The Shear Wave Velocity And Radial Anisotropy Structure Beneath The Eastern Qinlin-Dabie-Sulu Orogenic Belt

The eastern Qinling-Dabie-Sulu orogenic belt,located in the central China,is a tectonic combination zone of the North China Block and the Yangtze Block.As one of the largest utrahigh pressure?UHP?metamorphic rock zone in the world,the eastern Qinling-Dabie-Sulu orogenic belt is an important geological unit to study the exhumation mechanism of UHP metamorphic rocks and other dynamic problems.The eastern Qinling-Dabie-Sulu orogenic belt is also an ideal region for studying the formation and evolution of collisional orogenic belts.Geologists have done a lot of work in the eastern Qinling-Dabie-Sulu orogenic belt,which provides important data for understanding the evolution process of the orogenic belt.At the same time,many ambiguities still exist in the seismic structures of the eastern Qinling-Dabie-Sulu orogenic belt,especially about the shear wave velocities and anisotropy structures of the orogenic belt.In order to construct a better seismic model of the eastern Qinling-Dabie-Sulu orogenic belt,I use three component seismic waveform data obtained from the National Seismic Network and the Regional Seismic Network,which consist of 352 seismic stations located in the orogenic belt and the surrounding area.The data was recorded from 1^st January 2009 to 30^th June 2014.Using finite-frequency ambient noise tomography?FFANT?and teleseismic two-plane-wave tomography technique?TPWT?,I obtain phase velocity maps of Rayleigh wave and Love wave at 8 to 83 s periods.In order to obtain seismological evidence for the dynamic processes such as formation and evolution of the orogenic belts and the formation and exhumation of UHP metamorphic rocks,I construct 3-D lithospheric shear wave velocity structures and crustal radial anisotropy structures by inverting dispersion curves of Rayleigh and Love waves.The main work and results of this thesis are as follows:?1?Most traditional ambient noise tomography is based on ray theory.In this paper,I develop a finite frequency ambient noise tomography method,which considers the finite frequency effects of surface waves in tomography.FFANT can obtain higher resolution surface wave phase velocity map in broadband periods?10¹50s?based on ambient noise data.As a case study for the western United States,by comparing the phase velocity maps obtained by FFANT to those obtained by ray-theory-based ambient noise data?20s and 30s?and teleseismic data?60s and 70s?,I prove the reliability of the FFANT method to obtain higher resolution phase velocity maps at boardband periods.In addition,I improve the TPWT method to make it capable to image Love wave phase velocity maps.Taking the western United States as an example,I demonstrate the reliability of Love wave phase velocity maps obtained using my improved TPWT method.?2?I construct 3D models of the crust and uppermost mantle shear wave velocity structure and the crust radial anisotropy structure beneath the eastern Qinling-Dabie-Sulu orogenic belt.I use the FFANT method and TPWT method obtained the Rayleigh wave and Love wave phase velocity maps at 8 s to 83 s periods in this study region.By inverting local dispersion curves of Rayleigh and Love waves extracted from phase velocity maps,I construct the shear wave velocity model of the crust and upper mantle and the radial anisotropy model of the crust.?3?The shear wave velocity and radial anisotropy models show the zonal features corresponding to the orogenic belt and its surrounding regions.In the upper crust?>10km?,the shear wave velocity structures are closely related with the surface geological features,the Weihe Basin,the North China Basin and the Jianghan Basin are characterized by low shear velocities and strong positive radial anisotropy.The UHP metamorphic rock zone in the Dabie-Sulu orogenic belt are characterized by high shear wave velocities and weak radial anisotropy.Combined with the geological and geophysical data available in the study region,I infer that the thickness of the UHP metamorphic rocks should not exceed than 10 km.In the middle crust?15²5 km?,the shear wave velocity of Macheng-Luotian-Yuexi are higher than those of the surrounding area,which is related to the lithospheric delamination and the asthenosphere upwelling.In the lower crust and uppermost mantle?25⁴5 km?,the area to the west of the Taihang-Wuling mountain gravity gradient zone shows low shear wave velocity anomalies and weak radial anisotropy,while the area to the east of this gradient zone shows high shear wave velocity anomalies and strong positive radial anisotropy.The east-west transition of shear wave velocity and radial anisotropy may be related to the rapid westward subduction of the Pacific plate and the westward migration of the subduction zone since the late Jurassic.In the lithospheric mantle?50⁷0km?,the shear wave velocities in the lithosphere are not uniform in the study region.The shear wave velocity at 70 km depth is still higher in western Hubei,implying that the lithospheric thickness in this area is more than 70 km.There are obvious higher shear wave velocity anomalies in the southern margin of the Dabie orogenic belt,which is related to the subduction of the Yangtze plate to the North China plate and the exhumation of the ultrahigh pressure metamorphic rocks.?4?The shear wave velocity and radial anisotropy structure across the Qinling orogenic belt show that the upper crust has low shear wave velocities and negative radial anisotropy,I think the upper crust is a brittle,viscoelastic and ductile tectonic deformation zone.Low shear wave velocities and strong negative radial anisotropy are observed in the middle crust.It is believed that this region is a slipping-nappe belt introduced by the thrust nappe from the Qinling Block to the North China Block and the Yangtze Block.The middle and lower crust exhibits positive radial anisotropy,which may be related to the upwelling of asthenosphere.The deformation zone with horizontal rheological characteristics is formed because the middle and lower crust is heated and melted.In the lithospheric mantle,the lithospheric thickness of the North and South Qinling Mountains is not uniform,which may be related to the lithospheric subsidence.The lithospheric thickness of the Yangtze Block and the Qinling orogenic belt is obviously different.It is inferred that the Xiangfang-Guangji fault may be the collision suture zone between the Yangtze Block and the Qinling orogenic belt.?5?The exhumation mechanism of Dabie-Sulu UHP metamorphic rocks.The Eastern Qinling-Dabie-Sulu orogenic belt bears the trace of subduction of the Yangtze Block underneath the orogenic belt.In terms of tectonic deformation,the Qinling orogenic belt retains information of crustal deformation during the collision orogenic period.A series of crust-mantle interactions occurred in the Dabie-Sulu area during the post-orogenic period.The S-wave velocity profile across the Dabie Sulu orogenic belt shows that there is a northward high velocity slab in the lithospheric mantle of the orogenic belt,which provides important constraints for distinguishing different exhumation dynamics models.Combined with previous studies,I infer that the UHP metamorphic rocks experienced many reentry periods:?1?during the middle and late Triassic,the Yangtze plate subducted to the North China Block,which caused the break-off of the Yangtz lithosphere and further caused the rising of the UHP metamorphic rocks from the mantle lithosphere to the middle and lower crust.In the late Triassic and early Middle Jurassic,the two major blocks continued to converge,and The UHP metamorphic rocks can be traced to the middle and upper crust along the subduction tunnel under buoyancy.?3?In the Cretaceous,the lithosphere of the Dabie orogenic belt was torn apart,resulting in a massive magma upwelling and forming a fornix structure in the orogenic belt.Thus,the UHP metamorphic rocks are further exhumed to the surface of the earth.