Shear wave velocity structure of the crust and upper mantle in Southeastern Tibet and its geodynamic implications

Southeastern Tibet,which has complex topography and strong tectonic activity,is an important area for studying the subsurface deformation of the Tibetan Plateau.Through the two-station method on 10-year teleseismic Rayleigh wave data from 132 permanent stations in the southeastern Tibetan Plateau,which incorporates ambient noise data,we obtain the interstation phase velocity dispersion data in the period range of 5–150s.Then,we invert for the shear wave velocity of the crust and upper mantle through the direct 3-D inversion method.We find two low-velocity belts in the mid-lower crust.One belt is mainly in the SongPan-GangZi block and northwestern part of the Chuan-Dian diamond block,whereas the other belt is mainly in the Xiaojiang fault zone and its eastern part,the Yunnan-Guizhou Plateau.The low-velocity belt in the Xiaojiang fault zone is likely caused by plastic deformation or partial melting of felsic rocks due to crustal thickening.Moreover,the significant positive radial anisotropy(VSH>VSV)around the Xiaojiang fault zone further enhances the amplitude of low velocity anomaly in our VSVmodel.This crustal low-velocity zone also extends southward across the Red River fault and farther to northern Vietnam,which may be closely related to heat sources in the upper mantle.The two low-velocity belts are separated by a high-velocity zone near the Anninghe-Zemuhe fault system,which is exactly in the inner and intermediate zones of the Emeishan large igneous province(ELIP).We find an obvious high-velocity body situated in the crust of the inner zone of the ELIP,which may represent maficultramafic material that remained in the crust when the ELIP formed.In the upper mantle,there is a large-scale low-velocity anomaly in the Indochina and South China blocks south of the Red River fault.The low-velocity anomaly gradually extends northward along the Xiaojiang fault zone into the Yangtze Craton as depth increases.Through our velocity model,we think that southeastern Tibet is undergoing three different tectonic modes at the same time:(1)the upper crust is rigid,and as a result,the tectonic mode is mainly rigid block extrusion controlled by large strike-slip faults;(2)the viscoplastic materials in the middlelower crust,separated by rigid materials related to the ELIP,migrate plastically southward under the control of the regional stress field and fault systems;and(3)the upper mantle south of the Red River fault is mainly controlled by large-scale asthenospheric upwelling and may be closely related to lithospheric delamination and the eastward subduction and retreat of the Indian plate beneath Burma.

Inversion of Q value structure beneath the Tibet Plateau

A total of 11 earthquakes with 15 Rayleigh wave paths, recorded at 11 broadband digital PASSCAL seismometers installed in the Tibet Plateau by the Sino-U. S. joint research group, were used to determine the phase velocity and attenuation coefficient of surface waves in periods of 10-130 s. The average shear wave velocity andquality factor Qβ structures in the crust and upper mantle were obtained in this region. The result shows the average oP is low and there exists a high attenuation (Qβ= 93- 141 ) layer in the crust. The depth range of the lowoP value layer (16-42 km) is consistent with the range of low velocity layer (21-51 km) in the crust. Below63 km in the lower crust, oP decreases with depth from 114 to 34 at depth of 180 km. The low shear wave velocity and low value of Qβ at the same depth range in the crust indicate that the rocks in the range is probablymelted or Partially melted. According to the shear wave velocity structure, the average thickness of the crust is about 71 km and a clear velocity discontiniuty appears at the depth of 51 km. The low-velocity zone (4. 26 km/s) at depth of 96-180 km may be corresponding to the asthenosphere.

Ambient Noise Tomography of Jinan:The Migration of Groundwater and the Formation of Geothermal Water

Jinan is an important city in eastern China,with rich groundwater in the region.There are four famous springs in the urban area and an abundance of geothermal water in the northern part,which makes the migration of groundwater in this area a very important issue.To study the shallow shear wave velocity structure and groundwater migration in Jinan,we utilized almost a month of continuous waveform data from 175 short period seismometers deployed by the Chinese Academy of Geological Sciences,in order to calculate the cross-correlation function.We picked 7749 group dispersion curves and 6117 phase dispersion curves with a period range of 0.2–2 s.Through inversion,we obtained the fine threedimensional shear wave velocity and azimuthal anisotropy structure(0–2.4 km).Combining the results with local geological and hydrological data,the following conclusions were reached.(1)There are widespread high velocity anomalies in the region between the Qianfoshan and Wenhuaqiao faults,as well as to the east of the Wenhuaqiao Fault,which may be related to the intrusive gabbro known as the Jinan Intrusive Rock.(2)The two distinct high velocity anomalies in our model(referred to as west and east Jinan Intrusive Rock in this paper)may indicate that the Jinan Intrusive Rock was broken through crustal movement.(3)There is an obvious low velocity layer under the intrusive rock,which could be the channel of groundwater migration.The precipitation in the southern mountain region seeps down into the ground,then is blocked by the Jinan Intrusive Rock and can only progress downwards to a deeper part,where the groundwater is heated by the geothermal gradient.The heated water finally arrives at the northern part and forms geothermal water.(4)The depth of the low velocity layer beneath the Jinan Intrusive Rock varies laterally,which may indicate that the depth of the groundwater migration is different beneath the west and east Jinan Intrusive Rock.(5)There is strong azimuthal anisotropy in southern Jinan,with nearly E-W fast orientation,which may be related to the tilt limestone layering structure.