Fault weakening by water in the brittle-plastic transition zone of the Wenchuan fault zone (Longmenshan, China):a study through deformed exhumed granites

The seismogenic fault of the 2008 Mw 7.9 Wenchuan earthquake showed a high-angle reverse slip, and the hypocenter lies in the middle crust. High pore fluid pressure and/or low friction coefficient related to water-rock reaction may have contributed to fault sliding. The current technologies cannot give us the water content in the deep fault. We determined the water content in deformed granites at the outcrops of two ductile shear zones found in the southern and middle sections of the Yingxiu-Beichuan fault, and estimated the deformation temperature and flow stress that the samples experienced in early geological history to explore the past earthquake mechanisms in the Longmenshan region. The microstructures of deformed granites showed that inhomogeneous ductile deformation occurred in the deep fault. The deformation temperature is ~400 to 500°C , obtained by the deformation fabrics of quartz, and the flow stress ranges from 15 to 80 MPa. Using Fourier Transform Infrared spectroscopy (FTIR), the trace amount water in quartz and feldspar include hydroxyl in crystals, grain boundaries water and fluid inclusions. The quartz water content ranges from 0.001 wt% to 0.031 wt%, and that of feldspar from 0.004 wt% to 0.103 wt%. Trace-amount water increases with the strain of rocks, and water contents with fluid inclusions decrease with deformation. Based on the study of fluid inclusions with the composition of NaCl-H2O using micro-thermometer and Laser Raman micro-spectrometer, the temperature of fluid capture is estimated to range from 330 to 350°C, corresponding to a depth of 18 to 19 km in the Longmengshan fault zone, which indicates that the fluid pressure coefficient ranges from 0.15 to 0.9, implying that the pore fluid pressure varied form low to high. Both high pore fluid pressure and minerals with low friction coefficient generated by water-rock reaction could weaken the fault significantly and increase the depth of the brittle-plastic transition zone according to the reconstructed rheological structures of the Longmenshan region. All of these could lead to a brittle fault slip on top of the brittle-plastic transition zone. In addition, we built a preliminary model with respect to water transition in a seismic cycle using the data of water content and fluid pressure across the deep fault.