Lithospheric Density Structure Beneath The Tien Shan And Surroundings And Its Geological Implications

Individual terranes separated by Paleozoic continental suture zones exhibit disparate rates and styles of deformation.In particular,the Tarim Block and Junngar Block are apparently remaining stable and essentially undeformed amidst widespread tectonism.In contrast,the deformation of Tianshan continued until the Mesozoic time.The tectonic and seismic activity resumed at about 20–25 Ma,which has been attributed to be a far-field consequence due to the collision of the Indian and Eurasian plates since about55 Ma ago.The geodetic measurements revealed that the Tianshan is currently undergoing significant crustal shortening at a rate of ?20 mm/yr in the north-south direction.However,the north-south shortening rate of the whole Tianshan gradually decreases from west to east,and the Eastern Tianshan region is relatively stable.This situation is still not understood completely.All density perturbations occurring in the lithosphere are the result of geodynamic processes,such as mountain building and plume upwelling.Therefore,understanding the variation of lithospheric density is very important to understand the tectonic deformation of the Tianshan orogenic belt.It is helpful to better understand the deformation and evolution process of the Tianshan orogenic belt by probing the properties of the earth's continental circle and understanding the composition of the continental crust.To this end,we combine the S-velocity model and gravity data to develop a 3D density model of the crust and upper mantle beneath the Tien Shan and its surroundings.In this paper,the lithospheric density model of Tianshan area is constructed by using the latest high-resolution shear wave velocity model and crustal thickness model,combined with petrophysical data,and using the shear wave velocity-density conversion relationship.The gravity effect of the sediment is calculated by using the established crustal model,and then the lithospheric gravity anomaly is obtained.The density structure of the lithosphere in the Tianshan region is retrieved by using the three-dimensional density inversion algorithm in the spherical coordinate system,and the density model based on seismic tomography model is selected as the reference model to constraint the inversion results.The inversion results show that the West Tien Shan have the characteristics of high density mantle.Due to the subduction of the Asian lithosphere,the upper mantle beneath the West Tien Shan is dense and high-speed.Due to the subduction of the Asian lithosphere,the West Tien Shan lithosphere is relatively weak.A large scale negative density anomaly has been observed in the Central Tien Shan.This paper suggests that the negative density anomaly is related to the upwelling of mantle hot material.The most significant negative density anomalies,which complement the initial model,are observed in the Eastern Tien Shan and southwestern Tarim.The conversion factor indicates that these anomalies are associated mainly with the mantle material depleted in Fe components.The early Permian mantle plume impinged upon the base of the lithosphere beneath southwestern Tarim,melting already refractory mantle lithosphere in the southern and western parts of the block.Such depleted mantle would then have lower density but higher viscosity than the rest of Tarim and thus be more resistant to transmitting stress from the ongoing Indo-Eurasian collision.However,it is still controversial whether the melting of the East Tianshan lithosphere is caused by mantle plume or subduction slab break off.Previous studies show that the strength of mafic crust is higher than that of felsic crust.In this paper,the compressional wave velocity,shear wave velocity,magnetism and density characteristics of the crust of each tectonic unit are analyzed,and compared with the crustal structures of different tectonic units in the world.The Tarim and Junggar basins have the characteristics of oceanic plateau crust with relatively thick lower crust with high density,high wave velocity and high Vp/Vs ratio.The Central Tien Shan has a thick felsic lower crust and a typical orogenic crustal structure,while the Eastern Tien Shan crustal structure is very different from that of the typical orogenic belt.Combined with petrophysical data,the possible rock composition of the lower crust of each tectonic unit is further analyzed.The lower crustal composition of Tarim and Junggar basins is equivalent to gabbro,the lower crustal composition of Central Tien Shan is equivalent to diorite,and the lower crustal composition of Eastern Tien Shan is equivalent to amphibolite and quartz assemblage.In this paper,we suggest that the Junggar-Turpan-Hami block lithosphere may have originated from the oceanic plateau.The Junggar Basin has not undergone the subduction of the late period,so the original lithosphere has been preserved,while the Eastern Tien Shan has undergone the subduction of the Paleo-Asian Ocean,and the lower crust of the Eastern Tien Shan has been transformed.Based on the above research results,the lithospheric tectonic models of the Tien Shan has been established.This paper suggests that the India-Eurasia collision and the subduction of the Eurasian lithosphere may be the important reasons for the formation and evolution of the West Tien Shan.The deformation of the Central Tien Shan may be related to lithospheric detachment and upwelling of hot mantle materials.Because of its strong mafic crust and depleted mantle lithosphere,the Eastern Tien Shan remains relatively stable during the tectonic movement.