Abstract: The Pengxian blind fault is a typical active fault in the central Longmen Shan front belt. It has important reference value for understanding the growth mode and process of the eastern Tibetan plateau. Because the fault is covered by the thick Upper Cenozoic strata in the western Sichuan Basin, its three-dimensional spatial distribution, structural style and formation mechanism remain unclear. In this paper, based on several high-resolution 3-D seismic reflection profiles, together with near-surface geological data and borehole data, we investigate the structural geometry of the Pengxian blind fault and build a 3-D model based on the results. We analyze the shape and scale of underground spatial distribution of the fault through a three-dimensional fault model. According to the theory of fault-related fold and fold-accommodation fault, this paper discusses the forming mechanism of the Pengxian buried structures. The shallow tectonic deformation in front of Longmen Shan is closely related to the detachment layer of the Middle and Lower Triassic, and this detachment layer f1 horizontally propagates into the Longquanshan anticline in the western Sichuan Basin. The Pengxian buried fault is a typical fault-bend fold and the f1 horizontally propagates into the western Sichuan Basin with a fault slip of 3.5km. The Pengxian blind fault is a high angle(50°~60°)thrust fault developed in the front wing of the kink-band zone, striking NE-SW, with a total length of~50km; But the fault is not connected with the Dayi buried fault in the south section of Longmen Shan. They are two different faults, and this defines the scale of the Pengxian blind fault. This limitation makes sense for analyzing and evaluating the magnitudes of potential earthquake. All above study provides research basis for further analysis of the potential seismic risk in this area. The Pengxian blind fault is parallel to the anticlinal axis with small amount of offset as a fold-accommodation fault. We believe that the fault formation is related to the fold deformation of the fold front limb. The study reveals the geometry, kinematics and formation mechanism of the Pengxian active fault, and provides a basis for further analysis of fault activity and hazard. Therefore, there is little possibility of strong earthquakes at the Pengxian blind fault due to its formation mechanism of the fault which is generally characterized by fold deformation and shortening deformation. In this paper, we discuss the location of Pengxian blind fault in the middle of Longmen Shan and Sichuan Basin. Because the Pengxian buried structures are in the transition area, the shortening amount in Pengxian indicates that the absorption in the basin is quite limited. It reflects the blocking effect of Sichuan Basin. In the study, we find that the relationship between folds, faults and sediments is an important part of tectonic interpretation; the theory of fault-related fold and fold-accommodation fault is well used for analysis. This would have great significance for the study of structural deformation, which can help to build a three-dimensional model of fault.

Key words: seismic interpretation, Pengxian blind fault, fault-related folding, fold-accommodation fault, 3-D model, central Longmen Shan

摘要： 彭县隐伏断裂是龙门山中段山前的一条典型活动断裂，其构造特征对认识龙门山构造带向四川盆地生长的方式和过程具有重要的参考价值，但因其被四川盆地晚新生代较厚地层所覆盖，该断裂的构造样式、三维空间展布、形成机制以及发震能力等问题尚不明确。文中利用多条高精度三维地震反射剖面，结合区域地质与钻井资料，对彭县隐伏断裂的几何结构进行综合分析，建立了三维断层模型；同时根据断层相关褶皱与褶皱调节断层原理，对彭县隐伏构造的形成机制进行了探讨。研究结果表明，龙门山山前的浅层构造变形与中下三叠统的滑脱层f₁密切相关。该滑脱层一直向E延伸至四川盆地西部的龙泉山，彭县隐伏背斜为该滑脱层f₁之上的低角度断层转折褶皱，彭县隐伏断裂则发育在彭县隐伏背斜膝折带的前翼，是一条高角度（50°~60°）的逆断裂，走向NE-SW，总长约50km；彭县隐伏断裂与龙门山南段大邑隐伏断裂并不相连，非同一条断裂。彭县隐伏断裂与彭县隐伏背斜褶皱轴的走向一致且断距较小，该断裂的形成与背斜前翼褶皱变形有关，分析认为是一条褶皱调节性断层。文中系统地揭示了彭县隐伏活动断裂的几何学和运动学特征，对进一步分析断层活动性和危险性提供了依据。

关键词: 地震解释, 彭县隐伏断裂, 断层转折褶皱, 褶皱调节性断层, 三维模型, 龙门山中段