Dilatancy and pore pressure effects during fracture and shear of antigorite-rich serpentinite

Abstract:

Geophysical observations show evidence of near-lithostatic pore pressure in fault zones that experience episodic slow slip events. In addition, dilatancy hardening is one of several hypotheses proposed to explain the transient self-sustaining nature of slow slip. Thus, pore pressure evolution is possibly an important control on slow slip behavior. Phyllosilicates are abundant in regions of subduction zones and along the deep San Andreas Fault where slow slip is observed, yet previous work on phyllosilicate-bearing rocks indicates that fracture and frictional sliding occur with little dilation of the pore space. We present experimental results of pore pressure effects during fracture and frictional shear of antigorite-rich serpentinite.

Triaxial deformation experiments were conducted on intact cores 25.4 mm in diameter and 53.0 mm in length and ~1.5 mm thick gouge zones sandwiched between porous sandstone or steel sawcut forcing blocks. Experiments were conducted at room temperature, confining pressures from 50 to 100 MPa, pore pressure from 0 to 95 MPa, and axial displacement rates from 0.016 to 4.0 μm/s (shear rates from 0.02 to 5 μm/s). Foliated and macroscopically isotropic intact serpentinite samples were tested; fabric orientation is strongly correlated with both strength and dilatancy. When foliation is oriented parallel to σ₁, strength and dilatancy are greatest and similar to those for crystalline rocks. When foliation is oriented 45 ° to σ₁, however, dilatancy and strength are both greatly reduced. During frictional shear we observe a transition from rate-strengthening to rate-weakening behavior at ~0.5 μm/s, which is within range of typical slow slip rates and consistent with previous studies. This transition occurs concurrently with a transition from slip-induced compaction to dilation. Higher pore pressure correlates with overall higher friction coefficient, and also with lower compaction rates and higher dilation rates in the rate-strengthening and rate-weakening regimes, respectively. These preliminary results show that serpentinite exhibits many of the physical properties that promote slow slip behavior at room temperature. Ongoing studies at temperatures up to 400 °C will test whether these properties persist with the onset of thermally activated processes.