Deep Structures In The Longmenshan Tectonic Belt And Its Constraint On The Wenchuan Earthquake Surface Ruptures

The Longmenshan tectonic belt is located at the transition zone between the Eastern Tibet and the Sichuan basin, where the landform, crustal thickness, magnetic and other geophysical characteristics change abruptly. The altitude difference in about50km distance across this belt can reach as much as4000-5000m. The steep topography and complex lithology of this belt makes it still lacks an accurate and comprehensive understanding about its deep structure. As a result, people can not well understand the seismogenic mechanism and the deep structure of the Wenchuan earthquake(Ms8.0), which happened on May12,2008, along this belt. The longmenshan tectonic belt is mainly composed of three faults, which are Wenchuan-Maoxian fault (Back Longmenshan fault), Yingxiu-Beichuan fault (Central Longmenshan fault), and Guanxian-Anxian fault (Frontal Longmenshan fault), respectively.The Wenchuan earthquake (Ms8.0) ruptured along both the Yingxiu-Beichuan fault and the Guanxian-Anxian fault, and produced two surface rupture zones, which are240-270km and～80km long respectively. The former rupture zone is characterized by thrusting accompanying right-lateral strike-slip motion. The latter is a pure thrust. One thrust type earthquake event ruptured two different faults and produced such a complex surface rupture zone, it is the first time that this phenomenon has been reported in the world. How does the complex rupture zone formed? Deep structure of the seismogenic faults is the key to understand the earthquake mechanism and the formation of the complex surface rupture zones.This dissertation is based on the "Wenchuan Earthquake Fault Scientific Drilling" of the National Science and Technology Planning Project. By analyzing the structures in the cores, comprehensive logging, interpretation of seismic reflection profiles that are deployed after the earthquake and old profiles before the earthquak, deep structures (～10km underground) of the Longmenshan tectonic belt are studied here. The relationship between the deep structure and the surface rupture is also discussed. The main conclusions of this study are listed as follow.(1) By cores study and comprehensive geophysical logging of the WFSD-1, WFSD-2, and WFSD-3boreholes, we found that the fault rocks in WFSD-1and WFSD-2(located in the hanging wall of the Yingxiu-Beichuan fault) are intensively distributed in Triassic Xujiahe formation, with a steep fault angle, whereas fault rocks in WFSD-3(located in the hanging wall of the Guanxian-Anxian fault) are loosely distributed in Triassic Xujiahe formation, with a more gentle fault angle. What we found indicate the activity of Yingxiu-Beichuan fault is much stronger than the Anxian-Guanxian fault.(2) The overlapping and repeated occurrences of Pengguan complex and Xujiahe formation in WFSD-2demonstrate thrust sheets are developed. Crustal shortening is intensive. and may be cut by the high angle Yingxiu-Beichuan fault.(3) Interpretation of seismic profiles reveal that the north segment of the Yingxiu-Beichuan fault dip～50°-70°to NW in～10km underground. The fault offset the Tangwangzhai nap for about3km. This indicates the north segment of the Yingxiu-Beichuan fault is a high angle thrust fault, which is coincided with the kinermatics of the surface ruptures, distribution of aftershocks, and focal mechanism of aftershocks.(4) Geophysical profiles revealed that the south segment of the Yingxiu-Beichuan fault is a high angle oblique thrust fault, with NW dipping angle of～45-56°at～7-8km depth. This south segment of the Yingxiu-Beichuan fault has main thrust movement feature, with slight horizontal movement. This fault character is coincided with the kinermatics of the surface ruptures, distribution of aftershocks, and focal mechanism of aftershocks.(5) Geophysical profiles revealed that the Guanxian-Anxian fault has characters of "fault ramp-fault flat-fault ramp" and low angle extends at<～10km depth, and this fault connected with the Yingxiu-Beichuan fault at～15km depth or more. The two faults extending to a decollement surface at～20km depth, which is the possible reason for the simultaneous activity during the Wenchuan Earthquake:(6) The high angle geometric feature at>10km depth of the Yingxiu-Beichuan fault not only constrain the linear distribution of the Wenchuan Earthquake surface rupture zone, but also confirmed the horizontal movement of the rupture zone and the symmetric deformation characteristics of the two sides of the fault; The low angle geometric feature at>10km depth of the Guanxian-Anxian fault constrain the pure thrust movement properties. the no-linear distribution of the Wenchuan Earthquake surface rupture zone by the topographic change, and asymmetric deformation characteristics of the two sides of the fault.