Research On Crustal Deformation And Dynamic Background Of Indochina Block

The Indochina block is located on the southeastern margin of the Tibet Plateau and the eastern side of the collision zone between the Indian plate and the Eurasian plate.With the process of uplift and thickening of the Tibetan Plateau,the geological structure of the region is complicated,with high frequency of earthquakes and shallow focal depth.It is one of the regions with strongest crustal deformation and seismic activity in the mainland of China.This paper selects 126 mobile stations in the Indochina block(the area to the west of the Jinshajiang-Honghe Fault),longitudes ranging from 97°E to 105°E,latitudes ranging from 21°N to 30°N,data recorded from May 2011 to December 2013.The three-components teleseismic data is used to extract P receiver functions.And then investigating the anisotropy parameters beneath stations by both the arrival time variance minimizing of Moho Pms of grid searching method and azimuthal stacking method.Finally we obtained 118 splitting parameters of Pms phase beneath stations to analysis the anisotropy in the Indochina block by comparing with GPS velocity field and previous research findings.The following conclusions are mainly obtained:1.The splitting time of the Pms phase in the Indochina block region is in the range of 0.08-1.44 s and the average split time is 0.47 s,which is much larger than the anisotropic splitting time of the Pms phase in the upper crust 0.15 s.Therefore,we speculate that the anisotropy of crust in the study area comes mainly from the lower crust.2.In the whole Indochina block,we did not observe that the lower crust flow of the southeastern Tibetan plateau crossed the Jinshajiang-Honghe fault and it does not show any obvious clockwise rotation around the eastern Himalayas.Therefore,we speculate that the lower crust flow was cut off by the Jinshajiang-Honghe fault during the flow from the eastern Tibetan Plateau into the southwestern Yunnan region.3.In the western part of the Indochina block,there are a number of stations with the fast direction NW or NE-SW.This paper speculates this phenomenon is caused by the subduction of the Indian plate beneath Burma in the region and the lower crust of southwestern Yunnan was pressed northeast..4.In the Indochina block area,the Pms phase fast-wave positive direction does not agree with the direction of the surface GPS velocity field.In view of the complex tectonic stress field affected by the Indochina block,we speculate that the upper crust is coupled from lower crusts of the Indochina block.5.Comparing with XKS splitting,we speculate that the lower crust is decoupled from the upper mantle in Indochina block.6.The Pms phase splitting fast-wave polarization direction distribution at active faults and stations near the fault is almost parallel to the strike of the fault,indicating that the anisotropy of the crust is greatly affected by the fault,and the deformation of the crust is controlled by the fault activity.We also speculate that the reason for the formation of anisotropy in(or near)the Jinshajiang-Honghe fault,the lancangjiang fault and the Jiali-Nujiang fault is different from inside the Indochina block.