SLOW FAULT PROPAGATION IN SERPENTINITE UNDER CONDITIONS OF HIGH PORE FLUID PRESSURE (Invited Presentation)

Evidence for near-lithostatic pore fluid pressure exists along plate boundary faults that exhibit rupture propagation and slip rates that are up to 8 orders of magnitude slower than seismic rates. There are several hypotheses for the occurrence of these slow slip events and most are focused on processes that limit the acceleration fault slip. We investigate the role of high pore fluid pressure in limiting the rate of fault rupture propagation using experimental rock deformation. We faulted intact cores (1” diameter) of antigorite-rich serpentinite in a triaxial configuration at low effective stress (10 MPa), confining stress from 10 to 130 MPa, pore fluid pressure from 0 to 120 MPa, and at temperatures to 100 °C. In these experiments, the ratio of pore fluid pressure to confining stress ranges from 0 ≤ λ ≤ 0.92. For λ = 0 and at all temperatures tested, strain weakening associated with fault localization and propagation occurs rapidly (~2 s) and audibly. With increasing λ and temperature the rate of rupture propagation decreases significantly and faulting is silent. At the most extreme conditions, stress drops occur over 150 s. Our results show that the serpentinite exhibits no net dilation prior to peak strength, consistent with previous results for serpentinite. We observe an increase in pore volume during strain weakening, indicating that the serpentinite does dilate during rupture propagation. We infer that this dilation locally reduces pore fluid pressure and slows the rate of fault growth. We use the mechanical results and microstructural analyses of deformed samples to constrain a slip-weakening model of microcrack growth and coalescence and discuss how these results for intact rock can inform us of processes in mature fault zones.