| The Dabie-Sulu orogenic belt (111°E-124°E、31°N～40°N) located in eastern China, and it is one of the major structural belts. This work includes two research fields:seismic anisotropy and ambient tomography.The analysis on seismic anisotropy is divided into two models-single anisotropic layer and multiple anisotropic layers. For the case of single anisotropy, we have analyzed observations of low frequency SKS waves recorded on Shangdong, Jiangsu, Anhui, Henan and Hubei digital seismograph network and China Broadband Digital Seismograph Network (2006.05-2011.05). With rotation correlation and transverse component minimization methods, we obtaine splitting parameters (φ,δt) beneath Dabie-Sulu and its adjacent areas. The results show that the delay times between fast and slow wave range from0.5s to1.63s, which indicate that the anisotropic structures are at the depths about57km-187km, including the effects of lithosphere and asthenosphere. The fast wave polarization directions beneath this area are divided into four groups. The fast wave polarizations beneath North China prefer nearly E-W. According to the geology results, we infer that the upper mantle anisotropy beneath North China caused by the convection of the asthenosphere. The anisotropy results beneath Dabie orogenic belt show a good relation with the strike direction on the surface, which reflect the north-south-trending compression and collision of Yangtze block and North China block. In Yangtze, the results near the south of Dabie show nearly perpendicular to the Dabie orogenic belt, and the results near the east of Dabie show NEE-SWW trends. The anisotropy beneath Sulu orogenic belt prefers NEE-SWW, which shows differences between the strike direction on the surface and the absolute plate motion. The upper mantle anisotropy can be explained by the combined effects of asthenosphere convection and the so-called "fossile anisotropy" frozen in the lithosphere during the Indosinian-Yanshanian period.It has no relationship beween splitting parameters and incoming polarization in the case of single anisotropic layer. On contrast, in the presence of multiple anisotropic layers, this backazimuthal variation takes the form of a periodic variation in both φ and δt. It has long been recognized that the presence of complex anisotropy will result in variations in apparent splitting parameters with incoming polarization azimuth, and this will open doors for new work on imaging complex anisotropy structure at depth. The complex anisotropic structure beneath Dabie-Sulu orogenic belt can be diagnosed by the previous results, which inspire us to focus on further in-depth studies. In the study, we calculated the apparent parameters (φ, δt) variations as a function of incoming polarization with π/2based on multiple layers. Compared to the result of SplitLab software, we find that the result calculated in our program is in good agreement with SplitLab on two-layer model, which strongly support our program. In the three-layer model, we changed the order of the layers and calculate the apparent splitting parameters for each model. We find that the order of the layers strongly effects the apparent splitting parameters (φ, δt), but the period is consistent of π/2. The experimental data obtained from the four different structures are analyzed and the results illustrate as bellow:the model for Dabie includes fast-axis at N97°E and N46°E for the top layer and the bottom layer, with delay time1.1s and0.31s; the model for Sulu includes fast-axis at N88°E and N51°E for the top layer and the bottom layer, with delay time1.0s and0.4s; the model for North China includes fast-axis at N105°E and N60°E for the top layer and the bottom layer, with delay time0.6s and0.88s; the model for Yangtze includes fast-axis at N110°E and N68°E for the top layer and the bottom layer, with delay time0.5s and1.1s.In order to obtain the Rayleigth wave structure beneath the study region, we present ambient noise tomography using nearly two years’continuous vertical-component seismograms recorded by Shandong, Jiangsu, Anhui, Henan and Hubei seismograph networks and China Broadband Digital Seismograph Network (2009.05～2011.05). We obtain Green functions after cross-correlation and stacking between all possible station-pairs. Then we retrieve nearly4000Rayleigh wave dispersion curves by using multiple filtering method. Group velocity maps at10-25s are reconstructed with SIRT method. The results of ambient noise group velocity maps reveal that there are evident Rayleigh velocity differences among different tectonic regions:(1) Tanlu fault exhibits higher velocity at periods of10-15s, and its surrounding areas including Luxi block and Jiaobei block appear higher velocity in the crust, which may be caused by the enrichment of the high velocity and higher density rocks in the crust during the extensional environment;(2) higher velocity beneath Sulu high pressure metarmophosed at all periods of10s-25s, which may implies us that the metarmophosed zone extends probably down to the lower crust. On the other hand, it is hard to define the range of Dabie high pressure metarmophosed, because only high velocity appears at the periods of10s-15s;(3) the subducted Pacific slab destabilized mantle convection beneath the eastern NCC, and it is resulted in cratonic destruction in the eastern NCC, which might be the reason of lower Rayleigh velocity beneath NCC and Yangtze block. |
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