Quantitative Study Of Blind Active Faults In Compressional Tectonic Environment

Since Cenozoic,the Longmen Shan has experienced compressive tectonic evolution because of the collision between India plate and Eurasia plate.A large number of thrust faults are developed in the Longmen Shan,and there are many hidden active faults in the Longmen Shan front belt.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.Due to 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 remains unclear.In this paper,we rely on several high-resolution 3-D seismic reflection profiles,together with near-surface geological data and borehole data to investigate the structural geometry of the Pengxian blind fault,and to build a3-D model based on the results;We analyze the shape and scale of underground space distribution through 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.Finally,we analyze the impact of the Pengxian blind fault on the eastward extension of the eastern margin of the Tibetan plateau.Main conclusion of this essay includes:The shallow tectonic deformation in front of the Longmen Shan is closely related to the detachment layer of the Middle and Lower Triassic,and this detachment layer f1 horizontally propagated into the Longquanshan anticline in the western Sichuan Basin.The Pengxian buried structure is a typical fault-bend fold and it shows that the f1 horizontally propagated into the western Sichuan Basin with a fault slipped 3.5 km.The Pengxian blind fault developed in the front wing of the-kink-band zone,and it 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.Therefore,there is little possibility of strong earthquakes at the Pengxian buried fault because of the formation mechanism of the fault.The Pengxian blind fault is a high angle(50°-60°)thrust fault developed in the front wing of the-kink-band zone,strike to NE-SW with a total length of~50 Km.The Pengxian blind fault did not cut through the basement,and it extends down 8 km,which was connected to the detachment layer of the Middle and Lower Triassic.According to the 3-D model of the fault plane,the fracture area of the fault can be calculated to be about 450km~2.The Pengxian blind 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 the analying and evaluating the magnitudes of potential earthquake.All above study provides research basis for further analysis of the potential seismic in this area.The Pengxian blind structure is located in the transition zone between the Longmen Shan and the western Sichuan Basin.With the result of fault kinematics and growing strata analysis,there are at least two large-scale tectonic deformations.In the late Miocene 10-15Ma,the Longquanshan anticline began to rise,and movement time of the Pengxian buried structure should be earlier than the initial exercise of Longquanshan anticline.The Pengxian anticline slides 3.6-4km along the detachment layer f1,and the fold deformation is mainly the migration of the knee fold;At the Late Pliocene(3.6Ma),the Pengxian anticline rises slowly again and is deformed mainly by the rotation of the wing.