Jennifer Wilson, Laurel Goodwin, and Claudia Lewis
Faults in ignimbrite sequences are of interest for a variety of reasons, including their potential impact on flow and transport. Issues of groundwater recharge and contaminant transport through thick vadose zones in the faulted Bandelier Tuff of Los Alamos, NM, and Calico Hills Tuff at Busted Butte, NV, are of particular societal concern. We have investigated small-displacement faults in these sequences in an effort to determine controls on deformation, characteristics of the resulting fault-zone structures, and the effects of these structures on fault-zone permeability and fluid-fault interaction. Two basic types of fault-zone structures exist at both study sites: shear fractures and deformation bands. The primary control on which of these structures forms in a given unit appears to be the strength and number of contacts between lithic fragments, phenocrysts, and glass shards, which is directly related to degree of welding and inversely proportional to porosity. Low porosity welded units deform by transgranular fracture, resulting in shear fractures. In contrast, high porosity non-welded units deform by cataclastic grain crushing and pore collapse within deformation bands. The magnitude of grain-size reduction in deformation bands in non-welded tuffs is significantly greater than has been documented in sandstone, commonly producing clay-sized fault gouge. Previous work in sand indicates that deformation bands have higher unsaturated permeability than protolith material in dry desert environments, allowing the faults to act as preferential flow paths in the vadose zone. We interpret variably altered fault-zone material in both the Bandelier and Calico Hills tuffs as evidence of preferential wetting of deformation bands above the water table. Deformation bands have locally been cemented by calcite and/or altered to smectite. Fine-grained, low porosity deformation bands may wick fluids into the fault zone, promoting the alteration of unstable volcanic glass to clay. Precipitation of calcite can be facilitated by the high surface area/volume ratio of clay-sized grains within deformation bands.