SPATIAL DISTRIBUTION OF DEFORMATION BANDS IN THE PAJARITO FAULT ZONE, NEW MEXICO: IMPLICATIONS FOR VADOSE ZONE FLUID FLOW

Though previous studies have shown that in the Pajarito fault system (PFS), there are proportionally more small displacement faults than large displacement faults (Carter and Winter, 1995), the distribution of these small displacement faults has not been widely studied. Of particular concern in desert environments are deformation band faults, found in high porosity nonwelded tuffs in the PFS (Wilson et al., 2003). Deformation bands are narrow zones in which grain-size reduction, grain-boundary sliding, and pore collapse accommodate mm-cm displacements. In contrast to fractures, deformation bands reduce porosity and saturated permeability with respect to protolith material and are likely to increase fault-parallel unsaturated permeability in the vadose zone (Sigda et al., 1999). Where found in large enough concentrations, deformation bands may enhance vertical fluid flow to a degree that might raise concerns in areas where contaminants are present in the near-surface environment. Such areas of concern include Los Alamos National Laboratory, located in the Espanola Basin of the Rio Grande Rift (New Mexico). The Pajarito fault (PF) is an active normal fault that bounds the west side of the Espanola basin. Its many splays, extending 50 km along strike, dissect the Pleistocene Bandelier Tuff beneath the Laboratory, while the main fault extends south through Bandelier National Monument. Associated with the PF are numerous small displacement faults, including fracture-based faults in welded tuffs and deformation bands in nonwelded tuffs. Extensive field mapping shows that the concentration of deformation bands is high adjacent to the PF (up to 250/50 m lateral distance), but decreases significantly (~10/50 m) within a kilometer of the fault. As nonwelded units of the Bandelier Tuff are extensive beneath the Pajarito Plateau, the high spatial density of deformation bands in the vicinity of faults with at least 12 m displacement could greatly enhance fluid flow into the subsurface.