Two dimensional (2-D) numerical simulations were conducted using the distinct element method (DEM) to explore the influences of particle size distribution (PSD), defined by 2-D power law exponent D, and of interparticle friction μp on mechanical behavior and strength of granular shear zones. The value of D ranged from 0.81 (a coarse breccia) to 2.60 (fine-grained gouge); μp was assigned to 0.10, 0.50, or 0.75; normal stresses on the shear zone walls, &sgr;n, varied from 40 to 140 MPa; assemblages were sheared to 200% strain. Fault friction, defined as the ratio of shear to normal stress, μf=&tgr;/&sgr;n, was quite low for all experiments. Low μp suites yielded μf≈0.20--0.25, while higher μp values resulted in only slightly higher values for μf≈0.25--32. The stress-strain response of the latter experiments was similar to that of overconsolidated granular assemblages: a peak strength was reached by about 10% strain, followed by a period of strain weakening to 30--50% strain, and finally stabilizing at a residual strength for the rest of the experiment. The transitional phases were accompanied by increasing shear zone dilation of up to about 1.5%. The low μp suites behaved more as normally consolidated assemblages; they showed no noticeable strain weakening and relatively minor dilation of about 0.2%. The anomalously low strengths of the simulated assemblages can be explained largely by high degrees of particle rolling. Periodic drops in shear strength during residual deformation phase of the experiments correlated directly with reduced rates of dilation and the localization of strain. Fault strength also showed second-order variations with D: the low μp suites showed a steady decline in maximum residual strength μfmax with increasing D due to the importance of interparticle sliding in all configurations; in the higher μp suites, μfmax decreased for D values less than a characteristic value of 1.60, then leveled out for increasing D. This may be explained by the increasing importance of particle rolling as small particles became more abundant with increasing D;the particles began to self-organize and strain became more localized. Although the simulations lack particle fracture, they offer insight into how micromechanics control the mechanical evolution of granular shear zones. ¿ 1999 American Geophysical Union