An Experimental Study On Frictional Sliding Of Fault Rocks From Longmenshan Fault Zone Under Hydrothermal Conditions

To investigate the frictional sliding behavior of typical fault rocks as related todynamics of faulting under different temperature and pressure conditions, wecollected natural gouge samples from fault exposures ruptured during the2008Wenchuan Mw7.9earthquake and phyllosilicate-rich mylonite from a ductile thrustzone of the Gengda-Wenmao fault, respectively, and conducted frictional slidingexperiments under hydrothermal conditions. According to the XRD results of naturalgouge samples, five samples of different mineralogical assemblies are chosen forexperiments with initial confining pressure of50MPa and at different temperaturesfrom25°C to150°C. For the fault mylonite, we applied experiments under highertemperature in the range of100°C-600°C and effective normal stress of200MPa and300MPa to represent the deeper portion conditions (elevated temperature and pressure)of fault zone. In order to obtain velocity dependence of friction, shearing rates arestepped up and down in the range of0.0488μm/s-6.1μm/s.In the experiments of natural gouge samples, we found that the frictioncoefficient of most samples shows a decreasing trend with increasing clay content.But the Pingxi black gouge sample (PX-3) shows a much lower value than theprediction by the general trend, indicating further strength reduction due to the~9%organic matter included. We also found a significant effect of temperature on thefrictional sliding stability of organic-matter-bearing and carbonate-rich gouges. Whentemperature increases to150°C, the Pingxi black gouge (with organic matter) exhibitsunstable oscillation that indicates velocity-weakening frictional sliding behavior, incontrast to the velocity-strengthening behavior at lower temperatures. The velocitydependence of carbonate-rich samples with more than50wt.%carbonates is alsosensitive to temperature and shearing rates, and all samples have smaller velocitydependence of friction (a-b) at higher temperatures. The (a-b) value of the PX-2sample (with~80%carbonates) even decreases to the velocity neutral point at150°Cand slow shearing rates.At higher temperature and pressure conditions, the frictional coefficient ofmylonite exhibits systematic increase with increasing temperature (100°C-600°C).The frictional behavior of mylonite shows a transition from initialvelocity-strengthening behavior to velocity-weakening behavior at about300°C, and then transitions back to velocity-strengthening behavior as temperature is elevated.The velocity dependence of mylonite also shows strong pressure sensitivity. When theeffective normal stress is increased to300MPa, the stable frictional behavior issignificantly enhanced with larger (a-b) compared to that under the lower pressurecondition. The transition from velocity weakening to velocity strengthening frictionalbehavior may be controlled by competition between pressure-solution healing anddilation weakening processes in the mylonite gouge.In the framework of rate and state friction constitutive law, the effects offrictional properties of natural gouge and mylonite on faulting dynamics are discussed.We found that fast slip nucleation is possible for faults associated with theorganic-matter-bearing gouge at150°C, whereas triggered seismic slips are possiblein faults that contain the carbonate-rich samples which have a small (a-b) value at150°C. The temperature-dependent sliding behavior of carbonate-dominant andorganic-matter-bearing gouges probably provides a new clue to the problem oftransition from the shallow aseismic zone to the seismic zone on the subductioninterfaces of oceanic plates. From experimental results of mylonite, at200MPaeffective normal stress unstable slip events may nucleate in mylonite gouge under250oC-550oC temperature conditions, and transitional behavior is found at600oCwhich is rate dependent. At300MPa effective normal stress, the temperaturecondition for the nucleation of unstable slip events is constrained to260oC-320oC(corresponding to15-19km depth of Longmenshan fault zone), and rate dependent,transitional behavior is also found at400oC. For velocity-strengthening cases under100oC-600oC temperature conditions, a large loading disturbance from neighbouringarea may cause triggered slip events at the corresponding depth.