The Research On Crust And Upper Mantle Structure By Integrated Geophysical Methods In Songliao Basin And Its Adjacent Area

Songliao Basin are located between Siberian Plate and North China Plate. Its basement is composed of many microplates. Over the years, the structure in this region is very complex with the multiple tectonic movements. Its basement features and deep interface structure become very complicated. There are many geological problems to be solved including the nature of its basement, the distribution of fractures and its deep dynamic mechanism. The geophysical evidence is in urgent need to corroborate the hypothesis that is put forward by Scholars.In this paper, we combined the gravity, magnetic and seismic data to study the characteristics of the geophysical fields in Songliao Basin and its adjacent area. In the aspect of gravity and magnetic inversions, the grid spacing of the data is 0.5°×0.5°, we use the wavelet multi-scale decomposition, logarithmic power spectrum analysis and Parker-Oldenburg modeling and inversion method to study the structures of the basement and the deep interfaces of Songliao Basin. In the aspect of seismic inversion, we use seismic tomography and P-wave anisotropy to study the three-dimensional seismic velocity structure and the azimuthal anisotropic beneath the study area based on the P-wave travel time records of earthquakes in the Northeast China in the past 10 years.The feature of gravity and magnetic fields has a significant distribution in NNE direction and proves an obvious mosaic tectonic background. According to the feature of gravity field, we divide the study area into six sub-regions. It shows an obvious feature of deep collage-type construction. The same with the magnetic field, the study area is also divided into six sub-regions. It reflects the distribution of basins in the study area. With the method of Wavelet Multi-scale Decomposition, the gravity and magnetic fields are decomposed into 4th separation order to extract the anomalies caused by the basement, Moho discontinuity and Curie isothermal surface. The apparent depth of anomalies in each order is determined by the power spectrum. Then the 2th and 3th order wavelet detail anomalies are used to study the basin’s basement. The inversion results show that the basement is in concave and convex distribution. The depth of Songliao Basin’s basement is from 8.5 km to 12.3 km. We delineate 13 basal depressions. According to the distribution characteristics of wavelet multi-scale decomposition in EW, SN, NE-SW and NW-SE directions, we identify 22 basement faults. Through the inversion of the 4th order gravity approximation anomalies, we obtain the map of Moho discontinuity. The Moho discontinuity is obviously elevated while the average depth is about 34.5 km. The uplifting center is right in the middle of Songliao Basin. Based on the inversion result of magnetic anomaly in the 4th order approximation, we get the map of Curie isothermal surface. Its depth distributes in ups and downs forms. In the area of Songliao Basin, it appears to be a central uplift, its shape shows an EW and NW-trending, that correlates with the feature of Moho discontinuity.The inversion result of regional seismic tomography shows that in the shallow crust, the P-wave velocity anomalies are related to the topography features. Low-V represents basin area and the high-V represents the mountains.In the upper crust, velocity anomalies are related with the topography of the metamorphic basement and the distribution of faults and lithology. In the lower crust, Songliao basin area appears to be high-V affected by the upwelling of substance mafic magma. In the area of volcanoes the velocity anomaly is low-V. The distribution of earthquakes is also affected by the P-wave velocity anomalies.In the upper mantle, the area of Yakeshi volcano, the Wudalianchi and Tianchi shows low-V anomalies. Correspondingly, the P-wave anisotropy of the upper crust matches up with the distribution of the basal faults. The anisotropy in the lower crust and upper mantle is in larger scale and has a good districts character. We determine the direction of the main flow of the mantle zone is in NNE, NEE and SW directions according to the features of P-wave anisotropy, GPS velocity data and the position of the uplifted mantle.By integrated interpretations of geology and geophysics methods, we discuss several problems including the corresponding characteristic between gravity anomaly with P-wave velocity structure, the nature of the basement of Songliao Basin, the ascertaining of northward extension belonged to Tan-Lu fault zone and the geodynamics of Songliao Basin. We get conclusions as follows:1) In the upper crust, the gravity anomaly is in good correspondence with the P-wave velocity anomaly. As the depth increases, the gravity anomaly reflects the different nature of deep blocks, while the P-wave velocity anomaly does not reflect the overall characteristics of deep velocity structure because of the lack of seismic data.2) By the corresponding analysis of the basal gravity and magnetic anomalies, we predict the lithology of the basement. For the consideration of the coupling characteristics between the basement lithology and depth of depression, we make some corrections about the depth of depression of the basement and the distribution areas of lithology.3) From the aspects of morphology, structure evolution and earthquake distribution along the faults of Dunhua-Mishan and Yilan-Yitong, we get the conclusion that the Yilan-Yitong fault is the north extension of the Tan-Lu deep fault.4) By analyzing the maps of Moho, Curie and the P-wave anisotropy in 40 km, we consider the formation of Songliao Basin is the result of multiple tectonic movements. The movement of the collage of microplates in the Northeast China provides the material basis, the subduction of the Pacific plate to the Eurasian plate is the main driving force, and the collision between Siberian plate and North China plate plays an important role in controlling the basin’s scale.