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Shunyou Cheng, Northwest University (China)
Xiaoduo Wang, Northwest University (China)
Xiaoqiang Xu, Northwest University (China)
Yanwei Qu, Northwest University (China)
Xianghui Jing, Northwest University (China)
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We take the observed surface elevation, remove the contribution of the crust to it according to Airy isostatic model and use the residual topography (Mooney et al.,2003;Gvirtzman et al., 2001) and geological and geophysical data to analyze lithosphere structure and regime for Sichuan basin in western China continent.
Refracted seismic profilings indicated that crustal thickness of the Sichuan basin (Upper Yangtze craton) and ORDOS basin (North China craton) is 42km and 40km and elevation above sea level <500m and <1200m, respectively. Given global crustal average density (2.67g/cm3) and thickness (34km), we carry out residual topography analysis for the Sichuan and ORDOS basins. The result shows that average residual topography in the Sichuan basin is less than -1000m and but only -200m in the ORDOS basin and is accordance with global cratons being generally depressed by 300 to 1500m in comparison with predictions from pure crustal isostasy (Mooney, et al., 2003). Further more, calculated an effective elastic thickness of the lithosphere in the Sichuan basin is more than 30km and a maximum value in whole Southern China continent and concords with thickened and fast velocity lithosphere root by seismic tomography (Lebedev,2003;Huang,et al., 2003) and relative variable range of lithosphere geoidal anomalies (<10m for the Sichuan basin and <1m for the ORDOS basin) and big negative satellite gravity, as well as positive high satellite magnetic anomalies. The findings mentioned above suggested that there is a thickened and weight lithospheric root and more cold (lower temperature) and less depleted chemical composition than suggested by studies of shallow mantle xenoliths beneath the Sichuan basin in comparison with tectonic units round it, thus it has held big negative buoyancy force for long period of time.
Since Mesozoic the lithospheric root was being in sinking regime with different degrees. As a result, the Upper Yangtze craton underneath the Sichuan basin had subducted into underneath orogenic belt and tectonic zones about it and suggest that lateral range of old lithosphere beneath the Sichuan basin is much larger than present Sichuan basin. In comparison with the Sichuan basin, lithospheric root of the ORDOS basin is shallower, with lower velocity and was modified by strong tectonic process during Mesozoic-Cenozoic. However, combination of lithospheric roots underneath the Sichuan and ORDOS basins had turned into a transitional tectonic unit (such as S-N tectonic belt) or a firm rock in midstream between the eastern and western parts with different structural characteristics within the China continent. Both cratons with thickened lithospheric root and more cold and fast velocity, in turn, less depleted chemical composition, play an important role during China continental formation and evolution and geodynamics.
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