VARIATIONS IN SMALL FAULT DENSITIES AND STRUCTURAL CHARACTER WITH WELDING IN THE BANDELIER TUFF, NM, AND IMPLICATIONS FOR VADOSE-ZONE FLUID FLOW

The distribution and character (i.e., fractures versus deformation bands) of small faults within and outside the damage zone surrounding a major fault must be determined to accurately establish fault zone architecture and permeability structure. We have documented the spatial distribution and orientation of small faults associated with the Pajarito fault zone, which cuts the Bandelier Tuff in NM. The Pajarito fault zone extends across the town of Los Alamos, as well as Los Alamos National Laboratories (LANL). The Bandelier Tuff includes mechanically distinct cooling units: welded units fail predominantly by fracturing, glassy non-welded units fail through the formation of deformation bands, and crystallized non-welded units form both fractures and deformation bands. We focused on the distribution of structures with distance from the major splays of the Pajarito fault zone in the end-member welded and glassy non-welded units. Data from these units indicate that small-displacement faults and fractures are not only more numerous, but also strike parallel to the main faults within damage zones. Outside the damage zones, small faults are both more widely spaced and less regularly oriented. Further, field mapping shows that small displacement faults and fractures are distributed differently within different mechanical units. Although small faults are clustered around main fault splays in both the glassy non-welded and welded units, damage extends up to 350 m from the main faults in the mechanically weaker non-welded units but only 120 m in the welded unit studied. Also, mapping shows a distinct damage zone asymmetry in the welded units: hanging wall damage zones are significantly wider than footwall damage zones. Two dimensional hydrologic modeling incorporating the fault density data indicates that within 100 m of the main Pajarito fault splays, small faults occur in high enough densities to affect vadose-zone fluid flow. Models show that high densities of small faults enhance flow rates above host rock values, and provide preferential flow paths that could potentially facilitate contaminant transport from LANL to the regional aquifer.