Processing And Interpretation Of Gravity And Magnetic Anomalies In Eastern Sichuan And Tibet And Research On Crustal Structure

The eastern Sichuan-Tibet area is located at the junction of three active blocks,with complex tectonic activities and frequent seismic activities.The deep structure of the eastern margin of the Tibet in the area and its continental dynamics have always been hotspots in the field of international geosciences.Predecessors have carried out geophysical work such as earthquakes and magnetotelluric.However,a single method has some shortcomings for large-scale regional structure research overall understanding.To obtain a more precise and reliable information of the crustal structure in eastern Sichuan and Tibet,this paper combines the results of previous earthquakes and magnetotelluric soundings on the shallow and deep crustal structure of the research area,and uses 1:250,000 measured gravity anomaly data to obtain the regional field and the local field by the upward continuation separation.After processing and analyzing the local field,it is found that the eastern Sichuan and Tibet can be divided into two geological units,which are the Western Sichuan Plateau,characterized by an orogenic area,and the Sichuan Basin,characterized by a stable sedimentary basin.The regional tectonic framework is controlled by the fault zone systems which trend are NW,NEE,and nearly NS.Different from the previous inversion of the research area as a whole,this paper establishes a variable density index model,collects seismic sounding data as a constraint,and uses the Parker-Oldenburg interface inversion method to calculate the regional gravity field obtained by the separation of the potential field.According to the thickness of the crust,it is divided into left and right areas,and different parameters are set to obtain more accurate Moho undulations in the study area.The overall depth of the Moho gradually rises from northwest to southeast.Combined analysis of crustal physical properties and deep tectonic movement in the eastern Sichuan-Tibet area,it is speculated that the geophysical properties of the plateau and basins have different.The obstruction of the crust in the Sichuan Basin caused the accumulation of material,thickening the Songpan-Ganzi crust,and simultaneously caused the density interface to sink,forming a gradient zone of the depth of the Moho.Using the Parker interface inversion method to calculate the aeromagnetic anomaly data after variable dip angle polarization,The Curie isothermal of the Longmenshan area is uplifed.The density model calculated by the seismic sounding velocity profile is used as the initial model,combined with the identified structural units and faults,constrained the Moho undulations by the variable density interface inversion,The profile inversion obtain two density structure traversing the Longmenshan fault area.The density structure profile shows that the difference in the density structure of the crust on both sides of the Longmenshan between the two profiles is obviously.The density of the lower crust of the plateau is lower than that of the Sichuan Basin,and the thickness has increased significantly.The tectonic deformation of the crust caused by the accumulation of the crust caused by the accumulation of low intensity substance which obstructed by hard blocks during the eastward migration.The low-density layer about 20 km below the surface is consistent with the inner depth,which may represent the local melting of the crust.