Micromechanisms of creep in clay‐rich gouge from the Central Deforming Zone of the San Andreas Fault

Abstract

We report the strength and constitutive behavior of gouge sampled from the Central Deforming Zone (CDZ) of the San Andreas Fault. Layers of flaked CDZ gouge were sheared in the triaxial saw cut configuration using the stress relaxation technique to measure the gouge strength over 4 orders of magnitude in shear strain rate and at rates as low as 5 × 10 −10 s −1 and 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• C) at the sampling depth of 2.7 km. Gouge was sheared dry and with brine pore fluid at 25 MPa pore pressure. Dry gouge is stronger and more rate strengthening than brine-saturated gouge. 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 ∼ 5 × 10 −9 s −1 . At strain rates greater than ∼ 5× 10 −9 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× 10 −9 s −1 indicates a change in the rate-controlling deformation mechanism. The magnitude of the friction rate dependence parameter, a, and the temperature sensitivity of a are consistent with crystal plasticity of the phyllosilicates. We hypothesize a micromechanical model for the CDZ gouge whereby a transition from fracture and delamination-accommodated frictional flow to crystal plasticity-accommodated frictional flow occurs with decreasing strain rate.

Localized Slip and Associated Fluidized Structures Record Seismic Slip in Clay‐Rich Fault Gouge

“…In clay‐rich fault rock, records of frictional heating during seismic slip are challenging to preserve unless the rock melted, in part because dehydration is a reversible process and decomposition products can be difficult to identify as such. Recent detailed structural analyses of carbonate fault gouges and rocks have shown that dynamic weakening processes can be recorded in the fabric of the fault rock (Bullock et al, ; Rempe et al, ; Smith et al, ), and herein we present additional evidence that the structures that form in clay‐rich fault rocks at seismic slip rates and low effective normal stress are distinct from those that have been shown to develop at subseismic deformation rates (e.g., French et al, ; Logan et al, ).…”

“…In fault gouge that has rate‐strengthening constitutive properties, like the CDZ gouge (Carpenter et al, ; Coble et al, ; French et al, ; Lockner et al, ), coseismic shear heating and resultant thermal pressurization can cause the apparent slip weakening and rate weakening observed in the CDZ gouge (French et al, ; Rice, ). Such thermally driven coseismic processes can also cause the localization of inherently slip‐ or rate‐strengthening fault rocks to a localized slip zone that has finite thickness (Rice, ; Rice et al, ).…”

“…Using optical microscopy, we determined our CDZ sample to be 90–95 vol % phyllosilicate matrix and 5–10 vol % clasts that are primarily serpentinite, quartz, calcite, and oxides (French et al, ), consistent with other samples of CDZ gouge whose composition is reported (Bradbury et al, ; Ikari et al, ; Moore, ). Our CDZ gouge was taken from the same block studied by Coble et al () and French et al () and has an X‐ray diffraction profile identical to the pattern reported by Lockner et al (). The intact CDZ gouge has a scaly fabric with shiny striated shear surfaces and is semicohesive (Schleicher et al, ); it holds its shape but can be broken apart by hand.…”

“…Previously, we measured the constitutive behavior of fault gouge from the CDZ at seismic slip rates (French et al, ) and at extremely low slip rates (French et al, ) to constrain the potential for seismic slip along the creeping segment of the SAF. In the 2014 study, we related the strength of the fault rock as a function of slip and slip rate to the thermal evolution of the gouge during shear and qualitatively described the concurrent evolution of microstructures.…”

Deformation in cemented mudrock (Callovo–Oxfordian Clay) by microcracking, granular flow and phyllosilicate plasticity: insights from triaxial deformation, broad ion beam polishing and scanning electron microscopy

“…Predictions of mechanical and transport properties over long timescales are essential for the evaluation of subsurface integrity. For this, it is generally agreed that a multiscale experimental approach that combines measurement of bulk mechanical and transport properties with microstructural study to identify deformation mechanisms is required to develop microphysics-based constitutive equations, which can be extrapolated to timescales not available in the laboratory, after comparison with naturally deformed specimens (Morgenstern and Tchalenko, 1967;Tchalenko, 1968;Lupini et al, 1981;Rutter et al, 1986;Logan et al, 1979Logan et al, , 1987Logan et al, , 1992Marone and Scholz, 1989;Evans and Wong, 1992;Katz and Reches, 2004;Niemeijer and Spiers, 2006;Colletini et al, 2009;Haines et al, 2009Haines et al, , 2013French et al, 2015;Crider, 2015;Ishi, 2016).…”