DEVELOPMENT OF SHEAR LOCALIZATION IN SIMULATED QUARTZ GOUGE - EFFECT OF CUMULATIVE SLIP AND GOUGE PARTICLE-SIZE

Frictional sliding experiments were conducted on two types of simulate d quartz gouge (with median particle diameters 5 mu m and 25 mu m, res pectively) at confining pressures ranging from 50 MPa to 190 MPa in a conventional triaxial configuration. To investigate the operative micr omechanical processes, deformation texture developed in the gouge laye r was studied in samples which had accumulated different amounts of fr ictional dip and undergone different stability modes of sliding. The s patial patterning of shear localization was characterized by a quantit ative measurement of the shear band density and orientation. Shear loc alization in the ultrafine quartz gouge initiated very early before th e onset of frictional sliding. Various modes of shear localization wer e evident, but within the gouge zone R(1)-shears were predominant. The density of shear localization increased with cumulative slip, whereas the angle subtended at the rock-gouge interface decreased. Destabiliz ation of the sliding behavior in the ultrafine quartz gouge correspond ed to the extension of R(1)-shears and formation of boundary Y-shear s egments, whereas stabilization with cumulative slip was related to the coalescence of Y-shear segments to form a throughgoing boundary shear . In the coarse quartz gouge, the sliding behavior was relatively stab le, probably because shear localization was inhibited by distributed c omminution. Two different models were formulated to analyze the stress field within the gouge zone, with fundamentally different predictions on the orientations of the principal stresses. If the rock-gouge inte rface is assumed to be bonded without any displacement discontinuity, then the maximum principal stress in the gouge zone is predicted to su btend an angle greater than 45 degrees at the interface. If no assumpt ion on displacement or strain continuity is made and if the gouge has yielded as a Coulomb material, then the maximum principal stress in th e gouge zone is predicted to subtend an angle less than 45 degrees. If the apparent friction coefficient increases with overall slip (i.e., slip-hardening), then the Riedel shear angle progressively decreases w ith increasing shear strain within the gouge layer, possibly attaining a zero value which corresponds to a boundary Y-shear. Our quantitativ e data on shear localization orientation are in reasonable agreement w ith this second model, which implies the coefficient of internal frict ion to be about 0.75 for the ultrafine quartz gouge and 0.8 for the co arse gouge. The wide range of orientations for Riedel shear localizati on observed in natural faults suggests that the orientations of princi pal stresses vary as much as in an experimental gouge zone.