Unstable (and potentially seismogenic) sliding of gouge- bearing shear zones has been correlated in lab studies with the generation of localized zones of slip and shear zone dilation. The micromechanics responsible for these kinematic responses are poorly documented in the lab, but can be revealed by numerical simulations of the discrete mechanics of granular shear zones using the distinct element method (DEM). Two-dimensional (2D) simulations of shear were conducted on compact granular assemblages to explore thecorrelations between strength, dilation, and deformational response. Second order effects introduced by variations in particle size and particle size distribution (PSD) were examined as well, in order to explore the evolution of cataclastically deforming gouge. Scaled shear zones about 1 cm thick were filled with power-law distributed particles (radii of 500, 250, 125, and 62.5 Ám), and sheared to 200% strain to reach residual strength conditions. The 2D power law exponent, D, varied from 0.81 to 2.60; normal stresses ranged from 40 to 140 MPa.
The numerical experiments reveal a direct correspondence between shear strength and dilation rate (i.e., changes in volume with strain) which correlates well with deformational behavior. Episodes of peak strength correlate to high rates of dilation, and are accompanied by interlocking of the shear zone and distributed shear. Abrupt drops in strength correspond to decreases in dilation rate followed by eventual contraction, as localized zones of slip form and propagate through the shear zone. These observations are consistent with predictions of others (e.g., Marone et al., 1990, J. Geophys. Res., 95, 7007; Beeler et al., 1996, J. Geophys. Res., 101, 8697), based on energy considerations, that shear strength is sensitive to the work performed against normal stress during dilation. Variations in numerically determined shear strength also suggest second order dependencies on net dilation and PSD. The former results from changes in porosity, interparticle contact abundance, and magnitude and orientation of contact forces, all of which influence particle mobility. The PSD effects appear to arise from the partitioning of deformation between interparticle sliding and rolling. Enhanced interparticle rolling occurs in high D assemblages, leading to the self-organization of rolling particles into localized slip bands and a reduction in shear zone strength.