Abstract: The Daxing Fault is an important buried fault in the Beijing sub-plain, which is also the boundary fault of the structural unit between Langgu sub-sag and Daxing sub-uplift. So far, there is a lack of data on the shallow tectonic features of the Daxing Fault, especially for the key structural part of its northern section where it joins with the Xiadian Fault. In this paper, the fine stratigraphic classifications and shallow tectonic features of the northern section in the main Daxing Fault are explored by using three NW-trending shallow seismic reflection profiles. These profiles pass through the Daxing earthquake(M6¾)area in 1057AD and the northern section of the main Daxing Fault. The results show that seven strong reflection layers(T₀₁—T₀₃, T_Q and T₁₁—T₁₃)are recognized in the strata of Neogene and Quaternary beneath the investigated area. The largest depth of strong reflection layer(T₁₃)is about 550~850ms, which is interpreted as an important surface of unconformity between Neogene and Paleogene or basement rock. The remaining reflection layers, such as T₀₁ and T_Q, are interpreted as internal interfaces in Neogene to Quaternary strata. There are different rupture surfaces and slip as well as obviously different structural features of the Daxing Fault revealed in three shallow seismic reflection profiles. The two profiles(2-7 and 2-8)show obvious rupture surfaces, which are the expression of Daxing Fault in shallow strata. Along the profile(2-6), which is located at the end of the Daxing fault structure, a triangle deformation zone or bending fracture can be identified, implying that the Daxing Fault is manifested as bending deformation instead of rupture surfaces at its end section. This unique structural feature can be explained by a shearing motion at the end of extensional normal fault. Therefore, the Daxing Fault exhibits obviously different tectonic features of deformation or displacement at different structural locations. The attitude and displacement of the fault at the shallow part are also different to some extent. From the southwest section to the northeast section of the fault, the dip angle gradually becomes gentler(80°~60°), the upper breakpoint becomes deeper(160~600m), and the fault displacement in Neogene to Quaternary strata decreases(80~0m). Three shallow seismic reflection profiles also reveal that the Daxing Fault is a normal fault during Neogene to early Quaternary, and the deformation or displacement caused by the activity of the fault reaches the reflection layer T₀₂. This depth is equivalent to the sedimentary strata of late Early-Pleistocene. Therefore, the geometry and morphology of the Daxing Fault also reveal that the early normal fault activity has continued into the Early Pleistocene, but the evidence of activity is not obvious since the late Pleistocene. The earthquakes occurring along the Daxing Fault, such as Daxing earthquake(M6¾)in 1057AD, may not have much relation with this extensional normal fault, but with another new strike-slip fault. A series of focal mechanism solutions of modern earthquakes reveal that the seismic activity is closely related to the strike-slip fault. The Daxing Fault extends also downwards into the lower crust, and may be cut by the steeply dipping new Xiadian Fault on deep seismic reflection profile. The northern section of the Daxing Fault strikes NNE, with a length of about 23km, arranged in a right step pattern with the Xiadian Fault. Transrotational basins have been developed in the junction between the northern Daxing Fault and the southern Xiadian Fault. Such combined tectonic features of the Daxing Fault and Xiadian Fault evolute independently under the extensional structure background and control the development of the Langgu sub-sag and Dachang sub-sag, respectively.

Key words: the north section of Daxing Fault, shallow seismic reflection profile, Neogene-Quaternary, tectonic feature, fault activity

摘要： 跨大兴断裂北段的3条浅层二维地震反射剖面, 揭示了大兴断裂北段所在区的新近系—第四系结构与断裂结构特征。 研究结果表明: 大兴断裂北段所在区域的新近系—第四系内可识别出T₀₁—T₀₃、 T_Q和T₁₁—T₁₃ 共计7组强反射同相轴, 与其下的古近系及更早期的地层呈不整合接触。 大兴断裂是一条地壳深度的断裂, 其在浅部不同构造位置的产状、 断距等有所差异, 宏观呈现由SW向NE倾角减缓(80°~60°)、 上断点埋深变深(160~600m)、 新近系断距(80~0m)减小的特征。 在断裂NE末端表现为与三剪运动有关的变形加宽三角带, 而不是一条持续的断层。 断裂的几何结构和形态揭示其于新近纪—早第四纪仍延续了古近纪的伸展正断活动, 晚更新世以来无明显活动, 现今沿线发育的地震可能与此构造的关系不大。 大兴断裂的北段走向NNE, 长约23km, 与夏垫断裂呈右阶排列, 形成一个长约13km的断裂重叠区。 这种断裂组合在伸展构造背景下独立演化, 分别控制廊固、 大厂次级凹陷的发育, 并最终在断裂交接重叠区形成成熟型的转换斜坡。

关键词: 大兴断裂北段, 浅层地震, 新近纪-第四纪, 构造特征, 断层活动性