| Since the 1960s, the plate tectonics theory has been very successful in interpreting oceanic crustal structure and deformation, and accepted by the geoscientists around the world. However, in the meantime people realized that such a theory did not explain structure and deformation of the continents well, so studies about inner continental structure and deformation has been one of foci in geosciences. Over the past 20 years, many theories and hypotheses have been proposed to describe the deformation and dynamics of the continents. These theories and hypotheses, when applied to the study of deformation pattern of the Chinese continent, could be categorized into two groups of theories, namely "continental extrusion" and "crustal thickening".The two groups of theories have coexisted, been debated, and reached to no consensus over the years, one of the reasons was the lack of observational verification of present-day crustal deformation. With the applications of GPS technique to crustal deformation monitoring in recent years, the situation has been improved. This thesis is focused on the study of present-day crustal deformation of the Chinese continent, based on high precision analysis of GPS data.GPS data used in the study ware mainly collected by the Crustal Movement Observation Network of China, and some other densification data observed in some regions and surrounding areas of the Chinese continent. When using the GAMIT software to process GPS data, a relaxation mode of the satellite orbits is used to process the local data as well as the global data, and in doing so not only ensuring the consistency of the models and methods adopted, but also avoiding systematic biases from using IGS precise orbits of different periods. Meanwhile, the newest research findings in geophysical models and algorithms, such as the absolute phase center models of the satellite and receiver antennae, the global meteorologic models and troposphere mapping functions, and a new algorithm to efficiently solve for ambiguities for long baselines, are adopted.GPS observed crustal deformation usually includes both tectonic and non-tectonic deformation signals, and it is vitally important to detect and remove the non-tectonic deformation signals in the data in order to effectively use GPS observations for tectonic deformation studies. Using the Earth satellite data and geophysical models, non-tectonic crustal deformation caused by the ocean tide loading, atmospheric mass loading, snow and soil moisture mass loading, and non-tidal ocean mass loading are calculated. Based on the quantitative analyses, the effects due to non-tectonic crustal deformation for the position time series of GPS fiducial stations from the Crustal Movement Observation Network of China are studied and corrected. The study shows that these effects on the vertical components of station positions are remarkable, especially ones from the atmospheric mass loading and snow and soil moisture mass loading. Using these geophysical models to correct for the non-tectonic deformation, the RMS of the station vertical positions can be reduced by about 1 mm, which is about 11% of the total RMS, and the amplitudes of annual vertical position variations are also reduced by about 37%. Moreover, the position time series corrected using these models followed by an empirical fitting of annual and semi-annual variations are smoother than that corrected using the empirical fitting of annual and semi-annual variations only, indicating that the geophysical model corrections can not be substituted by pure empirical fitting in removing the non-tectonic deformation effects.Based on high precision analysis of GPS data and evaluation of the effect of non-tectonic deformation, the velocity field of crustal motion of the Chinese continent is obtained using the QOCA software, with the coseismic and postseismic displacements of large earthquakes being modeled under constraints. The result indicates that, except for Northeast China and South China, most part of the Chinese continent has been uplifting, particularly in North China and the Tibet plateau, where the uplifting rates reach 5 mm/a.Starting from the rigid block motion model, a deformable block motion model is developed. Based on prior information from geologic, seismologic, and geodetic studies, the Chinese continent and its surrounding areas are partitioned into 27 tectonic blocks initially, and the F-test is used to justify whether a block is rigid or deforming, and whether two neighboring blocks are independent or should be merged together. Finally, the process has yielded 22 independent tectonic blocks. The result reveals 3 categories of deformation patterns in the Chinese continent. The first category, associated with the region east of the north-south seismic belt of China and within the Tarim and Junggar blocks, is characterized with stable interior having no relative motion between internal GPS sites and weak seismic activities, and the sizes of these blocks are usually greater than a thousand kilometers. The second category, associated with the borderland of the Tibetan plateau, such as the Alxa, Xining, and southwest Longmenshan blocks, is characterized with block-like motion with deformation mainly accommodated along the block boundaries, but with limited block sizes and moderate internal tectonic activities. The third category, associated with the interior of the Tibetan plateau, the Tian Shan orogenic belt, and the Sichuan-Yunnan region, is characterized with broadly distributed deformation within the regions, including compression, extension, and shear motion, which cannot be reasonably described by relative motion of rigid blocks of large sizes. Based on the analysis of regional lithospheric structures and deformation patterns above, it is inferred that the deformation modes of the Chinese continent are mainly controlled by the crustal structure. The crust of eastern China, Tarim, and Jungger regions is mechanically strong, and its deformation takes the form of relative motion between rigid blocks. On the other hand, the northward indentation of the India plate into the Asia continent has created the uplift of the Tibetan plateau and the Tian Shan Mountains, thickened their crust, and raised the temperature in the crust. The lower crust is likely developed with low seismic velocity and high electric conductivity layers and deforms visco-plastically. The brittle part of the crust, driven by the visco-plastic flow of the lower crust, deforms extensively at all scales. The regions of the second category located at the borderland of the Tibetan plateau are the transition zone between the regions of the first and the third categories in term of the crustal structure. Driven by the lateral boundary forces, their deformation mode is also between the two, in the form of block motion and deformation with smaller block size and less internal strength.Synthesizing the horizontal and vertical deformation fields derived from GPS measurements and the lithospheric structures elaborated above, a new model is proposed for deformation mode and mechanism of the Tibetan plateau and its surrounding regions.(1) the NNE indentation of the India plate into the Asia continent has caused the horizontal convergence and vertical uplift of the Himalayas, and the convergence and uplift rates are 12-15mm/a and 5mm/a, respectively. (2) In the region north of the Himalayas and south of the Bangong-Nujiang suture zone, the deformation at the surface shows about the equal amount of N-S compression and E-W extension at the rate of 15-20×10-9 strain/a, and the northward under-thrusting of the India plate in the lower crust has resulted in the surface uplift at the rate of about 2 mm/a. (3)the Qiangtang region located north of the Bangong-Nujiang suture zone undergoes strong N-S compression (at a rate of～25×10-9 strain/a) and E-W extension (at a rate of～20×10-9 strain/a), the visco-plastic lower crust and upper mantle is thickened resulting from the northward indentation of the under-thrusting India plate front, driving rapid uplift at the surface at a rate of about 5 mm/a. (4) The Qaidam region located in the northeast of the Tibetan plateau has been compressed in the N-S direction at a rate of～25×10-9 strain/a, and stretched in the E-W direction at a rate of～4×10-9 strain/a, respectively. Large difference between the two components is interpreted as the result of mechanic coupling of the lithosphere, causing N-S convergence and thickening of the whole lithosphere, and the surface uplift at a rate of 1-3 mm/a.The Chinese continent is located in the junction region of the Eurasia, India, Pacific, and Philippine Sea plates. It is generally agreed that deformation of the western part of the Chinese continent is predominantly driven by intensive collision between the India plate and the Eurasia plate and the northward push of the former into latter. On the other hand, it is still controversial what the dominant driving force is for deformation of the eastern part of the Chinese continent. In this study, the result of block motion model reveals that the blocks located in the eastern part of the Chinese continent have a consistent ESEward motion and the motion rates increase gradually from north to south. Meanwhile, the blocks of North China and Northeast China show counter-clockwise rotation rates of about 0.7-1.7×10-9 radian/a, suggesting progresssive change of dynamic boundary conditions along the eastern margin of the Eurasia plate from north to south. In light of the above result, combining with previous geologic and seismologic findings and mechanic modeling result, it is concluded that deformation of the eastern part of the Chinese continent is mainly the result of tensional and ocean-ward stresses caused by the eastward retreat of western trenches the Pacific and Philippine Sea plates, which increases progressively from north to south.The 12 May 2008 Wenchuan, Sichuan earthquake ruptured the Beichuan-Yingxi-u and Guanxian-Jiangyou faults, and produced surface rupture of～240 km and～70 km in length along the two faults, respectively. In this study, the coseismic displacement field is derived using GPS observations collected before and after the quake, and then is used to invert for the fault geometry and slip distribution of the rupture. The result shows that the Beichuan-Yingxiu fault dips to the northwest at a moderate angle of～41°at the southwest end, and the fault plane gets progressively steeper northeastward along strike, reaching a dip angle of～73°at Qingchuan. The averaged width of fault plane is 12-15 km. Slip caused by the earthquake is characterized mainly by thrust motion with a modest right-lateral strike slip component at the south segment of the fault. As the rupture travelled farther northeastward, the thrust component tapers down gradually, and the dextral component becomes dominant at the northeast end of the rupture. The slip distribution on the Beichuan-Yingxiu fault shows two high-slip concentrations of up to 7.9 m and 7.5 m, respectively. The two high-slip concentrations are just in the neighborhood of Yingxiu Beichuan cities,which suffered the greatest fatalities and structure damages during the quake.The seismic moment release is estimated 6.78×1020Nm, corresponding to an Mw 7.9 earthquake. |
PDF Full Text Request