The Structure Of Crust And Upper Mantle Beneath Northern North China By Receiver Function

The Central Asian Orogenic Belt (CAOB) between the Siberia paleo-continent and the North China paleo-continent has a close relationship with the evolution of the Paleo-Asian Ocean. The Xing'an-Mongolia Orogenic Belt (XMOB) as a part of the CAOB locates in North China. It is generally considered that the closure of the Paleo-Asian Ocean result in the formation of the XMOB, but there still remain some disputes about the position, manner of the Paleo-Asian Ocean final closure and the formation and evolution of the XMOB due to complex evolution process, wide Cenozoic sedimentary coverage and the lack of high resolution deep exploration data. In recent years, receiver function method develops fast and become an effective method for studying the structure of the crust and upper mantle and for meeting the need of the high resolution imaging of deep structure, moreover the stacking and migration methods have been widely used in the receiver function study. Funded by the National Natural Science Foundation of China, Institute of Geology, Chinese Academy of Geological Sciences deployed 41 portable broadband seismic stations along a longth of 650 km profile from Huailai county at northern north China to Bayinondor town near the boundery of China-Mongolia. The profile extends roughly along the reflection and refration profile funded by Sinoprobe project, across the northern margin of the North China Craton and southern margin of the CAOB from south to north. The stations space is 15 km on average. The temporary seismic array run over a year and 421Gb data have been collected.We obtained 2366 P-wave receiver functions by time domain interative deconvolution method, cruatal thicknesses, Vp/Vs ratios and Poisson ratios beneath the profile were got by H-? stacking method, then the Moho and mantle transition zone image by CCP stacking method. The result are as follows:(1)The Moho depth on average is 40 km along the profile. The crustal thickness changes little across different tectonic blocks but it shows evident variation at the boundaries between the blocks. The Moho dips southward beneath the Ondor sum belt and the Bainaimiao belt, while dips northward beneath the Baolidao belt and the Hegenshan belt, The Linxi fault seems to be as the boundary dividing the northern margin of North China Craton (NCC) and the southern Central Asian Orogenic Belt. (2) The poisson ratio shows a correlation will of the tectonic unit along the profile. The averaged Poisson ratio in the northern margin of North China Craton is larger than the southern part of Central Asian Orogenic Belt. The Poisson ratio changes little within the blocks except nearby the boundaries of different blocks. (3) There is a negative correlation between the crustal thickness and the Poisson ratio that hint the lateral heterogeneities of the crust structure resulted from the tectonic deformation. (4) Poisson ratio of the crust along the profile presents an approximately nonlinear synmetric shape to south margin of Solonker suture zone. (5) The 410km and the 660km discontinuities extend normally along the profile according to the IASP91, only south of 39.5? the thickness of the mantle transition zone is thicker than 250 km because of the 660km discontinuity droping. The mantle transition zone thickening could be related to the effect of the Pacific Ocean subduction.We have the preliminary conclusions below:(1) The lithospheric destruction of the NCC caused by the subduction of the Pacific Ocean is limited to the south of Yanshan Orogenic Belt (2) The Moho geometry by CCP stacking method and the Poission ratio distribution by H-? method hint the Paleo-Asian Ocean has a bidirectional subdection pattern (northward and southward) and the ocean closed at the Linxi Fault.