Study the Structure of Crust and Upper Mantle in Eastern Margin of Qinghai-tibet Plateau and Western Margin of Yangtze Platform by a Seismological Passive Source Experiment
In this study we use data from broadband digital seismograph stations of Earthquake Administration of Sichuan Province and mobile seismic broadband stations co-operated by the Chengdu University of Technology,the French Institute of Geophysics and Ecole Normale Super Paris to study the junction zone between eastern margin of Qinghai-Tibet Plateau and westtern margin of Yangtze platform by the method of Receiver Function.Located at the junction zone between eastern margin of Qinghai-Tibet Plateau and westtern margin of Yangtze platform,the Sichuan region bears with very complex geological and tectonic settings.Previous researches have revealed that the northward moving of the Indian plate is blocked by the Eurasian plate.The collision leads to a rapid uplift of the Qinghai-Tibet plateau,with eastward and southeastward horizontal extrusion.The eastward extrusion is blocked by the stable Sichuan basin.As a result,the deformation is transformed into thrust movement in the middle part of the Longmenshan fault zone.Accordingly,a series of NE-striking or approximate NS-striking and NW-striking mountains appear at the boundary of Qinghai-Tibet plateau and Sichuan basin.In these areas,lower crust thickens and low-velocity layer can be found.On the contrary,the Sichuan basin has relatively strong crust and upper mantle,but no low-velocity layer in crust.The results showed that:(1)The eastern margin of Qinghai-Tibet plateau and westtern margin of Yangtze platform in study area presents obvious difference in crust and upper mantle velocity structure,the velocity of crust beneath Qinghai-Tibet plateau is significantly lower than that of Sichuan basin.S velocity of upper crust beneath Sichuan basin can reach 3.0-3.5 km/s,the value of middle crust is 3.6-3.8 km/s the value of lower crust is 4.0-4.3 km/s and the value of upper mantle is 4.5-4.8 km/s.And there is no low velocity layer in the crust,indicating strong block on lithology and a typical Craton structure.The most notable feature of the eastern margin of the Qinghai-Tibetan Plateau is that the velocity structure is significantly lower,and the presence of low-velocity layer is not continuously distributed.The velocity of the eastern margin of the Qinghai-Tibetan Plateau is about 2.8 km/s or so near surface,while the S velocity of upper crust is about 2.8-3.4 km/s,the middle crustal Vs velocity is about 3.0-3.4 km/s,the lower crustal Vs velocity is about 3.4-4.1 km/s and the upper mantle Vs velocity is about 4.1-4.5 km/s.The S-wave velocity structure shows that a low velocity layer is widely observed below the eastern edge of the Qinghai-Tibetan Plateau at the depth of 20-40 km.In addition,low velocity layer is also found at the upper and lower crust from place to place.(2)The crust thickness of Sichuan basin is about 38-45 km;and the crust in the Qinghai-Tibet region is about 52-66 km thick.The variation depth of Moho is very large on the border of the junction of the Qinghai-Tibet plateau and the Sichuan basin.it shows that there are the complex geological structure in the borderof the junction.(3)The Poisson ratio of the eastern margin of Qinghai-Tibet is different from Sichuan basin.The Poison ratio of southern and northern of eastern margin of Qinghai-Tibet reaches middle to high value(0.23-0.31),indicating that the crust of these two regions are mainly formed of weak ultramafic or mafic rocks that can be easily deformed.In the central part of the eastern margin of Qinghai-Tibet plateau,the Vs in the middle crustal Vs reaches the lowest(2.8-3.4 km/s),Poisson ratio reaches highest(0.28-0.32).Studies have shown that when the temperature reaches or exceeds the solid phase,Vs dropped dramatically,Poisson ratio will raise sharply.According to the research results and experiment of petrology and the results of electromagnetic detection in the research area,it can be considered that there are some conditions for partial melting of the crust in the study area and especially in the salt bearing fluid is easier to participate in the partial melting.The light colored granite(about 10Ma)in Gongga Mountain may be caused by partial melting of this crust.Most of the eastern margin of the Qinghai-Tibetan Plateau area have moderate to high values of Poisson’s ratio(0.23-0.32).The inhomogeneity of crustal rock,as well as the emergence of crustal partial melting or water fluids may result in significantly different Poison’s Ratio.Below the Sichuan Basin block its Poisson’s ratio is 0.23-0.28,in most areas are equivalent,reflecting the crust and upper mantle is more uniform,and has a rigid rock,there may be rich in silicon qualitative plutonic.(4)Receiver function inversion of the broadband seismic data yielded Vs velocity structure of the crust and upper mantle of eastern margin of Qinghai-Tibet Plateau and Sichuan basin.The Vs velocity distribution images in different depth together with the distribution of low-velocity layer reveal a picture of the spatial distribution and dynamic effect of the crustal flow can be obtained.Below the eastern margin of the Qinghai-Tibetan Plateau,the crustal flow does not exist everywhere,but is only confined to a limited area and distributes mainly along the north-west trending active fault zone.It flows from the Qiangtang in central Qinghai-Tibet Plateau.The mainstream moves along the north-west trending Xianshuihe fault to north-south trending.And it moves along the Anning River and Xiaojiang fault to south trending.Another branch moves along the Lancang River,Nujiang River fault zone to south trending.In the northern part of the study area,a north-east and east-west crustal flow converges to the Longmen-Shan Mountain.Crustal flow is mainly distributed in the middle crust(depth 20-40 km),some of them are up to 50 km of the lower crust.Blocked by the crust and upper mantle beneath the Sichuan basin,the crustal flow appears to split up or down branches.Upward branch caused crustal uplift and results in a steep mountain.Downward branch caused Moho sinking and the crust thickening.(5)Study of the lithosphere and the asthenosphere is an important aspect of study of the Earth’s internal structure.Its findings are of great significance to understand the composition of matter and movement of the substance within the Earth.CCP migration stack of receiver function methods shows a clear boundary of Moho and 410 km discontinuities,but also shows some discontinuously distributed discontinuity boundary at the bottom of the lower crust,lithosphere interface and 410 km boundary.The 410 km discontinuity is deepping slightly from Sichuan Basin to Tibet Plateau.Below the Sichuan Basin the depth of this boundary is about 400 km,while in the eastern margin of the Qinghai-Tibetan Plateau the depth is about 420 km.In the upper mantle,there are a number of discontinuous westward tilted layering structures.Between the LAB and 410 km discontinuity there are many discontinuous layers with less westward tilting and almost horizontally distributed.Beneath the eastern margin of the Qinghai-Tibetan Plateau and the Sichuan Basin,there is a clear distribution of low velocity layer below the LAB.But there are differences between them.Below Sichuan Basin,the low velocity layer distribution is more complete.Below the eastern margin of the Qinghai-Tibetan Plateau,There are also some high-velocity discrete layers visible.These results suggest that: in the plateau region due to crustal thickening,small part of crust may enter the top part of the asthenosphere.But this phenomenon does not appear below the Sichuan Basin.