Fault zone deformation overprints permeability of nonwelded ignimbrite; Chalk Cove Fault, Bishop Tuff, Bishop, California

Deformation-induced secondary heterogeneities associated with a steeply-dipping normal fault in the nonwelded Bishop Tuff were examined in a multidisciplinary study using in situ gas permeability tests, structural mapping, and laboratory analyses as analogs for fault deformation features within the poorly exposed Paintbrush nonwelded hydrogeologic unit at Yucca Mountain, Nevada. Three features of fault-zone deformation were identified that may act to constrain lateral flow in unsaturated nonwelded ignimbrites. First, development of a fault gouge, with a decrease in grain size caused by grain comminution, decreases the permeability of this element with respect to the host rock. Second, long open fractures paralleling the fault have an increased permeability with respect to the host rock. Third, small-scale fractures and grain rotation in adjacent matrix blocks induce an interconnected porosity not seen in host rock. Intrinsic permeability variation beyond what is observed in the host rock reflects the effect of small-scale deformation in the matrix adjacent the fault. These features can reduce the continuity of potential capillary or permeability barriers, and thus limit any redistribution of percolation that would result from lateral flow diversion.