Insights on Post-Emplacement Deformation of the Bandelier Tuff, Rio Grande Rift, New Mexico

Many studies of the Tshirege Member of the Bandelier Tuff (1.25 Ma), erupted from the Valles caldera in New Mexico, have attributed deformation of the cooling units almost exclusively to tectonic structures of the Rio Grande rift. We present data collected from a field site, a large excavation at Los Alamos National Laboratory, which indicates the faults, fractures, and fissures within the Bandelier Tuff at this location developed through degassing and cooling of the tuff. The tuff exhibits fissures that are commonly funnel- shaped and upward-flaring, varying in width from centimeters to meters, some with fines-depleted textures characteristic of fossil fumaroles. The unit overlying the funnel-shaped fissures shows intense fracturing in all cases, and units over the fissures also demonstrate evidence of fumarolic alteration through the presence of clays and the reduction of cristobalite and feldspars in the fractured zone. These fossil fumaroles are the focus of most observed faulting with vertical displacements up to 36 cm. Comprehensive analysis of fractures indicates orientations do not match a regional structural grain. All data indicate that the lower unit began actively degassing following emplacement and continued fumarolic activity during the early stages of cooling. Subsequent deposition of younger pyroclastic units sealed the underlying fumarolic pipes. Localized overpressuring, and perhaps some small-scale phreatic explosive activity, occurred, causing partial opening of the fumarolic pipes and disrupting the pipe interiors and the overlying units. Field relations require that all of the units had to be sufficiently cooled by this time to deform in a brittle manner. Thus, the sites of fumarolic activity provided weaker spots in the cooling tuff that accommodated differential settling of the deposits in the form of small-displacement faults. Earliest-formed fractures typically associate with gas-escape structures such as fumaroles, which occur with observable alteration and mineralization. Degassing of ash-flow tuffs may be short lived, but studies of similar thick (100 m) pyroclastic deposits in Nevada show that tuff compaction and welding cease, in most cases, within about 10 years of emplacement. Based on these previous studies, cooling of the upper unit at our field site to temperatures allowing brittle failure may have only taken weeks to months as the unit is only 5-10 m thick. Thus, the fissures, fumaroles, faults and fractures observed in the excavation all likely formed in the first 10 years, and possibly in only a few months, after deposition of the Tshirege Member of the Bandelier Tuff while it degassed, cooled, and compacted.