关键词: 郯庐断裂带新沂段, 初至波成像, 地震反射剖面, 地壳结构和构造, 第四纪活动性

Abstract: The Tan-Lu fault zone is the largest active tectonic zone in eastern China, with a complex history of formation and evolution, and it has a very important control effect on the regional structure, magmatic activity, the formation and distribution of mineral resources and modern seismic activity in eastern China. Xinyi City has a very important position as a segmental node in the Shandong and Suwan sections of the Tan-Lu fault zone. Predecessors have conducted research on the spatial distribution, occurrence and activity characteristics of the shallow crustal faults in the Suqian section of the Tan-Lu belt, and have obtained some new scientific understandings and results. However, due to different research objectives or limitations of research methods, previous researches have either focused on the deep crustal structure, or targeted on the Suqian section or other regions. However, the structural style and deep-shallow structural association characteristics of Xinyi section of Tan-Lu belt have not been well illustrated, nor its activity and spatial distribution have been systematically studied. In order to investigate the shallow crustal structure features, the fault activities, the spatial distribution and the relationship between deep and shallow structures of the Xinyi section of the Tan-Lu Fault, we used a method combining mid-deep/shallow seismic reflection exploration and first-break wave imaging. Firstly, a mid-deep seismic reflection profile with a length of 33km and a coverage number greater than 30 was completed in the south of Xinyi City. At the same time, using the first arrival wave on the common shot record, the tomographic study of the shallow crust structure was carried out. Secondly, three shallow seismic reflection profiles and one refraction tomography profile with high resolution across faults were presented. The results show that the Xinyi section of Tan-Lu fault zone is a fault zone composed of five concealed main faults, with a structural pattern of “two grabens sandwiched by a barrier”. The five main faults reveal more clearly the structural style of “one base between two cuts” of the Tan-Lu fault zone. From west to east, the distribution is as follows: on the west side, there are two high-angle faults, F₄ and F₃, with a slot-shaped fault block falling in the middle, forming the western graben. In the middle, F₃ and F₂, two normal faults with opposite dip directions, are bounded and the middle discontinuity disk rises relatively to form a barrier. On the east side, F₂ and F₁, two conjugate high-angle faults, constitute the eastern graben. The mid-deep and shallow seismic reflection profiles indicate that the main faults of the Xinyi section of Tan-Lu fault zone have a consistent upper-lower relationship and obvious Quaternary activities, which play a significant role in controlling the characteristics of graben-barrier structure and thickness of Cenozoic strata. The shape of the reflective interface of the stratum and the characteristics of the shallow part of the fault revealed by shallow seismic reflection profiles are clear. The Mohe-Lingcheng Fault, Xinyi-Xindian Fault, Malingshan-Chonggangshan Fault and Shanzuokou-Sihong Fault not only broke the top surface of the bedrock, but also are hidden active faults since Quaternary, especially the Malingshan-Chonggangshan Fault which shows strong activity characteristics of Holocene. The results of this paper provide a seismological basis for an in-depth understanding of the deep dynamics process of Xinyi City and its surrounding areas, and for studying the deep-shallow tectonic association and its activity in the the Xinyi section of the Tan-Lu Fault.

Key words: Xinyi section of Tan-Lu fault zone, first-break traveltime imaging, seismic reflection profile, crustal structure and tectonics, Quaternary activity