关键词: 布格异常, 小波分析, 构造特征, 大别造山带

Abstract: Bouguer gravity anomaly is a comprehensive reflection of deep and shallow density disturbances of the Earth’s internal mass. Important tectonic information of internal structure at different depths can be obtained by source separation of Bouguer gravity anomaly. Bouguer gravity anomaly from ground-based observations and Bouguer gravity anomaly from EGM2008 of the eastern Dabie Orogen were merged based on least-squares collocation. By model construction of massive bodies and data experiments, optimal wavelet base(sym6)and the corresponding optimal level(6)for gravity anomaly decomposition of the study area were confirmed. Two-dimensional discrete wavelet transform method was applied to obtain low-frequency components and high-frequency components of merged Bouguer gravity anomaly of the study area, and average depths of disturbed surfaces of the wavelet decomposition results were determined by spectrum analyses. In combination with data of crustal structure, geologic structure, effective elastic thickness of lithosphere and seismic activities, the deep structure and shallow structure of the crust were analyzed, and the structural background of seismic activities was discussed. The result shows that steep gradient belts of low-frequency components of Bouguer gravity anomaly outline the density transfer zones of deep structure between the eastern Dabie Orogen and surrounding blocks. It is speculated that the suture zone between the eastern Dabie Orogen and the North China Block locates at the front edge of Qingshan-Xiaotian Fault(the eastern part)and Meishan-Longhekou Fault(the western part), the structure transfer zone between the Dabie Orogen and the Yangtze Block locates at the Tancheng-Lujiang fault zone(the eastern part)and 20km north of Xiangfan-Guangji Fault(the southern part). The interior of the eastern Dabie Orogen is characterized by significant low gravity anomaly, which means an obvious depression of Moho surface, and the steep gradient belts of gravity anomaly inside the eastern Dabie Orogen indicate the imperfection of deep structure. High-frequency components of Bouguer gravity anomaly reveal that density structure of the mid-upper crust was influenced by regional faults such as Feizhong Falut, Lu’an-Hefei Fault, Meishan-Longhekou Fault and Tancheng-Lujiang fault zone. The distribution of high-frequency Bouguer gravity anomaly shows that Luo’erling-Tudiling Fault has obvious effect on density structure of the mid-upper crust, and the range of influence breaks northward through the NWW-orientated Qingshan-Xiaotian Fault and Meishan-Longhekou Fault, and may extend to the front edge of Feixi-Hanbaidu Fault. Further analysis combined with seismic activities shows that plate contaction occurred along the suture zones(front edge of Qingshan-Xiaotian Fault)of deep structure between the eastern Dabie Orogen and the North China Block. Besides, deep and shallow structures of this area are both imperfect, and not strong enough for long-time stress accumulation. Therefore, rocks tend to break at weak points(locations where faults intersect and shallow structure transfers)and release stress frequently, which are main reasons why small earthquakes concentrated at Huoshan area.

Key words: Bouguer gravity anomaly, wavelet analysis, tectonic features, Dabie Orogen