Seismic Study Of Crust And Upper Mantle Structure And Tectonics In Middle-to-Lower Yangtze Craton And Its Adjacent Regions

China was assembled by several Precambrian cratons which were welded together along gen-erally east-west oriented sutures in different time periods of the Mesozoic. However, a prominent feature in today’s landforms of China is a nearly NS oriented topographic step which runs for about3000km from Xing’anling through Taihangshan to Wulingshan. It cuts through the Amuri-an block, the North China craton, and the Yangtze craton and seperates high-rise terranes in western China from lowlands in eastern China. Several large Cenozoic rift basins (e.g., the Songliao Basin, the North China/Bohai Basin, and the Nangxiang-Jianghan Basin) have developed east of the step which are in sharp contrast to plateaus and mountain ranges of Mesozoic ages in the west. The step also marks drastic changes in geophysical potential fields on the surface and crustal-upper mantle structures below. Its location coincides with a strong lineament of Bouguer gravity anomaly gra-dient which is believed to reflect crustal thickness change from40-45km in the west to30-35km in the east as revealed by seismic studies. Studies also showed that the thin crust in eastern China is underlain by a low-velocity upper mantle. These observations together with other geological and geochemical studies suggest that the eastern parts of the North and South China cratons have lost much of their thick mantle lithosphere (decratonization) since the Late Mesozoic. Without protection from its cratonic mantle lithosphere, the crust in eastern China has been extensively stretched and reworked, as demonstrated by the development of rift basins, significant reduction in its thickness, and voluminous magmatism.In this study, We used three-component waveform data of131permanent broadband seismic stations in the Middle-to-Lower Yangtze Craton and its adjacent areas from2009.01to2010.12and obtained detailed velocity structure and tectonic information using teleseismic P-wave travel-time tomography and teleseismic P-wave receiver functions. The results shed light on dynamics of southeastern China. The main results of this study are summarized in the following.(1)We performed a high resolution P-wave tomography down to510km beneath the131per-manent seismic stations in the Middle-lower Yangtze Craton, using totally564teleseismic events (Mw>5.0) in2009and2010. The results show a high P-wave velocity anomaly north of29°N that represents a stagnant slab in the mantle transition zone (MTZ). The MTZ south of29°N shows low P-wave velocity anomalies, which indicates that the southern edge of the Pacific stagnant slab is located at29°N. A big mantle wedge (BMW) is formed above the subducting western Pacif-ic plate and the stagnant slab in the MTZ, producing low P-wave velocity anomalies in the area. High P-wave velocity anomalies mainly exist in the upper Yangtze Craton located to the west of the north-south-oriented Bouguer gravity anomaly gradient lineament. The obtained crustal velocity structure explains well the observed travel-time residuals of seismic stations. Both the areas of Jianghan Basin and North Jiangshu Basin show low P-wave velocity anomalies beneath. Cross-sections of the velocity model show a up-welling of mantle materials. EW-oriented profiles show the BMW north of29°N N including beneath the Lower Yangtze Craton, the Qinling-Dabie orogenic belts and the North China Craton. They clearly show that the center of the up-welling is right underneath the Dabie Mts. The SW-oriented profile shows that the Pacific stagnant slab may partially block a vertical up-welling of lower mantle beneath it, causing a up-welling beneath the Southeastern China and its northward migration.(2) We determined depth variation of the410-km and660-km discontinuities beneath south-eastern China by CCP stacking of P-wave receiver functions of121permanent seismic stations and then combined the results with seismic velocity variations to estimate temperature and water content variations in the MTZ of the region. Previous tomographic studies have shown a stagnant slab in the mantle transition zone in eastern Asia that is connected to the subduction of the western Pacific. Our obtained temperature variation clearly outlines the shape of the stagnant slab, with its western edge at113.5°N and the southern edge at28.5°N. The good correlation between the loca-tion of stagnant slab in MTZ and surface tectonics suggests that the Cenozoic extension in eastern China is closely associated with the subduction of western Pacific and its eastward migration. The water content inside the stagnant slab is lower than the surroundings, suggesting that the water has already been released from the subducting slab in the upper mantle.(3) We used P-wave receiver functions to obtain the crustal thicknesses beneath121perma-nent stations in the Middle-lower Yangtze craton and its adjacent areas, using nearly700teleseis-mic events (Mw>5.0) in2009and2010with epicentral distance from30°to95°. We combined previous crustal thickness results and the results show a V-shaped crustal thinning zone in the re-gion. The north-south-oriented left branch transverses the Xiangzhong Basin and Jianghan Basin in the north and extends to the Nanxiang basin located at the southern boundary of the North China Craton (NCC). The NNE-SSW-oriented right branch extends from Ganzhou of Jiangxi province to the eastern areas of the Tanlu fault in the north. The two branches merge in a crustal thinning belt offshore of South China. We attribute the V-shaped crustal thinning to different stress environ-ments between north and south of29°N. The area to the north is under extension due to an eastward migration of the Pacific subduction and nonuniform crustal thinning occurs because of weak zones such as rift basins and large faults. On the contrast, the area to the south is in a compressional stress regime due to the subductions of the Eurasia plate under the Philippine sea plate. The high angle eastward subduction in the Manila trench may cause a tense mantle flow in the back of the subducting slab which could lead a northward mafic/ultramafic intrusion beneath southern China and produce the V-shaped crustal thinning.(4) Western Hubei Province is in the southern end of a3000-km-long north-south-oriented Xing’anling-Taihangshan-Wulingshan topographic step in China, which separates high-rising plateaus and mountain ranges in the west from low-elevation plains in the east. We calculated teleseismic P receiver functions of32permanent broadband seismic stations in Hubei Province and estimated crustal thicknesses under them using the Ⅱ-κ method. We also obtained detailed crustal structural images along three profiles using the CCP stacking method. The results show an east-west crustal thickness increase in the study area from30-35km to45-50km in less than20km of horizontal distance. most likely in a step-wise fashion. The thin crust beneath the Nanxiang and Jianghan basins in eastern Hubei extends into the interior of the Wuling Uplift and the Huangling Massif in western Hubei. The lack of mirror symmetry between the Moho and surface topography suggests that part of the mountain ranges in western Hubei is either compensated by non-Airy-type isostasy models or not in isostatic equilibrium but supported by the strength of the lithosphere. The brittle deformation in the lower crust as indicated by the abrupt Moho steps seems to be decoupled with brittle deformation in the upper crust. The CCP images also reveal an apparent double Moho beneath the Wudang Mts. which is interpreted to be due to a partially eclogitized lower crust after the original cratonic mantle lithosphere was replaced by warm and hydrated mantle materials in eastern China in the Late Mesozoic. The Moho steps were formed when a segment of eclogitized lower crust became gravitationally unstable and foundered into the mantle.To sum up, we estimated the eastward migration speed of western Pacific subduction to be approximately3.3cm/yr, and proposed a dynamic model of crustal and lithosphere thinning of southeastern China in three different stages. In the first stage, the western Pacific trench was lo-cated near the present-day North-South-oriented gravitational gradient lineament and started the eastward migration in the late Cretaceous (nearly96Ma). In the second stage, the continue east-ward migration of the trench caused wide-spread EW extension of mainland China east of the lineament until the collision of the Philippine sea plate and nearly east-west-oriented island arc of southern Japan in15Ma; In the final stage, the Philippine sea plate subducted northward beneath the Eurasia plate to the northeast of Taiwan. To the south of Taiwan, the Eurasia plate subducted under the Philippine sea plate along the Manila trench. This led to the cessation of extension in South China and produced a mantle flow behind the Eurasia subducting slab and crustal thinning of Southeastern China.