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Deformation bands in nonwelded ignimbrites: Petrophysical controls on fault-zone deformation and evidence of preferential fluid flow

¹ Earth and Environmental Science Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA
² Earth and Environmental Sciences Division (EES-9), Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

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.

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