Seismic Faulting Mechanisms In The Longmen Shan Tectonic Belt

Active faults include seismic faults and aseismic faults, here in this thesis, we focus on a seismic fault, recognize its structural characteristics and discuss the related seismic faulting mechanisms. The2008Wenchuan earthquake (Mw7.9) occurred in the eastern margin of the Tibetan Plateau in the Longmen Shan area, producing270km and80km-long co-seismic surface ruptures with different kinematics features along the Yingxiu-Beichuan and Guanxian-Anxian faults, respectively. The Wenchuan Earthquake fault Scientific Drilling project (WFSD), as a rapid response to the large earthquake, was carried out on November6,2008. This thesis focuses on the drilling core from the first borehole (WFSD-1) and the outcrop of the Yingxiu-Beichuan fault in Bajiaomiao village, Hongkou town, Dujiangyan city. Based on detailed fields surveys and drilling core records compiling, microstructure observations via optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM), were carried out on the samples that were systematically collected from both outcrop and the WFSD-1drilling cores. The physical properties and chemical composition of the samples were determined by Multi Sensor Core Logger systems (MSCL), powder X-ray diffraction and in situ synchrotron X-ray analyses. We identified the location of the Wenchuan earthquake Principal Slip Zone (PSZ) in the WFSD-1drilling cores, confirmed the composition and internal structure of the Yingxiu-Beichuan fault zone, and discussed the fault mechanisms in the Wenchuan earthquake fault zone. Followings are the main results that we have obtained:(1) The Yingxiu-Beichuan fault zone is confirmed to be～240m-wide from the Hongkou outcrop, corresponding to a～105m fault zone in the WFSD-1drilling cores. The Yingxiu-Beichuan fault zone possesses the characteristics of a multiple cores model and consists of5different fault rock units, from NW to SE:cataclasite unit, black fault gouge-breccia unit, gray fault breccia unit, dark-gray fault breccia unit and black fault gouge-breccia unit. The cataclasite unit consists of pseudotachylytes and cataclasite in the Pengguan complex, and the others consist of fault breccia and/or fault gouge in the Upper Triassic Xujiahe Formation.(2) Fault gouge microstructures and clay mineral distributions, combined with the characteristics of borehole temperature profiles, geophysical loggings, drilling mud gas monitoring and chemical compositions, the Wenchuan earthquake PSZ was determined to be located at-589.2m-depth in the WFSD-1drilling cores.(3) Viewed from the Bajiaomiao outcrop, the co-seismic surface rupture is located in the lower part, close to the bottom of the Yingxiu-Beichuan fault zone. There is an intersection angle of about15°～25°between the strikes of the surface rupture and the Yingxiu-Beichuan fault zone; the Wenchuan earthquake PSZ is located in the upper part, close to the top of the Yingxiu-Beichuan fault zone in the WFSD-1drilling cores. These indicate that the Wenchuan earthquake PSZ crosses the Yingxiu-Beichuan fault zone obliquely, implying that the Wenchuan earthquake fault and the Yingxiu-Beichuan fault may not be one same fault system.(4) Gray, dark-gray, brown and black pseudotachylyte veins with thicknesses ranging from several mm to～10cm are visible in the Yingxiu-Beichuan fault zone. Spherulites, microlites, flow textures, honeycomb-like vesicle structures, irregularly shaped and deep embayed clasts in pseudotachylytes, suggest that they are of melt-origin. Different colored veins and their overprinting relations observed at both macroscopic and microscopic scales, demonstrate that seismic faulting events and the associated generations of pseudotachylyte occurred repeatedly along the Yingxiu-Beichuan fault zone.(5) Frictional melt with masses of irregular fractures and no vesicle was observed in the WFSD-1cores at～732m-depth, indicating that large amounts of fluid invaded during earthquake. Frictional melt lubricated the fault, mading it easy to slip, with plenty of fluid flux cooling the melt rapidly, strengthening the fault and promoting a cessation of slip. This illustrates that vast fluid flux may strengthen the fault during co-seismic slip. This is the first evidence recorded in natural material showing fault weakening to strengthening during earthquake process, which is an important theoretical basis for understanding earthquake rupture process.(6) The Yingxiu-Beichuan fault zone consists of～105-240m-wide fault rocks and has the characteristics of a multiple cores model, indicating that large earthquakes occurred repeatedly along the Yingxiu-Beichuan fault. Cumulative offsets caused by numerous earthquakes promoted the Longmen Shan rapid uplift. Thermochronometry research indicates that the activity of the Yingxiu-Beichuan fault has controlled the rapid uplift of the Longmen Shan since it formed15～10Ma ago. The internal structure of the5different fault rocks combination in the fault zone may be related to the Longmen Shan uplift.(7) The pseudotachylytes in the Pengguan complex and the melt in sedimentary rocks show that frictional melt lubrication occurred during earthquake. Thermal pressurization and mechanical lubrication commonly exist during fault slip. These indicate that multiple fault weakening mechanisms may work during a single co-seismic event. Graphite locally enriched in the Wenchuan earthquake PSZ implies that graphitization occurred, as it is associated with strong dynamic weakening in the experiments, we infer that Yingxiu-Beichuan fault was extremely weak at borehole depths of-589.2m during the Wenchuan earthquake. The enrichment of graphite along localized slip zones could be used as an indicator of transient frictional heating during seismic slip in the upper crust, as well as an indicator for large earthquakes.