Study On The Structural Characteristics And Genesis Mechanism Of The Tangwangzhai-Yangtianwo Compound Syncline

The Longmenshan structural belt is located in the northwestern margin of the Sichuan Basin and spreads from northeast to southwest,which is an important structural belt in Southwest China.The most striking feature of the Longmenshan structural belt is its unique thrust nappe structure and the accompanying klippe group.Although predecessors have done a lot of research on various aspects of the Longmenshan structural belt and the klippe group,there are still some differences on the understanding of the structural geological model of the Tangwangzhai-Yantianwo compound syncline located in the northern section of Longmen Mountain.Based on the field geological survey,drilling data and seismic profile interpretation,and using balanced profile technology and sandbox modeling,this paper mainly studies the structural characteristics and evolution law of the study area,and analyzes the formation mechanism of Tangwangzhai-Yantianwo compound syncline.We got the following understanding and the results:?1?The Tangwangzhai-Yangtianwo compound syncline spreads from northeast to southwest,and the axis is inclined to southeast.This compound syncline shows extremely asymmetrical characteristics,and the deformation and displacement strength of the two synclinal wings are quite different.The deformation of the northwest wing is weak;the Devonian is inclined to the southeast with a gentle dip angle,and the strata are well developed and thick.The deformation of the southeast wing is strong;the Devonian are inclined to the northwest with steep dip angle,and the thickness of the stratum is thin and incomplete.?2?The structural style of the study area has the characteristics of layering and zoning.On the plane,the surface structure is bounded by the Majiaoba fault and the T/J unconformity interface,which can be divided into thrust nappe,piedmont fold belt and Sichuan Basin.On the profile,the piedmont fold belt can be divided into upper complex fold belt and lower complex fold fault anticline by No.1 fault.The detachment layers in the study area are mainly the Cambrian Gaotai Formation gypsum salt rock,Silurian shale and gypsum rock formations of the Middle and Lower Triassic layers.According to the distribution characteristics of detachment layer,the structure of the hinterland can be divided into three structural deformation layers:below the Cambrian detachment layer,the structure style is relatively simple and stable;between the Cambrian detachment layer and the Triassic detachment layer,the structural deformation is complicated,and the fault is extremely developed;the strata above the Triassic detachment layer has few faults and shows passive deformation.?3?By restoring the balance profile and combining with the predecessors to divide the structural evolution of the study area,it is believed that the study area has mainly undergone the following tectonic evolution stages:a.From Caledonian to Hercynian,it is mainly manifested as the extensional dynamics background and small synsedimentary normal faults are developed;b.Indosinian period,the study area was in the differential uplift stage under the extensional background,forming the prototype of the Tangwangzhai-Yantianwo compound syncline and its front thrust nappe structure;c.Himalayan period,structural dynamics background changes from tension to extrusion.The structural features are mainly thrust nappe,and the structural styles formed in the early period are transformed and finally formed.?4?Through sandbox modeling and previous research results,it is believed that the formation of complex structures in the study area was due to the differential uplift in the evolution from sea to land during the Indosinian period?after T₂l deposition?,forming the basin-ridge coupling relationship of plateau-slope-basin.The shear dynamic environment appears in the slope zone,and the extrusion dynamic environment appears in the basin area.The Tangwangzhai-Yantianwo compound syncline and its front thrust nappe structure,located at the transitional part of the slope zone and the basin,were formed under this tectonic environment.After the northwest-southeast compression of the Himalayan period,the previous structure was transformed and finally formed.