Crustal And Upper-Mantle Velocity Structures Beneath Southeastern Margin Of The Tibetan Plateau Based On Body-Wave Tomography

As an important passageway of material migration,the southeastern margin of the Tibetan Plateau(SETP),which presents high level of seismicity,strong tectonic activity and active volcanic acitivty,has been the focus of numerous studies on geoscience.To reveal the geodynamic model of SETP and understand the seismicity,tectonic activity and volcanic acitivity in this reigion,this dissertation present the 3-D velocity models based on the body-wave tomographic inversion of P-and S-wave arrival times.The main contribution and conclusions of this dissertation are listed as follows:(1)This study is based on a continuous waveform data collected by the Earthquake Administration of Yunnan Province from 49 seismic stations in the 2008to 2016 observation period.We selected the arrival times of P waves and S waves according to the catalogue from the China Earthquake Data Center and obtained an initial data set of 42440 P picks and 27324 S picks from 4633 earthquakes that occurred in Yunnan Province and the surrounding areas.Finally,we obtained the 3-D P-wave and S-wave velocity models of SETP via tomographic inversion of arrival times.(2)We revealed a new geodynamic mechanism beneath the SETP according the P-wave velocity structure and other studies.Durring the migration from Tibetan Plateau to southeast,the mantle materials were stopped and the crustal materials continued to slide southward near 25�N.Meanwhile,the crustal materials were separated into two material migration passageways.The two boundaries of southeastern passageway corresponded to the Xiaojiang Fault and Red-river Fault,and the ones of southwestern passageway corresponded to Lancangjiang Fault and the border between China and Myanmar.More than 100 large earthquakes were distributed in the two passages for the migration of material.In addition,the volcanic activities of Tengchong volcanic field were impacted by the southwestern passageway.(3)On the whole,the distribution of large earthquakes is controlled by the two material migration passageways.Almost all the large earthquakes occurred in the two passageways.The earthquakes that occurred in the interior of the block,where there are no obvious faults,are caused by the collision of the interior materials in the passageways;and more earthquakes along the main strike-slip faults,such as Xiaojiang Fault and Red-River Fault,are caused by the interactions between material motion and the resistance of the rigid block.(4)The tectonic evolution of SETP is controlled by the two material migration passageways.Here,the right-lateral Red-River Fault and left-lateral Xiaojiang Fault are caused by the southeastern passageway,and Lancangjiang Fault is caused by the southwestern passageway.Because the high velocity of Simao Block is different from Sichuan-Yunnan Diamond-shaped Block,the Simao Block may belong to the Indochina Block.(5)The mantle magma beneath Tengchong volcanic field may ascend into the crust and reach to the surface through cracks caused by southwestern passageway.This is because the Tengchong volcanic field is located on the western boundary of the southwestern passageway,and there is a low-velocity anomaly corresponding to mantle magma at the top of the mantle.(6)The continuous waveform data used in this study was collected by the Earthquake Administration of Yunnan Province from ten seismic stations during the2012 to 2017 observational period.Nine of these stations belong to the Tengchong Earthquake Monitoring Network,and the other one belongs to Yunnan Province Earthquake Network.We manually selected the arrival time of P and S waves and obtained an initial data set of 6975 P picks and 6685 S picks from 1235 earthquakes that occurred in the Tengchong volcanic field and surrounding areas.The 3-D structures of Vp,Vs and Vp/Vs were obtained through local earthquake tomography.(7)We revealed three magma chambers with higher Vp/Vs ratio,higher Vp and lower Vs beneath Tengchong volcanic field according to the 3-D structures of Vp,Vs and Vp/Vs,the relative geothermal distribution and the studies of other volcanic fields in the world.Meanwhile,the magma chamber beneath Rehai spring,which corresponds to high He~3/He~4 ratio,may be fed by mantle magma.Two other chambers corresponding to lower He~3/He~4 ratio may be fed little or not fed by mantle magma.(8)Rehai spring with low Vp/Vs,low Vp and low Vs is suggested as a geothermal reservoir filled with boiling water,sufficient water steam and other gas.The cluster of earthquakes beneath Rehai spring is caused by magmatic activity.Dayingshan volcano connects to two magma chambers,and may erupt in the future.Maanshan volcano is separated from the magma chamber and may be a dead volcano.Heikongshan volcano connects to one magma chamber with little input of mantle magma,and may have little chance for eruption in the future.In summary,this dissertation comprehensively described the geodynamic feature of the SETP and revealed the geodynamic mechanism of high-level seismicity,the formation mechanism of strong tectonic activity(especially the large faults)and active volcanic acitivty in this reigon according to the 3-D velocity structures.In addition,it also presents the distribution of magma chambers beneath Tengchong volcanic field and the condition of Rehai spring,Dayingshan volcano,Maanshan volcano and Heikongshan volcano according to the 3-D structures of Vp,Vs and Vp/Vs and geochemical studies in this region.