The geometry, kinematics of Tongnanba anticline and its structural model
Chen Longbo1, He Dengfa1, Wen Zhu2, Mei Qinghua1, Li Yingqiang1
1. Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation Mechanism, Ministry of Education, China University of Geosciences, Beijing 100083;
2. Key Laboratory of Shale Gas Exploration, Ministry of Land and Resources, Chongqing Institute of Geology and Mineral Resources, Chongqing 400042
Tongnanba anticline is located in the eastern part of Micang Mountain front in the north-eastern Sichuan Basin. It is the second largest anticline bearing oil-gas in the Sichuan Basin with comparatively lower prospecting degree than the rest parts in the basin. The NW-SE thrusting from Micang Mountain greatly influenced the origin of Tongnanba structure. Yet, the deformation feature and mechanism is still not fully understood. To discuss the geometry and kinematics of Tongnanba structure is of great value to reveal the origin of the anticline and to future guild oil-gas prospecting. Based on high-quality 3D seismic profile covering most of the anticline, we depict its structural geometric and kinematic feature by employing fault-related-fold theory and balanced restoration, and then present the structural model and discuss the origin of the anticline. Our study shows that the general feature of Tongnanba structure is: 3 segments, 4 structural layers and 1 structural superposition. Specifically, 1)Bounded by 3 detachments: The gypsum salt layers in Triassic Jialingjiang Formation, the mudstone layer in the top of Silurian and metamorphic rocks in the Precambrian serving as a weak detachment, 4 vertical structural layers and multi-detachments deformation system are therefore recognized(i.e., the upper structural layer, the middle structural layer, the lower structural layer and the deep structural layer), with the middle structural layer being greatest deformed. 2)The deep structural wedges are developed across the entire anticline, the number and geometry of these wedges directly affect the geometry overlying anticline and the difference in different segments. 3)Blocked by the rigid basement in the center of Sichuan Basin, the displacement of the wedge tip is unable to accommodated forward, but have to be accommodated by secondary fold accommodated faults developed by the rotation of the forelimb of the anticline. 4)Tongnanba anticline received NE-SW structural superposition from the thrusting from Daba Mountain which results in a series of structural duplex and imbricate structure developed within the middle structural layer in the northeastern segment of the anticline.