| The Qinghai-Tibet Plateau is not only the highest plateau in the world,but also the continent-continent collision orogenic belt with the largest deformation scale and the latest formation age.Up to now,the Qinghai-Tibet Plateau is still in the process of continuous orogeny,which causes frequent earthquakes in its interior and surrounding areas.According to the statistics of China Earthquake Networks Center(CENC),more than 200 earthquakes with magnitude 3 or above occur annually in this region,indicating that the Qinghai-Tibet Plateau is still in a strong tectonic activity and is therefore considered to be an natural experimental area for continental dynamics research.The Gangdese metallogenic belt,located in the front of the collision of the Plateau,is an important area for studying the subduction pattern of the Indian lithosphere and is conducive to understanding the mechanism and dynamics of the uplift of the plateau.The study of seismicity inside the plateau is helpful to understand the relationship between different blocks in the Tibetan Plateau and to reveal the tectonic evolution of the Qinghai-Tibet Plateau.In this study,the data from 128 portable stations set up by the Chinese academy of geological sciences in the Qiangtang platform and the Gangdese metallogenic belt are used,combined with the data of permanent seismic stations are used to carry out the following research:First,we relocated the earthquakes in and around the plateau.Our results show that the maximum horizontal position deviation between relocation and catalogue reaches 200 km and the relocated earthquakes are more clustered.The results of relocation in the middle part of Qiangtang terrane show that: 1)compared with the results obtained by CENC,the earthquakes are more concentrated and the focal depths are concentrated in the range of 0-8 km,indicating that the tectonic activity of the upper crust in the Qinghai-Tibet Plateau is active;2)8 earthquakes with magnitude 3.0 and above occurred in June 2017,and the relocated epicenters were concentrated in the vicinity of Gradandong,which was quite different from the results reported by the CENC.The results of small earthquakes with M < 3.0 before and after relocation also show obvious differences.In addition,the results of M ? 3.0 earthquakes relocation in other areas of the plateau show that: The epicenters were concentrated in the Qiangtang volcanic rock area,the eastern margin of Jinshajiang suture zone,the north-south trending fault zone of the plateau and the southeastern margin of the plateau.Based on all the relocation results,we believe that the EW-trending extension in the whole Qinghai-Tibet Plateau is the main dynamic source of frequent earthquakes in this region.Second,we analyzed the focal mechanism of earthquakes with magnitude 3.0 and above occurring in Qiangtang terrane from June 2016 to June 2017.The inversion results of focal mechanism coincide well with the surface tectonic properties,which verifies that the plateau is still strongly compressed by the Indian Plate.Among these earthquakes,8 earthquakes occurred near Gradandong in June 2017 are of normal faults or strike-slip and normal faults with some similarities,and it is presumed that these earthquakes are surface responses of intense crust-mantle activity.Third,we used the Pn wave differential imaging method to obtain the P-wave velocity structure at the top of the upper mantle in the eastern segment of the Gangdise metallogenic belt.The following results are obtained: 1)The P-wave velocity of the upper mantle ranges from 7.7 km/s to 8.5 km/s,and the maximum amplitude of velocity variation is 4.9%.The whole study area presents bands of high velocity alternated with low velocity anomalies in the East-West direction.2)The eastern side of the study area shows the East-West differentiation with 92.5 E as the boundary,and the two sides of the study area are Middle Gangdise and Eatern Gangdise.It has been considered that this is caused by the different sources of mantle materials.3)The Yadong-Gulu rift,Xietongmen-Shenzha fault zone and Linzhou volcanic basin show low-velocity anomalies.The Yadong-Gulu fault zone extends southward across the Yarlung Zangbo suture zone.4)Mineralization spots mainly distribute on the boundary of low velocity anomaly area,while the electrical structure of the crust shows that the high conductors in the middle crust correspond well with the ore-controlling structures.It is deduced that the metallogenic background of this area is consistent.Due to the limitation of imaging area scale,the existence of plate tear in this area still needs further verification... |
PDF Full Text Request