[1] We investigated the frictional sliding behavior of simulated quartz-clay gouges under stress conditions relevant to seismogenic depths. Conventional triaxial compression tests were conducted at 40 MPa effective normal stress on saturated saw cut samples containing binary and ternary mixtures of quartz, montmorillonite, and illite. In all cases, frictional strengths of mixtures fall between the end-members of pure quartz (strongest) and clay (weakest). The overall trend was a decrease in strength with increasing clay content. In the illite/quartz mixture the trend was nearly linear, while in the montmorillonite mixtures a sigmoidal trend with three strength regimes was noted.Microstructural observations were performed on the deformed samples to characterize the geometric attributes of shear localization within the gouge layers. Two micromechanical models were used to analyze the critical clay fractions for the two-regime transitions on the basis of clay porosity and packing of the quartz grains. The transition from regime 1 (high strength) to 2 (intermediate strength) is associated with the shift from a stresssupporting framework of quartz grains to a clay matrix embedded with disperse quartz grains, manifested by the development of P-foliation and reduction in Riedel shear angle. The transition from regime 2 (intermediate strength) to 3 (low strength) is attributed to the development of shear localization in the clay matrix, occurring only when the neighboring layers of quartz grains are separated by a critical clay thickness. Our mixture data relating strength degradation to clay content agree well with strengths of natural shear zone materials obtained from scientific deep drilling projects.

[1] Compaction bands are a compactant failure mode in porous rock, forming thin tabular structures normal to the maximum compressive stress with negligible shear offset. We investigated the conditions involved in the development of compaction bands in sandstone, including the influence of composition and the geometric attributes of the bands across a range of length scales. To extend beyond existing laboratory data on the relatively pure quartz Bentheim sandstone, a suite of triaxial experiments were conducted on Diemelstadt and Bleurswiller arkoses. Mechanical data and microstructural observations demonstrate that compaction bands can develop in compositionally heterogeneous rock and are the dominant failure mode in the transitional regime from brittle faulting to cataclastic flow. Synthesis of field and laboratory data on band dimensions in five sandstones over four orders of magnitude revealed a quadratic scaling relation between the thickness and length of compaction bands, wherein thickness is proportional to the square root of the band length. Using an anticrack/antidislocation fracture mechanics model, we obtained a scaling relation in which the stress level is inversely proportional to band thickness. We show that this relation provides a mechanical basis for interpreting discrepancies between laboratory and field data. Together, the laboratory and field data constrain the critical strain energy release rate in the model to be on the order of 2-80 kJ/m 2 , comparable with laboratory measurements.

[1] To investigate how strain localization develops from a structural and stress heterogeneity, we introduced a V-shaped circumferential notch in cylindrical samples of Bentheim and Berea sandstones and conducted triaxial compression tests at confining pressure optimum for compactive failure. The critical stresses for initial yield map out a cap with a negative slope in the stress space, and the presence of the notch enhanced the local stress which induced damage to occur at remote stresses significantly lower than in the unnotched sample. Our mechanical and microstructural data demonstrate the spectrum of failure modes and depict the initiation and propagation of a localized structure. In the Bentheim sandstone we observed discrete compaction bands that propagated through the sample cross section with episodic force drops, and in the Berea sandstone, we observed diffuse bands accompanied by strain hardening. Compactive yield was marked by an upsurge in acoustic emissions, corresponding to the formation of a process zone at the notch tip. To probe the initial yield behavior and geometry of the process zone, we developed a micromechanical model using linear elastic fracture mechanics, which predicts a process zone extending 0.3-0.5 of the notch depth, in accord with the microstructural observations. Analysis of the stress path reveals the potential activation of multiple localization modes around the notch tip. The energy required to develop a compactive deformation band was inferred to range from 6 to 43 kJ m À2 . In the Bentheim sandstone the energy was observed to be inversely dependent on the confining pressure.