The Structure Of The Crust And Upper Mantle In North China Craton From Teleseismic Receiver Function

Lithosphere "de-rooting" beneath the North China Craton (NCC) is a representative phenomenon of reactivation of the craton. Studying the dynamics of the NCC can help to understand the mechanism of de-rooting.In this thesis, I start with a introduction to models of mantle dynamics and previous studies of the NCC geology, lithosphere, upper mantle anisotropy, the transition zone, and mantle convection models from seismic tomography. I then describe the basic principles of receiver function analysis and waveform modeling, and the H-?stacking and CCP stacking methods.Receiver functions are calculated from teleseismic waveform data recorded by 200 broad-band seismic sensors deployed in the eastern, central, and part of the western NCC. They are used to reveal structures of sedimentary basins by waveform inversion. The H-?stacking and the CCP stacking methods are used to obtain crustal and upper mantle structures. The goal is to understand mantle dynamic mechanism through studying structure beneath the NCC.The results show that:1) The structures and the evolution of the two basins in the NCC—the Bohai Bay Basin (BBB) and the Datong basin (DB) are different. Though both are the rift basins, the structure of the BBB is more complex. The subsidence is different from one block to another in the basin. The western and eastern sides across the fault in the basin are not symmetric; the sedimentary thickness of the eastern side is much thicker than the western side, which shows the characteristic of horst (half horst)-graben (half graben) pattern. The uplifts and depressions alternate in the BBB. Thick sedimentary layers always have high average S velocity with low Vp/Vs ratios. This may be caused the Palaeogene strata of higher S velocities and lower Vp/Vs ratios than the newer strata. There is a good correlation between the distribution of geothermal areas and high Vp/Vs ratios. In contrast, the DB has two depositional centers, one in Xiawozhai and another in Housuo. The sedimentary thicknesses near the centers are much thicker than the areas around. And thick sedimentary areas have high Vp/Vs ratios, suggesting that the DB was formed by stretching caused by upwelling of mantle.2) The crustal thickness of NCC is thicker in the western part than the eastern part. The relationship between the crustal Poisson ratio and thickness is very complex. In the central part the Poisson ratio is negatively linear proportional to crustal thickness while the relationship around the BBB is more complex. There the Poisson ratio increases rapidly with the decrease of crustal thickness. This suggests that the thinning of NCC in the eastern part is caused by stretching without underplating of mafic mantle material. There was underplating of mafic magmas from partially molten upper mantle in the central block.3) The stagnant of subducting Pacific slab reaches the bottom of the transition zone beneath the north-south gravity lineament. The transition thickness around Fanshi is very thin, which may be caused by the upwelling mantle material generated by the crack of the stagnant slab down to the lower mantle. The upwelling of the mantle material in the direction SEE caused the extension of the BBB and the expansion of the DB.