Tectonic Evolution Of Northwestern Margin Of Yangtze Plate Based On The Analysis Of The Seismic Section

Longmen Mountain-Micang Mountain area locates in the northwestern Sichuan Basin and northwestern edge of Yangtze plate.It is connected to the Qingling-Dabie orogen in the north,the Daba Mountain orogen in the east and Songpan-Ganzi orogen in the west.The special tectonic location of this area makes its evolution closely related to the orogens around.This paper,using the new application and new theory of modern Structural Geology,Seismology,Balanced-cross Section,Foreland Basin Analysis and Basin Mechanism,processes and interprets the high-resolution 2-D seismic sections in this area systematically,and talks about the tectonic evolution of this area by choosing classic seismic sections to do structural balanced recoveries.The primary conclusions are listed as below:1.The Front-mountain belt of Micang Mountain and northern Longmen Mountain develops a series of anteriror-exhibitioned imbricate thrust faults,which extends down 20 km to the detachments in the pre-Sinian crystalline basement and terminates in the detachments of Middle-Lower Triassic,forming passive roof dual structures.And there are also anteriror-exhibitioned imbricate thrust faults formed in the middle part of Longmen Mountain,but most of these thrust faults convergence downward in the detachments of Middle-Lower Triassic and extend upward to the surface.2.The deformation degree of folds and faults in the southern Micang Mountain increases from east to west,and we speculate that the main reason for this lies in the amount of north-south compression of the left-lateral strike slip,coming from NE-SW trending faults,and the south-eastward thrusting in the mountain-front faults of Northern Longmen Mountain decreases from west to east in Micang Mountain area.3.Based on Sequence Stratigraphy and Seismology,we build a model for the basin tectonic evolution according to the contact relationship of faults and the analysis of basin filling sequences:the foreland basin of Longmen Mountain started to form in the late period of Late-Triassic when Songpan-Ganzi orogen thrusted upwards the Western Yangtze plate,and developed giant-thickened wedge accumulation of Xujiahe Formation.During Middle-Jurassic,a unified foreland basin was produced in the northern edge of middle-upper Yangtze as the final collision of North China Cration and Yangtze plate.And the depocenter transferred to the northern edge of Micang Mountain from the middle Longmen Mountain,forming colossal-thickened Shaximiao Formation.4.Based on previous research results and the inversion and recovery by the application of the balanced-cross section technique into our research area,we divide the evolution of the research orogenic belt into 4 stages:1.Passive margin developing period before Late-Triassic;2.Foreland basin developing period between the late period of Late-Triassic and the early period of Early-Cretaceous;3.The intro-continent deformation and superposed deformation period between the late stage of Early-Cretaceous and the early stage of Eocene;4.The overall uplifting period after the later Eocene.