Study On The Crust And Upper Mantle Structure In Jiangsu Province And Its Adjacent Areas

Jiangsu and its adjacent area is located in the intersection of three main blocks: Sino-Korea Block, Yangtze Block, and Qin-Qi-Kun Fold Zone. There are Tanlu Fault extending to Lithosphere, Ultra High-Pressure Metamorphic (UHPM) Belt in Dabie-Su-Lu Orogenic Zone in the reseach area that shows complicated tectonic features.In the paper, the geological background and existing deep structure results are introduced, and the new results about deep structure in the research area are systematically obtained by multi-methods. The innovations in the paper are:1. The earthquakes in research area with both the Hypo-DD and GA methods were relocated. Thus, the limit of using one method can be refrained. By analyzing relocated seismic events, some new results are available, such as focal depths ,the depth differences in different tectonic elements, and the position of interested faults.2. The 3D image results of crustal velocity structure is obtained by traveltime inversion using relocated seismic events, which improves the deep structure research in Jiangsu and southern Yellow Sea.3. On the basis of teleseismic Body-wave Receiver-Function method, deep structures below stations in different tectonic parts are studied. The depth of lithosphere in the research area is available for the first time, which is useful for geo-dynamic research.4. The differences in deep structure of three sub-blocks in research area are analyzed with several parameters, such as respectively interval velocity, weighted RMS velocity, Moho depths, focal depths, and the depths of lithosphere.The main results in the paper are:1. The accuracy of seismic locations is greatly improved by relocating calculation, the seismic events are more focused on some certain areas, and the focal depths are more reasonable.2. The focal depths are similar in Lower Yangtze Block (Jiangsu part) and North China Basin. The events are mainly located in the depths of 10～11km, 15～17km, corresponding to the bottom of upper crust and middle crust respectively. The focal depths in Dabie Orogenic Belt are quite different from another two blocks. The shallow earthquakes (less than 10km) are quite predominate in Dabie Orogenic Belt, especially in the depths of 6～7km, possibly relating to high-velocity area, but the difference is not obvious deeper than 10km.3. The 3D crustal velocity image is obtained in 40×40km. The inversion results are consistent with HQ-13 and Gaochun—Nanjing DSS profiles, which means that the accuracy in the paper is quite good.4. The Moho depths are different from the east to the west, and the Moho depths are shallower in eastern and northern research area, deeper in western and southern research area. The Moho depths are quite different in different tectonic element. The Moho depths in Dabie Orogenic Belt are deeper in surrounding area, while the Moho depths in North China Basin are shallower in Lower Yangtze Block. Sulu sub-block shows obvious differences in Moho depths (about 32km), which is uplifted.5. The position of great earthquake is related to the P velocity abnormality. Great earthquake usually occurred in the transition area of higher and lower velocity, but not in higher or lower velocity. The magnitude of seismic event is not quite related to velocity gradient slope.6. On the basis of teleseismic Body-wave Receiver Function, it is shown that the thickness of lithosphere (about 90 km) in North China Basin is a little less than it is in Lower Yangtze Block. The thinnest lithosphere (about 76km) in the research area is below Suqian station, in Tanlu Fault, which hints that uplift phenomenon is observed. The thickest lithosphere (about 128km) is below Hefei station in Dabie-Su-Lu Orogenic Zone.7. The deep structure is quite different for different tectonic unit in the research area. Dabie Orogenic Belt shows deepest crust, shallow focal depths, highest interval velocity & RMS velocity, and thick lithosphere. Sino-Korea Block shows comparatively shallower crust, deeper focal depths, moderate interval velocity & RMS velocity, shallower lithosphere than Lower Yangtze Block. Lower Yangtze Block shows deeper crust than Sino-Korea Block, shallower crust than Dabie Orogenic Belt, similar focal depths with Sino-Korea Block, lowest interval velocity & RMS velocity, similar lithosphere thickness with Sino-Korea Block, a litter deeper than Sino-Korea Block.