Micro-mechanisms of creep in clay-rich gouge from the Central Deforming Zone of the San Andreas Fault
Melodie E. French, Frederick M. Chester, & Judith S. ChesterPublished 2015, SCEC Contribution #1959
We report the strength and constitutive behavior of gouge from the Central Deforming Zone (CDZ) of the San Andreas Fault. Layers of flaked CDZ gouge were deformed in the triaxial sawcut configuration; the stress relaxation technique was employed to measure the gouge strength over four orders of magnitude in shear strain rate, and at rates (5 x 10-10 s-1) as low as within an order of magnitude of in-situ rates. Deformation conditions correspond to the in-situ effective normal stress (100 MPa) and temperature (65 to 120 ◦10 C) at the sampling depth of 2.7 km. Gouge was sheared with brine pore fluid at 25 MPa pore pressure, and also dry. Brine-saturated CDZ gouge strengthens with increasing strain rate and decreasing temperature, and the dependencies of strength on strain rate and temperature increase at rates below 5x10-8 s-1. Dry gouge is stronger and more rate-strengthening than brine-saturated gouge. At strain rates greater than 5 x 10-8 s-1, the rate-dependence is consistent with previous studies on the CDZ gouge conducted at even higher rates. The increase in rate-dependence below 5 x 10-8 s-1 indicates a change in the rate-controlling deformation mechanism. The magnitude of the friction rate-dependence parameter, a, and its temperature sensitivity are consistent with dislocation glide in phyllosilicates. We hypothesize a micromechanical model for the CDZ gouge whereby a transition from fracture and delamination accommodated frictional flow to dislocation glide accommodated frictional flow occurs with decreasing strain rate.
Citation
French, M. E., Chester, F. M., & Chester, J. S. (2015). Micro-mechanisms of creep in clay-rich gouge from the Central Deforming Zone of the San Andreas Fault. Journal of Geophysical Research: Solid Earth, 120(2), 827-849.
Related Projects & Working Groups
Fault and Rupture Mechanics
