deformation.The low-velocity,low-resistance layer(ductile deformation layer)existed in the 20-30 km depth of the Longmenshan and Songpan-Ganzi block is not only the main detachment layer of the upper crust deformation,but also plays an important role in the decoupling during the subduction of Songpan-Ganzi lower crust to the Sichuan Basin.The crust-scale structural wedge is an important factor for the intracontinental deformation of the Longmenshan.The combination of structural wedge and thrust nappe in sedimentary layers is the main cause for upper crust shortening and uplift of the Longmenshan.Fault-related folds and fold-accomodation faults are important tectonic deformation styles of the fold-thrust belt in the frontal Longmenshan.The lateral heterogeneity of the detachment layers and the multiple transformations of the tectonic stress field during the Mesozoic-Cenozoic have important influence upon the differential structural deformation.On September 30,2017,an unexpected Mw 5.4 earthquake occurred in the Qingchuan County at the northern Longmenshan.Two rupture activities occurred in the Qingchuan earthquake.The reactivation of pre-existing low-angle thrust fault and the formation of newly formed right-lateral strike-slip rupture as well as the spatial overlap indicated the potential risk in the piedmont zone of the Longmenshan.Our results revealed that the Longmenshan is a typical intra-continental deformation zone,which underwent extension-convergence cycle within the continental crust,and still maintains strong tectonic activity in the present day.