HETEROGENEOUS PERMEABILITY INDUCED BY FAULT ZONE DEFORMATION IN NONWELDED IGNIMBRITE: CHALK COVE FAULT, BISHOP TUFF, BISHOP, CALIFORNIA

Three features of fault zone deformation were identified that may act to constrain lateral flow in unsaturated nonwelded ignimbrites: (1) development of a fault gouge, with a decrease in grain size caused by grain deformation, leads to a marked decrease in permeability over that of host rock, (2) long open fractures paralleling the fault impede lateral flow, and (3) small-scale fractures and grain rotation in adjacent matrix blocks induce an interconnected porosity not observed in host rock. The effect of heterogeneity on lateral flow caused by capillary or permeability barriers in a gently sloping ignimbrite is a relevant issue for a potential high-level radioactive waste and spent nuclear fuel repository at Yucca Mountain, Nevada. Deformation-induced secondary heterogeneities adjacent a near-vertical fault in the nonwelded Bishop Tuff (Bishop, California) were studied through in-situ gas permeability, structural mapping, and laboratory analyses as analogs for fault deformation features within the poorly exposed Paintbrush nonwelded hydrogeologic unit at Yucca Mountain. Increases in permeability magnitude and variability above that of the host rock reflect the effect of small-scale deformation in the matrix adjacent the fault. Such features can reduce the continuity of potential capillary or permeability barriers, and thus any redistribution of percolation through lateral flow diversion.

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