Application Of Receiver Function Method For Crust And Upper Mantle Structure Beneath Sichuan And Yunnan Region

In this work we select teleseismic waveform data recorded by stations from threesources:(1) portable stations deployed by the Sino-French cooperation experimentand independent observations in Sichuan region in the past8years;(2) portablestations deployed by the Massachusetts Institute of Technology in2003to2004inSichuan and Yunnan region;(3) permanent stations of Sichuan and Yunnan seismicnetwork. We use time domain iterative deconvolution technique to extract receiverfunctions and further to obtain the crust and upper mantle S-wave velocity structuredown to100km. We also calculate crustal thickness and crustal average Poisson’s ratiobeneath the stations of the Sichuan-Yunnan region by linear inversion method andscanning receiver function method (H-K method).Our results show:(1) S-wave velocity structure beneath each station in the easternmargin of Tibetan Plateau presents a most notable feature of the speed being very low;S-wave velocity values of the middle crust is basically about3.0~3.4km/s; while theupper mantle S-wave velocity values is about4.0~4.5km/s; low-velocity layer iswidespread in the crust, most of the low-velocity layer is located at the depth between20~40km of the middle crust. In Xiaojin and Heishui area the thickness can be up to30km; there are also a small amount of low-velocity layer in the upper crust at about10~20km deep. In the lower crust at about40~60km deep, crustal thickness varies from50~66km in this area and reaches a maximum in the area from Shimian to Xiaojin inthe middle of the east margin of the Tibetan Plateau. Northward and southward crust arethinning gradually. This may illustrate that the interior materials of Tibetan Plateaumainly move through the central part of the eastern margin to Longmenshan regionwhere it encountered rigid Sichuan basin, and on the one hand some substance in thecrust move along the barrier resulting in the rise of Longmenshan and the formation ofthe Longmenshan nappe structure.At the same time, it accumulates a lot of stress at thebottom of the Longmenshan; Meanwhile, the horizontal movement of substance in thecrust changes direction due to the blocking by Sichuan basin and moves laterallynorthward and southward along the boundary between eastern margin of the TibetanPlateau and the Sichuan Basin.(2) The results of the Sichuan basin present a sharpcontrast to the eastern margin of the Tibetan Plateau, due to the presence of near surface sediments in Sichuan basin, S-wave velocity is about2.4~2.8km/s. At the rest of thedepth the S-wave velocity is relatively high. S-wave velocity in the lower crust canreach up to4.0km/s; at the depth of100km, S-wave velocity in the lithosphere mantleof most parts of the region is4.6~4.8km/s, showing a typical cratonic rigid block. Thecrustal thickness of this region is40~44km and there is no low velocity layer in thecrust.(3) There is a big lateral change in S-wave velocity and crustal thickness inYunnan region from west to east, from north to south. S wave velocity near surface isgenerally characterized by low speed, only existing a high speed region of3.4~3.6km/sin the central part of Yunnan. Western and Eastern region of Yunnan is lower than3.2km/s, northeast and southeast of Yunnan can be as low as2.6km/s. At the depth of20km of the middle crust, the velocity of most part is relatively low compared toSichuan basin and the velocity varies very largely, about3.0~3.5km/s. At30km depth,western and eastern Yunnan have entered into the lower crust which is a high S-wavevelocity region, speed value is generally greater than3.6km/s, and it is up to4.0km/s insouthwestern Yunnan. Crustal thickness in northwestern Yunnan is about50~60km,thinning eastward and southward to less than40km in Wanding and Jinghong. For themost part in Yunnan region, lithospheric thickness is less than80km. These could bedue to western Yunnan move as clockwise rotation along the East Himalayan syntaxisunder the effects of the extrusion produced by plate motion and the region interiorbrittleness material move laterally leading to the earth’s crust thinning.The crustal flow pattern, spatial distribution and influence on the earth’s crust andupper mantle has undoubtedly become one of the mainstream topics of the deepgeophysical and geodynamics research. Based on the receiver function inversionresults of Sichuan and Yunnan region in this article, combined with the existing GPSmeasurements in the study area, we believe crustal flow in the eastern margin of theTibetan Plateau and the Yunnan region is not universal and uniform in spatial, which isdistributed only in limited regions. It is mainly originated in the central Qiangtangplateau: one branch flows southeastward along the Xianshuihe fault and is blocked bythe Sichuan basin, leading to the split and decoupling; one branch flows alongBangong-Nujiang suture zone, through Lijiang, Mangshi, Wanding etc. in Yunanregion; one branch flows along the Anninghe-Xiaojiang fault, through Malong, Jinpingetc. The last two are mainly controlled by the East Himalayan syntaxis, showing acontinuous clockwise rotation.