INFLUENCE OF FLUID CHEMISTRY ON THE FRICTIONAL BEHAVIOR OF ULTRAMAFIC ROCKS

Ultramafic rocks are juxtaposed against more silica-rich rock types along subduction zone thrust faults and along continental faults such as the San Andreas fault that cut ophiolitic rocks. The contrast in rock chemistry, when combined with pore fluids at elevated temperatures, may promote fluid-assisted reactions that influence mechanical properties of these faults. To investigate this possibility, we are conducting triaxial experiments under hydrothermal conditions to determine the effect on the frictional properties of gouges of ultramafic rock caused by shearing them between granite driving blocks. Experiments to date were conducted at a fluid pressure (initially deionized water) of 50 MPa and effective normal stress of 100 MPa, at temperatures between 200 and 350C and shearing rates of 3.6-360 cm/yr. We have completed a series of experiments on gouges of antigorite-rich and lizardite-rich serpentinite and have initiated work on dunite gouge. All three gouges are markedly weaker when sheared between the granite blocks than they are when sheared between blocks of ultramafic rock. The reductions in strength are greatest at the highest temperatures and slowest shearing rates tested, with strength losses of as much as ~40% for serpentinite gouges and ~60% for dunite gouge. We consider the weakening to be driven by shearing in the presence of fluids whose chemistry was modified by interaction with the granitic wall rocks. The weakening mechanism may differ for the different gouges. Over the time span of the serpentinite experiments (1-14 days), the cause of the weakening appears to be a solution-transfer process involving dissolution and reprecipitation of serpentine, with the dissolution rate of serpentine varying significantly with fluid chemistry (e.g., pH). In contrast, weakening of the dunite gouge in experiments lasting <3 days is attributed to the growth of weak, Al-bearing, Mg-smectite clays on the shear surfaces. Minor amounts of the metastable Mg-smectite clay also crystallized during the longest-duration serpentinite experiments, and a reaction-weakening mechanism may dominate in the long term for the serpentinite gouges, as well. These results emphasize the influence of the chemical environment, particularly fluid chemistry, on the mechanical behavior of a fault containing ultramafic rocks.