Water In Granites From The Brittle-Plastic Transition Zone Of The Longmenshan Fault And Experimental Simulation Of Cracks Healing

The seismogenic fault of the Mw7.9Wenchuan earthquake showed a high-anglereverse slip. Based on the analysis of the mechanics for high-angle reverse slip, thehigh pore fluid pressure mightly exist in the deep fault, which is favorable for thesliding of the fault. The current technologies can not provide us the fluid status ofthe fault in the middle crust. The Longmenshan fault zone shows an overthrustfeature, and this made the Pengguan Complex lifted up to the surface, withsignatures of the status of deep fluid and deformation conditions in the earlygeological history. The deep environment in the early geological history can beexplored by studying the deformed granites at the outcrop in the surface to know thedeep seismogenic mechanism in the past, and this helps to understand the currentmechanism of the large earthquakes occurrence in the Longmenshan region, such asthe Mw7.9Wenchuan earthquake.Geological surveys were carried out along the surface ruptures of the Wenchuanearthquake. Deformed granites were collected in two ductile shear zone found in thesouthern section and middle section of the Yingxiu-Beichuan fault respectively. Thedeformation temperature and the flow stress were estimated by the deformationfabrics of quartz. Using Fourier transform infrared spectroscopy (FTIR),the types ofwater, water contents and its distribution were measured. The fluid compositions, icemelting temperature and homogenization temperature in fluid inclusions weremeasured to calculate the depth of the fluid capture and the pore fluid pressure usingmicro-thermometer and Laser Raman microspectrometer. The rehological structuresof the Longmenshan region were constructed to discuss the role of the high porefluid pressure on the fault slip. The simulating experiments with the Carrara marblewere carried out using the3GPa molten salt medium triaxial apparatus. This studyfocused on the dynamic recrystallization and the effect of the water and strain on themicro-cracks healing. The experimental simulatation of micro-cracks healing usingCarrara marble with the similar mechanism to the samples in the deep crust could beused to discuss the fractures healing by the dynamic recrystallization of the quartz in the brittle-plastic transition zone.The studies showed that inhomogeneous ductile deformation occurred in thedeep of the Yingxiu-Beichuan fault. The quartz and feldspar in the ductile shearzone were deformed by the subgrain rotation recrystallization, indicating thedeformation temperature is from400to500oC and the flow stress range from15to80MPa. The grain size of the quartz in the center of the fault is reduced by thedynamic recrystallization, which leads the deformation mechanism of thefine-grained quartz in the fault core to be the diffusion creep. The trace amountwater in quartz and feldspar include the hydroxyl in crystals, grain boundaries waterand fluid inclusions water. Water content of quartz is from0.001wt%to0.031wt%,and that of feldspar is in range of0.004wt%to0.103wt%, and water contentsincrease with the strain of rocks. Based on the study of fluid inclusions with thecomposition of NaCl-H2O, the temperature of fluid capture is from330to350℃,corresponding to the depth from18to19km in the Longmengshan fault zone, whichindicate the fluid pressure coefficient from0.15to0.9, implying sublithostatic porefluid pressure existed locally. High pore fluid pressure could weaken the faultsignificantly and increase the depth of the brittle-plastic transition zone according tothe rheological structures of the Longmenshan region which considering the porefluid pressure. All of these could help to trigger the brittle fault slip on the top of thebrittle-plastic transition zone.The microcracks-healing simulating experiments with Carrara marble showedthat a lot of brittle micro-cracks were developed during the loading under roomtemperature and high strain rate. However, most of the micro-cracks were healed bythe dynamic recrystallization of the calcite under high temperature and low strainrate subsequently. Water and larger strain could enhance the mico-cracks healing.The micro-carcks healing by dynamic recrystallization of quartz was found in theductile shear zone in the southern section of the Yingxiu-Beichuan fault. Themechanism of cracks healing in field is similar with the simulation experiments,which indicates dynamic recrystallization is one of the major fault healingmechanisms in the brittle-plastic transition zone probably. The cracks healing process could help to form high pore fluid pressure in local fault zone.