Continental Crustal Petrological Structure: Several Case Studies

The Central Asian orogenic belt serves as a key tectonic unit for continental dynamics research and is used to explore the history of continental crust proliferation and tectonic evolution.The North China Craton is a key area for studying cratonization of the earth.In order to better understand the composition of the crust of the Chinese mainland,especially the rock composition of the middle and lower crusts,the Daxingan Mountains(narrow),Jihei fold belt,Xingmeng orogenic belt,Hailar Basin,Yinshan fold belt,Yanshan belt and Alxa block to understand the petrological composition of the continental crust.According to the crustal P-wave velocity structure of each tectonic unit in the study area,according to the wave velocity(Vp)-density(?)conversion method,the density of the middle and lower crust and the corrected P-wave at 600 MPa and 20 ? Wave velocity structure,the wave velocity is compared with the laboratory rock wave velocity,and the composition of the middle and lower crust rocks in each structural unit is obtained.At the same time,according to the correction elevation and heat flow values of each structural unit,project a "standard" thermal equilibrium curve,and then judge its thermal equilibrium state and briefly analyze the causes of thermal isostasy.The thermal equilibrium analysis results show that the Greater Xing'an Mountains(narrow),Jihei fold belt and Yinshan fold belt are in thermal isostatic state,while the Xingmeng orogenic belt,Hailar Basin and Yanshan belt are not in thermal isostatic state.There are three reasons for the thermal not isostasy: the difference between the actual and standard geothermal lines;the difference in the density of the lithospheric mantle;the dynamic factors caused by "drag" and "buoyancy".The comparison results of P-wave velocity and density show that the middle crustal composition of each tectonic unit in the eastern Central Asian orogenic belt and the northern North China Craton is a neutral-basic transition stage,while the lower crustal material composition is basic.At the same time,using the method proposed by Behn and Kelemen(2003)to analyze the relationship between the crustal composition and the seismic P-wave wave velocity,the lower crustal wave velocity value is about 6.8 km/s,and the comparison with the laboratory rock wave velocity also shows that the lower crustal lithology basic,which is different from the conclusion made by Gao Shan that the lower crust of mainland China is neutral.