The Effective Elastic Thickness Over China And Surroundings And Its Lithosphere Dynamic Implication

Under surface and subsurface loading, the lithosphere should respond isostatically by subsidence or uplift. The nature and degree of these responses or compensation-local or regional, fast or slow, are determined by the thermal and mechanical properties of the lithosphere. These responses can be used to estimate the effective elastic thickness (Te), which shows the deformation-resistant capacity of the lithosphere subjected to the long-term (>105yr) tectonic loading, and is also an effective tool to study the large-scale tectonics of continent and dynamics of lithosphere. The spatial variations of Te are of great importance to address lithospheric mechanical behavior, evolution and dynamics of China Mainland with complex geological structure and deformation.The Chinese Mainland is not a single giant craton; it comprises a number of the stable blocks and several active orogens. Several major processes control its tectonics:the rapid collision of the India-Australia plates with Eurasia in the southwest, the subduction of the Pacific and Philippine Sea plates in the east, and the continental extension in the Baikal rift in the north. Due to such complex structure and dynamics, there must be interactions amongs different blocks. Thus, it is necessary to investigate in detail Te distribution for the whole Chinese mainland, in order to study the destruction of the North China Craton and the uplift of Tibetan Plateau, and to better understand the whole continental tectonic deformation and lithopheric dynamics.Based on the topography and Bouguer gravity anomaly data, this thesis has obtained the spatial variations of Te in China and surroundings with high resolution using multitaper and wavelet coherence methods. Some new understandings of the geotectonics and dynmanics are presented on the basis of the new Te map. Firstly, to evaluate the performance of the multitaper and wavelet coherence methods, the synthetic model tests with the given Te are carried out. The model tests clearly indicate that both methods are capable of resolving Te efficiently. Secondly, based on the topography and gravity data from the new combined satellite-terrestrial model, Te variations over China and surroundings are estimated using these two methods. The results show that the Te values estimated by the multitaper are agreement with those of the Fan wavelet; and the variations of Te are significant over Chinese Mainland. Generally, Te is high in the cold stable cratonic blocks and low Te generally correspond to the young Phanerozoic orogens, however significant lateral changes also exist within these structures. Finally, combined with the information from geology, GPS, and seismic tomography, the new understandings of the geotectonics and dynmanics in China and surroundings are as follows:(1) Comparison of the Te spatial variations and distribution of earthquakes indicates that there are close link between elastic thickness and seismicity over Chinese Mainland. The stable areas with high Te are characterized by a lack of seismicity, which suggests that the stable tectonic provinces with high Te effectively resist deformation. Most of the earthquakes are situated in the low Te areas (10-40km) or steep Te gradient zones, which indicates that the weak lithosphere and areas with steep change of Te are prone to accumulate and then release tectonic stresses causing earthquakes.(2) The quantitative analysis by comparing the lateral variations of Te over China and surroundings with the thermal age of tectonic provinces, heat flow and shear-velocity for the mantle indicates that the thermal structure of lithosphere has great influence on Te variations over China Mainland, and Te positively correlates with the mantle S-velocity at100km. Area of high Te exhibit low heat flow and high mantle seismic velocities at～100km, and vice versa. However, due to the special geological structure, complex lithospheric evolution and anisotropy over China Mainland, the correlations between Te and other lithospheric proxies may be scattered, resulting in some degree of uncertainty.(3) The spatial variations of Te in the NCB show that Te values of the Ordos craton are much higher than the average crustal thickness (-42km). This suggests that the high Te in the old stable craton largely relates to a strong and thick lithospheric mantle. Compared with the stable West Block with remarkably higher Te, the Te values within the East Block are lower and vary dramatically. This suggests that the originally strong Archean lithosphere of the eastern NCB has been weakened, but the localized high Te zones indicate that the Archaean mantle relics are likely preserved in some parts.(4) Combining with the tomography models, The Te variations of South China Block show that the Sichuan basin is not a uniform rigid craton, but is obviously anisotropic; the old cratonic nucleus of the Yangtze Block may exist in the eastern Sichuan Basin, northern Guizhou, Chongqing and northwestern Hunan. Low Te (<20km) prevail over most areas of the middle and eastern parts of the SCB. Combining with the surface tectonic features and mantle themal structure, we propose that the lithospheric strength of the SCB (especially in the eastern part) has been reduced by the processes related to the subduction of the old oceanic lithosphere. In turn, the weakened lithosphere tends to deform under the surrounding strong convergence stresses, leading to the broad fold belts and a wide range of magmatic activity observed today. (5) Within the Tibetan Plateau, the prevailing low Te values in the middle, eastern and southeastern parts might be associated with the ductile crustal flow, hot upper mantle and high thermal regime. The prevailing low Te values of the Tibetan plateau indicate that the deformation of the Tibetan lithosphere might be not only sistributed in the suture or fault zone, but also coherently distributed within the Plateau, particularly in the middle, eastern and southeastern parts.(6) The variations of Te over the NCB, SCB, and Tibet Plateau together with the geological, GPS, and seismic tomography data indicate that the lithospheric strength of China Mainland is a result of various interacting tectonic processes, which continuously modified and reworked the lithosphere structure. In particularly, hydration and thermal dynamic processes associated with the subduction of the ancient Pacific plate under Eurasia, and the rapid convergence of India and Eurasia in the southwest during Mesozoic-Cenozoic are the dominant processes, which modified (mainly weaken) the lithosphere of the China Mainland.