Investigation Of The Structure And Deformation Of The North China Craton Based On Ambient Noise And Earthquake

The North China Craton(NCC),one of the oldest craton in China,developed a continental root in the Archean,and was reactivated then partly destructed especially in the eastern block since the Mesozoic.In the last decade,numerous studies have addressed the timing and geometry of the lithospheric thinning and destruction with geophysical and geochemical methods,however,still with debates on details of the destruction processes and dynamic mechanisms.The lithospheric thinning was accompanied with strong crustal extension and deformation as well as massive volcanism,which can be recorded by seismic velocity anomalies and anisotropy in the crust and upper mantle.This dissertation has investigated the crustal and upper mantle lithospheric with several methods,providing new constraint to the lithospheric destruction processes and potential dynamic mechanism of the NCC.In the past decades,benefit from the globally wide deployment of dense seismic arrays,tomographic techniques based on dense seismic array have been quickly developed.In particular,surface wave tomography based on ambient noise cross-correlations has been widely applied to obtain high-resolution crustal structures of earth.In this study,the continuous vertical component waveforms of North China Seismic Array in the NCC are used to perform surface wave azimuthally anisotropic tomography to constrain the 3-D crustal and uppermost mantle structure and deformation in the NCC,in order to further reveal the lithospheric deformation pattern and the processes associated with the evolution and destruction of the NCC.My results show that the fast axes of upper crust azimuthal anisotropy(period of surface waves T<15s)generally follow the orientation of fault strikes,and the isotropic speed variations show a good correlation with the shallow geological structures.My results also indicate very weak azimuthal anisotropy within the middle-to-lower crust(T = 15-20s)where strong radial anisotropy was reported in several previous studies in the same region,which probably results from sub-horizontal alignment of anisotropic aggregate minerals,such as mica,due to strong regional extension.In the uppermost mantle(T>= 30s),my results show quite strong azimuthal anisotropy,which is obviously different from that of middle-to-lower crust.And my results reveal quite complex isotropic variations within the uppermost mantle.My results reveal significant variations in both isotropic phase speed and azimuthal anisotropy within the lithosphere beneath the NCC probably resulted from laterally heterogeneous modifications.Body waves and surface waves are complementary due to their different spatial sensitivities to crustal and uppermost mantle velocity structures.In this study,the first arrival times of body waves and surface wave dispersion data are jointed to directly invert the 3D Vp and Vs model with both datasets well fitted.Joint inversion of body wave and surface wave data can better constrain the 3D Vp and Vs model than just body wave or surface wave data alone,with great improvements in resolution and reliability.The results of joint inversion show great consistence with geological features.The shallow velocity structures are mainly controlled by surface geological structures:high speed in the mountain areas and low speed in the sedimentary basins.The lowest crust shows high speed in the eastern part and low speed in the western part bounded by the North-South Gravity Lineament.The refined crustal velocity model from joint inversion is critical for understanding the geological structures and its evolution within my study region.In addition,joint inversion results can provide a better initial model for further full waveform inversion.It is generally hypothesized that reactivation and destruction of the NCC was triggered by the continuous westward subduction of the Paleo Pacific plate,which stagnant within the mantle transition zone beneath the NCC.In order to investigate the influence of the Paleo Pacific plate subduction on the evolution of the NCC,we need to know the influence of the Paleo Pacific plate subduction on the mantle transition zone and the lateral variations of 410km and 660km discontinuity structures.Here we present how to image the lateral variations of 410km and 660km discontinuities with dense seismic array observation using a combination of techniques including the common-middle-point stacking,phase-weighted stacking,and travel time correction from 3D velocity structure.The result from the reflected P phases extracted from ambient noise cross-correlations was compared with that from teleseismic Ps conversions at the same region.Both results indicate shallower depth of 410km discontinuity in the northeastern part of the study region and deeper depth in the southwestern part.The overall pattern of the topography of 660km discontinuity is similar to 410km discontinuity but more complicated.The similarity between the results from these two methods proves the reliability of this interferometric method.In addition,relative amplitude ratio between the 410km and 660km discontinuity reflected P phases varies in different regions,which may imply the lateral changes of sharpness of these two discontinuities.In summary,this paper studies the crustal and upper mantle structure beneath the NCC in various aspects.My results help to understand the impact of the westward subduction of the Paleo Pacific plate on the evolution of the NCC and provide new observational constraints on the crustal deformation and the crust-mantle interaction beneath the NCC in response to the destruction and thinning of the cratonic lithosphere during the Mesozoic to Cenozoic.