The role of fault zone fabric and lithification state on frictional strength, constitutive behavior, and deformation microstructure

Abstract

[1] We examine the frictional behavior of a range of lithified rocks used as analogs for fault rocks, cataclasites and ultracataclasites at seismogenic depths and compare them with gouge powders commonly used in experimental studies of faults. At normal stresses of ∼50 MPa, the frictional strength of lithified, isotropic hard rocks is generally higher than their powdered equivalents, whereas foliated phyllosilicate-rich fault rocks are generally weaker than powdered fault gouge, depending on foliation intensity. Most samples exhibit velocity-strengthening frictional behavior, in which sliding friction increases with slip velocity, with velocity weakening limited to phyllosilicate-poor samples. This suggests that lithification of phyllosilicate-rich fault gouge alone is insufficient to allow earthquake nucleation. Microstructural observations show prominent, throughgoing shear planes and grain comminution in the R1 Riedel orientation and some evidence of boundary shear in phyllosilicate-poor samples, while more complicated, anastomosing features at lower angles are common for phyllosilicate-rich samples. Comparison between powdered gouges of differing thicknesses shows that higher Riedel shear angles correlate with lower apparent coefficients of friction in thick fault zones. This suggests that the difference between the measured apparent friction and the true internal friction depends on the orientation of internal deformation structures, consistent with theoretical considerations of stress rotation.Citation: Ikari, M. J., A. R. Niemeijer, and C. Marone (2011), The role of fault zone fabric and lithification state on frictional strength, constitutive behavior, and deformation microstructure,

Shear behavior of DFDP-1 borehole samples from the Alpine Fault, New Zealand, under a wide range of experimental conditions

“…We do not observe a significant difference in frictional properties between the three samples tested as intact cylinders compared to disaggregated samples (Figs. 6, 7) which is somewhat surprising considering that rock fabric can have a significant effect depending on the intensity of the foliation and whether phyllosilicate minerals are present (e.g., Collettini et al 2009;Ikari et al 2011). For our samples, the predominant rock fabric dips 35°-65° based on visual core description, consistent with earlier field observations indicating that the foliation of Alpine Fault-related rocks dips ~40°-50° (Sibson et al 1981).…”

Heterogeneous strength and fault zone complexity of carbonate-bearing thrusts with possible implications for seismicity

“…at greater depth (e.g. Ikari et al, 2013;Verbene et al, 2013). Earthquake nucleation is therefore facilitated in cataclastic, calcite-rich materials because of the elastic energy accumulation, due to high strength and high healing rates, combined with the tendency for unstable slip behaviour…”

A “slice-and-view” (FIB–SEM) study of clay gouge from the SAFOD creeping section of the San Andreas Fault at ∼2.7 km depth

“…That foliated clay gouge can be significantly weaker than equivalent powder samples has been demonstrated in rock deformation experiments (Collettini et al, 2009;Haines et al, 2013;Ikari et al, 2011a) indicating that weak faults can occur at low concentrations of clay minerals, as long as they form an interconnected fabric. Based on electron microscopic observations of various mudrock-based gouges from the creeping zone of the SAFOD borehole, it was also proposed that smectite-coated foliated slip surfaces occur at the nanometre-scale, and were termed "clay nanocoatings" (Schleicher et al, 2010).…”