Study On The Crust And Upper Mantle Structure Beneath The Northern Part Of North China And Northeast China

Global and large-scale regional tomographic models have clearly imaged that the Pacific plate is subducting beneath the Eurasian plates,and it is currently stagnant in the mantle transition zone beneath eastern China.A big mantle wedge(BMW)has formed in the upper mantle above the subducting Pacific slab and the stagnant slab.Dehydration of the stagnant slab and corner flow in the BMW may cause the hot and wet asthenospheric materials upwelling,which lead to the formation of intraplate volcanoes in Northeast China and reactivation of the North China Craton.Hence it is very important to study the crust and upper mantle structure beneath the northern part of North China and Northeast China for understanding deep dynamic processes caused by the deep subduction and stagnancy of the Pacific slab in the mantle transition zone.For these reasons,the following three tasks have been carried out.In the study of crustal structure beneath the northern part of North China,by using double-difference seismic tomography and jointly inverting 183909 absolute arrival times and 495753 differential arrival times from 10461 local events recorded by permanent seismic network and North China Seismic Array,we determined a 3D P-wave velocity model of the crust under the northern part of North China(37.5°-41.5°N,111.5°-119.5°E)and obtained hypocenter parameters of 9108 earthquakes relocated firstly.The new understangdings are obtained as follows:(1)Our results show that earthquakes relocated are more concentrated along faults,and most of focal depths are within 0-22 km.The distribution of hypocenters along the Zhangjiakou-Penglai fault zone is linear and nearly vertical,which reflects the deep fault of steep dip angle.There are two clusters of earthquakes in the north section of Tangshan fault.It is speculated that there is a steep dip angle fault with NE strike on the east side of Tangshan fault.(2)The majority of hypocenters are located in the low velocity anomalies or in the conjunctional areas of low and high velocity anomalies.(3)Significant low velocity anomalies are found in the source areas of the 1976 Tangshan earthquake(M 7.8)and the 1679 Sanhe-Pinggu earthquake(M 8.0),which are considered to be associated with fluids.(4)Combining previous studies with our present model,we think that the deep subduction and stagnancy of the Pacific slab have resulted in deep slab dehydration and upwelling of hot asthenospheric materials,and the asthenospheric materials have reached the top of uppermantle and broken into crust along the deep faults which may cut through the crust,then the hydrous minerals in the top of uppermantle and lower crust are dehydrated to produce fluids,and these fluids further cause the weakening of the seismogenic layer in the middle and upper crust.Hence,we believe that the crustal heterogeneity and the seismotectonics under the northern part of North China are closely related to a series of deep dynamic processes associated with the deep subduction and stagnancy of the Pacific slab under East China.Secondly,we collected oil seismic stack velocities of 40 survey points in the North China basin and processed them to obtain curves which reveal the variations of velocity depend on the depth at each point,digitized 9 profile results of Deep Seismic Sounding(DSS)to get discrete velocities under the each profile,built up high precision 3D P-wave velocity model above 10 km depth under the North China basin(115.50°-117.6°E,38.40°-40.75°N)by using above data and geologic modeling software.The new understandings are listed as follows:(1)Our results show that the North China basin is an area with alternating uplifts and depressions,they are Huanghua depression,Cangxian uplift,Jizhong depression from east to west,and the upper crust velocity structures are quite complex.(2)The depth of crystalline basement varies dramatically under the North China basin.Under the Jizhong depression,the depth of crystalline basement can reach to 10 km at the deepest part,and it is deep in the southwest and sallow in the northeast along the structural orientation as a whole.Under the Cangxian uplift,the depth of crystalline basement is about 2-4km,and under the Huanghua depression,it can reach to 9km at the deepest part.The great relief of basement indicates the difference of settlement of these three secondary tectonic units and the strong extension tectonic movement since Mesozoic and Cenozoic era.(3)The crystalline basement is shallower under the Taihangshan uplift and Yanshan uplift than under the North China basin,which suggests the raising tectonic movement of uplift area since Cenozoic era.(4)It is the first time to use oil seismic stack velocities on the establishment of the crust velocity model under the North China basin in this study,comparing with models obtained from DSS profiles,our present model are better to reflect the complex upper crust structures under the North China basin.This result can provide a reliable velocity model for the study of strong ground motion forecasting in North China basin,and support earthquake prevention and disaster reduction,so it is significant.In the study of upper mantle structure beneath Northeast China,we selected waveform data from 223 teleseismic events recorded by seismic stations from Heilongjiang,Jilin,Liaoning,Inner Mongolia regional permanent networks and NECESSArray temporary network,obtained 22569 P-wave relative travel-times by multi-channel cross correlation measurement,calculated travel-time sensitivity kernels,carried out finite-frequency tomographic inversions to image the three-dimensional velocity structure of the mantle down to a depth of 600 km beneath Northeast China.We also analyzed reliability and resolution of inversion results by using spatial distribution of sampling density and checkerboard test.We obtained following new understandings:(1)Our results reveal a low-velocity anomaly extending from 80 to at least 200 km depth beneath the Songliao basin,which corresponds to the characteristics of high temperature and low density in this depth range,so this low-velocity anomaly may imply partial melting mantle.(2)The velocity structures on both sides of the North-South Gravity Lineament(NSGL)are obviously different,and this difference can reach to 200 km depth,it is suggest that NSGL may be a variation belt or a boundary of internal structures in the upper mantle beneath Northeast China.(3)There is a wide range of low velocity anomaly under the Changbaishan volcano,which extend from the shallow part of upper mantle to the mantle transition zone and it may indicates that the upwelling of hot material derived from the mantle transition zone.We speculate that the formation of the Changbaishan volcano may be the result of the concerted efforts of several factors,such as the upwelling of hot material from the mantle transition zone,the stagnant and dehydration of the Pacific slab,and the backflow of the hot and buoyant sub-lithospheric mantle through a gap in the subducting slab.