Constitutive behavior of gouge from the Central Deforming Zone of the San Andreas Fault approaching in-situ strain-rates

Well-foliated smectite-rich fault gouge was recovered from the actively creeping Southwest Deforming Zone (SDZ) and the Central Deforming Zone (CDZ) during drilling of the San Andreas Fault Observatory at Depth (SAFOD). The SDZ and CDZ have a combined thickness of ~4m and are believed to accommodate the majority of aseismic creep (20 mm/yr) along the central segment of the San Andreas Fault (SAF). Assuming distributed shear, the in-situ shear-strain rate here is ~10 ^-10 s^-1. This study investigates the constitutive behavior of the well-foliated Mg-smectite CDZ gouge recovered from 2.7 km depth. Previously published laboratory friction experiments and microstructural studies of CDZ gouge have proposed several explanations for the fault creep including: inherently low strength of saponite, slip along films of nanometer sized clay-particles that line the clasts and shear surfaces that define a scaly fabric, and pressure solution creep. To test these hypotheses, we conducted stress-relaxation experiments on the CDZ gouge; stress-relaxation allows measurement of strength over several orders magnitude of strain-rate, and at strain-rates lower than typically achieved in constant stress or constant displacement-rate experiments. The gouge was gently fragmented to ~800 micrometers to preserve the clay microfabric. Gouge layers ~2 mm thick were sheared at 100° C and 100 MPa effective normal stress between sawcut cylinders using a triaxial deformation apparatus. We report the results of 5 experiments during which we conducted 17 stress-relaxation tests lasting from 1 to 30 days. Relaxation tests achieved strain rates from 10^-9 to 10^-5 s^-1 (approaching in-situ rates). The gouge was deformed either room-dry or saturated with a brine similar in composition to the in-situ pore fluid chemistry at SAFOD; for one experiment temperature was stepped between 60-100° C. Future experiments will be conducted on finely powdered gouge to test the effects of microstructure. In order to constrain the deformation mechanisms that accommodate fault creep, the mechanical data is analyzed using both a power-law flow and temperature-dependent friction constitutive relations. The brine-saturated gouge shows a marked reduction in strength (i.e. an increase in rate-dependence) at strain rates less than 5×10^-7 s^-1, indicating a change in the rate-controlling deformation mechanism. The CDZ gouge strength is best described by a temperature dependent friction law, which indicates temperature weakening and strain-rate strengthening behavior at near in-situ temperature, normal stress, and strain-rate conditions. Dry gouge is stronger and more rate-strengthening than brine-saturated gouge at all experimental conditions tested. These results suggest that, although the strength of the CDZ gouge is weaker and more strain-rate strengthening at in-situ rates than previously reported, the primary deformation mechanism cannot be described with a pressure solution creep law and may be best described as frictional flow.