Crustal And Upper Mantle Structure Of The Tien Shan Orogenic Belt

The Tien Shan,as a significant part of the Central Asian orogenic belt,is one of the most active and youngest intra-continental orogenic belts in the world.The Tien Shan orogenic belt is bounded by the Kazakh Shield and Junggar Basin to the north,and the Tarim Basin to the south.Those tectonic blocks not only affect the evolution of different geological tectonic units,but also control the activities of the strong earthquakes in the Tien Shan area,forming a special tectonic environment.The tectonic and seismic activity resumed at about 20-25 Ma,which has been attributed to be a far-field consequence to the collision of the Indian and Eurasian plates since about 55 Ma ago,generating a large number of E-W trending thrust faults and intensive earthquakes.Therefore,it is significant to investigate the crustal and mantle structure for understanding the dynamic process of the Tien Shan,and is also an important scientific issue of the current continental dynamics.In this study,based on the ambient noise data,we have carried out the following studies to reveal the deep structure of the Tien Shan orogenic belt.(1)High-quality empirical Green's functions(EGFs)are derived from the cross-correlation waveforms of the vertical components of continuous waveform data of 108 stations in the entire Tien Shan region from January 2012 to December 2014.The stations used in this study spread across the eastern,central and western Tien Shan,which greatly improves the ray-path coverage in the entire Tien Shan.In the pre-process of the ambient noise data,we use two different normalization methods to improve the signal-to-noise ratio of empirical Green's function.Firstly,the Rayleigh wave phase velocity dispersion data with a period of 8-50 s are obtained by the temporal normalization method,which are used to study the azimuthal anisotropy variation of the Tien Shan.Furthermore,the empirical Green's functions of Rayleigh wave with periods of 7-200 s are obtained by frequency-time-normalization method,which are used for full-wave ambient noise tomography beneath the Tien Shan region.Compared to traditional normalization method,the frequency-time-normalization method can significantly improve the signal-to-noise ratio of the EGFs and extract longer-period Rayleigh waves.(2)The two-dimensional Rayleigh wave phase velocity and azimuthal anisotropy distributions at different periods are constructed using the continuous regionalization and the generalized inversion scheme method.Our new tomographic model reveals prominent lateral heterogeneities beneath the entire Tien Shan orogenic belt.The shallow structure in the study area shows a good spatial correlation with surface tectonics.Low velocity anomalies and high velocity anomalies are imaged beneath the basins and surrounding mountain ranges,respectively.The eastern Tien Shan exhibits distinct low velocity anomalies from the middle crust to the lower crust,and high velocity anomalies with the N-S fast wave directions are observed on the margin between the Tarim Basin and Junggar Basin,which may indicate that the lithosphere of the Tarim Basin and Junggar Basin has subducted down to the upper mantle beneath the eastern Tien Shan.Prominent low velocity anomalies beneath the central Tien Shan extend from the middle crust and the upper mantle,and the directions of fast wave are complicated,thus we can infer that the lithospheric structure of the central Tien Shan is weak.The low velocity anomalies are observed beneath the western Tien Shan and the directions of fast wave are NW-SE,which may represent the continental crust of the subducting Eurasia lithosphere that has been pulled down into the mantle beneath the western Tien Shan.The lower velocity anomalies beneath the Tarim Basin suggest that the Tarim basin is not a stable cratonic lithosphere but has been eroded and destructed by the upwelling hot low velocity materials.(3)We construct a well-defined shear wave velocity model of the crust and upper mantle with full-wave ambient noise tomographic method.High-quality empirical Green's functions at periods of 7-200 seconds are extracted from the cross-correlation of the vertical component of continuous seismic data at 108 stations during 2012-2014.Firstly,we apply finite-difference method to simulate the Rayleigh surface wavepropagation in three-dimensional media,and the phase delays between synthetics and EGFs are measured with cross-correlation method.Then,the Rayleigh wave sensitivity kernels to Vp and Vsv are calculated with the scattering-integral method.Finally,the damped least squares method is used to jointly invert P-and S-wave velocity.Our tomographic results show remarkable velocity variations between and within the major tectonic units from the crust down to the upper mantle.We observe very slow upper crust beneath the Tarim and Junggar sedimentary Basins.The interior of the Tarim Basin demonstrates strong seismic heterogeneities.The high-velocity mantle lithosphere of the Tarim Basin underthrusts northward toward the central Tien Shan.Lithosphere underthrusting could trigger intrusion of hot mantle material and partial melting,in correspondence with the prominent low-velocity anomalies observed in the lower crust and uppermost mantle of the central Tien Shan.In contrast,the high-velocity upper mantle of the eastern Tien Shan may reflect the convergent effect between the Tarim and Junggar Basins,which consequently prevents the asthenosphere upwelling.We observe an arc-shaped low-velocity anomaly in the upper mantle of the western Tien Shan,which might indicate subduction of the continental crust of the Eurasia lithosphere.The observed structural variations along the Tien Shan orogenic belt suggest different tectonic mechanisms for the lithosphere formation and modification of the three segments along strike.