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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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Of interest to the Underground Nuclear Explosion Signatures Experiment are patterns and timing of explosion‐generated noble gases that reach the land surface. The impact of potentially simultaneous flow of water and gas on noble gas transport in heterogeneous fractured rock is a current scientific knowledge gap. This article presents field and laboratory data to constrain and justify a triple continua conceptual model with multimodal multiphase fluid flow constitutive equations that represents host rock matrix, natural fractures, and induced fractures from past underground nuclear explosions (UNEs) at Aqueduct and Pahute Mesas, Nevada National Security Site, Nevada, USA. Capillary pressure from mercury intrusion and direct air–water measurements on volcanic tuff core samples exhibit extreme spatial heterogeneity (i.e., variation over multiple orders of magnitude). Petrographic observations indicate that heterogeneity derives from multimodal pore structures in ash‐flow tuff components and post‐depositional alteration processes. Comparisons of pre‐ and post‐UNE samples reveal different pore size distributions that are due in part to microfractures. Capillary pressure relationships require a multimodal van Genuchten (VG) constitutive model to best fit the data. Relative permeability estimations based on unimodal VG fits to capillary pressure can be different from those based on bimodal VG fits, implying the choice of unimodal vs. bimodal fits may greatly affect flow and transport predictions of noble gas signatures. The range in measured capillary pressure and predicted relative permeability curves for a given lithology and between lithologies highlights the need for future modeling to consider spatially distributed properties.

[1] Deformation band faults in nonwelded ignimbrites of the Bandelier Tuff record unsaturated fluid flow and transport. More than two thirds of the faults studied display diagenetic minerals virtually absent from adjacent protolith. Stable isotope analyses indicate that smectite enrichment and calcite cementation result from low-temperature meteoric fluid-fault interaction. Fault zone microstructures, rare earth element signatures, and mineralogy indicate that smectite was introduced to deformation band fault zones by a combination of translocation from the surface and in situ alteration of fault gouge. Rod-shaped calcite microcrystallites and the close association of calcite with plant roots suggest a combination of pedogenic, biologically mediated, and physicochemical precipitation. Enrichment in smectite and calcite modified major oxide and trace element contents of deformation bands with respect to protolith. Collectively, these observations indicate that these faults have served as, and may still be, zones of preferential vadose zone fluid flow and transport. These processes alter fault rock permeability; affect the transport of solutes via processes such as dissolution, precipitation, and adsorption, and they change the mechanical properties of the fault zone. Smectite and calcite enrichment results in localization of deformation, effectively halting the further development of deformation bands. Vadose zone alteration therefore is one way to produce a clay-rich fault core at early stages in a fault's history, resulting in fault zone weakening and localization of slip. Progressive burial of growth faults can subsequently bring fault cores into the seismogenic zone.