The impact of faults on fluid flow and transport through thick vadose zones depends in part on the nature of fault-zone deformation. Both fractures and deformation bands occur in ignimbrite sequences at Los Alamos, New Mexico, and Busted Butte, Nevada. The primary controls on mode of failure are grain-contact area and strength, which are directly related to degree of welding and crystallization and inversely proportional to porosity. Low-porosity welded units deform by transgranular fracture; high-porosity, glassy, nonwelded units deform by cataclasis within deformation bands. Moderately high porosity, nonwelded units that have undergone devitrification and/or vapor-phase crystallization form either deformation bands or fractures, depending on local variations in the degree and nature of crystallization. Grain- and pore-size reduction in deformation bands commonly produces indurated, tabular zones of clay-sized fault material. Many of these bands are locally rich in smectite and/or cemented by carbonate. Preferential wetting of deformation bands is inferred to promote alteration and cementation. We therefore interpret variably altered fault-zone material as evidence of preferential fluid flow in the vadose zone, which we infer to result from enhanced unsaturated permeability due to pore-size reduction in deformation bands.